FAQ on accoustics

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• Basic accoustics
• Vibration
• Architectural and building acoustics
• Noise

Basic accoustics

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What is sound?

Sound is produced by vibrating objects and reaches the listener's ears as waves in the air or other media. When an object vibrates, it causes slight changes in air pressure. These air pressure changes travel as waves through the air and produce sound. To illustrate, imagine striking a drum surface with a stick. The drum surface vibrates back and forth. As it moves forward, it pushes the air in contact with the surface. This creates a positive (higher) pressure by compressing the air. When the surface moves in the opposite direction, it creates a negative (lower) pressure by decompressing the air. Thus, as the drum surface vibrates, it creates alternating regions of higher and lower air pressure. These pressure variations travel through the air as sound waves.

 Following table lists the approximate velocity of sound in air and other media. In gases, the higher the velocity of sound, the higher the pitch will be

Approximate Speed of Sound in Common Materials

The hearing mechanism of the ear senses the sound waves and converts them into information which it relays to the brain. The brain interprets the information as sound. Even very loud sounds produce pressure fluctuations which are extremely small (1 in 10,000) compared to ambient air pressure (i.e., atmospheric pressure). The hearing mechanism in the ear is sensitive enough to detect even small pressure waves. It is also very delicate: this is why loud sound may damage hearing.

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 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. 

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What is sound wave?

Sound is transmitted via the movement of the particles in a medium, such as air or water. Energy is transferred from one region to another via a series of compression and tension cycles: the motion of the particles is parallel to the propagation direction. The acoustic disturbance can be represented as a wave, with the x-axis representing time, and the y-axis the displacement of a given particle in the medium from its rest position.

Increasing the strength of the sound source extends the displacement of the particle, and so the acoustic pressure will also increase. This is heard as an increase in loudness.

Exciting the sound more rapidly increases the frequency of the sound, and produces more cycles in a given period. This is heard as an increase in pitch.

Two basic quantities that can describe the nature of a sound are frequency and amplitude (of displacement or acoustic pressure). Sounds can be formed by a simple harmonic mixture of frequencies (as produced by a guitar string), an intentional mixture of frequencies and amplitude (music) or a seemingly random mixture of frequencies and amplitudes (noise).  

Sound wave	Increased volume sound wave		Increased frequency sound wave

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What are pitch and frequency?

Frequency is the rate at which the source produces sound waves, i.e. complete cycles of high and low pressure regions. In other words, frequency is the number of times per second that a vibrating body completes one cycle of motion. The unit for frequency is the hertz (Hz = 1 cycle per second. Low pitched or bass sounds have low frequencies. High-pitched or treble sounds have high frequencies. A healthy, young person can hear sounds with frequencies from roughly 20 to 20,000 Hz. The upper frequency limit decrease with age, and so the older a person gets, the less well they can hear high notes. Also, the male hearing range decreases more quickly than the female, and so women can generally hear higher pitch notes than men of similar age. The sound of human speech is mainly in the range 300 to 3,000 Hz.

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What is a decibel (dB)?

The decibel scale is a logarithmic scale applicable to any parameter, used to make quantities with a wide range of values more manageable. In the measurement of sound, we are concerned with the amplitude of the acoustic pressure, measured in pascals (Pa). The range of acoustic pressures that the human ear can detect is very wide - from the lower limit of hearing at around 20 micro Pa (2 x 10-5 Pa) to the threshold of pain at around 20 Pa. This very wide range of values is unwieldy, so it is converted into a logarithmic scale. This changes the range of values shown above to the more manageable range of 0 dB to 140 dB. Thus 0 dB is roughly the lowest level a normal person can hear, but it is not the lowest level possible! 

 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.

2) The human ear interprets loudness on a scale much closer to a logarithmic scale than a linear scale.

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How is sound measured?

The measurement of sound determines how loud something is, whether it is too noisy, or even whether it is safe to be near. 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. 

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

The sensitivity of the human ear to sound depends on the frequency or pitch of the sound. People hear some frequencies better than others. If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This occurs because people hear high frequency noise much better than low frequency noise.

Noise measurement readings can be adjusted to correspond to this peculiarity of human hearing. An A-weighting filter which is built into the instrument de-emphasizes low frequencies or pitches. Decibels measured using this filter are A-weighted and are called dB(A). Legislation on workplace noise normally gives exposure limits in dB(A). Table 2 lists examples of typical noise levels. 

 A-weighting serves two important purposes:

1. gives a single number measure of noise level by integrating sound levels at all frequencies
2. gives a scale for noise level as experienced or perceived by the human ear 

Typical Noise Levels

 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. 

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How do sound levels add?

Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] are based on a logarithmic scale (see Appendix A). They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB. 

 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. A simple way to add noise levels:

Addition of Decibels

For instance, using the example of two machines each emitting a noise level of 90 dB:

• Step 1: The numerical difference between the two levels is 0 dB (90-90= 0), using the first row.
• Step 2: The number corresponding to this difference of 0, taken from the right hand column, is 3.
• Step 3: Add 3 to the highest level, in this case 90. Therefore, the resulting noise level is 93 dB.

