EFFECT OF STIMULUS SPEED ERROR ON MEASURED ROOM ACOUSTIC PARAMETERS
Hak, Jan · 2007
Impulse response measurements based on deconvolution techniques normally require a connection between the stimulus generator and the response recording device. This is inconvenient for long distance measurements, in particular of room acoustics. Playing the excitation signal from a CD generally introduces errors due to a speed mismatch between the CD player and the response recorder. It is investigated which speed differences are to be expected, and how common room acoustic parameters are affected. It is found that MLS stimuli are not really usable. On the other hand, sweeps allow large speed errors and can be used in a broad range of situations. Parameters based on short initial energy intervals are prone to errors when measured with stimuli played too fast rather than too slowly. INTRODUCTION Many room acoustic parameters are derived from the room impulse responses. Examples of such parameters are the reverberation time, which is related to the energy decay rate, the clarity and definition, which both are related to early to late energy ratios, and the speech intelligibility, which is related to the energy modulation transfer characteristics of the impulse response. Impulse response measurements are often based on convolution techniques [1][2], using maximum length sequences (MLS) [3] or sweeps [4] as stimulus signals. These techniques basically require the room response to be captured synchronously with the generated excitation signal. In some cases it is necessary to play the stimulus signal and capture the response asynchronously. For instance with speech intelligibility measurements in railway stations, the source may be a CD player in an announcer’s booth in one city, while the receiver is a microphone connected to a PC on a platform in another city. Normally, the stimulus playback speed will then differ slightly from the response recording speed, which affects details of the resulting impulse response, hence the derived acoustic parameter values. It is investigated how and to what extent this effect occurs.