norman@lasspvax.UUCP (Norman Ramsey) (08/05/85)
In article <1286@hound.UUCP> pearse@hound.UUCP (55131-S.PEARSE) writes: > >Recently there has been talk about "fast" vs. "slow" woofers. Someone >indicated this is only a function of limited bandwidth of the woofer. >Could someone explain how a, well, bandpass or lowpass system results >in a time delay? Is it just like how an equalizer introduces phase >distortions? Surely an equalizer doesn't introduce audible delays, >does it? There is a wonderful theorem of mathematics (vis a vis acoustics, that is) which relates frequency response to time response. In linear response theory, a system can be described either by its frequency transfer function (i.e. "frequency response", which is what audiophiles like flat) or by its response to some (time) singular excitation (usually at delta function or step function at t = 0). These two are related by some reasonable complex equations full of Fourier transforms. Basically the idea is this -- the better the high frequency response of a system (read: larger bandwidth), the more quickly it will jump when belted with a single pulse at time zero. I think this is all that's going on. As far as phase distortions go, I am very uninformed about what an equalizer does. I do know that any kind of simple filter (like one I am capable of designing) has a characteristic frequency. Below that frequency there is usually no phase shift, while far above it there is usually a 180 degree phase shift. The phase shift changes continuously from zero to 180 as the frequency goes from zero to infinity, with most of the change occurring near the characteristic frequency. I have no idea what can be done with complicated banks of filters; it's black magic to me. If I could draw I would draw you a picture of the frequency response (magnitude and phase) of a simple RC filter. So the frequency response is a complex number with two components -- a magnitude (which is what you always see quoted in dB) and a phase (which you don't hear much about). Ideally you want the phase and magnitude to be constant over the useful range of the device. Hope this helps. This sort of thing is discussed on a very practical level in a book by Horowitz and Hill called _The Art of Electronics_ -- it should be in your engineering library. -- Norman Ramsey ARPA: norman@lasspvax -- or -- norman%lasspvax@cu-arpa.cs.cornell.edu UUCP: {ihnp4,allegra,...}!cornell!lasspvax!norman BITNET: (in desperation only) ZSYJARTJ at CORNELLA Never eat anything with a shelf life of more than ten years