Acoustic Treatment in Home and Project Studios
by Simon Duggal
In an untreated room reflections or echos off the walls, ceiling, floor and furniture can arrive at the ear very soon after the direct sound from the speakers. If these reflections arrive within 20 milliseconds of the speaker sound, the brain can’t distinguish a difference between them. So what you think is the sound of your speakers is actually a combination of direct and reflected sound.
Different frequencies reach the ear at different times depending on what type of surface the sound has bounced off and how far away from the listening position that surface is. For example, you’ve recorded a bass part and all the notes in the part sound fine except for one which is a great deal louder than the others. You might try automating a level reduction whenever this note sounds or try compressing the bassline in an attempt to squash this note back into place. After spending some time dealing with this troublesome note you finish your mix and play it in your car only to find that now you can’t hear that troublesome note at all. The note wasn’t actually louder in the first place, it was being reflected off surfaces in your room and arrived at your ears at the same time as the note from the speakers, effectively increasing its amplitude. The reflections in the room fooled you into thinking the note was louder and consequently you reduced the level of the note when you didn’t need to. This happens not just with one note in a bass part but at different frequencies right across the spectrum making it impossible for you to judge anything accurately in an untreated room. Equally, a reduction in volume can occur causing you to turn a particlular note, sound, frequency, instrument or voice up because you believe it can’t be heard. Once again your room is decieving you.
The subject of small room acoustics is deep and complex and I’m not about to get into the depths of it here. It is possible however, to create an accurate listening position or sweet spot in your home studio without having to study physics!
Below are some guidelines that will help you create a reflection free zone (RFZ) where you can trust that what you hear is an honest representation of what is actually going on in your recording.
Broadband Bass Absorption
Sound is energy. The deeper or lower the sound the more energy it creates. A kick drum will create more energy than a hi hat cymbal for example.
Depending on the dimensions of your room, particular low frequencies reflect back and forth overlapping each other producing standing waves which cause the room to sound boomy. This makes some bass notes much louder than others. It’s impossible to get rid of this effect known as room modes though it is possible to even out the sound by placing broadband bass traps strategically around the room. Remember, the notes aren’t actually any louder on your recording, they just appear to be louder because of the effect of room modes.
Bass energy tends to build up mostly in corners. In a rectangle room there are 12 corners including where the floor meets the walls and where the walls meet the ceiling. Broadband bass traps absorb this energy thereby reducing or even eliminating these reflections.
The first step in treating your room is to place broadband bass traps into as many corners as you can. This may not be possible in some places such as where there’s a piece of furniture or a door but treating as many of the other corners will undoubtedly make a big difference. Another key area to treat with broadband bass traps is the rear wall behind the mix position.
Retail or DIY Broadband Bass Traps?
Broadband bass traps (or bass absorbers) can be bought off the shelf from many pro audio music stores or online. If you are on a budget and have some basic diy skills, or know someone who has who can help you out, you can build your own for a fraction of the retail cost. If you decide to buy retail traps don’t waste your money on foam products. They do nothing for frequencies below 500Hz as they are simply not dense enough to absorb bass energy. Foam bass traps are more likely to suck out a big hole in your lower mid range causing you to add this frequency range back into your recordings and mixes making them sound muddy and dull.
Below are a couple of great videos on how to build your own broadband bass traps:
Once you have treated your room with broadband bass absorption, you will need to deal with the early reflection points. These are the points on the side walls to your left and right when seated at the mix position (see the diagram above). If left untreated the sound from your speakers will bounce back and forth between these walls causing all kinds of problems such as phasing and comb filtering. Ask a friend to hold a mirror flat against the wall and move it around. Any position where you can see a reflection in the mirror of either of your speakers should be treated with absorption. Using broadband bass traps at the early reflection points is a good idea as reflections from the side walls can extend down into the bass frequency range.
Another common problem in small studios is low ceilings. An early reflection is created as sound bounces off the ceiling and reaches the sweet spot at the same time as the direct sound from the speakers. An effective treatment is to hang broadband absorbers above the listening position. 4ft x 2ft x 4″ broadband bass absorbers should be used and spaced 2 inches off the ceiling. Cover at least the square area above you from speaker to speaker and to your listening position.
As I said earlier, the subject of small room acoustics is deep and I couldn’t possibly go into fine detail in one article. If however, there is something specific that you would like some information on, feel free to drop me a line or post your comments and questions below and I will reply asap.
For a more indepth look at the science of room acoustics I recommend you read articles by Ethan Winer – a highly respected authority on the subject. You can find detailed information, videos and graphs on his website at: http://www.realtraps.com/info.htm
Disclaimer: Record, Mix and Master is not affilated with Realtraps.com in any way though we do fully endorse Ethan Winer’s expertise in this field.