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Acoustics FAQ

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Archive-name: physics-faq/acoustics
Last-modified: 7th September 1997
Version: 0.09

                    *** ACOUSTICS FAQ ***


In order to allow maximum compatibility only ASCII symbols are used

     * To make acoustics accessible to a wider public
     * To encourage cooperation within the acoustics community 

Changes since previous version 
     1.2  Web site revision & additions
     1.3  software revisions
     2.1  addition
     2.9  added and qs following renumbered
     2.10 revised
     2.11 revised
     6.1, 6.4  revised
     6.7  musical intervals added, following renumbered (inc ref 6.10)
     9    address & e-mail additions and revisions

1] Resource Pointers

1.1  What acoustics related news groups and FAQs are there ?
1.2  What World Wide Web sites are there ?
1.3  What acoustics software is available on the Net ?
1.4  What acoustics books and journals are there ?

2] Basic Acoustics

2.1  What is sound ?
2.2  What is a decibel (dB) ?
2.3  How is sound measured ?
2.4  What does dB(A) or "A-weighted" mean ?
2.5  How do sound levels add ?
2.6  How does the ear work ?
2.7  At what level does sound become unsafe ?
2.8  What is sound intensity ?
2.9  How does sound decay with distance ?
2.10  What is the sound power level ?
2.11  What is the speed of sound in air, water .. ?
2.12  What is meant by loudness?

3] Vibration

3.1  What is vibration?
3.2  How is vibration measured ?
3.3  How is vibration isolated and controlled ?

4] Architectural & Building Acoustics

4.1  What is reverberation time ?
4.2  What is the sound absorption coefficient ?
4.3  What is the difference between insulation & absorption ?
4.4  How is sound insulation measured ?
4.5  How do I improve the noise insulation of my house/dwelling ?

5] Reserved

6] Miscellaneous Questions

6.1  What is active noise control ?
6.2  What causes a sonic boom ?
6.3  Can you focus sound ?
6.4  What is sonoluminescence ?
6.5  Why does blowing over a bottle make a note ?
6.6  What is pitch ?
6.7  What are musical intervals?
6.8  What causes "helium voice" ?
6.9  What is structural acoustics ?
6.10 What is the Doppler effect ?
6.11 What is white noise, pink noise ?


8] Various Tables

8.1  Formula for A weighting

9] List of National Acoustic Societies  

1]  Resource Pointers

***  1.1  What acoustic related news groups and FAQs are there ?

news groups

news:alt.sci.physics.acoustics - started by Angelo Campanella - now the
principal group for discussion of acoustics topics. Ang's CV is at URL

news:sci.physics - general physics but occasionally acoustics related
questions are posted. - includes discussion on audio equipment, speakers
etc. There are other groups which may be of interest. and - groups
for sufferers of these complaints

news:bionet.audiology - matters relating to hearing and hearing loss

news:bit.listserv.deaf-l  news:uk.people.deaf  news:alt.society.deaf
- usenet seems an ideal communication medium.

news:comp.dsp - the group for people interested in computing digital
signal processing solutions, FFTs FIRs IIRs etc. 

news:comp.speech - speech recognition and simulation - various discussion of use of
internal soundcards in IBM compatible computers.

The main archive site for all usenet FAQs is

A list of sites (including html) for the Acoustics FAQ is at
The Active Noise Control FAQ by Chris Ruckman is at
The Tinnitus FAQ deals with a range of hearing disorders. It is
maintained by Mark Bixby and available at
The Audio FAQ, with everything you ever wanted to know about the
subject, from preamplifiers to speakers and listening room acoustics.
It is located in the pub/usenet/* directories
The comp.speech faq maintained by Andrew Hunt has information on speech
processing and some software links
***  1.2  What World Wide Web sites are there ?

Many acoustical web resources can be found from links in the first two
locations or the "search engines" listed below.
     (virtual lib for acoustics & vibration with useful links) 
     (wide selection of acoustics related links)
     (science questions and answers)
     (simple acoustics introduction from David Worrall)
     (theoretical basic acoustics lecture notes; difficult stuff like
     the wave equation etc, in hypertext for browsing, or gzipped    
     Postscript format for downloading)
     (Acoustical Society of America home page with several links and 
     comprehensive career section, book lists and Society info etc)
     (Angelo Farina has published a variety of papers - some are
     available in zipped MSWord format)
     (European Acoustics Association)
     (Institute of Noise Control Engineering home page)
     (Steve Ekblad's extensive audio related BBS and Internet list)
     (Technical societies, conferences etc etc but not specifically
     acoustics related)
     (main ISO standards page)
     (national standards organizations addresses)
     (official ANSI site)

The Digital Equipment Corporation has an extremely powerful Advanced
Search facility at:

alternatively try searches on: (can also be used as Usenet posting gateway)

or use your nearest Archie site to look for files you want.

***  1.3  What acoustics software is available on the Net ?

A range of programs available for downloading from the Simtel archive.

Spectrogram 3.2 - Accurate realtime Win95 spectrum analysis program
(freeware) by Richard Horne is at a few sites including:

The comp.speech faq has several links to speech related software
including speech recognition and text to speech programs.

There are a few programs for various platforms listed at URL
The programs listed are mainly for sound analysis and editing.

Some software is available for audio systems design at URL  

Odeon is a program for architectural acoustics. A demonstration version
is available by ftp. The demo includes a large database for
coefficients of absorption. A web page at URL
describes the capabilities of the program and gives the ftp address.

Also some interactive acoustics software (eg room acoustics, RT,
decibel conversion etc) is available at a couple of sites.

***  1.4  What acoustics books and journals are there ?

There is a large range of books available on the subject. Generally the
choice of book will depend on which approach and subject area is of
interest. A few books are listed below:

>>Introduction to Sound
>>Speaks, C
Good foundation for acoustics principles 

>>Acoustics Source Book
>>Parker, S (editor)
Basic introductory articles on many topics discussed in the
alt.sci.physics.acoustics group. Old book - technology a bit dated.

