Basic principles of room acoustics

The physics of sound propagation is quite complicated. However, since sound is an everyday part of our lives, it is not at all difficult to realize all the basic principles of how sounds behave in a room. This beginner's guide will help you understand the basics of room acoustics and explain why it is important to address room acoustics not only in music studios and rehearsal rooms but also in homes or commercial spaces.

Room acoustics

The acoustics of the room as a discipline involves studying and analyzing direct and reflected sound. Appropriate acoustics are essential in all spaces where the sound is transmitted to the listener, which includes both speech and music. The room acoustics design criteria are ideally set according to the intended use of the room. For example, it is advisable to use more absorbent surfaces in music studios than in offices or living rooms, in which we are not primarily focused on achieving the best musical experience. Of course, it is possible to create a suitable acoustic solution for both speech and music in the same space, but this is often achieved with some compromise.

Propagation of sound waves in a room. They can be reflected from walls, floor and ceiling, absorbed by sound-absorbing material and scattered from diffusor.

Basic principles

The main difference between indoor and outdoor sound propagation is in the level of reflected sound. Naturally, the indoor environment creates a more reflective sound than the outdoor environment because the sound waves bounce off the walls of the room and the equipment used there. The way the sound is reflected depends on the shape of the room and the structure of the materials used.

Sound reflection

For direct reflections, the angle of incidence is equal to the angle of reflection, so the sound waves thus reflected usually occur on relatively smooth surfaces (on walls). However, when a sound wave strikes a rugged surface, the sound is further reflected in a diffuse manner. In this case, the reflection is fragmented into many reflections of lower intensity, which are scattered at a wide-angle and create a uniform sound field in which we achieve the best listening experience.

Since the sound waves reflect from all the suitable surfaces of the room (mostly walls), we use absorbent materials to attenuate some of them to decrease the effect of reflected sound on the sound we can hear. To get more insights into the effect of acoustic treatment on the propagation of sound waves, visit our acoustic helper.

Behavior of the sound wave on the contact with the bare wall, absorptive panel and diffusor.

Echo

An important element in room acoustics is an echo. This phenomenon has been encountered since forever, most often in large rooms, caves, tunnels, etc. Echoes are reflections that can be heard clearly and separately from the direct sound - it is the sound that arrives at the listener with a delay after the direct sound. Usually, the echoes are heard due to intense reflections coming in 40 ms and more after the direct sound signal reaches the listener. In other words, the difference in path length between direct sound and reflected sound is at least 13.8 meters, which corresponds to the propagation time of 40 ms or more.

Note that echoes are most often detected in the front rows of the auditorium and on stage. This is because the front row is furthest from the rear wall, creating the largest difference in length between the direct sound and the sound reflecting directly from the rear wall, or a combination of the ceiling and the rear wall. Sometimes only an interpreter or lecturer can perceive an echo in this case! You can already imagine a basic acoustic solution for this problem: the use of absorbent or diffuse materials on the rear wall, from which the sound waves would not bounce back to the stage.

Illustration of how echo can be heard on the stage of the auditorium. The presenter hears their own speech with a delay that is caused by the reflected sound from the back wall and ceiling.

A fluttering echo occurs where sound moves back and forth between two parallel surfaces and is attenuated much more slowly than reflections from other surfaces. We often register echoes of this type in small rooms in which the walls are only a few meters apart and at frequencies of 250 Hz and above.

An example of fluttering echo: in a small room with parallel bare walls, the sound waves are reflected many times from the opposite surfaces. The sound wave takes a significantly longer time to decay than in a room where the sound waves are absorbed by acoustic treatment.

Reverberation time

Reverberation time is a measure used to quantify the reflection of sound waves. Reverberation is the time required for the sound reflections to decay by 60 dB, ie. until the sound is no longer audible and is lost in space. The reverberation time is directly proportional to the volume of the room and indirectly proportional to the amount of sound-absorbing material. The more reflections of sound waves in the room we observe, the longer reverberation time can be measured. The reverberation time can be most easily reduced by using an acoustic treatment.

Sound-absorbing materials

All materials have certain sound-absorbing properties. Sound waves that are not absorbed must therefore be reflected, transmitted, or scattered. The sound-absorbing properties of a material can be described as the sound absorption coefficient in a certain frequency range.

The coefficient can be considered as a percentage of the absorbed sound, where 1.00 is complete absorption (100%) and 0.01 is minimal (1%). The most effective sound-absorbing materials are used in acoustic absorbers, which are most suitable for acoustic treatment for music studios, home theaters, listening rooms, or offices.

Acoustic absorbers are widely used on more than 50 % of the wall area in music studios.