The Perception of Musical Tones in the Statistical Sound Fields of Rooms.

The perception of musical tones in a room is discussed in light of the room's chaotic sound transmission characteristics. We measure the auditory system's sensitivity to sound source level differences in a room for sine wave and harmonic tone signals. Using the method of adjustment, we derive just noticeable differences, 2.8 dB for the sine wave and 3.2 dB for the tone. The related difference limens derived using a forced choice methodology are 3.4 dB for the sine wave and 3.0, 3.1, and 2.8 dB for three varieties of a tone. The mutually agreeing JND and DL for the sine wave imply that the auditory system processes the equivalent of at least four samples of the room's statistical soundfield. The comparable discriminability of a harmonic tone, whose soundfield's std. dev. is already reduced due to its multiplicity of frequency samples, suggests that the value 3 dB can be considered a round number baseline criterion of discrimination in a room, independent of signal spectrum. Three routes by which the auditory system may access information about a sound source in a room are: (1) motion about the soundfield, (2) the room's onset transient and (3) binaurality. In a related loudness discrimination experiment, we find that with either of the first two of these routes, discrimination is nearly the same as with both; however, with neither, correct discrimination is by chance. We studied the third route by measuring the cross correlation of a random soundfield sampled at the two ears. Binaurality provides two independent sound samples at excitation frequencies above 500 Hz. This value represents a reduction of the two-microphone correlation frequency by a factor of two due to diffraction of sound by the head. With a masking (in a room) experiment, we find that the auditory system's ability to detect a sine wave in the presence of noise is improved by around 5 dB, when the masked tone is presented via the room's random transmission and the masker noise via earphones, as compared to the thresholds measured with all other combinations of these masker/masked-tone presentations. This singular result suggests that detection via the peaks in the fluctuating statistical soundfield occurs only when coupled with auditory segregation of the room/earphone communication paths.