>>The Science of Sound
>>Rossing, T
Introductory book on acoustics, music and audio

>>Fundamentals of Acoustics
>>Kinsler, L  Frey, A  et al.
Good overall coverage of acoustics but includes lots of theory

>>Acoustics ...
>>Pierce, A
Classic advanced text - lots of theory

>>Engineering Noise Control
>>Bies, D & Hansen, C
Practically biased with examples. Partially updated and corrected.  

>>Handbook of Acoustical Measurements and Noise Control
>>Harris C  (editor)
Comprehensive practical reference book. 

A list of recently reviewed noise-related books is at URL

Some Journals
Journal of the Acoustical Society of America (monthly)
Noise Control Engineering (US - every 2 months)
Acoustics Bulletin (UK - every 2 months)
Acta Acustica (P.R.China)
Acta Acustica / Acustica (Europe - 6 per year)
Journal of the Acoustical Society of Japan (E) (English edn - 2 months)
Acoustics Australia (3 per year)
Journal of Sound & Vibration (UK - weekly)
Journal of the Audio Engineering Society (US - 10 per year)
Applied Acoustics (UK - 12 per year)


|  Definitions used:
|  10^(-5) indicates 10 raised to the power of minus 5
|  1.0E-12 indicates 1.0 x 10^(-12)
|  1 pW indicates 1 picowatt i.e. 1.0E-12 Watt
|  W/m^2 indicates Watts per square metre
|  lg indicates logarithm to base 10
|  sqrt indicates the square root of
|  pi = 3.142                                          
|  Lw is sound power level, the w is subscripted 

2]  Basic Acoustics

***  2.1  What is sound ?

Sound is the quickly varying pressure wave within a medium.
We usually mean audible sound, which is the sensation (as detected by
the ear) of very small rapid changes in the air pressure above and
below a static value. This "static" value is atmospheric pressure
(about 100,000 Pascals) which does nevertheless vary slowly, as shown
on a barometer. Associated with the sound pressure wave is a flow of
energy. Sound is often represented diagrammatically as a sine wave, but
physically sound (in air) is a longitudinal wave where the wave motion
is in the direction of the movement of energy. The wave crests can be
considered as the pressure maxima whilst the troughs represent the
pressure minima.

How small and rapid are the changes of air pressure which cause sound?
When the rapid variations in pressure occur between about 20 and 20,000
times per second (ie at a frequency between 20Hz and 20kHz) sound is
potentially audible even though the pressure variation can sometimes
be as low as only a few millionths of a Pascal. Movements of the ear
drum as small as the diameter of a hydrogen atom can be audible! Louder
sounds are caused by greater variation in pressure - 1 Pascal, for
example, will sound quite loud, provided that most of the acoustic
energy is in the mid-frequencies (1kHz - 4kHz) where the ear is most

What makes sound?
Sound is produced when the air is disturbed in some way, for example
by a vibrating object. A speaker cone from a hi-fi system serves as a
good illustration. It may be possible to see the movement of a bass
speaker cone, providing it is producing very low frequency sound. As
the cone moves forward the air immediately in front is compressed
causing a slight increase in air pressure, it then moves back past its
rest position and causes a reduction in the air pressure (rarefaction).
The process continues so that a wave of alternating high and low
pressure is radiated away from the speaker cone at the speed of sound. 

 ***  2.2  What is a decibel (dB) ?

The decibel is a logarithmic unit which is used in a number of
scientific disciplines. In all cases it is used to compare some
quantity with some reference value. Usually the reference value is the
smallest likely value of the quantity. Sometimes it can be an
approximate average value.

In acoustics the decibel is most often used to compare sound pressure,
in air, with a reference pressure. References for sound intensity,
sound power and sound pressure in water are amongst others which are
also commonly in use. 

Reference sound pressure (in air) = 0.00002 = 2E-5 Pa (rms)
     "      "   intensity         = 0.000000000001 = 1E-12 W/m^2
     "      "     power           = 0.000000000001 = 1E-12 W
     "      "   pressure (water)  = 0.000001 = 1E-6 Pa  

Acousticians use the dB scale for the following reasons:

  1) Quantities of interest often exhibit such huge ranges of
  variation that a dB scale is more convenient than a linear
  scale.  For example, sound pressure radiated by a submarine may
  vary by eight orders of magnitude depending on direction.
  2) The human ear interprets loudness on a scale much closer to
  a logarithmic scale than a linear scale.

***  2.3  How is sound measured ?

A sound level meter is the principal instrument for general noise
measurement. The indication on a sound level meter (aside from
weighting considerations) indicates the sound pressure, p, as a level
referenced to 0.00002 Pa.

          Sound Pressure Level = 20 x lg (p/0.00002) dB

Peak levels are occasionally quoted. During any given time interval
peak levels will be numerically greater, and often much greater than
the (rms) sound pressure level.

***  2.4  What does dB(A) or "A-weighted" mean ?

Noise was not of particular concern at the beginning of the century.
The first electrical sound meter was reported by George W Pierce in
Proceedings of the American Academy of Arts and Sciences, v 43 (1907-8)
A couple of decades later the switch from horse-drawn vehicles to
automobiles in cities led to large changes in the background noise
climate. The advent of "talkies" -  film sound - was a big stimulus to
sound meter patents of the time, but there was still no standard method
of sound measurement.

The first tentative standard for sound level meters (Z24.3) was
published by the American Standards Association in 1936, sponsored by
the Acoustical Society of America. The tentative standard shows two
frequency weighting curves "A" and "B" which were modelled on the ear's
response to low and high levels of sound respectively.

The most common weighting today is "A-weighting" dB(A), which is very
similar to that originally defined as Curve "A" in the 1936 standard.
"C-weighting" dB(C), which is used occasionally, has a relatively flat
response. "U-weighting" is a recent weighting which is used for
measuring audible sound in the presence of ultrasound, and can be
combined with A-weighting to give AU-weighting. The A-weighting formula
is given in section 8 of the FAQ. 

In addition to frequency weighting, sound pressure can be weighted in
time with fast, slow or impulse response. Measurements of sound
pressure level with A-weighting and fast response are also known as the
"sound level". 

Some sound level meters can measure the average sound level of a noise
over a given time. It is called the equivalent continuous sound level
(L sub eq) and is A-weighted but not time weighted.

