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Re: Tom Danley at AES Chicago

Rapid

The point you raise (i.e.: the asumption that direct radiator speakers are minimum phase) is exactly what Tom Danley has called into question and provided evidence against. If you take a theoretical speaker on an infinite baffle, it will show an increasing acoustic load with increasing frequency. To make the driver measure nominally flat, the mass of the moving system will have to be increased to the point that it counteracts this tendency. At this point the driver can modeled as an electrical analogous circuit, or an R/C filter to be specific. This is where the R=the voice coil resistance lumped with the amplifier source impedance, and the C is the moving mass of the driver reflected through the driver motor. This C behaves like a capacitor (but is a seperate factor from the actual voice coil capacitance of course). So basicly what you have in this model is an R/C crossover which adds phase shift. Now let's say that you increase the strength of the magnet to a great degree (or conversely drasticly lighten the moving mass) to the point that you have an "overdamped driver", or where the magnet becomes dominant. You then get a rising high frequency response as the mass becomes less dominant. You can see this in a typical big magnet musical instrument driver (which is over damped) as the bass will roll off typicaly over an octave above Fs or, to look at the other way, a rising high frequency response. The large magnet Lowther drivers are typical of this family too, and their rising high freq. response is generally noted (however their whizzer cones are a factor in this too). Adding a horn to this type of driver introduces an acoustic load which is not like a capacitance in that it is (theoreticly if done correctly) consistant over the horn's pass band, and not frequency dependent. So you are substituting the R/C filter effect for the more consistant acoustic resistant load of the horn ( and some bandwidth too, no doubt). Hopefully Tom will come in here if I have mangled any of the concepts.

Paul


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