Subject: Acoustics FAQ
Date: Tue, 16 Jan 96 00:30:12 GMT
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                    *** ACOUSTICS FAQ ***


DISCLAIMER - NO WARRANTY OF ANY KIND WHATSOEVER IS MADE FOR THE FITNESS
OF THE CONTENTS OF THIS FAQ.

In order to allow maximum compatibility only ASCII symbols are used



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


Contributors
============

     Michael Carley (mjcarley@maths.tcd.ie)
     Johan L Nielsen (nielsen@tele.unit.no)
     Chris Ruckman (ruckman@oasys.dt.navy.mil)
     Asbjoern Saeboe (saeboe@tele.unit.no) 
     Jesper Sandvad (js@kom.auc.dk)
     Andrew Silverman (Enviro@measure.demon.co.uk)


Changes since previous version 
==============================
     various revisions to news group and FAQ info
     revision of Web sites
     added altavista and other search facilities
     revision of journals and books
     swapped sections 6.1 & 6.4 
     added sections 6.5,6.6,6.7
     added INDEX (section 7)
     added formulae for A weighting
     revision of ASA e-mail address

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  What is the sound power level ?
2.10 What is the speed of sound in air, water .. ?
2.11 What is meant by loudness?
2.12 How is vibration measured ?

3] Reserved

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 can I improve the noise insulation of my house ?

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 bottle make a note ?
6.6  What is pitch ?
6.7  What causes "helium voice" ?

7] INDEX

8] Various Tables

8.1  A, C & U Weigtings (with 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
http://www.Point-and-Click.com/Campanella_Acoustics/angelo.htm

news:sci.physics - general physics but occasionally acoustics related
questions are posted.

news:rec.audio.tech - includes discussion on audio equipment, speakers
etc. There are other rec.audio groups which may be of interest.

news:alt.support.hearing-loss and news:alt.support.tinnitus - groups
for sufferers of these complaints

news:bionet.audiology - all matters relating to hearing

news:bit.listserv.deaf.l  and  news:uk.people.deaf - groups primarily
for the 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

news:comp.sys.ibm.pc.soundcard.misc - various discussion of use of
internal soundcards in IBM compatible computers.


FAQs
----
This Acoustics FAQ is posted to alt.sci.physics.acoustics, alt.answers
and news.answers. It is available at URLs
http://www.mme.tcd.ie/~m.carley/Acoustics/faq.html (hypertext version)
ftp://rtfm.mit.edu/pub/usenet/news.answers/physics-faq/acoustics
and at a number of other worldwide mirror sites.
It can also be obtained by sending an e-mail to:
mail-server@rtfm.mit.edu
with message: send usenet/news.answers/physics-faq/acoustics     

                         --------------
The Active Noise Control FAQ is maintained by Chris Ruckman and posted
to groups including news.answers and alt.sci.physics.acoustics. It is
available at URL
ftp://rtfm.mit.edu/pub/usenet/news.answers/active-noise-control-faq
and most worldwide usenet mirror sites
or by e-mail to 
mail-server@rtfm.mit.edu
          (send usenet/news.answers/active-noise-control-faq)
                         --------------
The Tinnitus FAQ was started by Mark Bixby and is now maintained by Lee
Leggore and posted to some news groups including alt.support.tinnitus
and news.answers. The FAQ also has information on a range of other
hearing disorders. It is available at the following URLs
ftp://rtfm.mit.edu/pub/usenet/news.answers/medicine/tinnitus-faq 
http://www.cccd.edu/faq/tinnitus.html
                         --------------
The Audio FAQ is also available (in 4 parts) at rtfm 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/rec.audio.* directories

 

***  1.2  What World Wide Web sites are there ?

Almost all important acoustical web sites can be found from links in
the first two locations listed below. 

http://web.mit.edu/org/a/avlab/www/vl.home.html
     (virtual lib for acoustics & vibration with many useful links   
     including the MIT Acoustics and Vibration Lab)
http://capella.dur.ac.uk/doug/acoustics.html 
     (wide selection of acoustics related links)
http://www.mme.tcd.ie/~m.carley/Acoustics/notes.html
     (theoretical basic acoustics lecture notes; difficult stuff like
     the wave equation etc, in hypertext for browsing, or gzipped    
     Postscript format for downloading)
http://online.anu.edu.au/ITA/ACAT/drw/PPofM/INDEX.html
     (simple acoustics introduction from David Worrall)
http://asa.aip.org/
     (Acoustical Society of America home page with several links and 
     comprehensive career section, book lists and Society info etc)
http://www.qnx.com/~danh/audiobbs.html
     (Steve Ekblad's extensive audio related BBS and Internet list) 
http://www.techexpo.com/
     (Technical societies, conferences etc etc but not specifically
     acoustics related)
http://www.iso.ch/
     (main ISO standards page)
http://www.iso.ch/addresse/membodies.html
     (national standards organizations addresses)
http://www.ansi.org/
     (official ANSI site)