***  2.5  How do sound levels add ?

If there are two sound sources in a room - for example a radio
producing an average sound level of 62.0 dB, and a television producing
a sound level of 73.0 dB - then the total sound level is a logarithmic
sum ie

     Combined sound level = 10 x lg ( 10^(62/10) + 10^(73/10) )

                          = 73.3 dB

Note: for two different sounds, the combined level cannot be more than
3 dB above the higher of the two sound levels. However, if the sounds
are phase related there can be up to a 6dB increase in SPL. 

***  2.6  How does the ear work ?

The eardrum is connected by three small jointed bones in the air-filled
middle ear to the oval window of the inner ear or cochlea, a fluid-
filled spiral coil about one and a half inches in length. Over 10,000
hair cells on the basilar membrane along the cochlea convert minuscule
movements to nerve impulses, which are transmitted by the auditory
nerve to the hearing center of the brain. 

The basilar membrane is wider at its apex than at its base, near the
oval window, whereas the cochlea tapers towards its apex. Different
groups of the delicate hair sensors on the membrane, which varies in
stiffness along its length, respond to different frequencies
transmitted down the coil. The hair sensors are one of the few cell
types in the body which do not regenerate. They may therefore become
irreparably damaged by large noise doses. Refer to the Tinnitus FAQ for
more information on hearing disorders.

***  2.7  At what level does sound become unsafe ?

It is best, where possible, to avoid any unprotected exposure
to sound pressure levels above 100dB(A). Use hearing protection when
exposed to levels above 85dB(A), especially if prolonged exposure is
expected.  Damage to hearing from loud noise is cumulative and is
irreversible. Exposure to high noise levels is also one of the main
causes of tinnitus. The safety aspects of ultrasound scans are the
subject of ongoing investigation.

There are other health hazards from extended exposure to vibration. An
example is "white finger", which is found amongst workers who use hand-
held machinery such as chain saws. 

***  2.8  What is sound intensity ?

This may be defined as the rate of sound energy transmitted in a
specified direction per unit area normal to the direction. With good
hearing the range is from about 0.000000000001 Watt per square metre
to about 1 Watt per square metre (12 orders of magnitude greater). The
sound intensity level is found from intensity I (W/m^2) by:

          Sound Intensity Level = 10 x lg (I/1.0E-12) dB

Note: 1.0E-12 W/m^2 normally corresponds to a sound pressure of about
2.0E-5 Pascals which is used as the datum acoustic pressure in air.

Sound intensity meters are becoming increasingly popular for
determining the quantity and location of sound energy emission.

***  2.9  How does sound decay with distance ?

The way sound changes with distance from the source is dependent on the
size and shape of the source and also the surrounding environment and
prevailing air currents. It is relatively simple to calculate provided
the source is small and outdoors, but indoor calculations (in a
reverberant field) are rather more complex.

If the noise source is outdoors and its dimensions are small compared
with the distance to the monitoring position (ideally a point source),
then as the sound energy is radiated it will spread over an area which
is proportional to the square of the distance. This is an 'inverse
square law' where the sound level will decline by 6dB for each doubling
of distance.

Line noise sources such as a long line of moving traffic will radiate
noise in cylindrical pattern, so that the area covered by the sound
energy spread is directly proportional to the distance and the sound
will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow
the above laws because the spread of energy is less, and smaller
changes of sound level with distance should be expected.

In addition it is always necessary to take into account attenuation due
to the absorption of sound by the air, which may be substantial at
higher frequencies. For ultrasound, air absorption may well be the
dominant factor in the reduction.

***  2.10  What is the sound power level ?

Sound power level, Lw, is often quoted on machinery to indicate
the total sound energy radiated per second. The reference power is
taken as 1pW.

For example, a lawn mower with sound power level 88dB(A) will produce
a sound level of about 60dB(A) at a distance of 10 metres. If the sound
power level was 78dB(A) then the lawn mower sound level would be only
50dB(A) at the same distance.

***  2.11  What is the speed of sound in air, water .. ?

The speed of sound in air at a temperature of 0 degC and 50% relative
humidity is 331.6 m/s. The speed is proportional to the square root of
absolute temperature and it is therefore about 12 m/s greater at 20
degC. The speed is nearly independent of frequency and atmospheric
pressure but the resultant sound velocity may be substantially altered
by wind velocity.

A good approximation for the speed of sound in other gases at standard
temperature and pressure can be obtained from

               c = sqrt (gamma x P / rho) 

where gamma is the ratio of specific heats, P is 1.013E5 Pa and rho is
the density.

The speed of sound in water is approximately 1500 m/s. It is possible
to measure changes in ocean temperature by observing the resultant
change in speed of sound over long distances. The speed of sound in an
ocean is approximately:

c = 1449.2 + 4.6T - 0.055T^2 + 0.00029T^3 + (1.34-0.01T)(S-35) + 0.016z

T temp in degrees Celsius, S salinity in parts per thousand
z is depth in meters

See also CRC Handbook of Chemistry & Physics for some other substances
and Dushaw & Worcester JASA (1993) 93, pp255-275 for sea water.

***  2.12  What is meant by loudness?

Loudness is the human impression of the strength of a sound. The
loudness of a noise does not necessarily correlate with its sound
level. Loudness level of any sound, in phons, is the decibel level of
an equally loud 1kHz tone, heard binaurally by an otologically normal
listener. Historically, it was with a little reluctance that a simple
frequency weighting "sound level meter" was accepted as giving a
satisfactory approximation to loudness. The ear senses noise on a
different basis than simple energy summation, and this can lead to
discrepancy between the loudness of certain repetitive sounds and their
sound level.

A 10dB sound level increase is considered to be about twice as loud in
many cases. The sone is a unit of comparative loudness with 0.5 sone=30
phons, 1 sone=40 phons, 2 sones=50 phons, 4 sones = 60 phons etc. The
sone is inappropriate at very low and high sound levels where
subjective perception does not follow the 10dB rule. 

Loudness level calculations take account of "masking" - the process by
which the audibility of one sound is reduced due to the presence of
another at a close frequency. The redundancy principles of masking are
applied in digital audio broadcasting (DAB), leading to a considerable
saving in bandwidth with no perceptible loss in quality. 


