How your Room Affects the Sound of a Home Recording, and how to Stop it
Take an in-depth look at room acoustics and how reverb works, and come out with better recordings and superior playback
What’s the most important part of your audio signal chain? Is it your microphone? Your monitor speakers? Perhaps your computer itself? Actually, whether you’re recording or playing back audio, the most crucial link in the chain is neither electric nor digital – it’s the room the sound is located in.
Let’s say you’re hoping to get a great recording or aiming to get a pro-quality listening environment for playback. If the actual physical space that carries the audio isn’t up to standard, it won’t matter much what microphone you choose or how good your compression technique is – your audio quality won’t be up to scratch.
So why does the room affect the outcome of a recording so much? And what can we do to start making it better?
When you’re listening to anything through speakers, you’re not just hearing the sound coming through the speakers – you’re hearing that sound as changed by the environment you’re in at the time. The effect of the room depends on its size, its shape, its surfaces, and even your own position in it. This is exactly the reason that professional sound and music studios are outfitted with expensive acoustic treatment.
This change in sound is something we hear every day, although we may not think about it much. Just consider how different your voice sounds in the shower to how it sounds in your lounge. Music throughout history has often relied on the sound of the space it’s performed in to make it sound better – from choirs and chants in huge gothic cathedrals to perfectly designed vineyard-terraced concert halls.
Why do different spaces sound so different? The answer is all about the amount of sound that reflects around the room.
We’ve all been in spaces with an echo – under bridges, in large halls or churches, or even in steep valleys, for example. These places usually share a few characteristics: there’s a large distance between surfaces, and those surfaces are usually made of very hard material.
Those hard surfaces make it easy for the sound to reflect back at you, and the size means that the sound has taken long enough to come back at a noticeably different time. Actually, the same effects happen in much smaller rooms, too – it’s just less easy to notice. As we’ll see later in this article, the reflections within rooms have a huge effect on the sound.
In a room, compared to a larger space, one difference is that the reflections come back far quicker, as the surfaces are closer to you; another is that the sonic reflections off surfaces are more scattered (there are more objects filling the space to get in the way); and another difference lies in the amount of sound that’s absorbed by objects in the room.
The diagram above shows us what happens when a balloon is popped in a room. As the balloon pops, sound comes out in a sphere – at all angles. Some of that sound is directed at our listener and enters the ears. Sound moving in other directions instead goes and reflects off a nearby surface (a wall, the ceiling or the floor), and again ends up in the listeners’ ears.
Each one of these reflections is just like hearing a copy of the same sound again, a tiny fraction of a second after the first. The longer the reflection path, the longer the time a sound takes to hit. Most of the sound actually hits multiple surfaces before it reaches our listener. Below is the result of the original sound and the ‘copies of sound’ that our listener hears over time.
The green line represents the first sound that reaches the listener. This Direct Sound has not been reflected off any surface. It's the first time the sound is heard. In yellow, the Early Reflections are caused by the sound being reflected from the closest walls to the listener and the sound.
In red, the Late Reflections of sound are caused by other 'copies' of the sound that have reflected off multiple walls before reaching the listener's ears. After so many reflections, a lot of these reflections have lost energy and are quieter. There are so many late reflections that they start to blend together.
It’s a little easier to ‘hear’ the sound of any room you’re in by simply clapping. In different rooms, that short burst of sound will take a longer or shorter time to decay to nothing. The exact same thing is happening to any sound heard in that room – whether it’s people speaking, objects clattering, or sound coming out of speakers.
Measuring Reverb Time
There’s a generally agreed-on way to describe how reverberant a room is. By measuring how long it takes for the energy from a very short, very loud sound to die off, we end up with a measurement of the reverb time of the room in question. Specifically, we measure how long it takes the sound, in seconds, to fall by 60dB.
We’ll talk more later about reducing the reverb time of rooms, but there are more problems with sound in enclosed spaces that we need to know about first.
Every frequency of the audio spectrum has a corresponding wavelength. For example, a low musical C1 tone (the lowest C on a grand piano) has a frequency of 32.7Hz, and this frequency has a corresponding wavelength of about 10.5 meters. A higher note, G4, has a frequency of 392Hz, which has a wavelength of 0.875 meters.
If a 32.7Hz tone (10.5m) is played in a room that has two flat surfaces 10.5m apart, this frequency will be reinforced and will sound louder, a lot like how a standing wave is created in a brass or wind instrument when its length perfectly matches the incoming air flow.