The Digital Equipment Corporation has an extremely powerful search
facility at: 

http://altavista.digital.com/

alternatively try searches on:
http://www.lycos.com/
http://www.yahoo.com/
http://webcrawler.com/
http://wwww.cs.colorado.edu/WWWW/
or your nearest Archie site

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

There are a few programs for various platforms listed at URL
http://www.cisab.indiana.edu/csasab.html
The programs are mainly for sound analysis and editing of samples such
as animal sounds.

Some software is available for audio systems design at URL
ftp://ftp.uu.net/usenet/rec.audio.high-end/Software  

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
http://www.la.dtu.dk/~odeon/index.html
describes the capabilities of the program and gives the ftp address.

Sound Waves is a low cost educational program intended mainly for
schools (Macintosh only at present), and has been well received. The
program is written by Bill Moran at Lawrence Livermore National
Laboratory. It is not available for ftp but further information can be
obtained by reference to the Sound Waves FAQ at URL
http://www.physics.wisc.edu/~cross/FAQ/Sound_Waves.html

Physics demonstrations about oscillations and waves. The demos include
online gif picture outlines of experiments. See URL
http://www.physics.unc.edu/~labs/demo/waves/waves.html

***  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:

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

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

>>Engineering Noise Control
>>Bies, D & Hansen, C
Practically biased with lots of examples and tables. New 2nd edition

>>Handbook of Acoustical Measurements and Noise Control
>>Harris C  (editor)
Very good practical reference book. Covers most everything.


A list of recently reviewed noise-related books is at URL
http://gopher.allenpress.com/homepage/inceusa/inceusa_books.html


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 (Europe) due to merge with Acustica
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 pico Watt 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                                            |
     |________________________________________________________|


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.

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
sensitive.

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



***  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. Some tables are given
in section 8 of the FAQ. Values can also be calculated from
mathematical formulae in sound level meter standards. 

In addition to frequency weighting, sound pressure can be weighted in
time with fast, slow or impulse response. "Fast" is by far the most
common time weighting; 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.

ftp://rtfm.mit.edu/pub/usenet/news.answers/medicine/tinnitus-faq


***  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 usenet group bionet.audiology covers discussion
to do with hearing and hearing loss.

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 is defined as the amount of sound energy per unit area. 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, in
situations where a sound level meter would not be suitable.


***  2.9  What is the sound power level ?

Sound power level (L sub w) is often quoted on machinery to indicate
the amount of acoustic energy radiated. The reference power is taken
as 1pW.

Assuming the radiation pattern of sound is known, the sound level at
a distance from the machine can be calculated.

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.


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

The speed of sound in air at a temperature of 0 degC is 331.6 m/s
Since the speed is proportional to the square root of absolute
temperature and it is about 12 m/s greater at 20 degC. The speed is
almost independent of atmospheric pressure.

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.

See CRC Handbook of Chemistry & Physics for other substances.


***  2.11  What is meant by loudness?

Loudness is the subjective impression of the strength of a sound. The
loudness of a noise does not necessarily correlate with its sound
level. Loudness level is measured in phons by comparison with 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
hears 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.

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. 
  

***  2.12  How is vibration measured ?

This term often refers to the small oscillating movement of a solid
which is likely to be unwanted or undesirable. Some common examples are
the vibration of a building near a railway line or the vibration of the
floor caused by a spin dryer. 
 
Vibration is monitored with an accelerometer. This is a transducer that
is securely attached by some means to the surface under investigation.
The accelerometer produces a small charge output, proportional to the
surface acceleration, which is amplified by a charge amplifier and
recorded or observed with a meter. The apparatus is very susceptible
to extraneous effects. 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  

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 real 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.

***  4.5  How can I improve the noise insulation of my house ?

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 a competent person should be engaged to determine the
best approach for each circumstance. The following ideas may serve as
guidelines.

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.



-------------------------------------------------------------
-------------------------------------------------------------


6]  Miscellaneous Questions
    ----------------------

***  6.1  What is active noise control ?

This 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 original unwanted sound is added
to the electronically produced inverse wave the result is sound
cancellation. This method of noise control is becoming increasingly
popular for a variety of uses. It is claimed that in some circumstances
the technique can be more efficient in terms of energy usage than can
passive control methods. The topic is the subject of the Active Noise
Control FAQ which is available at a number of sites including:

ftp://rtfm.mit.edu/pub/usenet/news.answers/active-noise-control-faq


***  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 (kplotkin@access2.digex.net) 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 - 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 is idea is used in column speakers and commercial
systems for reducing noise levels outside the dance floor area of
discos. 