3]  Vibration

***  3.1  What is vibration ?

When something oscillates about a static position it can be said to
vibrate. The vibration of a speaker diaphragm produces sound, but
usually vibration is undesirable. Common examples of unwanted vibration
are the movement of a building near a railway line when a train passes,
or the vibration of the floor caused by a washing machine or spin
dryer. Floor vibration can be reduced with vibration isolators; however
there is often a penalty to pay in the form of a slight increase in the
machinery vibration and its consequent deterioration.

***  3.2  How is vibration measured ?
Vibration is monitored with an accelerometer. This is a device that is
securely attached by some means to the surface under investigation. The
accelerometer produces a tiny electrical charge output, proportional
to the surface acceleration, which is then amplified by a charge
amplifier and recorded or observed with a meter. The frequencies of
interest are generally lower than sound, and range from below 1 Hz to
about 1 kHz. 

It is sometimes more useful to know the velocity or displacement rather
than the acceleration. In the case of velocity, it is necessary to
integrate the acceleration signal. A second integration will provide
a displacement output. If the vibration is sinusoidal at a known
frequency, f, then an integration is easily calculated by dividing the
original by 2 x pi x f (noting that there is a phase change)

Example: A machine is vibrating sinusoidally at 79.6 Hz with an rms
acceleration of 10 m/s^2.
Its rms velocity is therefore 10/(2 x pi x 79.6) = 20 mm/s 
Its rms displacement is   10/(4 x pi^2 x 79.6^2) = 0.04 mm  

***  3.3  How is vibration isolated and controlled ?

Vibration problems are solved by considering the system as a number of
springs and masses with damping. It is sometimes possible to reduce the
problem to a single mass supported by a spring and a damper. 

If the vibration is produced by a motor inside a machine, it is usually
desirable to ensure that the frequency of motor oscillations (the
forcing frequency) is well above the frequency of the natural resonance
of the machine on its support. This is achieved by altering the mass
or stiffness of the system as appropriate.

The method of vibration isolation is very easy to demonstrate with a
weight held from a rubber band. As the band is moved up and down very
slowly the suspended weight will move by the same amount. At resonance
the weight will move much more, but as the frequency is increased still
further the weight will become almost stationary. In practical
circumstances springs are more likely to be used in compression than
tension, but the principles are exactly the same.

A further method of vibration control is to attempt to cancel the
forces involved using a Dynamic Vibration Absorber. Here an additional
"tuned" mass-spring combination is added so that it exerts a force
equal and opposite to the unwanted vibration. They are only appropriate
when the vibration is of a fixed frequency.

Active vibration control, using techniques akin to active noise
control, is now coming into use.

Intuitive attempts to reduce vibration from machinery can sometimes
instead aggravate the problem. This is especially true when care was
originally taken to minimize vibration at the time of design,
manufacture and installation.


4]  Architectural & Building Acoustics

***  4.1   What is reverberation time ?

Work on room acoustics was pioneered by Wallace Clement Sabine 1868-
1919 (see his Collected Papers on Acoustics, 1922).
The reverberation time, T, is defined as the time taken for sound
energy to decay in a room by a factor of one million (ie by 60 dB). It
is dependent on the room volume and its total absorption.

In metric units

                              0.161 x room Volume 
          T =  ----------------------------------------------
               sum of Surface areas x absorption coefficients

***  4.2  What is the sound absorption coefficient ?

The absorption coefficient of a material is ideally the fraction of the
randomly incident sound power which is absorbed, or otherwise not
reflected. It can be determined in two main ways, and there are often
variations in the results depending upon the method of measurement
chosen. It is standard practice to measure the coefficient at the
preferred octave frequencies over the range of at least 125Hz - 4kHz.

For the purposes of architectural design, the Sabine coefficient
(calculated from reverberation chamber measurements) is preferred.
Interestingly some absorbent materials are found to have a Sabine
coefficient in excess of unity at higher frequencies. This is due to
edge effects and when this occurs the value can be taken as 1.0 

The Odeon computer program includes a file of absorption coefficients.

***  4.3  What is the difference between insulation & absorption ?

There is often confusion between sound insulation and sound absorption.

Sound insulation is required in order to eliminate the sound path from
a source to a receiver such as between apartments in a building, or to
reduce unwanted external noise inside a concert hall. Heavy materials
like concrete tend to be the best materials for sound insulation -
doubling the mass per unit area of a wall will improve its insulation
by about 6dB. It is possible to achieve good insulation with much less
mass by instead using a double leaf partition (two separated
independent walls). 

Sound absorption occurs when some or all of the incident sound energy
is either converted into heat or passes through the absorber. For this
reason good sound absorbers do not of themselves make good sound
insulators. Although insulation and absorption are different concepts,
there are many instances where the use of sound absorbers will improve
insulation. However absorption should not be the primary means of
achieving good sound insulation. 

***  4.4   How is sound insulation measured ?

The measurement method depends on the particular situation. There are
standards for the measurement of the insulation of materials in the
laboratory, and for a number of different field circumstances. Usually
the procedures involve generating a loud sound of a specified type and
monitoring the transmitted noise.

It is very useful to have a single number to characterize the
insulation of a partition. Measurements are often conducted in third-
octaves, and the reduction plotted on a graph. A reference curve is
then fitted to the measurements using a specified procedure, and the
value of this curve at 500 Hz is taken as the figure. There is a slight
difference in procedure between the U.S. and ISO standards, but the
methods are basically similar. The same is also true for impact noise
transmission assessment, where a standard tapping machine is in use to
hammer floors. Sound pressure levels in the room below are monitored.

***  4.5  How do I improve the noise insulation of my house/dwelling?

This is one of the most commonly asked questions of noise consultants. 
Firstly you should consider whether better insulation is really
essential. The method of noise insulation will depend on the exact
situation, so the advice of a competent person should be sought at an
early stage. Sound insulation is most often asked for in order to keep
out unwanted noise, but is occasionally requested for the purpose of
minimizing disturbance to others. The following ideas may serve as

When the noise is from an external source such as a main road it may
be possible, if planning authorities permit, to screen with a noise
barrier. These can be effective providing that the direct line of sight
between traffic and house is concealed by the barrier.

