Subwoofer for Music - Yamaha YST-SW90

Current Setup

My stereo front Dynaudio Focus 220 MKII goes low but I wanted more low grunt, sub bass < 40Hz. The new Wyred4Sound MinT digital integrated amp has an Aux Out ideal for a RCA sub connection. This is often known as a 2.1 Channel system.

First Experiment with SW45

Without much research, I snapped up a Yamaha YST-SW45 on eBay to experiment(*). Focus 220 II is rated to 32Hz (hard to know what the manufacturer means but measured roughly to -6dB point). Comparing with Dynaudio Excite X32, I'd say the Focus 220 goes lower but is a bit flatter in its low level response. For all the setups, the high cut is set to the lowest possible 50Hz.

First impression was "Wow, that's nice". Deep rumbling bass gives a warmer feel I badly wanted. More tweaking of the SW45 and something seems missing. Strange, the lowest registers of the "Arrival To Earth" lost some of it's bone shaking feel - more fuzzy bass but not exactly lower. SW45 seemed to have cancelled out the sub bass coming out of Focus 220. The low frequencies can be recouped somewhat by turning up the sub volume but it was unsatisfactory. SW45 seems to "replace" rather than augment the deepest bass of the Focus 220 front.

Some of that was caused by phase cancellation. The situation improved further when I turn the sub around to face the back in order to effectively reverse the phase - without a phase switch - to match with rear ported speakers like the Focus. But the contribution and benefit of the SW45 was still minimal.

Enter Big Brother SW90

Then I was very happy to acquire a SW90 for even less and here are some interesting comparisons and audition results. Firstly I was surprised that the phase switch worked better at the "normal" setting when the SW45 effectively needed inverting. All the better that the sub can face the right way forward. And it is true that sub 30Hz frequencies are mostly felt rather than heard because the Focus 220 II already gives everything that can be heard.

A combination of the higher spec cone sitting higher up and extra bulk mostly hidden behind the curtain, so to speak, must have enabled the SW90 put out at least twice as much as bass as the SW45 and 7Hz lower too using the same 8 inch diameter. There are some good reason why it integrates so much better with my system (even without phase invert).

Analysis

Looking at the frequency response from Yamaha manuals, it’s strange to see the lower dB ceiling when set to 50Hz cut off for both subs. A bit of editing better illustrates the difference between the SW45 and SW90. Notice how much flatter the SW90 response is below the 50Hz cutoff.

The cutoff of the SW45 looks too similar to the Focus 220 which might explains why bass extension was not as obvious and the phase matching became a serious issue. Doing a logarithmic sum of the SW90 and Focus 220 should show a flatter extension while exhibiting a tiny bump at 45Hz but overall pretty flat and extends the range down to 25Hz (-3dB point). Definitely a very useful hands on experience experimenting with the two subs.

Focus 220 tower (purple trace) - The –3dB point is at 39 Hz, and the –6-dB point is at 31 Hz

In my book, 7Hz is huge by audio standard and and the sum of the output really shakes your innards to uncomfortable levels at times. Amazing for an 8 inch cone of 1999 vintage. More importantly, excellent choice for £30 in 2018.  Aesthetically it’s no more obtrusive than the SW45 and perfect for a medium sized room. For the same bass output, the required volume knob is also set lower than the SW45 (6 vs 8 of 10).

Doing the Math

Adding dB is roughly +3dB when doubling (i.e. 85dB + 85dB = 88dB).

Sum = 10*LOG(POWER(10, dB1/10)+POWER(10, dB2/10), 10)

It's reassuring when the maths and measurements confirm the audition and feel. I am happy with the overall sound and the plot seems to show a nice smooth extension of the low end without excessive bumps.

(*) With hindsight, based on the frequency response data of both front and sub, I shouldn't have bothered with SW45.