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The Audio Blog

Tips, tricks and fun for the recording musician

The Audio Blog is a set of thoughts, techniques, knowledge bits and the occasional rant about the wonderful world of audio and music recording. Follow me on the path to great sounding music, never a boring moment!

A good sounding room!

In my first post I talked a little on how - after performance and arrangement - the physical space where you record (the "recording room", or "room" for short) is the most important factor in the quality of the result - at least if you are using microphones.

Let's repeat it again: if you want to improve your recordings, the first and most important thing to look at is how your room sounds.

It may be a little unclear, however, what exactly does it mean.

Does a room sound like something? Normally, rooms aren't know to be very talkative...

Actually "room sound" is just jargon. It is a shortcut for "the way that the reflective properties of the room affect any sound which is produced therein, when perceived or captured at different locations in the room".

Which is quite a mouthful, hence the shortcut.

A room has a sound (i.e. affects sounds that is produced inside it etc), broadly dependent on its shape, its dimensions, the materials used for the walls and whether or not it's mostly empty or filled with stuff (and which kind of stuff). This sound is permanently imprinted in what you record, and cannot be easily equalized away.



Sound is simply a wave of energy propagating thru air- just like a sea wave is energy propagating thru water. Just like in water, this wave moves forward. In a room, it will hit a wall, and reflect. The reflection will then bounce back hitting your microphone.

If the wall doesn't absorb much energy, this reflection will have a very similar shape to the original, just a little bit less energy, and will be just so slightly delayed from the original sound (for example, since the speed of sound is 343 meters/second at 20c, it takes 0.02 seconds for a sound to bounce back from a wall 3 meters away).

When two energy waves meet each other, interference occurs. And when the waves are similar, but only slightly delayed from each other, the particular type of interference which happens (called comb filtering) mangles the sound big time, changing its timbre dramatically (the name is because the resulting waveform always looks like a bit like a comb).That is the sound that is captured by your microphone. And it's bad.

It's worth mentioning that interference cannot be easily EQ-ed away after the fact. Once filtering occurs, information from the original wave has been destroyed. So there's nothing to EQ (it is possible to make an "image" of a room using a known sound wave, looking at how the captured signal is with a very precise microphone and try to back-calculate what the original wave would have looked like... which is what room-correction systems attempt to do. However, it's difficult for a number of reasons, of which we'll talk another time).

And that's why most regular rooms aren't great for recording: they're too reflective.

Besides reflecting too much sound, "normal" walls reflect different frequencies at different rates. Everydays objects on the walls may absorb some mid-high and high frequencies, but usually do so only for relatively narrow bands (and depending on where the source points to), so that the mangling of the wave shape is even harder to predict.

Incidentally, this is also the reason why using egg cartoons or carpets to cover the walls is not really a great idea: these materials absorb only a very small band of mid/high frequencies, leaving the room even more unbalanced and sounding even worse than before.

All of this ensures that your microphone "hears" a very different timbre than the original source (for example, your voice).

What we need, then, to make a good recording - one that sounds like the original source?

A good sounding room

That is, one that absorbs and diffuse enough of all frequencies so that only a little bit of reflections are left (a natural and pleasant reverberation). In a room like that, the microphone will captures a sound which is very much like the original, with at most a little sense of space.

So why don't we hear "bad sound" in everyday rooms?

That's because of how our brain works (a psychoacoustic effect): when hearing sound in a room, our brain uses reflections (and how differently they arrive at our two ears) to understand the room size and shape... and then pretty much discards them. Therefore, we simply don't notice them at all, unless they're extreme (say tiled bathroom or empty room). At most, we get an headache.

But when reflections are captured by a microphone and printed into a recording, it's a different story: on playback, our brain will not filter them out at all (it's already busy dealing with the physical reflections due to the playback room), so we perceive the sounds as it is: bad.

Also, high frequency waves are short, so absorbing them is easy; while bass waves are very long, so hard to diffuse and absorb (a 20 Hz "note" is moving 17 a front of meters of air, and a 80 Hz sound has still a wavelength of 4.2 meters!). For bass, there's lots of air moving.

That means that, almost certainly, a regular room will reflect too much bass back to the mic (or your ears), very much changing how the actual source sounds.

So there you have it. A proper studio room is not a "studio room" because it contains a lot of glitzy and fancy kit (rack of compressors, big monitors, a huge Neve console)... but because its room designed or treated to tame frequency reflections (especially bass) so that the total reflections are just so, and they don't affect recordings in a bad way.

A studio room sounds good: little reflections, and absorbed more or less uniformly at various frequency.

That's really what you pay for when you go to a studio - not the kit, but the rooms!

Interestingly, different studio rooms will all have few reflections, but will all have slightly different reflective properties (due to materials, construction and design choices), so they will "sound" slightly different. And that's why you have famous studios rooms, known and loved by engineers because of their particular balance of reflections.

Is all lost?