The weak point for sound transmission to and from a building is most
often via the windows. Double glazing will usually afford noticeably
better protection than single glazing, but in areas of high external
noise it might be preferable to have double windows with a large air
gap and acoustic absorbent material in the reveals. A drawback of
improving external insulation is that, for some people, the resultant
lower background level can itself be disturbing; it can also make noise
transmission through party walls more apparent. The fitting of new
windows may reduce the level of air ventilation, and it will be vital
to compensate for this, if necessary with a noise attenuating system.

You may also need to consider noise penetration through the roof,
floors, ceilings and walls.

Noise through party walls can be reduced by the addition of a false
wall. This is constructed from a layer of sound insulating material,
commonly plasterboard, separated from the party wall by a large void
containing acoustic quilting. The false wall must not be connected to
the party wall because that would allow sound transmission paths. The
quality of construction is an important consideration if optimal levels
of attenuation are desired. It is advisable to contact an independent
noise consultant before allowing any building works to commence. 


6]  Miscellaneous Questions

***  6.1  What is active noise control ?

ANC is an electronic method of reducing or removing unwanted sound by
the production of a pressure wave of equal amplitude but opposite sign
to the unwanted sound. When the electronically produced inverse wave
is added to original unwanted sound the result is sound cancellation.

This method of noise control is becoming increasingly popular for a
variety of uses. It is sometimes considered a miracle "cure-all" for
noise problems which, at the present time, is not the case. For example
noise cancellation in 3D spaces, such as living areas, is very
difficult to achieve. However it can be more successful locally, eg for
a passenger sitting in an aircraft or car. There are many institutions
and companies around the world working on the technology to increase
the circumstances where ANC can be used effectively. The award winning
Active Noise Control FAQ is maintained by Chris Ruckman and available
at a number of sites worldwide including:

***  6.2  What causes a sonic boom ?

(from "Aircraft Noise" by Michael T Smith, Cambridge, 1989)

" ..   When the speed of an aircraft is supersonic, the pressure waves
cannot get away ahead of the aircraft as their natural speed is slower
than that of the aircraft. Slower, in this context, means just over
1200 km/hr at sea level and about 10% less at normal cruising altitude.
Because they cannot get away, the pressure disturbances coalesce and
lag behind the aeroplane, which is in effect travelling at the apex of
a conical shock wave. The main shock wave is generated by the extreme
nose of the aeroplane, but ancillary shocks are generated by all the
major fuselage discontinuities.  .. "

Ken Plotkin ( on 24th July 1995 wrote:

[snip] .. A body moving through the air pushes the air aside. Small
disturbances move away at the speed of sound.  Disturbances from a
slowly moving body go out in circles, like ripples from a pebble in a
pond. If the body moves faster, the circles are closer in the direction
of travel. If the body is supersonic, then the circles overlap.  The
envelope of circles forms a cone.  The angle of the cone is determined
by its vertex moving in the body's travel direction at the body's
speed, while the circles grow at the sound speed.  [snip]  The
existence of the "Mach cone", "Mach waves" and the corresponding angle,
was discovered by Ernst Mach in the nineteenth century. [snip]

***  6.3  Can you focus sound ?

Sound can be focused like light, but in the case of sound the "optics"
must be much larger because you are dealing with longer wavelengths.
The effect is heard in some domed buildings such as the Capitol in
Washington, and St Paul's Cathedral in London providing noise
background conditions permit.

Large parabolic reflectors can be used very effectively to send and
receive sound over significant distances. Check out your local science
museum or exploratorium - there may be a demonstration. It is also
possible to refract sound and focus it using a lens. The lens is
constructed from a large thin bubble, say 2 metres across, filled with
carbon dioxide. The effect is not very pronounced.

Sound can be directed by making use of constructive and destructive
interference. This idea is used in column speakers, and commercial
systems for reducing noise levels outside the dance floor area of

***  6.4  What is sonoluminescence ?

In the early 1930s Frenzel and Schultes discovered that photographic
plates became "fogged" when submerged in water exposed to high
frequency sound. More recent experiments have succeeded in suspending
a single luminous pulsating bubble in a standing wave acoustic field,
visible in an undarkened room. Generally sonoluminescence is light
emission from small cavitating bubbles of air or other gas in water or
other fluids, produced when the fluid is acted upon by intense high
frequency sound waves. The mechanism is not completely understood, but
very high pressures and temperatures are thought to be produced at the
centre of the collapsing bubbles.

See "Science" 14 October 1994 page 233, "Scientific American"
(International Edition) February 1995 Page 32 or "Physics Today"
September 1994 Page 22, all quite readable articles. 

See also the following URLs:

James Davison ( on 28th June 1995 wrote:

[snip] .. I have been sufficiently interested to reconstruct the
apparatus for producing this effect -- using a pair of piezoelectric
transducers, an old oscilloscope and a signal wave generator --
materials costing only a few hundred dollars.

I am proud to say that tonight I managed to reproduce this effect --
the tiny bubble has the appearance of a tiny blue star trapped in the
middle of the flask.  It is distinctly visible to the unadapted eye in
a dark room, and it is a very startling thing to see. [snip]

***  6.5  Why does blowing over a bottle make a note ?

Resonance in acoustics occurs when some mass-spring combination is
supplied with energy. Many musical instruments rely on air resonance
to improve their sonority. If you blow across the mouth of a bottle you
can often get a note. The bottle behaves as a Helmholtz resonator. The
main volume of air inside the bottle is analogous to a spring, whilst
the "plug" of air in the neck acts as an attached mass. The resonant
frequency is roughly given by:

               f =  { c sqrt (S/LV) } / 2pi
c is velocity of sound
S is the surface area of the neck opening
V is bottle volume
L is the effective length of the neck ie the actual length plus ends
correction. Ends correction ~ 1.5 times radius of neck opening

Example: A 75 cl (7.5E-4 m^3) wine bottle with neck diameter 19 mm, 
bottle neck length 8 cm, air temp = 20 degC 
calculated resonance = 109Hz (actual resonance was 105Hz)

Helmholtz resonators are sometimes employed as a means of passive noise
control in air conditioning ducts. They may also be hidden in the wall
design of auditoria and offices in order to improve the acoustics.

