Problems when recording "dry"

Multitrack recordings are generally made without the early reflections from the room. This is because we want isolation between instruments so that during mixdown we can control the quality and character of each individual instrument. To do this, we mike the instruments closely, and we have come to accept the resulting in-your-face, close-up quality of the basic tracks as our preferred and accepted sound quality. However, if the truth be known, these sounds are comparatively flat and one-dimensional compared to their true acoustical counterparts. When we hear them with delays added to compensate for the lack of early delays on basic tracks, we know we prefer them this way, without necessarily knowing why.

However, there are two problems. First, when a sound and its delay are recombined, the time of delay becomes the basis for a pitch that is generated as a function of the mix of the delay and original sound. This effect is called “comb-filtering,” because the spiky frequency response curve itself looks like the teeth of a comb. Without going into detail, a sound delayed by 1 millisecond (for instance) mixed with its original version will generate a frequency response curve with peaks at 1 KHz. and all whole number increments of I KHz. (2 KHz., 3 KHz., 4 KHz., etc.) and deep notches at 500 Hz., 1.5 KHz., 2.5 KHz., etc. The response curve will look like this:

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	Frequency response curve resulting from the mix of a signal at 0 dB and a 1 ms. delayed iteration of that same signal at 0 dB. Peaks will be at +6 dB and nulls will be <-100 dB. The graph is drawn with a linear horizontal axis for frequency, as opposed to the more conventional logarithmic or octave-based scale.

This “comb-filter” frequency response is dramatically different than the frequency response of the original signal, which is represented by the dotted line at 0 dB. Comb-filtering is clearly audible(!) and, as I said before, has a very clear pitch, which in this case is a pitch whose fundamental is 1 KHz. This pitch will impose itself on whatever sound is being delayed and recombined with itself.

Second, we also have to consider our auditory mechanism. When we are working with music, we tend to take this incredible system for granted. Making beautiful and powerful music is very difficult and requires so much concentration that we really don't have the time or mental resource left over to think about what is really going on in our neurons. However, this is a situation where I’ve found that a little extra knowledge really made a big difference in the quality of my recordings, so, listen up!

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