When the difference between two noise levels is 10 dB(A) or more, the amount to be added to the higher noise level is zero. In such cases, no adjustment factor is needed because adding in the contribution of the lower in the total noise level makes no perceptible difference in what people can hear or measure. For example if your workplace noise level is 95 dB(A) and you add another machine that produces 80 dB(A) noise, the workplace noise level will still be 95dB(A).

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What are basic rules of working with decibel (dB) units?

The decibel [dB, and also dB(A)] is a logarithmic scale. For mathematical calculations using dB units, we must use logarithmic mathematics (see Appendix A). However, in our day-to-day work we do not need such calculations.

The use of dB unit makes it easy to deal with the workplace noise level data provided we use a set of simple rules as summarized in the following table.  

Decibel (dB) basics

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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.

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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. 

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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 meter to about 1 Watt per square meter (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.

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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.

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What is sound pressure?

Sound pressure is the amount of air pressure fluctuation a noise source creates. We "hear" or perceive sound pressure as loudness.

Sound pressure also depends on the environment in which the source is located and the listener's distance from the source. The sound produced by the drum is louder two meters from the drum if it is in a small bathroom, than if it is struck in the middle of a football field. Generally, the farther one moves from the drum, the quieter it sounds. Also if there are hard surfaces that can reflect the sound (e.g. walls in a room), the sound will feel louder than if you heard the same sound, from the same distance, in a wide-open field.

Sound pressure is usually expressed in units called pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa. A gasoline-powered lawn mower produces about 1 Pa. The sound is painfully loud at levels around 20 Pa. Thus the common sounds we hear have sound pressure over a wide range (0.00002 Pa - 20 Pa).

It is difficult to work with such a broad range of sound pressures. To overcome this difficulty we use decibel (dB, or tenth (deci) of a Bel)). The decibel or dB scale is more convenient because it compresses the scale of numbers into a manageable range. More information about this "compressed", logarithmic scale is in Appendix A. The decibel is named after Alexander Graham Bell, the Canadian pioneer of the telephone who took great personal interest in the problems of deaf people.

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What is a sound pressure level?

Sound pressure converted to the decibel scale is called sound pressure level (Lp). Appendix A gives a detailed explanation of decibels and sound pressure levels. Figure 2 compares sound pressures in pascals and sound pressure levels in decibels (dB). The zero of the decibel scale (0 dB) is the sound pressure of 0.00002 Pa. This means that 0.00002 Pa is the reference sound pressure to which all other sound pressures are compared on the dB scale. This is the reason the decibels of sound are often indicated as dB re 0.00002 Pa.

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What is sound power?

The sound power is the sound energy transferred per second from the noise source to the air. A noise source, such as a compressor or drum, has a given, constant sound power that does not change if the source is placed in a different environment.

Power is expressed in units called watts (W). An average whisper generates a sound power of 0.0000001 watts (0.1 microwatt (µW)), a truck horn 0.1 W, and a turbo jet engine 100,000 W.

Like sound pressure, sound power (in W) is usually expressed as sound power levels in dB. Appendix B provides examples of sound power level calculations.

Figure 3 relates sound power in watts to sound power level in decibels. Note that while the sound power goes from one trillionth of a watt to one hundred thousand watts, the equivalent sound power levels range from 0 to 170 dB.

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What is a 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 meters. If the sound power level was 78dB(A) then the lawn mower sound level would be only 50dB(A) at the same distance.

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What is the relation between sound pressure and sound power?

Because the sound power of a noise source is constant and specific, it can be used to calculate the expected sound pressure.

The calculation requires detailed information about the noise source's environment. Usually a noise source with a lower sound power generates less sound pressure. The manufacturer can often provide the sound power of equipment. A number of international standards are available for labeling machines and equipment with their noise emission levels. From the sound power of a compressor, one can calculate the expected sound pressure and sound pressure level at a certain location and distance. This information can be helpful in determining possible noise exposures and how they compare to the noise guidelines.

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What is the speed of sound in air, water?

The speed of sound in air at a temperature of 0 ºC 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 ºC. 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.

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What is 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.

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Vibration

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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.

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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.

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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. 

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Architectural & building accoustics

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What is reverberation?

In an enclosed space, when a sound source stops emitting energy, it takes some time for the sound to become inaudible. This prolongation of the sound in the room caused by continued multiple reflections is called reverberation.

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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 

Reverberation time plays a crucial role in the quality of music and the ability to understand speech in a given space. When room surfaces are highly reflective, sound continues to reflect or reverberate. The effect of this condition is described as a live space with a long reverberation time. A high reverberation time will cause a build-up of the noise level in a space. The effects of reverberation time on a given space are crucial to musical conditions and understanding speech. It is difficult to choose an optimum reverberation time in a multi-function space, as different uses require different reverberation times. A reverberation time that is optimum for a music program could be disastrous to the intelligibility of the spoken word. Conversely, a reverberation time that is excellent for speech can cause music to sound dry and flat.

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What is reflection?