***  6.6  What is pitch ?

The term "pitch" has both a subjective and an objective sense.
Concert pitch is an objective term corresponding to the frequency of
a musical note A (at present 440Hz). Using such a standard will define
the pitch of every other note on a particular musical scale. For
example, with Equal Temperament each semitone is higher or lower in
frequency than the previous semitone by a factor of 2^(1/12). An octave
is a pitch interval of 2:1. Many sounds with no obvious tonal
prominence are considered by musicians to be of indeterminate pitch;
for example, the side drum, cymbals, triangle, castanets, tambourine,
and likewise the spoken word.
Pitch is also a subjective frequency ordering of sounds. Perceived
pitch is dependent on frequency, waveform and amplitude or changing
amplitude. Numbers can be assigned to perceived pitch relative to a
pure frontal tone of 1000Hz at 40dB (1000 mels) thereby establishing
a pitch scale.

Further info and examples on pitch from URL:

***  6.7  What are musical intervals ?

An interval is the ratio in frequency between musical notes. These
intervals are sometimes called a second, third, fourth, fifth etc.
which refers to the position on the scale that the note is to be found.
In the scale of C major: C D E F G A B C, the note 'E' is the third
note of the scale and the interval from C to E is therefore called a
third. For the scale D major: D E F# G A B C# D, the third will be F#.
The term 'interval' can also be used to indicate that the notes are
sounded together, in which case there are consonant intervals and
dissonant intervals.

The ratio of frequency intervals for Just Intonation is demonstrated
below in the scale of C major, though the same ratios apply to all the
major keys:
C         <- Octave

The interval between E & F and between B & C is a semitone, whilst the
other intervals are tones. The interval between any two notes above can
be found by multiplying the intervening ratios; thus if all the above
ratios are multiplied together the resultant is 2 because an octave is
twice the original frequency.

The notes of minor scales differ from their major counterparts; one
important difference being the flattened third. E flat is a minor third
above the note C. 

The use of Just Temperament causes serious problems of intonation when
music modulates between keys. Equal Temperament is nearly always used
as a compromise to the problem of tuning (see question 6.6).     

***  6.8  What causes "helium voice" ?

Many people, on hearing the voice of someone who has breathed helium,
believe that the person's speech pitch has increased.

WARNING - Breathing helium can be very dangerous.
A cavity will have certain resonant frequencies. These frequencies
depend on the shape and size of the cavity and on the velocity of sound
within the cavity. Human vocal cords vibrate non-sinusoidally in the
vocal tract, giving rise to a range of frequencies above the
fundamental. The vocal tract mainly enhances lower frequency components
imparting the recognizable voice spectrum.

The velocity of sound in helium is much greater than in air, so
breathing helium will raise the vocal tract's resonant frequencies.
Although the vocal cords' vibrational frequencies are little affected
by helium, the effect of higher cavity resonances is to alter
substantially the relative amplitudes of the voice spectrum components
thus leading to apparent pitch change. 

***  6.9  What is structural acoustics ?

Structural acoustics is concerned with the coupled dynamic response of
elastic structures in contact with non-flowing fluids.  (The fluid,
although non-flowing, undergoes small-amplitude vibration relative to
some equilibrium position.)  For heavy fluids like water, the coupling
is two-way, since the structural response is influenced by the fluid
response, and vice versa.  For lighter fluids like air, the coupling
may be either one-way (where the structural vibration affects the fluid
response, but not vice versa) or two-way (as occurs, for example, in
the violin).

Structural acoustics problems of interest involving water include the
vibration of submerged structures, acoustic radiation from
mechanically-excited, submerged, elastic structures; acoustic
scattering from submerged, elastic structures (e.g., sonar echoes);
acoustic cavity analysis; and dynamics of fluid-filled elastic
piping systems.  These problems are of interest for both time-harmonic
(sinusoidal) and general time-dependent (transient) excitations. Water
hammer in pipes can be thought of as a transient structural acoustics

Structural acoustics problems of interest involving air include
determining and reducing noise levels in automobile and airplane

Reference (for simple geometry problems):
"Sound, Structures, and Their Interaction," Second Edition, by M.C.
Junger and D. Feit, MIT Press, Cambridge, Mass (1986).

***  6.10  What is the doppler effect ?

When a sound source is moving, a stationary observer will detect a
different frequency to that which is produced by the source. The speed
of sound in air is approximately 340 m/s (see 2.11). The wavelength of
the sound emitted will be foreshortened in the direction of motion by
an amount proportional to the velocity of the source. Conversely the
wavelength of a receding sound source will increase. The doppler effect
may be noticed as a marked drop in pitch when a  vehicle passes at high

Example 1: A sound source, S, emits 1000 waves per second (1 kHz) and
is moving directly towards an observer, O, at a speed of 100 metres per
second (equivalent to approx 225 miles per hour).

After 1 second the wave front, which is travelling at the speed of
sound, will have travelled 340 metres from the original source
position. Also after that second the sound source will have moved 100
metres towards the observer.

  0 m                                            340 m
S |     |     |     |     |     |     |     |     |          O
  <--------------  1000 waves   ------------------>

                 100 m                           340 m
                S |   |   |   |   |   |   |   |   |          O
                  <-------  1000 waves   ---------> 

Therefore the same number of waves will occupy a space of 340-100 = 240
metres and the wavelength will be 240/1000 = 0.24 metres.
To the observer the frequency heard will be the speed of sound divided
by its wavelength = 340/0.24 = 1416.7 Hz. 

Example 2: An observer moving at 100 metres per second directly
approaches a stationary sound source, S, which is emitting 1000 waves
per second (1 kHz). In this example there is no change in wavelength.
In one second, the observer will hear the number of waves emitted per
second plus the number of waves which s/he has passed in the time
(1000+100/0.34) = 1294.1 Hz.

Note the interesting result - a stationary observer with moving source
will not hear the same frequency as a would a moving observer with
stationary source.