***  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. 


James Davison (TKGN58A@prodigy.com) 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 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 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). 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. 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 either URL:
http://lecaine.music.mcgill.ca/~welch/auditory/Auditory.html
http://gl15.bio.th-darmstadt.de/biologie/auditory/Auditory.html


***  6.7  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 the velocity of
sound 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. 


-------------------------------------------------------------
-------------------------------------------------------------

7]  INDEX
    -----

A-weighting 2.4 2.11 8.1
absorption coefficient 4.1 4.2
accelerometer 2.12
acoustic energy 2.1 2.8 2.9 4.1 4.3
Acoustical Society of America 2.4  http://asa.aip.org/
active noise control 6.1 
addition of sound 2.5
atmospheric pressure 2.1
audibility 2.1 2.11
column speaker 6.3
concert pitch 6.6
dB(A) 2.4 8.1
decibel (dB) 2.2 2.3 2.4
ear 2.1 2.2 2.6 2.7  http://lab9924.wustl.edu/ 
equal temperament 6.6
equivalent continuous sound level 2.4
focusing sound 6.3
frequency 2.1 2.4 2.11 6.6
hearing damage 2.6 2.7
Helmholtz resonator 6.5
historical notes 2.4 2.11
insulation 4.3 4.4 4.5
interference 6.3
Leq 2.4
logarithmic scale 2.2 2.3
loudness 2.1 2.2 2.11
loudspeaker 2.1 6.3
Lw 2.9
masking 2.11
mel 6.6
musical scale 6.6
pascal 2.1 2.2 2.8
passive noise control 6.1 6.5
phon 2.11
physical constants  http://physics.nist.gov/PhysRefData/contents.html
Pierce, George W 2.4
pitch 6.6 6.7  http://mambo.ucsc.edu/psl/audfaq.txt
resonance 6.5 6.7
reverberation time 4.1
Sabine, Wallace C 4.1
semitone 6.6
sonic boom 6.2
sonoluminescence 6.4
sound 2.1
sound absorption 4.1 4.2 4.3
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 2.11
sound level meter 2.3 2.4 2.8 2.11
sound power level 2.9
sound pressure 2.1 2.2
sound pressure level 2.3 2.4 2.5
speech 6.6 6.7
speaker 2.1 6.3
speed of sound 2.1 2.10 6.7
supersonic 6.2
tapping machine 4.4
tinnitus 2.6 2.7
velocity of sound 2.1 2.10 6.7
vibration 2.1 2.7 2.12
voice 6.6 6.7
wave 2.1
weighting 2.4 2.11 8.1
white finger 2.7


-------------------------------------------------------------
-------------------------------------------------------------


8]  Various Tables
    --------------

8.1            A, C & U Weightings (dB)

   Nominal         Exact
  Frequency      Frequency    A-weight  C-weight  U-weight

     10             10.00     -70.4     -14.3       0.0
     12.5           12.59     -63.4     -11.2       0.0
     16             15.85     -56.7     - 8.5       0.0
     20             19.95     -50.5     - 6.2       0.0
     25             25.12     -44.7     - 4.4       0.0
     31.5           31.62     -39.4     - 3.0       0.0
     40             39.81     -34.6     - 2.0       0.0
     50             50.12     -30.2     - 1.3       0.0
     63             63.10     -26.2     - 0.8       0.0
     80             79.43     -22.5     - 0.5       0.0
    100            100.00     -19.1     - 0.3       0.0
    125            125.9      -16.1     - 0.2       0.0
    160            158.5      -13.4     - 0.1       0.0
    200            199.5      -10.9       0.0       0.0
    250            251.2      - 8.6       0.0       0.0
    315            316.2      - 6.6       0.0       0.0
    400            398.1      - 4.8       0.0       0.0
    500            501.2      - 3.2       0.0       0.0
    630            631.0      - 1.9       0.0       0.0
    800            794.3      - 0.8       0.0       0.0
   1000           1000.0        0.0       0.0       0.0
   1250           1259        + 0.6       0.0       0.0
   1600           1585        + 1.0     - 0.1       0.0
   2000           1995        + 1.2     - 0.2       0.0
   2500           2512        + 1.3     - 0.3       0.0
   3150           3162        + 1.2     - 0.5       0.0
   4000           3981        + 1.0     - 0.8       0.0
   5000           5012        + 0.5     - 1.3       0.0
   6300           6310        - 0.1     - 2.0       0.0
   8000           7943        - 1.1     - 3.0       0.0
  10000          10000        - 2.5     - 4.4       0.0
  12500          12590        - 4.3     - 6.2     - 2.8
  16000          15850        - 6.6     - 8.5     -13.0
  20000          19950        - 9.3     -11.2     -25.3
  25000          25120                            -37.6
  31500          31620                            -49.7
  40000          39810                            -61.8