Lacking a properly designed (and probably oddly-shaped) studio room, a partial solution to this is room treatment which aims to bring a regular house room to a degree of absorption and response which is, if not perfect, acceptable. Such a room will seldom be as predictable as a purpose-designed one in a studio, but with enough attention it may be made to sound more than decent. I'll soon write a post on the subject.

So now you know why the room you record is more important than the microphone: because even the best microphone in the world will only capture what sound exists where it is placed, and a bad room ensure that the sound will be not good.

In other words, a very good microphone in a bad room will simply capture a bad sound extremely well. Whereas a mediocre microphone in a great room will capture a great sound in a okay-ish way.

As a final note: if you really want that big drum sound or your bass players don't like a DI-ed bass sound and insists in using a cabinet and his 500W amp, you can do worse than locate a room designed to handle reflections (typically a studio but also acoustically designed rehearsal rooms, for example) and rent it for a few hours to record there. It's usually worth it.

Good recordings!


More details

If you're interested, here's a little about the physics of reflections and how it affects the room sound.

Sound is energy travelling thru air, travelling at about 343 meters per second (at 20C/68F, a common air room temperature). A regular room is at most a few meters wide.

That means that for example, if vocal mic is 3 meters from the wall, the sound will have to travel 3 meters, bounce back and travel 3 meters, for a total of 6.. it will take about 0.02 seconds to do so. Very fast. If your mic is cardioid, the reflections hitting the back won't affect much, but the side reflections will (incidentally, handling of side sounds is what separates normal mics from great ones), and the reflection will again hit the wall behind the source and bounce back (a new reflection), adding a few milliseconds depending on the distance to the back wall).

Given their speed, all these reflections will hit your ears (or a microphone) almost together with the original sound, just a little bit delayed. Depending on how much energy is absorbed by the wall, the reflected sound will also have the same general shape, only a little bit flatter.

(as a note: walls usually absorb different frequencies at different rates, so the reflected wave will not have exactly the same shape as the original... but it will be in general rather similar, depending on what the wall is made of).

As I wrote above, when a sound wave and its slightly-delayed, slightly-less-energetic reflection meet, comb filtering occurs: certain frequencies (aka musical notes) are strongly reduced, while others are strongly increased. The result looks a little a hair comb, hence the name. Here's the same picture used above:

Comb filtering example
Comb filtering

It's easy to see that the timbre of the combined sound is seriously different than the original.

(by the way, interference between unrelated sound waves is nothing particular - it happens all the time and our brain is trained to split them up and distinguish, say, the TV sound from the phone that's ringing in the hallway).

A little filtering (i.e. interference between direct sound and reflections) is actually pleasant and necessary for us - that's what we normally call reverberation, something that our brain uses to understand the properties of the space we're in. A completely non-reflective room (an anechoic chamber, literally with no echos) is very unpleasant and disorienting. But large amounts of filtering are a no-no, as they radically alter the sound.

Another issue that affects filtering is that the different frequency bands (e.g. bass, low mids, mids, highs) have vastly different amount of energy, and are absorbed differently by your average wall.

Low frequency waves ("bass") are several meters long (a kick sound at 80 Hz repeats 80 times per second, and - being the speed of sound 343 meters/second - the energy wavelength is 323 divided 80, that is 4.2 meters). They also contain lots of energy, since they need to move a lot of air (think giant surfing-size sea wave).

Higher frequency waves ("highs") have short wavelength (a 5 KHz note repeats 5000 times per second, and 343 divided 5000 is 0.069 meters, that is 7 cm) and have little energy, since they don't need to move much air at all (think tiny ripples in a river).

This has two consequences: first, bass frequencies are pretty much omnidirectional - (their size is often comparable or larger than the size of the room, and have a lot of energy, so they bounce a lot before dissipating, and end up everywhere), while high frequencies are highly directional (they have little energy and they are short so they bound less before dissipating and the reflections tend to concentrate end in small parts of the walls) . They also are easily diffused (and hence direct reflections easily attenuated) for the same geometrical reason.

Second, bass frequencies are much harder to absorb (lots of energy to deal with!), while high frequencies are easily absorbed (they have little energy, so they dissipate quickly into silence).

That's simply because most materials are porous at some level - i.e. have tiny holes which are comparable in size to at least some of the tiny ripples of air that make high frequencies; whereas no normal wall material has holes big enough to absorb the giant bass waves, so every wall is pretty much a mirror for them.

Finally, the shape of the room itself dictates where the more directional frequencies end up, and how many bounces they need before the come back at the source (or if they come back at all).

That's how materials properties, the nature of sound as energy thru air and the geometry of a room contribute to its sound.

The net result is that while building up absorbers for mid and highs is relatively easy (and we just need to make sure they are broadband absorbers, i.e. they absorb evenly ranges of frequencies with declining amounts of energy), avoiding bass reflections requires specially designed walls (or section of walls) which, while still being walls, from the point of view of bass frequencies, look like having very large holes. These are called bass traps.

There's quite a few ways to build broadband absorbers and bass traps and I'm likely going to write a post on the subject sometimes!

#room #recording #treatment #technique #sound

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