Reflected sound strikes a surface or several surfaces before reaching the receiver. These reflections can have unwanted or even disastrous consequences. Although reverberation is due to continued multiple reflections, controlling the Reverberation Time in a space does not ensure the space will be free from problems from reflections.

<:od>Reflective corners or peaked ceilings can create a “megaphone” effect potentially causing annoying reflections and loud spaces. Reflective parallel surfaces lend themselves to a unique acoustical problem called standing waves, creating a “fluttering” of sound between the two surfaces. Reflections can be attributed to the shape of the space as well as the material on the surfaces. 

Domes and concave surfaces cause reflections to be focused rather than dispersed which can cause annoying sound reflections. Absorptive surface treatments can help to eliminate both reverberation and reflection problems.

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What is noise reduction coefficient (NRC)?

 The Noise Reduction Coefficient (NRC) is a single-number index for rating how absorptive a particular material is. Although the standard is often abused, it is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz rounded to the nearest 5%). The NRC gives no information as to how absorptive a material is in the low and high frequencies, nor does it have anything to do with the material’s barrier effect (STC).

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What is sound transmission class?

The Sound Transmission Class (STC) is a single-number rating of a material’s or assembly’s barrier effect. Higher STC values are more efficient for reducing sound transmission. For example, loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall.

The rating assesses the airborne sound transmission performance at a range of frequencies from 125 Hertz to 4000 Hertz. This range is consistent with the frequency range of speech. The STC rating does not assess the low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than speech, such as mechanical equipment or music.

Even with a high STC rating, any penetration, air-gap, or “flanking” path can seriously degrade the isolation quality of a wall. Flanking paths are the means for sound to transfer from one space to another other than through the wall. Sound can flank over, under, or around a wall. Sound can also travel through common ductwork, plumbing or corridors.

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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. 

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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.

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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. 

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

This is one of the most commonly asked questions. 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 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. One may also need to consider noise penetration through the roof, floors, ceilings and walls.

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

Noise

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What is hearing?

Of the five senses, hearing is one of the most important. Audible sounds enable communication, and they can tell us what and where things are. They certainly have a significant effect on how we feel. The human ear is an organ of complex design and function. The ear forms the receiver and transmission line to the brain, which then processes this information and converts the received signal into something that we can understand. The sound is then perceived as loud or soft, as a high or a low note, or on a more general level, perhaps as noise, or as music.

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What is noise?

Noise is an important form of pollution caused by unwanted sound. At low levels noise can be a nuisance, but exposure to sustained high levels, for example in a noisy workplace, can cause hearing loss. Impulsive noise, such as the sound of a pneumatic tool, or tonal noise, such as the whine of a machine, can be particularly irritating. But what some people consider as noise, others can tolerate, or may even like, and so the study of noise has to recognize these different subjective responses.

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What is the difference between sound and noise?

Sound is what we hear. Noise is unwanted sound. The difference between sound and noise depends upon the listener and the circumstances. Rock music can be pleasurable sound to one person and an annoying noise to another. In either case, it can be hazardous to a person's hearing if the sound is loud and if he or she is exposed long and often enough. 

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How Noisy Is.....?

Noise

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Why is noise an important workplace hazard?

Noise is one of the most common occupational health hazards. In heavy industrial and manufacturing environments, as well as in farms, cafeterias, permanent hearing loss is the main health concern. Annoyance, stress and interference with speech communication is the main concern in noisy offices, schools and computer rooms.

To prevent adverse outcomes of noise exposure, noise levels should be reduced to acceptable levels. The best method of noise reduction is to use engineering modifications to the noise source itself, or to the workplace environment. Where technology cannot adequately control the problem, personal hearing protection (such as ear muffs or plugs) can be used. Personal protection, however, should be considered as an interim measure while other means of reducing workplace noise are being explored and implemented.

As a first step in dealing with noise, workplaces need to identify areas or operations where excessive exposure to noise occurs. 

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How can I tell if my workplace is too loud?

If you answer yes to any of the following questions, the workplace may have a noise problem.

• Do people have to raise their voices?
• Do people who work in noisy environments have ringing in their ears at the end of a shift?
• Do they find when they return home from work that they have to increase the volume on their car radio higher than they did when they went to work?
• Does a person who has worked in a noisy workplace for years have problems understanding conversations at parties or restaurants or in crowds where there are many voices and "competing" noises?

If there is a noise problem in a workplace, then a noise assessment or survey should be undertaken to determine the sources of noise, the amount of noise, who is exposed and for how long.

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What are some properties of noise that can be measured?

The properties of noise which are important in the workplace are:

• frequency
• sound pressure
• sound power
•  time distribution

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What kinds of noise are there?

Noise can be continuous, variable, intermittent or impulsive depending on how it changes over time. Continuous noise is noise which remains constant and stable over a given time period. The noise of boilers in a power house is relatively constant and can therefore be classified as continuous.

Most manufacturing noise is variable or intermittent. Different operations or different noise sources cause the sound changes over time. Noise is intermittent if there is a mix of relatively quiet periods and noisy. Impulse or impact noise is a very short burst of loud noise which lasts for less than one second. Gun fire or the noise produced by punch presses are examples of such noise.

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