Introduction to Room Acoustics


In this new series, we would like to introduce some basic concepts in audio research in such a way, that it helps you to wrap your head around it easily. Nevertheless, we are of course only scratching the surface here. If you want to go more in-depth in audio research, we recommend you to read our publications:

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In this article we will focus on room acoustics and why they are such an important aspect of our immersive audio technology. Whenever we set up one of our systems, one of the first steps is to measure the acoustics of the room the system will be used in.

But let us start from the very beginning and have a look at what acoustic actually means. Acoustic is the science of sound. It centers around the emersion of sound, its propagation from source to receiver as well as perception of sound and its effects. Room acoustics is a subcategory of the field of acoustics. It specifically thematizes sound fields that propagate into a room. Every room has a certain set of characteristics, size, material of the surfaces, furnishing, etc., which influence sound events happening inside of it. Sound-absorbing or reflecting material, for example, can be added to a room to alter its acoustics.The room’s acoustics for one receiver-source combination inside the room can be roughly described by its room impulse response, which is usually structured in several characteristic parts. Whenever we talk about a sound coming straight from its origin to the listener’s ear we call this direct sound. Reverberation describes the effect that, in a closed space, sound waves are reflected from each surface.

Usually, unless the room is very large, we hear the reflections just as a modification of timbre, not as separately audible sounds. Reverberation includes everything that you hear apart from the original sound source. When we include reflections of reflections into this observation, the sound ‘travels’ through the room filled with air and while this happens, it repeatedly collides with different materials and the loudness decreases.

The time frame in which the loudness of the sound is reduced by 60dB from is origin is called the T60 reverberation time. The reverberation time increases with the size of the room and the amount of reflecting material inside of it. Sound-absorbing material, like furniture, reduces the reverberation time. In our Lab in Ilmenau for example, we use sound absorbing panels not only to alter reverberation time but also to reduce the so-called “flutter echoes” which can occur if sound travels back and forth between two reflecting surfaces rapidly without losing a lot of energy quickly. The resulting fluttering sound clearly stands out from the diffuse reverberation and is usually not favored.

Between the direct sound and the reverberation, there is another important aspect describing the room’s acoustics, the so-called early reflections. These include the first one to three reflections that reach the listeners’ ears, usually within 15 milliseconds. These are especially important, because of their psychological effects. Our brain is able to deduct information such as the room size and direction of the sound from the early reflections, even without seeing it.

To get all three parts – direct sound, early reflections, and the diffuse reverberation, of the room impulse response of a given room, we conduct measurements with an omnidirectional microphone, when setting up a system. This special type of microphone captures sound coming from all different directions, from front, sides, behind above and below.

    The incoming signals are recorded with equal strength, which makes the omnidirectional microphone an ideal tool to record the sound of a room. We play a logarithmic sinus sweep over a loudspeaker, which is then captured by the microphone, so we get the room impulse response containing the reflections of the room we are in. This room impulse response is then used by our algorithm, to simulate the virtual room acoustics. This process only takes a few minutes of setup time.