Room Acoustics
Originally published at The Null Device Blog. You can comment here or there.
I’ve been doing a lot of research on this topic, as every studio move I make means I have to re-treat a room. So I’m getting decent at doing this, usually on a budget.
What I can’t stress enough is just how important even just a little acoustic treatment is. Good acoustics can make the difference between a crappy recording and a good one - the best mics and monitors and consoles won’t help if there’s no way to know if the sound is accurate. A lot goes into it, but it’s not a tremendously complicated thing to do.
Acoustic treatment is not the same as soundproofing. Soundproofing gets you sonic isolation, and is another important facet, but not as critical to mixing and recording as one is often led to beleive. And most acoustic treatment products have little effect on the overall sonic isolation of a room - if you live in an apartment, hanging acoustic panels will not keep your neighbors from hearing things. Only heavy mass and airflow management can do that. Acoustic treatment may help with the neighbors a bit, inasmuch as you won’t need to monitor at high volumes to pick out detail.
The first instinct of a lot of people, myself included, is to start by buying a mess of cheap eggcrate foam and lining their room with it. That sort of soft batting does manage to do things like dampen slapback delay, trim reverb etc. Unfortunately it’s commonly used as a panacea, and as such the room ends up oddly anechoic with a lot of high frequency damping but no low frequency management. This isn’t going to help much - it’s a false sense of security. Also, most cheap commercial eggcrate foam doesn’t absorb an especially broad range, so it may just cut some slapback but not even control midrange. A few strategically-placed pieces of real acoustic foam or acoustic paneling, available from a lot of places online, will help a lot more than dousing the room with cheap eggcrate.
The first thing anyone who has a project studio should do is position, adjust, and decouple their monitors. Make sure that the tweeters are pointing towards your ears, equidistant from your head, from your normal sitting position. You don’t want them too close to you - that approaches mixing binaurally with headphones, which opens up loads of problems. Make sure there are no surfaces the speakers could reflect off of on the way to your ears - setting them way back on a desk ofent means there’s going to be sound waves bouncing everywhere and that can lead to phase cancellation, comb filtering, and other ugly sonic smears. Easy way to check is with a hand mirror - if you put the mirror down someplace and you can see from your normal seat a reflection of a speaker cone, that spot is a possible reflection point. Speaker stands help with this, as do desks with multiple levels, so you can elevate your speakers to a point where they’re too high to put you in a reflection path. Finally, decoupling your speakers from anything that can resonate is a good idea. Heavy, sand-filled stands are a popular option, albeit an expensive one. Industry giant Auralex makes MoPads, some molded foam pads that act as shock absorbers. There are other varieties of this, ranging in price from $20 to a few hundred, but even a few simple pieces of squishy packing foam can work well - just something so when the speaker vibrates, it doesn’t make your workspace or your floor vibrate too. I’ve heard of people trying to hang speakers from the ceiling to decouple them, but that’s not generally a good idea unless they’re heavy enough not to drift any - the kick from the woofer can push the speaker a few milimeters around, causing minor changes in phase and angle at a nearly constant rate. This would negate the benefit of any decoupling, and probably make things sound worse.
Bass traps? Bass traps. BASS TRAPS! No room used for mixing or recording should be without them. The corners of any room are going to be death for accurate monitoring - they reflect low frequency like mad, set up phase amplifications on some low frequencies and damp others. It’s bad. Bass trapping isn’t difficult to do. There are numerous decent commercial solutions - Auralex LENRD corner traps are pretty affordable, the stuff that the guys from RealTraps is without equal (the owner, Ethan Winer is an authority on studio acoustics who is kind enough to publish vigorously on the internet and moderate a number of studio acoustics forums), there’re some decent cheap ones from Acoustimac, there are a bunch of free plans on the internet for building your own fancy resontant-cavity traps. You can DIY bass traps low cost with a few slabs of Owens-Corning 703, 705 insulation or Mineral Wool wrapped in burlap. Even more cheaply, albiet not quite as effective, is a giant cardboard cement mold tube (get ‘em at home depot for about $4) packed with fiberglass insulation and sealed on both ends. Take any one of these solutions, stick them in room corners (especially the corners behind your monitors) and it’s amazing how quickly and dramatically the bass response on the monitors tightens up.
