ACOUSTIC GUIDE

Sound

Sound consists of acoustic oscillations which can take very complex and sophisticated form.

 

These periodic oscillations in fact are mechanical waves transmitted to our ears by particles of air. How does it work? Imagine the infinite network of microscopic balls bound with springs. In case even one or these balls gets pushed out of balance, it transmits kinetic energy in mechanical way – by the springs which represent molecular bond. The speed of oscillations is defined by frequency of sound which propagates from theoretical point source in every possible direction as straight lines, forming spherical shape. The speed of sound propagation for given medium is constant but it differs a bit according to the temperature. In the air of 20 °C it’s almost 344 m/s, so the sound travels almost 300 m in 1 second.

In case of high levels and low frequencies these oscillations affect human body also through vibrations transferred by solid materials.

Remember: Sound is acoustic wave which propagates in the same direction as it oscillates.

Acoustic properties

All our acoustical products are marked with dedicated icons to help you navigate through their different properties. We designed them to be picked up easily by specified function referring to acoustic behaviour explained below.

Absorption is ability to reduce sound level. It means sound wave – instead of further propagation – is damped because its kinetic energy is captured and dissipated inside specific material and structure, turning into heat. The quality and level of absorption is described as sound absorption coefficient? It ranges from 0 (no absorption) to 1 (whole energy absorbed). Most common examples are soft and porous materials such as textiles or sponge.

Reflection is totally opposite to absorption – sound wave bounces off the surface it strikes, similar to light wave reflecting from the mirror and creating light streaks on the wall. Sound behaves in the same way – it bounces off and propa¬gates in another direction. In case of flat and rigid surface sound wave is reflected at the same angle it hits that surface. Reflections take form of reverb or echo inside a room.

Attenuation is reducing sound level between different acoustic environments. It’s done by soundproofing, which is the most effective solution to common problem: noisy neighbourhood. Attenuating materials dramatically reduce external sound energy thanks to acoustic insulation, thus reducing unwanted noise.

Diffusion means multiple reflections. Once the hard surface takes waved, rugged and irregular shape, reflections are dispersed in different directions and sound wave becomes more chaotic and a bit lower. This results in lively and vivid sound that makes impression of bigger space.

Remember: Optimum acoustic conditions in your own environment can be achieved by proper balance between reflection, absorption and diffusion.

Acoustic parameters

On the basis of all phenomena explained above, there were some factors created in order to measure quality of materials impact on acoustical properties of the room.

Sound absorption coefficient (?) is the ratio of energy absorbed by a material to the energy hitting its surface. Ranging from 0 (no absorption at all, full reflection) to 1 (full absorption – whole energy is absorbed).

Scattering coefficient (s) is defined as the ratio of specularly reflected acoustic energy to the total of reflected acoustic energy, subtracted from 1.

Speech Transmission Index (STI) describes intelligibility of speech. Its numeric representation varies from value 0 – bad communication characteristics to 1 – excellent.

Materials inside a room have great impact on a parameter called a reverberation time (Rt). It defines how much time, after switching off the sound source is needed to achieve sound pressure level reduced by 60 dB in comparison to the state with sound source on.Shortly – the more absorptive materials in the room, there’s lower reverberation time Every acoustic environment has optimal suggested reverberation time which suits its intended function. Acoustic adaptation is based on this value and materials with particular sound absorption coefficients are proposed to reach this desired Rt.Remember: Reverberation time is the most universal parameter describing room acoustics.

Shortly – the more absorptive materials in the room, there’s lower reverberation time Every acoustic environment has optimal suggested reverberation time which suits its intended function. Acoustic adaptation is based on this value and materials with particular sound absorption coefficients are proposed to reach this desired Rt.

Remember: Reverberation time is the most universal parameter describing room acoustics.

 

Noise reduction

To understand what sound pressure level and is, look at the drawing below. It illustrates noise levels with simple, everyday life examples.

SDecibel (dB) is unit which is used to measure sound pressure level (SPL). It tells you how high it is by comparison of its temporary to reference value in logarithmic scale. Simply, it shows you how loud the sound is. Noise is a loud and unwanted sound which results in unpleasant effect. Depending on conditions it can be just a bit or much louder than the rest of acoustic background. In case of too high noise levels, acoustic insulation should be introduced. In other words, soundproofing means reducing sound pressure (volume) in receiving room with distance and objects affecting the unwanted sound.

It can be defined by Weighted Sound Reduction Index (Rw) or Weighted Level Difference (Dw).
Sound Reduction Index Rw is a difference in decibels [dB] between average sound pressure level in the source room and receiving room. It can be laboratory or field measured.

Remember: Once SPL is 3 dB higher, the sound is twice louder.

Reverberation vs echo

What happens when sound bounces back, unstopped by very poor acoustic conditions? Let’s take a look at two types of reaction on the drawing.

Reverberation is perceived when reflected sound wave reaches your ears in less than 0.1 second after the initial sound wave. There is no hearable time span between two sounds, they simply “merge” into one prolonged sound wave – reverberation.
Echo is perceived when the delay of reflected sound is longer than 0.1 s. You can expect it in larger spaces as sound can reach 30 m distance in 0.1 s.

Remember: Echo can be described as a longer reverb. In terms of proper acoustic conditions both phenomena are unacceptable and should be reduced.