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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.
Convolution relies on measured impulse responses to bring the sound of another space to our tracks. We need to recognize that every impulse response we load into our convolution engine was measured by someone else. Mistakes might be made, and a flawed impulse response will produce flawed reverb. We must seek out the best impulse responses and watch out for poorly measured ones. No single impulse response describes an entire hall. The impulse response, that signature pattern of reflections for a hall, depends on the location of the sound source and the location of the receiver and the state of occupancy of the hall.
Picture a concert hall with a stage for all the musicians and about 2,000 seats out on the orchestra floor for the audience. The impulse response is unique from each and every instrument location on stage to each and every seat location. So while someone might offer you the impulse response for your convolution reverb from some famous hall that you know and admire, there is no guarantee that the impulse response is from a good seat. You need to know something about the methods used when measuring the impulse response. Where do they put the sound source to trigger the pattern of reflections, and where do they put the measurement microphone to capture it? Moving either one of these, the sound source or the receiver position, gives you a different impulse response.
With 2,000 seats and, say, a 100 stage positions, there are 200,000 possible impulse responses. So we have this constraint on convolution. No single impulse response describes an entire hall. It's not enough to know you have an impulse response for a great hall; you need to also know that the impulse response is for a good seat for sound sources and a sweet spot on stage. And those coveted halls might sound great at a sold-out concert, but are the seats full when they grabbed the impulse response? Occupancy certainly influences the reflection pattern.
A seat with someone in it reflects sound differently than an empty seat. There is more. The measurement quality used to obtain the impulse response is also important. If shoddy equipment is used or poor technique is employed, it absolutely pollutes the impulse response, leading to a poor sounding reverb. Convolution reverb is only as good as the impulse response itself. For the impulse responses you use it really helps to know something about their measurement history, the placement strategies, the occupancy of the hall, the skills, and experience of the people who captured the impulse response, the gear they used, et cetera.
Be an informed user of convolution. Listen carefully for flaws every time you load up a new impulse response. Seek out the best reviewed impulse responses, so that you can bring the other spaces of the world into your mix with confidence and great-sounding results.
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