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Archive-name: physics-faq/acoustics
Last-modified: 7th September 1997
Version: 0.09
*** 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
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 ?
7] INDEX
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
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 - 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
news:comp.sys.ibm.pc.soundcard.misc - various discussion of use of
internal soundcards in IBM compatible computers.
FAQs
----
The main archive site for all usenet FAQs is
ftp://rtfm.mit.edu/pub/usenet/
A list of sites (including html) for the Acoustics FAQ is at
http://super.zippo.com/~consult/Acoustics_FAQ_mirrors.html
--------------
The Active Noise Control FAQ by Chris Ruckman is at
http://www.xis.com/~ruckman/
--------------
The Tinnitus FAQ deals with a range of hearing disorders. It is
maintained by Mark Bixby and available at
http://www.cccd.edu/faq/tinnitus.html
--------------
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/rec.audio.* directories
--------------
The comp.speech faq maintained by Andrew Hunt has information on speech
processing and some software links
http://www.speech.su.oz.au/comp.speech/
--------------
*** 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.
http://www.ecgcorp.com/velav/index.html
(virtual lib for acoustics & vibration with useful links)
http://capella.dur.ac.uk/doug/acoustics.html
(wide selection of acoustics related links)
http://www.campus.bt.com/CampusWorld/pub/ScienceNet/first.html
(science questions and answers)
http://online.anu.edu.au/ITA/ACAT/drw/PPofM/INDEX.html
(simple acoustics introduction from David Worrall)
http://www.mme.tcd.ie/~m.carley/Notes/
(theoretical basic acoustics lecture notes; difficult stuff like
the wave equation etc, in hypertext for browsing, or gzipped
Postscript format for downloading)
http://asa.aip.org/
(Acoustical Society of America home page with several links and
comprehensive career section, book lists and Society info etc)
http://pcfarina.eng.unipr.it/
(Angelo Farina has published a variety of papers - some are
available in zipped MSWord format)
http://eaa.essex.ac.uk/eaa/
(European Acoustics Association)
http://users.aol.com/inceusa/ince.html
(Institute of Noise Control Engineering home page)
http://super-highway.net/~wattsup/Audio%20related%20Site%20list.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 Advanced
Search facility at:
http://altavista.digital.com/
alternatively try searches on:
http://www.yahoo.com/
http://www.hotbot.com/
http://www.infoseek.com/
http://www.excite.com/
http://www.lycos.com/
http://www.dejanews.com/ (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:
http://tinker.winsite.com/info/pc/win95/sounds/gram32.zip
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
http://www.cisab.indiana.edu/CSASAB/index.html
The programs listed are mainly for sound analysis and editing.
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.dat.dtu.dk/~odeon/index.html
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
http://users.aol.com/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 / 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
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
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.
http://www.mankato.msus.edu/dept/comdis/kuster2/audiology.html
http://oto.wustl.edu/cochlea
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 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.
Important:-
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
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
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