***  6.11  What is white noise, pink noise ?

The power spectral density of white noise is independent of frequency.
Since there is essentially the same energy between any two identical
frequency intervals (for example 84-86Hz and 543-545Hz), white noise
narrow band FFT analysis will show as flat. However octave band
analysis will show the level to rise by 3dB per octave because each
band has twice the frequency range of the preceding octave.

Pink noise is often produced by filtering white noise and has the same
power within each octave. Narrow band analysis will show a fall in
level with increasing frequency, but third-octave band or octave band
analysis will be flat. 

see Joseph S. Wisniewski's Colors of noise FAQ at:-


A-weighting 2.4 2.12 8.1
absorption coefficient 4.1 4.2
accelerometer 3.1
acoustic energy 2.1 2.8 2.10 4.1 4.3
Acoustical Society of America 2.4
active noise control 6.1 
active vibration control 3.3
addition of sound 2.5
air absorption 2.9
ANC 6.1
atmospheric attenuation 2.9
atmospheric pressure 2.1 2.11
audibility 2.1 2.12
column speaker 6.3
concert pitch 6.6
dB(A) 2.4 8.1
decibel (dB) 2.2 2.3 2.4
Doppler effect 6.10
dynamic vibration absorber 3.3
ear 2.1 2.2 2.6 2.7 
elastic structures 6.9
equal temperament 6.6 6.7
equivalent continuous sound level 2.4
focusing sound 6.3
frequency 2.1 2.4 2.12 6.6 6.7
hearing conservation 2.7
hearing damage 2.6 2.7
Helmholtz resonator 6.5
historical notes 2.4 2.12
insulation 4.3 4.4 4.5
interference 6.3
interval (music) 6.6 6.7
inverse square law 2.9
just intonation 6.7
Leq 2.4
logarithmic scale 2.2 2.3
loudness 2.1 2.2 2.12
loudspeaker 2.1 6.3
longitudinal wave 2.1
Lw 2.10
major and minor keys 6.7
masking 2.12
mel 6.6
musical scale 6.6 6.7
ocean sound velocity 2.11
octave 6.6 6.11
pascal 2.1 2.2 2.8
passive noise control 6.1 6.5
peak level 2.3
phon 2.12
physical constants
Pierce, George W 2.4
pink noise 6.11
pitch 6.6 6.8
resonance 6.5 6.8
reverberation time 4.1
Sabine, Wallace C 4.1
semitone 6.6 6.7
sone 2.12
sonic boom 6.2
sonoluminescence 6.4
sound 2.1
sound absorption 4.1 4.2 4.3
sound cancellation 6.1
sound decay 2.9
sound insulation 4.3 4.4 4.5
sound intensity 2.2 2.8
sound intensity meter 2.8
sound level 2.4 2.5 2.12
sound level meter 2.3 2.4 2.8 2.12
sound power level 2.10
sound pressure 2.1 2.2
sound pressure level 2.3 2.4 2.5
speech 6.6 6.8
speaker 2.1 6.3
speed of sound 2.1 2.11 6.8 6.10
structural acoustics 6.9
supersonic 6.2
tapping machine 4.4
third-octave band 6.11
tinnitus 2.6 2.7
ultrasound 2.9
ultrasound scans 2.7
velocity of sound 2.1 2.11 6.8 6.10
vibration 2.1 2.7 3.1,3.2
vibration control 3.3
voice 6.6 6.8
wave 2.1
weighting 2.4 2.12 8.1
white finger 2.7
white noise 6.11


8]  Various Tables


A weighting can be found from the following formulae

For A-weighting: A(f) =

                              12200^2 f^4
(f^2 +20.6^2) (f^2 +12200^2) (f^2 +107.7^2)^0.5 (f^2 +737.9^2)^0.5

The weighting in dB relative to 1000Hz is now given by

          20 lg -------        note: A(1000) = 0.794

In tables, octave and third-octave frequencies are given as nominal
values, for example 1250 Hz or 2500 Hz. Ideally weightings should be
calculated for the exact frequencies which may be determined from the
formula 1000 x 10^(n/10), where n is a positive or negative integer.
Thus the frequency shown as 1250 Hz is more precisely 1258.9 Hz etc


9]  List of National Acoustical Societies

For standards organizations addresses see section 1.2

Please let me know if any information in this list needs amending.

Argentina Acoustical Association
Asociacion de Acusticos Argentinos
c/o Prof A. Mendez, Laboratorio de Acustica, Camino Centenario Y 506,
1897 - Gonnet, Argentina
Tel: +54 21 84 2686   Fax: +54 21 71 2721

Australian Acoustical Society
Private Bag 1, Darlinghurst, NSW 2010
Tel: +61 2 331 6920   Fax: +61 2 331 7296

Austrian Acoustics Association
c/o Prof Ewald Benes, Technische Universitat Wien, Institut fur
Allgemeine Physik, Wien, Austria
Tel: +43 1 58801-5587   Fax: +43 1 5864203

Belgian Acoutics Assosciation (ABAV)
Av. P Holoffe 21, 1342 Limelette, Belgium
Tel: +32 2 653 88 01   Fax: +32 2 653 07 29

Sociedade Brasileira de Acustica
Attn Prof Samir Gerges, Universidade Federal de Santa Catarina,
Departamento de Engenharia Mecanica, Campus Univeritario, C.P 476  
CEP 88040-900, Florianopolis - SC, Brazil
Tel: +55 48 2344074   Fax: +55 48 2341519

Canadian Acoustical Association
PO Box 1351, Station F, Toronto, Ontario, M4Y 2V9, Canada
Tel: +1 514 343 7559   or  +1 613 993 0102

Sociedad Chilena de Acustica
San Francisco # 1138, Santiago, Chile.
Tel/Fax: +56 2 555 63 66 or +56 2 551 79 20
e-mail:  with copy (Cc) to:

China (PRC)
Acoustical Society of China
17 Zhongguancun St., Beijing 100080, China 

Czech Republic
Czech Acoustical Society
Technicka 2, 166 27 Prague 6, Czech Republic.
Tel: +42 2 24352310  Fax: +42 2 3111786

