Acoustic concepts

An alphabetical overview of a number of key concepts in acoustics.


Acoustics is the science of sound. There are different areas of the acoustics field:

  • Environmental acoustics = INSULATION
  • Room acoustics = ABSORPTION
  • Building acoustics = INSULATION
  • Medical acoustics = INSULATION

Absorption coefficient (α)

The absorption coefficient is a measure of the amount of sound that is not reflected from the surface of an object as a proportion of the total amount of sound to which that surface is exposed.

The value is always between 0 (if all sound is reflected) and 1 (if all sound is absorbed). In practice, these extremes are not reached, sound is always partially reflected if it strikes a surface of a solid object during its dispersion through the air. It is a property of the material of which the surface is made.

In general, hard materials such as glass, steel and concrete have a low absorption coefficient (so they reflect a lot of sound) and soft materials such as wool, down and rubber have a high absorption coefficient (they reflect less sound). In other words, the less a material bounces, or dents and rebounds, when it is hit by the sound pressure exerted by the air, the more sound it reflects.


Sound consists of vibrations that propagate through a medium. In most cases this is air (airborne noise), but these vibrations can also propagate via contact surfaces (contact noise).

With airborne sound, the sound travels through the air. In case of contact noise, the sound spreads through materials. For example: stamping feet in a room causes contact noise because the solid materials of the construction are made to vibrate. The vibrating construction can then emit airborne sound in the space below, which can be heard there by an observer. In this example, the space below acts as a sound box and through which the vibrations of the construction are converted into audible sounds. Because of the initial spread of the sound through the construction in this example, this is referred to as contact noise.

Sound absorption

The conversion of acoustic energy into mechanical energy (vibrations) and / or thermal energy (heat). (The sound actually disappears in the material).
Sound absorption captures the sound waves within IN a room.
The amount of absorption is a property of a material and is expressed by the absorption coefficient α.

Sound insulation

Sound insulation or acoustic insulation is the shielding of a room against noise from adjacent spaces (or from the outside world). This isolation can also work in the opposite direction.

Sound insulation distinguishes between airborne noise and contact noise!

Sound insulation depends on the frequency and is normally measured between 100 and 3150 Hz. Airborne sound insulation can be expressed in single-number values, Dn, f, w, Rw or R'w. Contact sound insulation can be expressed in the single number value Ln, w or L 'n, w.

Sound vibration

Sound vibrations are the vibrations of the atoms or molecules of the medium in which the sound propagates.

These vibrations occur in the same direction as that in which the sound propagates. So sound propagates as a longitudinal wave, unlike light or water waves, which manifest as a transverse wave.

The sound vibrations that occur in the air cause a vibration in the eardrum, which enters the cochlea through the middle ear, after which it is heard as sound.

The quantized longitudinal vibrations that occur in solids at very high frequencies are called phonons.

Noise reduction coefficient (NRC)

A numerical value for sound absorption, ie showing the reduction of reverberation, in accordance with ASTM C 423, determined as the average of the absorption coefficients at 250, 500, 1000 and 2000 Hz.

Noise level (dB)

The sound level L is the logarithmic ratio of the sound pressure p and a reference value p0. It has been internationally agreed to use 1 * 10-5 as the reference value, which is approximately the intensity at the hearing threshold.

The noise level is thus expressed in decibels (dB). The noise level in a normal working or living environment varies between approx. 30 and 80 decibels.

Sound pressure level (dB)

The pressure variation caused by sound waves in the air is called sound pressure. The lowest sound pressure level that can be perceived is 0 dB, known as sound threshold. Sound pressure varies greatly in practice.

The sound pressure level of 120 dB is so high that the sound is experienced as painful, this is called the pain threshold.

Sound levels (dB)

Loudness is a popular term that is not well defined scientifically. In practice, this is a very subjective term and is perceived differently by each person: what is a disturbing strength for some is barely perceptible to others.

This volume depends on the power that the sound transmits to our ear. Since sound consists of small, rapid changes in pressure around atmospheric pressure, the power developed depends on the pressure.

Measured in dB (decibels). dB is measured in different frequencies. dB (A) (or LpA) is a single number value used for the total volume for all frequencies tuned to the sensitivity of the ear. dB (C) (or LpC) specifically targets low frequencies and better reflects how the sound is received by people with hearing impairment.


A silencer is a construction that aims to reduce sound pressure. A silencer is usually applied to an air outlet. This could be an exhaust duct of a fan or a combustion engine. But also a double wall or an earmuff can function as a silencer for, for example, ambient noise.

There are basically two different acoustic silencers:

  • acoustic absorption silencers: are lined on the inside with a material that converts the energy of sound vibrations into another form of energy, in almost all cases heat.
  • acoustic reflection silencers: lower the sound pressure for certain frequencies of the sound by making a transit time difference in the vibration. This creates interference, which decreases the amplitude in certain places. In fact, the total energy of the sound wave does not decrease, so that we cannot actually speak of silencing.

In practice, however, each silencer is to some extent hybrid, operating on both the absorption principle and the reflection principle.


(Post)reverberation is a sound phenomenon that arises from repeated reverberation of sound. The sound source itself may or may not be audible.

Reverberation time

(Post) reverberation time is the time that a sound phenomenon in a room can still be heard when the sound source has already stopped. The reverberation time depends on the frequency. Measuring the reverberation time allows us to calculate the total sound absorption in the room.

Speech frequency range

The average frequency range of speech is between 500 Hz and 2000 Hz, with children producing sounds that are more in the higher frequencies, men with deep voices producing sounds that are more in the lower frequencies and other adults are nicely in between.

Speech intelligibility

Speech intelligibility depends on the level of background noise, the reverberation time of the room and the shape of the room.

(reference: wikipedia)

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