Noise FAQ

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 sound cards in IBM compatible computers.


The main archive site for all usenet FAQs is

A list of mirror 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 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 has information on speech processing and some software links

1.2 What World Wide Web sites are there?
(virtual lib for acoustics & vibration with useful 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)
ISO Technical Committee 43 – all areas of acoustics and acoustical measurements.
Sunbcommittee 1 deals with measurements including sound power.
Subcommitte 2 deals with acoustical properties of buildings.
American Society for Testing and materials (ASTM) Committee E-33 “Environmental Acoustics”.
Deals with all aspects of building acoustics and some community noise measurements.

Some of the better search engines: (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 4.12 – Accurate real time 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 (e.g. room acoustics, RT, decibel conversion etc.) is available at a couple of sites.

Auralization – demo version of CATT-Acoustic (room acoustics prediction / auralization). A free download version is available on the Web site, but it lacks a small key file which can be transfered via e-mail in return for name, address and company/organization affiliation. See (4-98 per Bengt-Inge Dalenback * Mariagatan 16A * S-41471 Gothenburg * SWEDEN * phn/fax: +46 31145154)

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 (i.e. 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 tens of 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. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1kHz – 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.

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 high fidelity 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. Other examples are the Richter scale for earthquake event energy and pH for hydrogen ion concentration in liquids.

In all cases the logarithmic measure is used to compare the quantity of interest with a reference value, often 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 more easily interpreted with a logarithmic scale than with 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, calibrated on a decibel scale.

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

Often, the “maximum” level and sometimes the “peak” level of the sound being measured is quoted. During any given time interval the peak level will be numerically greater than the maximum level and the maximum level will be numerically greater than the (rms) sound pressure level;


2.4 What does dB(A) or “A-Weighted” mean? C-Weighted?

A sound level meter that measures the sound pressure level with a “flat” response will indicate the strength of low frequency sound with the same emphasis as higher frequency sounds. Yet our ear perceives low frequency sound to be of less loudness that higher frequency sound. The eardrum- stapes-circular window system behaves like a mechanical transformer with a finite pass band. In EE parlance, the “3 dB” rollover frequencies are approximately 500 Hz on the low end and 8 kHz on the high end. By using an electronic filter of attenuation equal to that apparently offered by the human ear for sound each frequency (the 40-phon response curve), the sound level meter will now report a numerical value proportional to the human perception of the strength of that sound independent of frequency. Section 8.2 shows a table of these weightings.

Unfortunately, human perception of loudness vis-a-vis frequency changes with loudness. When sound is very loud – 100 dB or more, the perception of loudness is more consistent across the audible frequency band. “B” and “C” Weightings reflect this trend. “B” Weighting is now little-used, but C-Weighting has achieved prominence in evaluating annoying community noises such as low frequency sound emitted by artillery fire and outdoor rock concerts. C-Weighting is also tabulated in 8.2.

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. “Noise” (unwanted sound) became a public issue.

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 modeled on the response of the human ear to low and high levels of sound respectively.

With the coming of the Walsh-Healy act in 1969, the A-Weighting of sound was defacto presumed to be the “appropriate” weighting to represent sound level as a single number (rather than as a spectrum). With the advent of US FAA and US EPA interests in the ’70’s, the dBA metric was also adapted by them, and with the associated shortfall in precision.

[Editor’s Note: A single number metric such as dBA is more easily understood by legal and administrative officials, so that promulgation, enforcement and administrative criteria and actions are understandable by more parties, often at the expense of a more precise comprehension and engineering action capability. For instance, enforcement may be on a dBA basis, but noise control design demands the octave-band or even third-octave band spectral data metric.]

The most commonly referenced weighting is “A-Weighting” dB(A), which is 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 this FAQ file.

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”.

Many modern sound level meters can measure the average sound energy over a given time. this metric is called the “equivalent continuous sound level” (L sub eq). More recently, it has become customary to presume that this sound measurement was A-Weighted if no weighting descriptor is listed.

2.5 How are decibel sound levels added?

If there are two uncorrelated 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 decibel sound level is a logarithmic sum i.e.

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 (“correlated”) 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 shell 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; the cochlea tapers towards its apex. Groups of the delicate hair sensors on the membrane, which membrane varies in stiffness along its length, respond to different frequencies transmitted down the spiral. The hair sensors are one of the few cell types in the body which do not regenerate. They can therefore be irreparably damaged by large noise doses. Refer to the Tinnitus FAQ for more information on associated hearing disorders.

2.7 At what level does sound become unsafe?

It is strongly recommended, to avoid unprotected exposure to sound pressure levels above 100dBA. Use hearing protection when exposed to levels above 85dBA (about the sound level of a lawn mower when you are pushing is over a grassy surface), and especially when prolonged exposure (more than a fraction of an hour) 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.

Health hazards also result 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?