The above may also be found from published mathematical formulae.
Frequencies in column 2 are exact to four places. The actual
frequencies are 1000 x 10^(n/10), where n is a positive or negative
integer.

The A weighting can also be found from the following formula

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

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


-------------------------------------------------------------



9]  List of National Acoustical Societies
    -------------------------------------

For standards organizations addresses see section 1.2

Please let me know if any information in this list is incorrect.

Argentina
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
e-mail: acustica@isis.unlp.edu.ar 

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

Austria
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

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

Brazil
Brazilian Acoustics Society
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 23444074   Fax: +55 48 2341524

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

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

Czech Republic
Czech Acoustical Society
c/o Ondrej Jiricek, Technicka 2, 166 27 Prague 6, Czech Republic.
Tel: +42 2 24352310  Fax: +42 2 3111786
e-mail: jiricek@feld.cvut.cz

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

Finland
Acoustical Society of Finland
c/o Helsinki University of Technology, Acoustics Laboratory,
Otakaari 5 A, SF-02150 Espoo, Finland
Tel: +358 04511 xt 2490
 
France
French Acoustical Society
Societe Francaise d'Acoustique
33 rue Croulebarbe, 75013 Paris, France
Tel +33 1 45 35 54 00   Fax: +33 1 43 31 74 26
e-mail: sfa@cal.enst.fr

Germany
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
e-mail: dega@aku.physik.uni-oldenburg.de

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

Hungary
Hungarian Society Optics, Acoustics and Film Technics
c/o Andras Illeny, Budaorsi ut 45, Budapest, Hungary H-1112
Tel or Fax: +361 185 1780
e-mail: h5369ill@ella.hu

India
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): Agrawals%npl@sirnetd.ernet.in

Italy
Italian Association of Acoustics
Associazione Italiana di Acustica
via Cassia 1216, 00189 Roma, Italy
Tel: +39 6 3765746   Fax: +39 6 3765341

Japan
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

Mexico
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
e-mail: SBERISTA@vmredipn.ipn.mx

Netherlands
Netherlands Acoustical Society
Nederlands Akoestisch Genootschap
Postbus 162, NL-2600 AD, Delft, Netherlands
Tel: +31 15 69 24 41   Fax: +31 15 69 21 11
e-mail: nag@tpd.tno.nl

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

Norway
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
e-mail: sverre.stensby@delab.sintef.no

Poland
Polish Acoustical Society
Polskie Towarzystow Akustyki
Instytut Akustyki, Uniwersytet Adama Mikiewicz, ul J.Matejki 48/49,
60-769 Poznan, Poland

Romania
Romanian Acoustical Society
Societatea Romana de Acustica
c/o Nicolae Enescu, University Politehnica, Splaiul Independentei nr
313, 77206 Bucuresti, Romania
Tel: +40 1 3120292   Fax: +40 1 3120188 

Slovakia
Slovak Acoustical Society
c/o Prof Stefan Markus, Racianska 75, PO Box 95, 830 08 Bratislava 38,
Slovakia
Tel: +42 7 254751   Fax: +42 7 253301
e-mail: markus@umms.savba.sk

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): pak03486@pixie.co.za

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

Sweden
Swedish Acoustical Society
Svenska Akustiska Sallskapet
c/o Tor Kihlman, Chalmers Univerity of Technology, Dept of Applied
Acoustics, S-412 96, Gothenburg, Sweden
Tel: +46 31 7722200   Fax: +46 31 7722212
e-mail: tk@ta.chalmers.se

UK
Institute of Acoustics
PO Box 320, St Albans, Herts, AL1 1PZ, UK
Tel: +44 1727 848195   Fax: +44 1727 850553
e-mail: Acoustics@clus1.ulcc.ac.uk

USA
Acoustical Society of America
500 Sunnyside Blvd., Woodbury, NY 11797, USA
Tel: +1 516 576 2360   Fax: +1 516 576 2377
e-mail: asa@aip.org
_____________________________________________________________

*** END ***
-------------------------------------------------------------
