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I'm reading the help for REW, and came across this bit in section 13 on Impulse Responses:
When an impulse response is measured by means of a logarithmically swept sine wave, the room's linear response is conveniently separated from its non-linear response. The portion of the response before the initial peak at time=0 is actually due to the system's distortion - looking closely, there are scaled down, horizontally compressed copies of the main impulse response there - each of those copies is due to a distortion harmonic, first the 2nd harmonic, then the third, then the fourth etc. as time gets more negative. The initial peak and its subsequent decay after time=0 is the system's response without the distortion.
I don't understand this at all.

Say we're talking about the impulse response of a speaker. Speakers are not perfect, so the sound that it produces will be a more or less distorted version of the electrical signal arriving at its input terminals. In particular, if an electrical impulse comes in, the sound that comes out will differ from an impulse in one way or another. But, surely, no matter how bad a speaker it is, it will not produce any sound before the electrical impulse arrives. How could it? It can't see into the future, after all.

So, what is the meaning of an impulse response that is nonzero at negative times?
 

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The placement of the distortion components is simply a convenient consequence of the way the response to a logarithmic sweep is processed into an impulse response. The actual (linear) response of the system being measured is the part that comes after those distortion images.
 
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