[DrWho]

Chrisbee wrote:

Interesting points. Though I have some further thoughts.

An IB cone is not limited to the front surface only. The back of the cone usually "sees" a finite rear enclosure. A pressure wave returning to the front of cone from the listening room is likely to be matched by a pressure wave returning from the rear volume. Only if the IB uses the great outdoors will there be no returning wave to match that arriving at the front. The phase of the returning pressure waves is very unlikely to match cone movement.

Unless the rear chamber and the front chamber are identical in shape, then the pressure waves are going to arrive at different times.

How is this "problem" related only to the IB? A reflex system below system resonance suffers driver unloading. Would it too not also suffer from returning pressure waves? Wouldn't the reflex cone drive its own reflex port into an infrasonic rumble?

It's not, but in a "small box" (relative to wavelength), you effectively see a static increase in pressure inside the cabinet that matches the movement of the cone. Below the system resonance, the leakiness of the port becomes the swamping issue.

Wouldn't these returning pressure waves be much reduced in comparison with the originating pulse from the driver cone?

It might be possible to monitor an IB or reflex system driver using the voicecoils as a low impedance microphone. While another IB (or VLF-capable sub) at the other end of the room plays a suitable signal.

Meanwhile: In at least one real-life IB situation: If the driver being monitored was an AE IB15 then I doubt much would be found emanating from the driver terminals with such stiff suspension. My very leaky IB enclosure is subject to the prevailing SW winds but no cone movement has been detected by its formerly worried owner so far.

I don't think we need worry too much about returning pressure pulses from the IB affecting itself. The change in atmospheric pressure from a returning VLF transient is simply too small to have any detrimental effect on sound quality.

I agree, the pressure in front of and behind the driver shouldn't move the cone around (in other words, that's not what I was saying). For a changing air impedance and a given voltage applied to the motor of the driver, you are going to realize different cone excursions and SPL. When the impedance of the air goes down, the acoustical damping on the cone is less as well as the SPL (the two are intimately related). But with an increase in impedance, the SPL goes up and the excursion goes down. In the modelling programs, the pressure infront of the driver is considered to be constant (normal air pressure in the room) and so the impedance is assumed constant. However, whenever the pressure in front of the driver goes down, the air impedance also goes down. It works in the other direction too, but a higher impedance isn't going to be the cause of a driver bottoming out

I believe The Thigpen Rotary Sub has yet to prove itself as silent as an IB when receiving no signal. Perhaps it needs a folded, tapered and heavily damped labyrinth of sufficient length to avoid unwanted pipe resonances?

lol - I was just referring to it in principle (increasing air pressure to increase the air impedance seen by the driver). Another equally effective approach is to use a horn - only they need to be insanely huge...

With an IB, you have no control over the impedance seen by the driver and it is certainly going to change with the dynamic response of the room. It is a nonlinear system making it very difficult to predict, but it happens. It just becomes a bigger issue with IB's because the cone excursion is an unstable system - compared to other designs where the cone excursion is better controlled (small sealed cabinets and vented systems above the port tuning)

Also, when cranking any type of system there are going to be peaks centered around the impedance peak of the system - usually the Fs of the driver. This is due to the heating of the voice coil adding a resistive component to the impedance response (reducing the output more at frequencies where the impedance is low). Peak excursion is generally going to happen around 15-20Hz, which isn't going to be very pleasant when the signal is riding along a huge 10Hz wave.