Sound intensity is expressed in decibels with respect to one pico-watt (10^-12 watts) per square meter. This is very nearly* numerically equal to the sound pressure level in decibels. This presumes no standing waves or reflections where the effective impedance can differ from that of free space air. In its complete form, intensity include the unit vector of the propagation direction, i.e. intensity is a vector quantity.

*For a plane wave, the sound power that passes through a surface of A square meters is defined as the ratio of the pressure squared to the air impedance

I = p^2/(rho*c)

When combined with the propagation unit vector, this defines the rate of sound energy transmitted in a specified direction per unit area normal to the direction. When measured in practical units, we can compute intensity after the relation that

Numerically, the sound intensity is related to the sound power as follows: In free air space, a source emitting Lw dB re 1 picowatt produces the sound pressure level Lp at a distance R feet as


At a one foot radius, that sound power is distributed over a surface of 4*pi = 12.57 square feet or (*.3048^2=.0920*) 1.17 square meters. 10log1.17=0.7dB. So within 0.1 dB, the coincidence exists that the sound intensity in picowatts per square meter is numerically equal to the sound pressure level in dB!

NOTE: This identity holds true only when the impedance, rho*c is exactly 400 mks rayls. This occurs for sea-level at 39 degrees C. For 22 C, rho*c = 412; a 0.13 dB difference arises. But at higher elevations, air density decreases for a given temperature. At an elevation of 840 feet above sea level, rho*c reduces to 400 at 22 C. (fortunate for much of Midwestern US!). The 0.13 dB difference at sea level is not usually significant for acoustical measurements.

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

2.9 How does sound decay with distance?

At distances large compared to the size of the source, sound intensity diminishes according to the inverse square law.

I = Io/D^2

It is relatively simple to reliably calculate provided the source is small and outdoors, but indoor calculations (in a reverberant field) are rather more complex.

If the observation position is at a distance that is small compared to the size of the source, sound level changes very little with location. One should be able to determine the “virtual center” of the whole sound field, whence inverse square law calculations can proceed in reference to that distance.

The surrounding environment, especially close to the ground and in the presence of wind and vertical temperature gradients have a great effect on the sound received at a distant location. Ground reflection affects sound levels more than a few feet away (distances greater than the height of the sound source or the receiver above the ground). Wind and air temperature gradients affect all sound propagation beyond 100 meters over the surface of the earth.

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?

(See ACCULAB Reference Sound Source on this site:

Sound power level, Lw, is often quoted on machinery to indicate the total sound energy radiated per second. It is quoted in decibels with respect to the reference power level. The reference level is 1pico-watt (pW) [1×10^(-12) watts]. One watt of radiated sound power is represented as “Lw=120 dB re one picowatt”. If the reported sound power is in terms of A-Weighted spectral weighting, a suffix, A, is applied to form dB(A).

The sound pressure level (SPL) resulting from sound power (Lw) being radiated into free space, e.g. over a paved surface, is computed from

SPL = Lw – 20*log(R) – 11 dB re 20 uPa (R in meters) SPL = Lw – 20*log(r) – 0.7 dB re 20 uPa (r in feet)

If instead the sound is emitted over a reflecting plane such as a hard surface, three (3) decibels are added to the SPL.

For example, a lawn mower with sound power level 100 dB(A) will produce at a sound pressure level (SPL) of about 89dB(A) at the operator (you) position over grass and 92 dB(A) when the mower is operated over a hard surface such as your driveway. At your neighbor’s yard 50 feet (15m) away, the SPL will be is 65 dBA.

2.10.1 How is sound power measured?

Sound power is usually measured indirectly as the sound pressure level found at a specific distance, and in every direction that sound can be radiated. The sound power emitted by Items that can be carried to a laboratory is usually measured in a hemi-anechoic room or a reverberation room.

Either the “comparison” or the “direct” method is used.

In the comparison method, the SPL that the item causes in that room is compared the SPL created by a standard “Reference Sound Source” (see the ‘Acculab’ portion of this web page) to determine the sound power emitted by the item. This is the most common and economical method.

In the direct method two processes may apply. For the hemianechoic method, the SPL is measured in every direction on a surface encompassing the test item, then combined to compute the emitted sound power. For the reverberation room, the SPL is measured at several locations in the that room, then the sound power is computed from

PWL = SPL + 10Log(A)-C.

A = absorption in the reverberation room, sabins or square meters.
C = 16.3 for A as sabins (square feet)
C = 6.2 for A in square meters.

See ISO Technical Committee Web Site for acoustical measurement information.

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

**** AIR ****

A convenient formula for the speed of sound in air is

c = 20*sqrt(273 + T), T in Centigrade and c in meters/sec


c = 49*sqrt(459 + T), T in Fahrenheit and c in feet/sec

The speed of sound in air at a temperature of 0 degrees C 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 degrees C. 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.

**** WATER ****

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 temperature 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 perceived 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 human 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.