Technical insulation is not only about heat — it is also about sound. We go through what sound and decibels are, the difference between sound insulation and sound regulation, the absorption classes A–E per ISO 11654, reverberation time, and practical noise reduction of machines.
What is sound, and what is it measured in?
What the ear perceives as sound is in reality pressure waves in the air. Sound is described by two quantities. The pitch (wavelength) is measured in hertz (Hz) and states the number of oscillations per second — a low frequency gives a long wavelength and a deep tone, a high frequency gives a short wavelength and a high tone. A healthy human ear can hear from about 20 Hz to 20,000 Hz; below 20 Hz it is infrasound and above 20,000 Hz ultrasound. The strength of the sound (the sound pressure) is measured in decibels (dB). It is the decibel level, not the frequency, that is most decisive for whether the sound is harmful. A further distinction is made between airborne sound, structure-borne sound (vibrations in solid bodies, e.g. impact noise) and noise from outside.
Decibels, frequency and A-weighting
Indoors, the ear perceives an increase of 10 dB(A) as a doubling of the loudness, and a reduction of 10 dB(A) as a halving — outdoors, 6 dB can make the same difference, because sound in the open is attenuated by about 6 dB each time the distance to the source doubles. A person with normal hearing can distinguish a change of 1 dB. The ear does not perceive all frequencies equally strongly; deep tones are heard more faintly than high ones. Therefore sound level meters are equipped with an A-filter that attenuates the deep tones, so that the measured value — dB(A) — corresponds to the way we hear.
- 01Bass: below 500 Hz — hardest to attenuate.
- 02Mid-range: 500–1,500 Hz.
- 03Treble: above 1,500 Hz — easiest to attenuate.
Two methods: sound insulation and sound regulation
A distinction is made between two methods. With sound insulation, you prevent the sound from propagating from one place to another. With sound regulation, you regulate the room reverberation (the reverberation time) by lining with sound-absorbing material. When a sound wave strikes a material, the energy is split into three: part is reflected, part is absorbed, and part is transmitted onward. For a typical sound-absorbing ceiling lining, about 70 % of the noise is converted into heat in the material, about 5 % is reflected, and about 20 % is transmitted onward.
Absorption classes per ISO 11654
For building materials, sound absorption is measured per 1/3 octave from 100 Hz to 5000 Hz. The absorption coefficient lies between 0.00 and 1.00, where 1.00 means that all sound is absorbed and 0.00 that all sound is reflected. ISO 11654 divides materials into absorption classes A to E by comparing the αp values with fixed reference curves. Class A is the best possible absorption.
| Class | Absorption coefficient αp |
|---|---|
| A | 1,00 – 0,90 |
| B | 0,85 – 0,80 |
| C | 0,75 – 0,60 |
| D | 0,55 – 0,30 |
| E | 0,25 – 0,15 |
| Not classified | 0,10 – 0,00 |
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