Isobaric Subwoofer

Rohit Balkishan Dubla

Introduction:

Here I describe the construction of my subwoofer meant to augment my new WMTMW speakers. These speakers have a lower -3 dB freq. of about 50 Hz and therefore a sub was mandatory. Since I already had 10" woofers from my older speakers I thought that if I can put these to good use then it would be great. After a bit of internet searches, I concluded that if cost saving, reuse (of the drivers and wood from older speakers) and small size are the goal then an isobaric subwoofer is the way to go. These woofers actually do need a large volume behind them (about 60-70 litres each) to hit about 40 Hz (-3 dB) - and then a bit of EQ to get them to go down till about 30 Hz. Therefore with these drivers in isobaric configuration would need a total volume of about 35 litres to get to the same unequalised -3 dB freq. of 40 Hz and then to go lower with EQ.

 Construction:

As I have reused the older speakers, it was much easy to build the isobaric enclosure - the older speakers already had the required cut-outs on their panels, which I simply cut to the required dimensions.  

I kept separate wires for the drivers which are terminated on their own connectors at the back. That is to give me the option of connecting them in series or parallel depending on what amp I make for them (single-ended or bridged).

To ensure no leakages, all edges (except one side - the one that came after the drivers were installed) are caulked with resin + hardener sealant from the inside as well as outside. Since there is no way to caulk the edges from inside for the side panel fitted after the drivers went in, I put double-sided rubber tape on top of each edge (including the inner panel's) before screwing that panel on top of them. This with the resin sealant from outside should ensure a completely air-tight box. Only thing is (perhaps) a small leakage due to insufficient seal with the inner panel since that was not (and could not be) caulked - something I guess I'll have to live with unless something warrants removing the front driver to caulk that panel. Further, before installing the drivers silicone was applied to the rims to ensure a good seal.

The subwoofer is driven using the 200W/4Ω power amplifier based on STK4192 and equalised by a 12 dB/oct peaking high-pass filter which extends its response to about 33 Hz and a 6 dB/oct low-pass filter cutting off at about 200 Hz. This, in conjunction with the subwoofer's own response peak at about 75 Hz yields a band-pass response that has bandwidth of 30-150 Hz approximately. The following graphs show the subwoofer's simulated response.

As can be seen, the subwoofer's response peak and the EQ bring the sub's response to match the woofer. Despite being inefficient, using the subwoofer's own resonance peak with the 2nd order peaking high-pass has helped create an overall response that has adequate SPL which blends well with the main speakers, without requiring too much amplifier power. Even at loud listening levels the amplifier hardly heats up - this is in part because of the subwoofer's own response peak, where it presents much greater impedance to the amplifier. Thus, as the amplifier is putting out power mostly in the vicinity of the subwoofer's response peak, the current and therefore dissipation is quite less.

 Results:

Subjectively the sub sounds rather uninteresting all by itself because of the overall band-pass response (30-150 Hz). But when operated with the main speakers, it not only adds the required bottom end to them, but also all but disappears from the scene. Room gain also helps a bit here and keeping the sub in a corner or close to the rear wall aids with the overall bass. There are no major suck-outs for a majority of positions within the listening area. Besides benefitting music, movies sound almost theatre-like - despite this being a primarily stereo setup: The subwoofer channel is extracted from the main left & right signals from the line-level crossover instead of the subwoofer output on the DVD player, and the DVD is set up for stereo mode instead of 5.1. Overall, a very satisfying experience, given the performance, sound quality and cost-savings that this has given me.