Acoustical Society of Denmark
c/o Department of Acoustic Technology, Bldg. 352 - Technical University
of Denmark, DK-2800 Lyngby, Denmark
Tel: +45 4588 1622   Fax: +45 4588 0577

Acoustical Society of Finland
c/o Helsinki University of Technology, Acoustics Laboratory,
Otakaari 5 A, FIN-02150 Espoo, Finland
Tel: +358 9 451 2499   Fax: +358 9 460 224
French Acoustical Society
Societe Francaise d'Acoustique
23 avenue Brunetiere, 75017 Paris, France
Tel +33 1 48 88 90 59   Fax: +33 1 48 88 90 60

German Acoustical Society
Deutsche Gesellschaft fur Akustik
c/o Department of Physics Acoustics, University of Oldenburg,
D-26111 Oldenburg, Germany
Tel: +49 441 798 3572   Fax: +49 441 798 3698

Hellenic Acoustical Society
Patision 147, 112 51 Athens, Greece
Tel or Fax: +30 1 8646 065

Hong Kong Institute of Acoustics
PO Box 7261
Hong Kong
Fax: +852 2886 3777

Scientific Society for Optics, Acoustics... (OPAKFI)
Fo utca 68, H-1027 Budapest, Hungary 
Tel/Fax: +36 1 202 0452
e-mail (c/o Andras Illenyi):

Acoustical Society of India
c/o Dr S Agrawal, CEERI Centre, CSIR Complex, Hillside Road,
New Delhi-110012, India
Tel: +91 11 5784642
e-mail (c/o National Physical Lab):

Italian Association of Acoustics
Associazione Italiana di Acustica
via Cassia 1216, 00189 Roma, Italy
Tel: +39 6 30365746   Fax: +39 6 30365341

Acoustical Society of Japan
Nippon Onkyo Gakkai
4th Floor, Ikeda Building, 2-7-7 Yoyogi, Shibuya-ku, Tokyo, Japan
Tel: +81 3 3379 1200   Fax: +81 3 3379 1456

Korean Republic
The Acoustical Society of Korea,
c/o 302-B, The Korean Federation of Science and Technology,
635-4, Yeoksam-dong, Kangnam-gu, Seoul-city, 135-080, Rep. of Korea
Tel: +82 2 565 1625   Fax: +82 2 569 9717

Mexican Institute of Acoustics    
Instituto Mexicano de Acustica
c/o Sergio Beristain, P.O. BOX 75805,
Col. Lindavista 07300 Mexico, D.F.
Tel +52 5 682 28 30   Fax: +52 5 523 47 42

Netherlands Acoustical Society
Nederlands Akoestisch Genootschap
Postbus 162, NL-2600 AD, Delft, Netherlands
Tel: +31 15 26 92 442   Fax: +31 15 26 92 111

New Zealand
New Zealand Acoustical Society
c/o  J. Quedley, CPO Box 1181, Auckland, New Zealand
Tel: +64 9 623 3147  Fax: +64 9 623 3248

Acoustical Society of Norway
Norsk Akustisk Selskap
c/o Lydteknisk senter-NTH Sintef Delab, N-7034 Trondheim, Norway
Tel: +47 73 59 43 36   Fax: +47 73 59 14 12

Acoustical Society of Peru
Sociedad Peruana de Acustica
Garcilazo de la Vega 163, Salamanca de Monterrico, Lima 3, Peru
Tel: +51 1 4351151   Fax: +51 1 4675625

Polish Acoustical Society
Polskie Towarzystow Akustyki
Instytut Akustyki, Uniwersytet Adama Mikiewicz, ul J.Matejki 48/49,
60-769 Poznan, Poland
Tel or Fax: +48 61666 420

Portuguese Acoustical Society
SPA - CAPS/Instituto Superior Tecnico, Av. Rovisco Pais
1096 Lisboa CODEX, Portugal
tel: +351 1 841 9393/39  fax: +351 1 352 3014

Romanian Acoustical Society
Societatea Romana de Acustica
c/o Nicolae Enescu, Universitatea Politehnica Bucuresti,
Splaiul Independentei nr. 313, 77206 Bucuresti, Romania
Tel: +40 1 4101615    Fax: +40 1 4104488 

Russian Acoustical Society
4 Shvernik ul, Moscow, 117036 Russia
Tel: +7 095 126 7401   Fax: +7 095 126 8411

Singapore Acoustics Society
c/o W Gan, Acoustical Services Pte Ltd
209-212 Nanyang Ave, NTU, Singapore 2263
Fax +65 791 3665

Slovak Acoustical Society
c/o Prof Stefan Markus, Racianska 75, PO Box 95, 830 08 Bratislava 38,
Tel: +42 7 254751   Fax: +42 7 253301

South Africa
South African Acoustics Institute
c/o Dr Fred Anderson, P.O. Box 912-169, Silverton, South Africa, 0127
Tel or Fax: +27 12 832857
e-mail (c/o Andersen Technology):

Spanish Acoustical Society
Sociedad Espanola de Acustica
Serrano 144, E-28006 Madrid, Spain
Tel: +34 1 5618806   Fax: +34 1 4117651

Swedish Acoustical Society
Svenska Akustiska Sallskapet
c/o Ingemansson AB, Box 47 321
S-100 Stockholm, Sweden
Tel: +46 8 744 5780   Fax: +46 8 18 26 78

Schweizerische Gesellschaft fur Akustique
Societe Suisse d'Acoustique
Postfach 251, 8600 Dubendorf
Tel: +41 1 823 4743  Fax: +41 1 823 4793

Turkish Acoustical Society - TAS
Y.T.U. Mimarlik Fakultesi
Yildiz, 80750, ISTANBUL/TURKEY
Tel: +90 212 259 70 70 ext: 2772
Fax: +90 212 26105 49

Institute of Acoustics
5 Holywell Hill, St Albans, Herts, AL1 1EU, UK
Tel: +44 1727 848195   Fax: +44 1727 850553

Acoustical Society of America
500 Sunnyside Blvd., Woodbury, NY 11797, USA
Tel: +1 516 576 2360   Fax: +1 516 576 2377


FAQ Contributors

Note: Please write to alt.sci.physics.acoustics newsgroup, not to the

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