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This installment of Foundations of Audio explains one of the most essential ingredients in audio mixing, reverb—the time it takes for sound to bounce, echo, and decay during a live performance or recording. Reverb gives a natural richness to your recordings, which is possible to reproduce. Producer and audio engineer Alex U. Case covers the acoustic, mechanical, and digital means for creating reverb, and charts the parameters (room size, density, etc.) you'll need to know to take advantage of the original recording space and enhance it in post. He then shows how to simulate reverb digitally with effects, adding timbre, texture, and contrast, and improve the sound of your mixes with a sense of space and depth.
These techniques can be practiced with the free Get in the Mix sessions, currently available for Pro Tools and Logic Pro.
Let's speak now of the audio equipment within the chamber. We've got loudspeakers and microphones. Loudspeakers are the sound source in the chamber, and microphones are the sound receivers. You might think that we need super tweaky high-end loudspeakers and microphones. While that would be great, history teaches us we can get away with using more average quality gear. For the speakers, dynamic range is more important than frequency response. That is to say, it's less important that the loudspeaker be accurate and flat from low to mid to high frequencies, and it's more important that the loudspeaker be capable of high sound pressure levels.
We choose loudness over accuracy. We need to energize the room fully, so sound reinforcement loud speakers are probably more useful than consumer hi-fi speakers. In fact, if you happen to have extra stage monitors or small loudspeakers on sticks used for small sound reinforcement situations, those could be ideal for setting up a chamber. A nice thing about live sound loudspeakers is that they are often horn-loaded drivers making them quite directional on how they radiate sound.
Having a directional radiation pattern makes it productive for you to re-aim the loudspeaker within the chamber and find a different sort of reverberant sound. So, any loudspeaker radiation pattern will work, but a directional horn-loaded loudspeaker makes it possible for you to tune the chamber by reorienting the speaker. There is an analogous situation with microphones. You can certainly use very high-fidelity, very flat omnidirectional microphones.
But directional microphones are useful because they also reward the engineer who is willing to experiment with placement. While any the microphone is fair game for a reverb chamber, the most popular ones these days are cardioid condensers, and more specifically ,small diaphragm cardioid condensers, like the Neumann KM 184s, AKG 451s, Shure KSM 137s, and Audio Technica 4041s. In terms of the placement of microphone and loud speakers you'll have to do some experimentation, but here are some rules.
First, you want to avoid a direct on-axis line-of-sight connection between the loudspeakers and the microphone. You don't want the loudspeaker to have a way to fire directly into the microphones; you want some sort of obstruction. Put a sound-reflective barrier in between or place them around the corner from each other if it's an L-shaped room. Otherwise, you can point the loudspeakers and microphones into different corners so that they face away from each other. The reason for this is that there's the risk of a direct acoustic connection between the loudspeaker and the microphones, which will give you not reverb, but essentially a direct sound with delay, which will cause comb filtering, plus the coloration to the sound introduced by your loudspeaker and your microphones.
When you're placing your speakers, orient them so that they energize the room. That is, think of the loudspeakers as a way to acoustically drive the space and position them so that you get the most scattering of sound as quickly as possible. Aiming the loudspeakers straight at a flat wall will energize that wall and the wall opposite, but aiming the loudspeaker in a corner will cause the sound to bounce around with more complexity, energizing more of the surfaces in the room sooner. Similarly, place the microphones in an orientation that you think captures the full complexity of the reflections and reverberation in the room without getting too much of the sound from the speaker directly.
It's going to take some experimentation. Of course, anything goes. All that matters is what sounds best. But as you explore the possibilities and capabilities of your chamber, stay oriented to the essential goal of having reverb returns from the chamber that are themselves 100% wet. That means they'll have as little direct sound from the loudspeaker to the microphone as possible.
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