Sound absorption, reverberation and acoustics
Spending long periods in rooms with poor acoustics is bound to have a negative impact on our health. It can lead to increased levels of stress, poor concentration and even depression. Likewise, sound quality will never be quite right in a room with poor acoustics. Comfortable acoustics are therefore indispensable. Problems with acoustics stem from levels of reverberation, which need to be neutralised where possible. This is best achieved by using soft, sound-absorbing materials. The more sound these materials absorb, the higher their sound absorption coefficient, represented with α. Sound absorption reduces the reverberation in a room, thereby creating perfect acoustics. Curtains, especially dimout curtains, absorb sound very well, sometimes even better than acoustic panels.
Noise or sound is a very natural phenomenon we experience as early as in the womb. There are two dimensions to sound: frequency and noise levels.
- The frequency determines the pitch (the tone) of sound, the number of times the sound vibrates per second. This is expressed in Hz (Hertz). Humans can hear frequencies between 20 Hertz and 20.000 Hz (20 kHz). Low-frequency sound refers to frequencies below 100 Hz (e.g. bass frequencies during a musical performance), whereas high-frequency sound refers to frequencies above 4000 Hz.
- Noise strength is actually the power, the volume.
Reverberation occurs when sound is reflected. In a church, for example, you hear the sound of the priest repeat itself after 1.5 seconds. This is the disturbing effect in the acoustics of a room. We are not used to this as our evolution has always taken place in the ‘open air’. If the time span of human existence were compared to a 24-hour clock, we would only have been working/living indoors for the past 30 seconds.
On the other hand, reverberation isn't always a bad thing either. It forms part of the sound we hear, giving it its particular timbre.
Reverberation magnitude refers to the reverberation or echo time, in other words the amount of time that a sound is still audible in a room after its source has stopped emitting sound.
It is therefore important to absorb the sound, thereby reducing its reverberation. The noise has to go somewhere! This is achieved by using soft materials such as foam, textiles, etc. By contrast, glass and concrete, materials that are often used in modern-day construction, are ideal ‘reflectors’. The degree to which sound is absorbed can be measured and is displayed as the absorption coefficient.
The acoustic absorption coefficient (αw) indicates the ability of a material to absorb sound wave energy. This coefficient ranges from 0 to 1. The higher this number, the more absorbent the material. For open windows, the value stands at 1 (or 100% absorption), for dimout curtains 0.7 (or 70% absorption), for rough concrete 0.1 (or 10% absorption) and for glass 0.05 (or 5% absorption). Anything that isn't absorbed is subsequently reflected!
The Sabine formula indicates that we can influence the reverberation time by making surfaces more or less absorbent!
The absorption coefficients of our HADES dimout fabric:
