Systemic Development > Speakers

what is a Proximity Woofer?

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malloy:
Saw this on YouTube. https://www.youtube.com/watch?v=4jyLsO1YD6k

The part about the 'proximity woofer' contributing to air pressure starts at around 19:00.

Can anyone explain in layman's terms what this is doing and how different is it from a subwoofer as commonly understood?  :?:

Thanks!

Nick B:
Paul,

I just subscribed to that guy‘s channel a few weeks ago. He sure is privy to some extraordinary gear. I watched two more videos so I could listen to the $1,000,000 speakers.

malloy:
Hi Nick,

Super exotic gear for sure. I thought I was familiar with most brands, but some of the brands he features, I've never heard of.

In the video I linked to, they even have a Berning prototype.

Paul

Nick B:
Hi Paul,

Fun to see such upscale gear. That Dohmann turntable is unique and has vibration control built in which I believe was designed for an electron microscope. I owned a McCormack amp some years ago. I visited his modest home in Vista a couple of times as he was doing some mods for me. A very nice man.

Nick

_Scotty_:
The explanation tendered for the operating principle of proximity woofer is not completely understandable as presented. It would be nice if a before and after graph of the frequency response
below 125hz at the listening position could be shown documenting its effectiveness.
 Adrian Celestinos at Aalborg University in Denmark did a Ph.D thesis a number of years ago on how to control room modes below 125Hz. See link https://vbn.aau.dk/ws/portalfiles/portal/12831869/AC-phd.pdf
This is a simplistic explanation of the principles involved.
 Optimal results were obtained in rectangular rooms when a planer wavefront in the bass frequencies is created by the front loudspeakers and propagates down the room length wise towards the back wall. In the normal course of events the planer bass wave impacts the rear wall and creates a series of standing waves which creates major frequency response aberrations in the bass frequencies.  Celestinos' approach is only intended to deal with room modes resulting from bass frequencies that propagate longitudinally from the front to rear of room. 
 His solution involves using sub-woofers built into to the wall at the rear of room whose output is reversed in
phase and delayed in time. The delay in milli-seconds of their output will correspond to distance the bass wavefront will travel to just before it can impact the rear wall and cause problems. The delay in adjustable
to account for the room size and the listeners position relative to the front loudspeakers and the rear of room.
 The object being to allow time for the bass to reach the listening position and then be canceled out before it hits the back wall. Under test room conditions he was able to produce a bass frequency response substantially better than plus or minus 3dB everywhere in the IEC and ITU test rooms.
 From a practical standpoint all that is required to implement this solution in ones own listening room is a sub-woofer with a bandwidth equal to or exceeding the bass extension of the main speakers and a digital delay unit adjustable between zero and 100 milliseconds, 20 milliseconds on the upper end would be more than adequate in most cases. 1 millisecond corresponds to about 1ft. of travel for a sound wave assuming a speed of sound of 1100 ft/sec. 
 In my case I purchased a QSC-DSP 30, 2 channel digital signal processor, XLR IN and OUT. This was being discontinued discontinued at the time because the chip it is based on was being discontinued by the mfgr.
I also purchased a pair Rythmik L12 sealed subs. Because bass frequencies are lost through the walls and ceiling of most listening rooms the actual bass output required to effectively cancel out the wave front before
it hits the rear wall will be much less than the output from the front speakers. Typically you have to be right on top of the sub before you can hear its output.
 The same thing can be accomplished with a sub-woofer or two operated out phase at the rear of room without the delay but the required size of in the rooms length needs to be long enough for the listeners position to be forward of the midpoint of the room which where the bass will start t to canceled out. The variable delay allows a great deal more freedom regarding the listening position relative to the rear wall of the room.
 My apologies for the long-windedness of the explanation. The proximity subwoofer has delay and phase adjustments but its position room  seems quite confusing. My understanding of bass response irregularities due to room modes below 125Hz. and their remedy is pretty much limited to the information from Adrian Celestinos.
 I will say that after implementing his approach to the problem, albeit in a somewhat compromised
form, the bass problems seem to be minimized and the imaging is much better. One can easily hear the improvement by simply switching subs on or off. The bass passages intruding into the living room upstairs are also much reduced when system is in operation.
 My friend Stan Warren has postulated that low frequencies provide a carrier function when the rest of the audio spectrum is superimposed on them as is the case when music is being listened to. When a standing wave is present in a listening room the carrier frequency is distorted, the greater the number the of bass frequency standing waves present the worse the imaging is. Even a partial reduction of these standing waves results in dramatic improvements in imaging easily heard in an A/B test when switching the system on or off.
 Scotty

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