Room Acoustics and Reverb
The space you use to record in can have a dramatic effect on the sound of your recordings. This article highlights some of the problems associated with recording in a domestic environment, and offers ways to improve acoustics.
Reverb is generated when a sound source produces energy which is reflected back to the listening position. The amount of reverb present in you recording space depends on a number of factors. The size of the room has a major impact, bigger spaces allow more reflections and therefore longer reverb times. For an instant impression of the reverb in your room, do a loud hand clap and listen how the sound fades away. In a small room the reverb may be hard to hear, but in a large rehearsal room or hall the reverb will be audible for several seconds.
The contents of the room also have a big effect. The reverb time in theatres is considerably shorter with a full audience present compared with the empty space. This is because all the bodies absorb a large amount of the sound energy. But back to the bedroom…
In small domestic spaces there is usually very little reverb to contend with. Any rooms in an average sized house usually have enough carpet and soft furnishings/curtains etc to make reverb time very short. If you plan to record in a garage or small hall type space there may be a lot of hard flat surfaces. These types of surfaces reflect sound back into the space unlike soft things which absorb it. In spaces such as this the room dimensions are relatively small and similar to each other. This means a mush of reverb will be created, as there will be many early reflections. In short it will sound like you recorded the drums in the garage.
This image shows the nicely spaced early reflections of a halls reverb which decays into densely spaced late reflections. 
The garage reverb has many early reflections but very few late reflections. 
So what to do if your mum won’t let you record drums in the front room and the only option is the garage amongst the un-used gym equipment? Well assuming you don’t want to spend big bucks on soundproofing professionally, the easiest way to reduce reverb time is with thick soft things. This could be duvets, pillows, couches, mattresses, thick curtains and any thing else you can find. Use some of the un-used gym equipment with soft stuff draped all over it and surround your drum kit, guitar amp and microphones. Put any spare soft stuff in corners and covering the walls, ceiling or floor. This technique will knock quite a bit off your mushy reverb. With a dry sound recorded you can add reverb with plug-ins or effects units that actually sounds good.
Room Modes
Another thing to consider apart from reverb are modal frequencies. These come in a variety of forms such as axial modes, tangential modes and oblique modes.
Axial modes occur between two parallel walls. The distance between two walls is equal to half the wavelength of the first axial mode.
Axial modes add significantly to the room’s frequency at their resonant frequency. In rooms with dimensions that are integer multiples of each other i.e 3m x 6m x 9m many axial modes occur in the same region and create a very large boost. The worst example of this would be a square room. The axial frequency can be calculated from the formula:
F = c / ( 2 x d )
Where F = 1st axial frequency (Hz), c = speed of sound (340 m/s) and d = distance between walls (m).
Therefore in a room 5 metres long, the first axial mode will be
F = 340 / ( 2 x 5)
F = 34 Hz
Axial modes will occur at multiples of this frequency i.e, 68 Hz, 102 Hz, 136 Hz etc. They will also occur between the other sets of parallel walls, assuming you are in a rectangular space.
Tangential and oblique modes are created by sound bouncing off more than two surfaces. For this reason they are absorbed by the walls more than axial modes and do not contribute as much to the rooms frequency response.
Generally room modes cause the biggest headaches in the bass region. If you are mixing in a room with closely spaced modal frequencies, the low end of mixes will sound very different to other rooms. Unfortunately there isn’t an easy way of moving walls. Pro studios often use bass ‘traps’ which absorb energy in a specific region. To overcome the problem at home you could try large furnature to break up the space, or consider moving to another room. Or if you’re feeling adventurous measure the response of the room and build a bass trap to suppress the problem area.
Posted on Sun 6th Sep 2009 12:18:27
