Acoustic Glossary of Terms


The amplitude is a physical measure related to the loudness of Acousticthe sound. The ear responds to sound pressure, which is measured in units of Newton/ m2 also known as Pascal (Pa). Calculations often start with another quantity, sound power, which is the rate of sound energy output of a source and can be measured in watts. Sound power is an inherent property of a sound source. Sound pressure is dependent on the surroundings. This is illustrated by considering the analogy with an electric fire. The heat 'amplitude' of an electric fire is rated in watts, but the temperature, which is experienced, also depends on the distance from the fire and the thermal insulation of the room. Similarly sound power in watts is the output from the grille, whilst the sound pressure, which is heard, depends on factors such as distance and sound absorption of the room.


Frequency corresponds to the pitch of a sound. High frequency sounds are heard as high pitched and low frequency sounds are heard as low pitched. The unit of frequency is the Hz and is defined as the number of cycles in one second.

The time taken to complete one cycle is defined as the time period and is measured in seconds.

Airborne Noise

The term airborne transmission or airborne sound is conveniently taken to mean sound that is transmitted mainly, but not necessarily exclusively, through the air. For instance, sound transmitted from one room to another may have to excite the adjoining wall into vibration.  Available sound paths, such as the walls, floor, ceiling, building framework and ductwork, connecting rooms etc, can transmit airborne sound. 

Sound Reduction Index Rw

Results according to ISO 717-1: Rw ( C ; Ctr ) 

C - adaptation term for pink noise
Ctr - adaptation term for typical traffic noise
Rw + C is the reduction of A-weighted sound pressure level for pink noise, Rw + Ctr for traffic noise.

Impact Noise

The most familiar impact source is footsteps upon a floor surface, particularly in multi-storey dwellings; other examples include water hammer in pipework, slammed doors and vibrational excitation originating at machinery, even light switches etc. What is needed to reduce impact transmission is a suitably large impedance between the source of impact and the radiating surfaces. This may be achieved by mass, by the use of resilient materials or a combination of both. Lightweight constructions (e.g. timber separating floors) are likely to cause problems because they need to be very stiff for structural reasons, with a consequential loss in sound insulation.

Noise Absorption

Noise absorption is expressed as a factor between 0 and 1.0.  If no sound were absorbed at a surface, its coefficient of absorption would be 0.  If all the sound is absorbed, the coefficient is 1.0. 

Depending on the type of room, the reverberation time within the room might be important. To control the reverberation time noise absorbing materials are used.

Reverberation Time

This is the time it takes for a sound to decay by 60 dB. Sound does not die away the instant it is produced but will continue to be heard for some time because of reflections from walls, ceilings, floors and other surfaces. It will mix with later direct sound and is known as reverberant sound.


Sound can be transmitted into a room by the surrounding structure.This can be through the:

  • separating wall
  • external wall
  • floor construction
  • ceiling construction

Airborne sound in the source room excites the separating partition into vibration, which directly radiates the sound into the receiving room. The amount of attenuation will depend upon the frequency of sound, the mass, fixing conditions and thus the resonant frequencies of the partition. 

  • Airborne sound in the source room may excite walls other than the separating one into vibration. The energy is then transmitted through the structure and re-radiated by some other partition into the receiving room.
  • Any wall other than the separating one may be excited. The sound is transmitted to the separating wall and then re-radiated by it.
  • Sound energy from the separating partition is radiated into the receiving room by some other wall.

Sound Absorbers

There are three main types of sound absorbers:

  • The porous or dissipative absorber
  • The membrane absorber
  • The cavity absorber

Each absorbs sound energy by converting it to heat, although the method is different in each case.  Also the frequency response of these types of absorber is different. 

The Decibel (dB)

The decibel (dB) scale - a scale based on the logarithm to the base 10, is often used because the ear is sensitive over an extremely large order of sound power values and does not tend to judge sound powers in absolute terms but does judge how many times greater one power is over another. This enables one source to be related to any other source.

Sound Power

Sound power is the rate of sound energy output of a source and is measured in Watts. Sound power is an inherent property of a sound source.

Sound Pressure

Sound pressure is that which is heard and measured and is dependent on the surroundings.

The heat amplitude of an electric fire is rated in Watts, but the temperature which is experienced, depends on the distance from the fire and the thermal insulation of the room.  Similarly, sound power in watts is the output from the grille, whilst the sound pressure heard depends on factors such as distance and sound absorption of the room.

Relationship between Sound Power Level & Sound Pressure Level

Although Sound Power Level and Sound Pressure Level are different, a given increase in Sound Power Level produces an equal increase in Sound Pressure Level.

Sound Transmission Class (STC)

The Sound Transmission Class (STC) is a single-number rating of a material 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.