Just like the wavelength of 10.5m (32.7Hz) will fit exactly between our two walls, the wavelength of 5.25m will fit exactly between those walls twice, reinforcing the corresponding frequency of 65.4Hz; and the wavelength of 3.5m will fit exactly three times, reinforcing the frequency 98Hz… and so on.
These frequencies are called Room Modes, and the overall effect on the sound is similar to that of using an EQ plugin on an audio signal – certain frequencies are boosted merely because of the room the sound is in.
Luckily, the effect of room modes only really creates problems under about 300Hz – above this frequency, things are so blurred that the individual resonances actually have less of an effect.
As if this problem wasn’t enough, this is only the situation created by two parallel surfaces. Not only does a standard room contain another two parallel surfaces (for a total of six, of course) but there are other dimensions at work…
Any two pairs of parallel surfaces also act together to reinforce more frequencies, and then any other combination of ‘bounces’ between any surface. What’s more, any of these ‘modes’ can congregate at different points around the room, so the frequency response can change simply by moving your ears or your microphone to different points.
However scary and chaotic the world of room acoustics seems, there’s one way to save a lot of frustration from the start. Rather than spending energy solving big problems in one room, is it possible to move your setup to another room that has better acoustic conditions from the start?
If the room is the most vital link in your recording chain, some rooms must be far better than others for getting the right sound. Some particularly bad acoustic properties of rooms include irregular shapes (L-shaped rooms or extra corners), non-parallel surfaces, curved surfaces that focus reflections into one space, and perfectly square rooms.
Ideally, your room should be a ‘shoebox’ shape, with its height, width and length as mathematically unrelated to each other as possible, in order to cut down the effects of those room modes.
With reverb time and room modes causing such problems to the sounds of recordings, playback and even just speech in a room, they’re prime candidates to be eliminated.
If there’s one prime mission when seeking to reduce reverb, it’s absorption. By ‘soaking up’ the sonic reflections in a room, you can start to get rid of almost every problem that they cause.
Absorption stops sound, but it doesn’t stop sound. Remember, surfaces like walls are so good at ‘stopping’ sound that it’s reflected back off them. To make absorption happen, we need a different strategy.
On a microscopic level, we need to take the kinetic (movement) energy present in the sound, and turn it into something else. If a surface is made up of a dense collection of tiny fibers, any sound that hits it will cause the fibers to rub together and create friction and heat. In other words, something soft will absorb a sound so that it reflects far less.
There are lots of options when it comes to stopping sound reflections, and many of these might be available to you already. You may have noticed how different a room sounds when the curtains are taken down, as these can be great absorbers, along with carpets.
Rugs and sheets also make great absorbers, as do clothes and towels. If you’re looking for an impromptu absorption setup for a quick voice recording, for example, it may be easiest to stack some folded materials and hang a few thick blankets close to the microphone. Many have also been known to stand mattresses against a wall to help deaden the sound of a room when recording.
Professional acoustic foam tiles are relatively cheap, and easy to place on walls, but proper placement is needed for best results (we’ll talk about this below).
Acoustic panels are a more heavy-duty solution than simple tiles. Made of a frame filled with mineral wool and covered with fabric, these can drastically reduce the reverb time of a room thanks to their large area and high absorption of sound.
Bass traps are foam pieces or panels placed at the corners of a room. The point here is to absorb extra sound at these corners, since they are areas of a room where mutliple modes can intersect and case even more problems.
Too complicated? To remove reverb after a recording has been made, check out ERA Reverb Remover!
There’s a key difference between ‘treating’ a room to make it sound better, and ‘isolating’ the room to stop sound escaping. Acoustic treatment won’t make much difference to your relationship with your neighbors, but it can certainly help you get better-sounding recording and playback environments for your work.
Just as having a long reverb time in a room can be bad for the sound inside, cutting it down too much can have a bad effect as well. A ‘dead-sounding’ room suffers from a lack of space and character. It’s possible to overdo the use of sound-absorbing materials.
Although a dead-sounding room is an unlikely situation for a home environment, one way to treat this is to swap some of your acoustic absorbers with acoustic diffusers. These panels are intricately designed to scatter any sound that hits them in many directions, breaking up the regularity of the surface and ‘scattering’ reflections rather than simply stopping them.