glossary & definitions


Sound is a mechanical pressure wave. When an object vibrates it creates a mechanical disturbance that is transferred through a medium such as air. The frequency of the sound waves depend on the frequency of the vibrating source. If the frequency is high then the sound wave will have a high frequency. Sound is measured by sound pressure level which is expressed as a power ratio and calibrated in decibels (dB). The ear has a large dynamic range in audio perception with the ratio of the quietest to the most immediately dangerous sound level (capable of causing permanent damage to the ear) being in the order of 1:1 trillion.


Noise is normally described as unwanted sound. What can be a pleasant sound to one person can be an unwanted noise to the next. 

sound levels & their perception

Noise is measured by sound pressure level which is expressed as a power ratio and calibrated in decibels (dB). The ear has a large dynamic range in audio perception with the ratio of the quietest to the most immediately dangerous sound level (capable of causing permanent damage to the ear) being in the order of 1:1 trillion.

Some typical values are given in the following table. Note that it is not a linear scale — going from 50dB to 100dB is not twice as loud but 16 times as loud and from 60dB to 30dB is not half as loud but one eighth as loud.

Regular exposure for more than one minute to 110dB risks permanent hearing loss, and prolonged exposure to any noise at or above 85dB can cause gradual hearing loss (NIDCD 2012).

Communities usually agree about what noise volumes are acceptable and what are not. The intensity of sound can be measured objectively in decibels, but our perception of what constitutes noise is affected by subjective factors. These include the type of noise (one person’s music might be another person’s noise), our mood, the time of day, background noise levels and our expectations. Sudden noises such as a motorbike exhaust or screeching brakes can be more disturbing than steady or expected noises. Frequency of noise may also have different impacts


Frequency is the rate, or number of times per second, that a sound wave cycles from positive to negative to positive again. Frequency is measured in cycles per second, or hertz (Hz). Humans have a range of hearing from 20 Hz (low) to 20,000 Hz (high). Frequencies beyond this range exist, but they are inaudible to humans.

decibel (dB)

The decibel measures sound pressure or electrical pressure (voltage) levels. It is a logarithmic unit that describes a ratio of two intensities, such as two different sound pressures, two different voltages, and so on. A bel (named after Alexander Graham Bell) is a base-ten logarithm of the ratio between two signals. This means that for every additional bel on the scale, the signal represented is ten times stronger. For example, the sound pressure level of a loud sound can be billions of times stronger than a quiet sound. Written logarithmically, one billion (1,000,000,000 or 109) is simply 9. Decibels make the numbers much easier to work with.

In practice, a bel is a bit too large to use for measuring sound, so a one-tenth unit called the decibel is used instead. The reason for using decibels instead of bels is no different from the reason for measuring shoe size in, say, centimetres instead of metres; it is a more practical unit.

The decibel unit is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. The decibel confers a number of advantages, such as the ability to conveniently represent very large or small numbers, and the ability to carry out multiplication of ratios by simple addition and subtraction. By contrast, use of the decibel complicates operations of addition and subtraction.

the UNSW School of Physics has published an interesting page with detailed definitions of decibels dB dB(A) dBA dB(C) dBV dBm dBi and more... see

sound transmission class (STC)

An internationally standardised method of rating the sound transmission loss of partition walls to indicate the sound reduction from one side of a partition to the other in the frequency range of 125 Hz to 4000 kHz. (Refer: Australian Standard AS1276 – 1979). Now not in general use in Australia see: weighted sound reduction index. (source: AAAC)

weighted sound reduction index, Rw

This is a single number rating of the airborne sound insulation of a wall, partition or ceiling. The sound reduction is normally measured over a frequency range of 100 Hz to 3.150 kHz and averaged in accordance with ISO standard weighting curves (Refer AS/NZS 1276.1:1999). Internal partition wall Rw + C ratings are frequency weighted to simulate insulation from human voice noise. The Rw + C is similar in value to the STC rating value. External walls, doors and windows may be Rw + Ctr rated to simulate insulation from road traffic noise. The spectrum adaptation term Ctr adjustment factor takes account of low frequency noise. The weighted sound reduction index is normally similar or slightly lower number than the STC rating value. . (source: AAAC)

sound transmission loss

The amount in decibels by which a random sound is reduced as it passes through a sound barrier. A method for the measurement of airborne Sound Transmission Loss of a building partition is given in Australian Standard AS1191 - 2002. (source: AAAC)

airborne noise

Airborne noise comes from common sound sources such as voices, TVs and radios. The noise performance of a building structure is called the Sound Transmission Class (STC). The higher the STC the better the structure is at isolating airborne noise. An STC rating of 45 means that the sound passing through the building is reduced by 45dB.

