Tuning your room – fixing acoustical issues
// March 12th, 2009 // 1 Comment » // Acoustics
This is a reprint from an earlier blog, now deceased. I still have quite a few incoming searches for it, so I thought it’d be worth carrying over. The information’s unchanged – the nice thing about acoustics is that unlike many other technology-driven fields, it really doesn’t change too much. The names change and the acronyms cycle, but the physics stays the same.
So you’ve bought your home theatre system, you’ve positioned your speakers, and you’ve calibrated your levels. That’s it, right? Everything’s hunky-dory now?
Not quite – the unfortunate reality of acoustics is that it’s real. Well duh, you say, but my point is that it isn’t a abstract model; the environment we’re in is affected by and affects the exact thing we’re interested in, namely how it sounds.
The core problem is that audio is a wave. Without making things too complex, think of your basic waveform – pretty much all of us have at some stage flicked a hose up and down with a friend. When we both flick the hose up at the same time, we end up with a really big peak, and when we flick them in opposite time, we cancel each other’s waves out.
Simplistically speaking, acoustics is exactly the same; our speakers produce various waveforms (acoustical energy), and they shoot out and bounce around the room. In the smallest room one can imagine, these waveforms bounce off the walls almost immediately after they’re produced by the speakers. As they reflect back and forth, gradually losing energy and sounding quieter, we hear these echoes and they cloud what we wanted to hear in the first place.
In an infinitely large room with no other sound sources, these waves never reflect back to us – they keep shooting out forever, so all we ever hear is exactly what the speakers produce. That sounds like paradise until one realises that the whole attraction of live music (and, to a lesser extent, studio music) is that they were recorded in situations where things did reflect off various things. The speakers at the front of the stage, the glass between the recording room and the microphone, and so on. So, the ideal situation is somewhere in-between that infinitely large room and a nutshell – something that allows some reflections, but not so many that they become overpowering.
The two major issues with any and every room are reflections and reverberation. Reflections are just what they sound like – echoes. While high frequency noises are technically waveforms, they tend to behave similarly to particles; think of them as billiard balls. A sound bounces off one wall and the echo hits your ears after the original sound. In practice, this can make it very difficult to clearly hear what you’re listening to – you lose clarity, and you try to turn up your system so you can hear it clearer. Typically annoying the hell out of your neighbours, straining your amp, and making you feel fatigued in the process due to the higher volume, of course, but hey, it’s all good, right?
Reverberation (more technically known as standing waves) is a little different – low frequencies tend to behave exactly like the waves they are. As the frequency gets deeper, the wave gets longer, and as the wave gets longer, the closer it gets to the length of your room. Think of your friend with the hose – if you’re unlucky enough to be sitting at the point where the reflection of the wave of a specific frequency cancels itself out, guess what? You can’t hear it. No matter how much you turn up the volume, you’ll never be able to hear it. Your neighbours, on the other hand, will hear it just fine.
What are the cutoffs between the two, you ask? For the technical, standing waves tend to be an issue below around 50hz, while reflections tend to kick in above around 200hz.
So, with all these potential problems, how do you know if you’ve got an issue? Reflections are easy – clap your hands. If you can hear a fluttery echo, you’ve got some issues. Reverberations are a little more difficult to detect; the simple test is to measure your room dimensions. If the width or length of your room is less than around 4m either way, you’ve got a problem. If both are less than 4m, I’ll guarantee you that you have pretty significant problems. You might be deaf by now as well, but at least you can still read this.
How significant, I hear you ask? In practice and in a bad room, certain frequencies can easily be up to twice as loud or twice as quiet as everything else. How does this make things sound? Like crap, to be blunt. It’s enough to ensure you can’t hear anything at all (having your explosions sound like a wet pop’s never much fun) or you get a headache and your windows break because of sudden spikes in specific frequencies. I’m not kidding, either – in my old room, sounds at a specific frequency were so much louder than everything else that even when I was listening at “normal” levels, my windows almost fell out of their frames because of the resonance.
Interested in a more technical approach to working out exactly where these problems exist (in terms of specific frequencies)? Room EQ Wizard is a brilliant program for automatically plotting your room’s frequency response. Writing a tutorial is a little beyond what I’ve got time for at the moment, but trust me – it’s worth playing with if you’ve got the time and inclination.
So, assuming you have a problem, how do you fix it?
The naive, but understandable approach, is to assume that one can use equalisation to fix any issues. And, to a certain extent, that will work. The catch is that it’ll only work for one spot in the entire room - if the characteristics of the room are affecting the acoustics produced by a set of speakers, boosting or decreasing specific frequencies will only add additional complexity to the problem. It may flatten the frequency response in one specific area of the room, but it’ll throw everything out even further everywhere else in the room.
The better way is to change the acoustical properties of the room. The good news is that it doesn’t have to be difficult to do so, but the bad news is that it may mean moving things around a fair bit and / or changing what’s in the room. The thing to bear in mind is that you’ll never achieve a “perfect” room – it’s impossible. So, the best you can hope for is to make things better. With that in mind …
Reflections are probably the easiest to fix. As they’re caused by waves bouncing around the room and hitting your ears, the logical thing is to either stop them bouncing or make them bounce in lots of different directions, primarily away from your ears. The reason for the second option is because every bounce makes the wave lose energy, making it quieter. The first thing to bear in mind is that hard things make the waves bounce while soft things dissipate the energy. So, the following are bad:
- wooden floorboards or concrete floors
- glass
- large open spaces of wall
- leather
- wood in general
What’s the easiest way to fix it? Try:
- adding bookshelves filled with books (the random shapes break the reflections)
- hanging an attractive rug on the wall
- laying some carpet or a rug in the room
- adding a fabric couch or footrest
If that’s not enough, the more advanced approach is to build some acoustical baffles. I’ll write some more on that soon, but the basic idea is to build something light and large that you can hang on the wall / ceiling to help dissipate acoustical energy by preventing it from reflecting. Egg cartons also work well, but look ugly – an interesting test is to simply hang eggshell foam behind the listening position. Typically, it’ll noticeably change the sound simply by preventing the waves from your speakers from reflecting off the wall behind you directly onto your ears.
The other approach is to build a large wooden board with varying sized pegs in random configuration to help reflect the sound away from your ears. They look good, but they require quite a lot of work.
Standing waves are a little harder to fix. Without getting into the specifics, lower frequency waves tend to behave a little differently, making typical solutions to reflections fairly ineffective. Plus, the room itself frequently acts a large speaker in its own right, reverberating and making certain frequencies every louder. So, with that in mind, how does one go about fixing it?
The easiest way is to build dampening baffles for the corners of the room. Corners tend to enhance low-frequency reverberation, so by packing an appropriate baffle into the corner, one can prevent the reverberation from occurring in the first place. Photos and instructions will follow soon …
The other, more technical option, is to build a resonator that traps specific frequencies. Think of blowing into a bottle – it makes a tone, tuned to the shape of the bottle. A Helmholtz resonator does exactly the same thing, except it traps specific frequencies by converting them into physical energy. The main problem with them is that they’re like a sniper rifle – they hit specific frequencies enormously effectively, but the trade-off is that they only hit specific frequencies. And, tuning them, to be blunt, is an absolute pain; it’s not for the faint of heart.
So, there’s your easy options as far as fixing a room goes.