Buchardt Audio S400

[KaiserSoze]

Actually @617, I've just realised our original misunderstanding due to my reading your earlier post too quickly. You were talking about the ratio of each driver's swept volume of air, not of each driver's surface area, which is much more useful - my apologies!

However, in that case the ratio is not 1.5:1. This is because for whatever reason SB gives the linear coil travel (p-p) for their woofers, as opposed to Xmax. This means that the SB17NBAC in fact has an Xmax of only 5.5mm, not 11mm, giving a swept volume of air for the woofer of only 64.5, and thus a PR:woofer ratio of around 3:1. This tracks well with the Hornresp simulation.

The passive radiator is similarly specified peak to peak, however for the passive radiator the specification is for the mechanical limit of excursion. Manufacturers routinely quote Xmax for active drivers based only on the distance the coil can move without reducing the coil/gap overlap, i.e., the linearity of the suspension stiffness is disregarded when calculating Xmax. If you use the mechanical limit of excursion (out of necessity) then the 5.5 mm value and the 11 mm value are consistent in the sense that both are +/- values.

However this isn't really starting off on the proper foot, because this "rule of thumb" is next to useless. This small passive radiator, with diaphragm area equivalent to a circular driver with actual diaphragm diameter slightly less than 6", ostensibly allows this speaker to play very low, with -3 dB at 33 Hz. Would an active driver about the same size (or even a little bigger) be able to do the same thing? Is the advantage in linear excursion a strong enough effect to explain how this is possible?

For equal SPL at two different frequencies, the ratio of swept volume is in proportion to the inverse square of the frequency ratio. Since the diaphragm area of the passive radiator is essentially identical to the active driver, the swept volume ratio will be the same as the ratio of linear excursion. At best the linear excursion of the passive radiator will be twice that of the active mid-woofer. Probably not this great, but let's be generous. Since the swept volume ratio will thus be 2:1, the frequency ratio will be 1.4:1. In other words, for the SPL at which the active mid-woofer exceeds its linear range at 65 Hz, the passive radiator will exceed its linear range at about 45 Hz (because .707 x 65 = 46). More likely it will be higher than 55 Hz, only a tiny bit lower than the frequency at which the mid-woofer exceeds its linear range at a given SPL. At 33 Hz (the quoted -3 dB point of the speaker), the passive radiator will exceed its linear excursion range at SPL much lower than the SPL at which the active mid-woofer exceeds its linear range at 65 Hz.

Since this speaker (like all speakers of roughly similar size) isn't going to produce real bass, not in the true sense of the word and at an audible level, and will therefore only ever be used when augmented by a subwoofer, what difference does it make whether it uses a port or a passive radiator?

 

[doorofnight]

I wonder if the cardioid tuning for the A700 is ready when the A700's are being delivered?