impact noise

Impact noise occurs when an object collides with another object, and you usually hear it from the floor above you. It could be footsteps, a chair sliding across a wood or tile floor, or an object falling on the floor. Impact noise travels freely through a structure and through air pockets.


Weighted, standardised impact sound pressure level, which is a single number rating of the sound level in a floor/ceiling assembly in terms of its ability to control structure-borne sound in a receiving room when excited by a standard impact testing machine, as measured under field conditions. The lower the L’nT,w, the better the acoustic performance.

L’nT,w +CI

Combined weighted, standardised impact sound pressure level plus spectrum adaptation term, used to rate the impact sound insulation performance of floor/ceiling systems. The lower the L’nTw+CI, the better the acoustic performance.


The Association of Australian Acoustical Consultants - see

table 4: AAAC star rating system
AAAC star ratingimpact isolation of floors Lnt,wAAAC descriptionapprox IIC rating
2 starLnt,w 65clearly audibleapprox IIC 45
BCALnt,w 62BCA clearly audibleapprox IIC 48
3 starLnt,w 55clearly audibleapprox IIC 55
4 starLnt,w 50clearly audibleapprox IIC 60
5 starLnt,w 45audibleapprox IIC 65
6 starLnt,w 40just audible or barely audibleapprox IIC 70
source: AAAC

We note that the Australian Association of Acoustic Engineers (AAAC) the IIC rating system has been replaced by a measurement called LnT,w (extract from AAAC literature):

Structure borne noise includes footsteps on hard floors, scraping chairs and dropping objects. For this type of noise, Australia and New Zealand have adopted the measurement called the weighted standardised impact sound pressure level LnT,w. A reduction in this parameter corresponds to an improvement in impact isolation. This replaces Impact Insulation Class (IIC), which is in common use in Australia. The AAAC rating system uses LnT,w. There is an approximate relationship between LnT,w and IIC. Either value can be subtracted from 110 to approximately indicate the other.


we have prepared our glossary to put the most important concepts first, use the search bar below if you need specific information - or contact us

table 1: sound levels and their perception
Sound level (dB)approximate loudness relative to ordinary conversationperception example
0dBdon't hear anythingthreshold of hearing
10dB1/32 as loudvery faint normal breathing
20dB1/16 as loudquiet room
30dB1/8 as loudquiet conversation / quiet office
40dB1/4 as loudmoderate quiet office / quiet rural office
50dB1/2 as loudquiet suburban area / dishwasher in next room
60dBordinary conversationaverage office / ordinary conversation
70dB2 x as loudloud busy street / vacuum cleaner at 3m
80dB4 x as loudnoisy office / passing car at 3m
90dB8 x as loudvery loud heavy traffic / passing bus or truck at 3m
100dB16 x as loudloud car horn / passing subway train at 3m
110dB32 x as loudpop group / night club with band playing
120dB64 x as loudextreme; jet take off at 100m
table 2: subjective perception of sound energy reductions
reduction in dBpercentagereduction in sound energy subjective perception
3dB50%barely perceptible
6dB75%sound appears to be reduced by about one quarter
7-9dB87%major reduction
10dB90%sound appears to be less than half
table 3: effect of sound transmission classes on speech perception
sound transmission classeffect on speech perception
25 normal speech can be heard easily
30loud speech can be heard easily
35 Loud speech can be heard but not understood
42Loud speech heard as murmur
45Must strain to hear loud speech
48 Loud speech can be barely heard
53Loud speech cannot be heard