Walking down halls ... (KEVIN AUSTIN )

Subject: Walking down halls ...
Date:    Fri, 31 Aug 2001 07:40:44 -0400

A was written ... >I have also noted degraded recordings from loudspeakers in enclosed >environments. This may not only be true of enclosed environments. > When I record speech from a loudspeaker and then play this >recording back to a listener on headphones I find it to be less >intelligible than a person simply listening to the loudspeaker in the same >environment. Does the listener perceive that what they are listening to is the recording of a loudspeaker? The headphones in this case become a 'second generation' transducer (ignoring the micrphones). > Even though I took considerable pains to calibrate and >balance the recording and playback equipment. Hmmm. From my experience, it is not possible to (seriously) calibrate the frequency response of a loudspeaker. While taken with octave or one-third octave, or even 1/12 octave equalization (meaning more than 96 filters ... at some 60 dB/octave cut-off etc etc), the loudspeaker may appear "flat", the examination of the frequency response characteristics of a loudspeaker are quite different when measured at (eg) 1200 divisions to the octave.IME, the peaks and dips can be several dB, spaced at under a few Hertz. This would mean that no loudspeaker is "flat", when examined with a response curve calibrated in cents (or even 10s of cents). The loudspeaker and the original 'cannot' sound the "same". >Unfortunately I don't have a direct answer, but in my case I suspect it is >related to imperfect recordings interacting with the acoustic environment. And also possibly the frequency-depemdent directional characteristics of the source. Which is ... Have a person stand still, outside (free field) and speak. Walk in a circle (at say 1 meter) around them. Listen to the _frequency response_ characteristics of the voice. Repeat with a (variety of) loudspeaker(s). Depending upon the loudspeaker, the dispersion pattern will produce greater and greater high frequency attenuation as one moves off-axis from the main drivers. Also, with very rare exceptions, there is low frequency sound coming through the walls of the loudspeaker to the sides and from the rear. The sound heard at the back of the head is not added to by the vibration of the skull ... in most cases. >For example the interaction of reverberation, Which leads to aspects of reverberation. The sound leaving the front of the loudspeaker has different frequency response characteristics than the sound that escapes through the walls of the enclosure. It is (effectively) a 'complex' sound. The voice is (almost) a point-source (in this generalized consideration). And, the speaker enclosure has added colorations of its own, which will be added to / embedded into the reverbertion. (A piano is another example of a 'complex source'. Placing microphones at different places (over the middle of the bass strings, over the middle octave hammers, in the various holes in the frame, under the tail, under the action, facing the (open) lid, a meter away from the lid ...) reflects that (up close), the piano is not a 'single source', and is anything _but_ a point source.) > noise, and the >directionality of the ears vs. microphones (in your case it may be related >to the directionality of the loudspeakers vs. human-speakers). I suspect >that better recording equipment and perfecting the >calibration/balancing/directionality of equipment might help and of course >reducing the noise. I would appreciate hearing about any answers you find >through your research. >On Wed, 29 Aug 2001, Michael S. Gordon wrote: >> Dear List - >> I am interested in capturing the acoustic flow field of a person walking >> down a hallway. Thus far I have made a couple of binaural recordings of (1) >> a perceiver walking past individuals reading text (live) and (2) a perceiver >> walking past a series of loudspeakers projecting an assortment of recorded >> sounds (e.g., human speech, >typewriter clicks, etc.). Those who have listened to these recordings >have found the first set a much more compelling >> indication of the >listener=B9s motion than the second. I wonder ... How wide was the hall? How far away were the speakers and the loudspeakers from the microphones? Would the results be tha same in free-filed conditions? (out of doors?) Is the same 'effect' present when the micropones remain still and the sources move past the fixed mics? (These are also issues for the artistic practice called "soundscaping".) Best Kevin kaustin(at)vax2.concordia.ca

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DAn Ellis <dpwe@ee.columbia.edu>
Electrical Engineering Dept., Columbia University