Ultra Low Frequency Sub-subwoofer

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A system for producing ultralow frequency (single digit Hz) with very little distortion is in the works. This system will use conventional drivers to drive 40” x 48” panels, four 8” drivers per panel. The system will be installed in the attic above the home theatre with openings through the ceiling to allow the air movement for operation as an infinite baffle. Four panels are planned which will provide this system with the equivalent surface area of about fifty two 15” woofers. This design is adapted from a similar design by Brian Elliot who’s system produces the cleanest lowest distortion bass I have ever encountered.

This drawing is an old drawing, the panels are 40" x 48", not 48" x 48". The cone to panel coupling has also changed.

The design is for ultralow frequency only with the most probably crossover point at about 18 Hz. The existing dipole arrays perform reasonably well to about 13 Hz but once this system is installed and operating, the dipoles will be rolled off at 18 Hz or possibly a little higher. This IB panel system will then extend to single digits.

Based on the performance of Brian Elliot’s dipole panel system and my own early experiments with this concept, 2nd and 3rd harmonic distortion components are expected to be in the -50 dB and -60 dB down range even at very high output.

While the concept of coupling conventional drivers to larger radiating surfaces is not totally unique to Brian, my project is very much inspired by his work. Brian is a very private individual and does not have a web presence, not even e-mail. I cannot share anything Brian tells or teaches me in private, I can only share concepts relating to this that I have come up with on my own that in some cases have been confirmed by Brian to be “on the right path”.

Problem with non linear distortion… You can’t filter it! Or can you??? Increasing the slope of your Low Pass filter does nothing to reduce the harmonic content in your subwoofer. It’s playing 20 Hz and creating 40 Hz, 60 Hz… harmonics and these have nothing to do with any signal input at those frequencies so no amount of filtering will help reduce them. If however there is a coupling that does not pass the harmonic content, it can be decreased. Essentially, a mechanical low pass filter.

I have experimented with DIY isolators using various types of rubber hose with bolts mounted on each side of the hose, one to mount to the driver cone and one to the suspended panel. The required stiffness of the rubber will be a critical factor and requires further experimentation to determine optimization of this. The idea of course is to optimize the couplers ability to transfer motion from the cone to the panel within the narrow ultralow frequency pass band and minimize its ability to transfer motion above the pass band thus reducing the transfer of harmonic content.

The panels I have chosen are 40” x 48” x 2” honeycomb shipping panels weighing about 5 lbs each. The high mass alone will make it difficult for these panels to produce much above 20 Hz so regardless of the effectiveness of the mechanical filter, harmonics will attenuate rapidly.

The 8” driver is the stamped frame version of the Tang Band W8-1747A which is no longer available from Parts Express. (I already have mine) As I understand it, this is not a discontinued product, just no longer carried by PE so it could become available from them again if a need were to arise. The cast frame version in the link above would also work, it’s just more expensive.

A remaining issue is the rubber surround. I have solutions, just no really good solutions. Ideally I would find a rolled edge like a normal speaker surround but in straight strips, not already cut in circles. I haven't finalized an option on this, I've ordered all sorts of pieces of rubber from McMaster Carr and several will work but I'm still looking for a better option.