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Filter theory question for speaker crossovers
Hi All,
Something that I've been thinking about for a while with the design of speaker crossovers, but don't have the necessary knowledge to calculate or figure out, is this: If you were to take, for example, an 18dB/oct Butterworth high pass/low pass crossover for a two way system, could you synthesize the same summed electrical response (and therefore the on axis response) in terms of amplitude and phase response if you were to lower the Q of the high pass section, and raise the Q of the low pass section from their standard 0.707, without changing anything else ? (In practice you would do this by altering the L/C ratio of the crossovers in opposite directions by a certain factor) Why would anyone want to do this ? Recently I've come to the slow realization of what may be one of the big downsides of having a crossover in the first place, when doing listening/measurements between a full range driver, and that same full range driver crossed over normally with a tweeter. Some classes of high pass and low pass filter have ringing at their crossover frequency whose phase is opposite to their complementary filter - so for example a symetrical high pass/low pass filter can have quite bad ringing at the crossover frequency if you take only one section on its own, but when the output of the two filters are summed together (accoustically) the ringing is largely negated. (My filter theory isn't strong enough here to know WHICH kinds of filter this applies to so hopefully someone can butt in here and correct me) It occurs to me that this cancellation of ringing only occurs on axis, and that even to the side on the horizontal axis this cancellation effect will be incomplete due to the different horizontal dispersion of the large driver vs the small one it is crossing over to. The end result, particularly if there is a large difference in dispersion between the two drivers as is often the case, is that the on axis response may contain very little ringing at the crossover frequency, and sound fine, but the ambient sound field in a typical reflective/reverberant room due to the off axis power response of the speaker near the crossover frequency will be predominately that of the high pass section, as the smaller driver has the widest dispersion. With the output from the low pass section unable to balance the output from the high pass section in these off axis directions, the ringing will not be canceled. The ambient sound field may contain undesirable artificats of ringing and in fact I think I can sometimes hear this characteristic in some of the tests I've done. Assuming that making the dispersion of the drivers more similar is not an option, it occured to me why not make the Q of the high pass section lower, so that the output of the high pass section which is dominating the ambient sound field near the crossover frequency is free of ringing, (or reduced) and then increase the Q (and/or do whatever other correction is necessary) of the low pass filter to give the same or nearly the same summed on axis response. Because the driver connected to the high pass section is bound to have much wider dispersion, you're not going to have a situation where the higher Q low pass section is heard on its own - at any angle you can hear that significantly, the output from the highpass will be audible too. Has anyone tried something like this ? Any gaping holes in my idea ? Regards, Simon |
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