Wikipedia defines acoustics as “qualities of a room that, together, determine its character with respect to auditory effects.” Essentially, it is how sound interacts with a space.

It is important to have an understanding of acoustics in order to know how to successfully capture sound.

Acoustics and Recording

There are four different things that can affect a recording in terms of room acoustics; room size, absorption, reflection and diffusion.

Room size

In a small room, you can get a build-up of low frequency sounds, which can cause muddiness in your recording.

In a large room however, due to surfaces being further apart from each other than they are in a small room, reflected sounds have longer reverberation times. This can reduce the clarity of a sound or recording. This mainly impacts frequencies from the 125-4,000 Hz range. This is the part of the frequency spectrum where vocals sit which is why they are normally recorded in a vocal booth to absorb reflections.

Absorption

A room with lots of absorbent materials in it will result in a ‘dead’ room, meaning that no sound can be reflected. This can good for avoiding recording extraneous noise or unwanted reflections. On the other hand, it can result in an unnatural sounding recording, which is where artificial reverb comes in. Too much absorption can also result in a lack of high end frequencies.

Reflection

Reflection of sound is when a sound ‘bounces’ off a surface back into a space. Recording in a live room where there are lots of reflective surfaces can result in phasing issues and a lack of clarity. This is due to the fact that the original sound and all the reflected sounds will arrive at the microphone at slightly different times.

Diffusion

Diffusion is the even spreading of sound energy in a space. A well diffused room is one in which the reverberation time is the same at any given point in the room. Very few rooms are completely diffusive, but the closer you can get to a fully diffusive room, the better your recording will be. This is due to the fact that differing reverberation times can cause your recording to lack clarity.

Controlling Acoustics

There are a number of ways of controlling the acoustics of a room; isolation booths, acoustic panels and reflection filters.

Isolation booths

As mentioned above, it is common for vocals to be recorded in isolation booths. This is also common for recording drums and guitar/bass amps too. Isolation booths help reduce unwanted reflections and external noise from being recorded. They can be especially useful for when an ensemble wants to be recorded all at the same time – having the vocalist in an isolation booth can reduce spill.

Acoustic panels

Acoustic panels are designed to absorb a range of frequencies in your recording room. They will differ in size and have different sized ridges to absorb different frequencies.

Reflection filters/shields

Reflection filters are used when recording vocals and are fitted around the microphone. They are designed, just like acoustic panels, to absorb any unwanted reflections, or rather any sound waves that don’t make it to the microphone before they become reflections.

Problems with acoustic treatment

One disadvantage of reducing the natural reverb of a room too much is that the recording can end up sounding unnatural. Some producers prefer to keep using the natural reverb of the room but install diffusion panels to try and reduce unwanted reflections.

Another way to add natural sounding reverb to your recording without too many unwanted reflections is to record in a ‘dead’ room but then send a copy of that signal to a reverb chamber or a room with some pleasant natural reverb, record the sound in that room and mix it in with the original signal again.

Acoustic Parameters

• Reverb tail: the reverb tail is the very last part of a reverberation. It is at this point that the reverb is slowly decaying. It will take longer for the sound to decay (meaning a longer reverb tail) in a large room than it will in a small room.

• Pre-delay time: this is the time, in milliseconds, between the original sound being produced and the early reflections being audible. The further the sound source is away from reflective surfaces, the longer the pre-delay time will be.

• Reverb time (RT60): reverb time is the time (in milliseconds) it takes for the sound to fade away. It is said that the sound has decayed when the sound pressure level has reduced by 60 dB.