Abstract
The use of moving volumes (people) instead of rotating vanes for sound field randomization has been proposed as a plausible and convenient technique. A perturbing volume Vp in a room of volume V0 shifts the frequency of the lmn mode by an amount (Δf/f)lmn = ∫vp(U − K)lmndV/∫v0(U + K)lmndV, where U and K are the modal potential and kinetic energy densities of the sound field. Computer experiments confirm that when Vp is moved throughout a room, the rms frequency shift is given by (Δf/f)rms = 3(Vp/V0)H(ka). For a cube of side a, H≃1 for ka⩽1, falling basically as (1/ka) above this. For Vp a thin square of side a, or bar of length a, the same asymptotic relations hold, but the transition to (1/ka) behavior is much less abrupt. If (Δf/f)rms⩾ the mean intermode frequency spacing (c3/4πV0f3), one expects adequate randomization of the room modes for frequencies above fp = c/(12πVp)1/3 provided Vp is displaced sufficiently between recordings of the spectral samples. Measurements of spectra using fixed source and microphone but moving Vp as outlined show however that the observed mean spectrum has a variance only slightly less than the ± 5.2 dB value associated with a single reading. That is, the expected randomization due to moving Vp hardly takes place. Other types of experiment confirm this. Possible reasons will be discussed. [Assisted by NSF.]
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