I had a disturbing dream the other night that I was modeling a reverb of an anechoic room. Been occupied a little bit too much by acoustics, reverberation and instruments samples, have we?
The Problem With Sampled Instruments and Reverberation
One problem with adding reverberation to instruments samples is that said samples normally already contain reverberation to some degree. Wet sampled instruments (at least historically including many orchestra libraries) contain a high degree of reverberation, even if many of them have been edited to contain an optional release tail. The same is true for instruments sampled dry, although they are sampled in rooms with very short reverberation times (a low RT60 value) so that the recording only will contain first and early reflections whose desirable levels are figured out during instrument and microphone placement.
But adding reverb to a sample already containing reverberation will cause comb filtering to some degree depending on sound source and reverb characteristics. One solution would be to sample the instruments in an anechoic chamber (e.g. Musical Instrument Samples from the Electronic Music Studios, University of Iowa) or at least in a hemi-anechoic or free-field chamber (see David A. Nelson, Acoustic Test Chambers and Environments, National Instruments, November 15, 2006).
The Problem With Acoustic Instruments and Radiation
But anechoic recording has its own set of limitations. For instance microphone placement in relation to the radiation of sound by acoustic instruments (Scott Wilkinson, “Resonance and Radiation”, Electronic Musician, September, 1994) need to be taken into account. In an anechoic chamber sound radiated as non-direct sound will not be reflected back and recorded by the microphone like in a studio or auditorium. This is especially troublesome with bowed strings having shifting radiation patterns depending on the tone being played.
Principal sound radiation patterns for the cello.
An omni-directional source, like a starter pistol or carbon arc, is regularly used as impulse during the recording of room impulse responses. After convolution with a sample the output will be analog to the sample being played back as an omi-directional sound source in the room. To be precise: the impulse response will contain all characteristics of the impulse source, room, microphone and signal chain. An instrument that naturally focus its sound forward (as principal radiation pattern), and being mic’ed accordingly when sampled, will then radiate the same principal sound level and frequencies in all directions in the reverberation model. This is however countered under normal circumstances by subjectively balance the dry signal, representing the direct sound, with the wet signal, produced by the reverb, in the mix.
With synthesized impulse responses it would be possible to take the orientation of the sound source and its radiation pattern into account (Perry R. Cook and Dan Trueman, A Database of Measured Musical Instrument Body Radiation Impulse Responses, and Computer Applications for Exploring and Utilizing the Measured Filter Functions, Princeton University, 1998) and filter the dry input to simulate the non-direct sound being radiated and reflected back to the microphone.