Diffusion vs Absorption always comes up. There are tons of products on the market that are foams or panels or suedes or fabrics that promise sound absorption and control. No sound absorber is perfect, though, so some sound is likely to get through and bounce around. This is particularly problematic in the area directly behind the listener. The answer there becomes diffusion - setting up something to keep the sound from reflecting directly back, scattering it in multiple directions. There are a number of commercial diffusers, and many are quite attractive as wall art. However, all a room really needs is something that takes up a lot of space, has different depths and angles, and a little mass. The easy answer is furniture. A bookshelf makes a remarkably good diffuser - the different depths of the books (plus the absorption factor of a lot of papery mass) and the shelves works well to diffuse random reflections. Plus it can hold all your myriad software manuals and books on mixing. If it’s big enough, a couch can also work, and that can provide absorption as well (and a place to sit) although usually they’re too low to offer enough help. Even randomly hanging different-sized pictures on the back wall can assist (but the bookshelf works better). The trickier bit is on the ceiling. Diffusion or absorption over the workspace can help reduce reflections and smear. Obviosuly, mounting a diffuser overhead is no mean feat, and you can’t put a bookshelf there. A cheap solution I’ve seen is to use a thin piece of plywood, bent so that it’s a convex surface, hung from the ceiling. Thin plywood can be tensioned with cheap steel wire so that it holds its shape, or it can be forced into a frame to hold it there too. That will help diffuse reflections enough that there shouldn’t be much reflective interference. Some sort of absorption above could work too.
Most sources I’ve come across recommend treating about 30-40% of the available wallspace. That’s sort of a mixed bag for most project/home studios - it can be expensive, and there’s the mere fact that this room is not a single-purpose control room, usually. Often it’s used for recording, mixing, mastering, rehearsals, and so forth. For something like recording vocals, it’s often nice to have a more “live” -sounding room to add a bit of natural space or air to the recording that a reverb can’t provide. And not everyone has a perfectly rectangular, purpose-built room at their disposal that’d be easy to treat. And some people rent, meaning bolting a few inches of OC705 to a wall is not an option. And, honestly, for those of us who don’t have a separate set of midfields and mains for previewing, just some simple treatment and bass management is enough. I’ve seen other more modular solutions as well - I once knew a guy who had a very dead room for most of his recording (back in the days of the 4-track) and he kept a big slab of plywood in his garage that he’d put on the floor before he did any vocals, allowing his dead room to temporarily liven up. Flexibility seems to be the key.
Isolation can be nice for recording a mic, but everything needs to be isolated - including any headphones. I’ve come across many vocal takes from people where you can tell they spent a lot of time isolating their vocal booth but the headphone bleed is painfully obvious. Vocal booth isolation is only really necessary if the engineer wants to monitor at the same time, or if there’s a lot of extraneous background noise. Over-isolate and the vocal takes can sound strangely flat. Really, what a good recording space needs is some degree of modularity - depending on the singer or instrument mic positioning is going to vary as is the mic’s position within the room. Things like RealTraps Portable Vocal Booth or sE’s ReflexionFilter are meant to prevent dicretional sound sources (vocals, instruments) from transmitting past and subsequently boucing off the far walls. They can be used behind the singer/instrument as well to prevent room sound from affecting the recording. These things do cost money, though. A simple and time-worn technique is the “wall of mattresses” or “blanket on a clothesline” - some big, heavy, soft material is suspended near the mic to deaden early reflextions or room sound. This is of course only if the sound needs it. If the room already sounds great for recording, there’s no reason to mess with success by adding a fancy isobooth contraption. You can convert a bathroom or a closet or a spare room into an isolation booth, but that’s a double-edged sword - unless the acoustics are right there, too, the vocals will end up sounding like they were recorded in a closet or bathroom. And of course when dealing with amps and all that, the rules all change, but I’ll leave that for a guitarist to detail.
Acoustics. Useful stuff to know.