Damping factor or watts?

These comments from recent audio shows are about rooms that used an amplifier having a damping factor of about 1.2 (that's "one point two"):

"Bass was excellent. I think I was told [the speakers] went down only to the mid 30s (I could be wrong) - sounded lower than that." - Paul Folbrecht on AudioCircle, RMAF '06

"The bass was powerful and tight while the overall presentation was wall to wall and incredibly precise." - Josh Ray, Sonic Flare RMAF '07 coverage

"Of all the rooms at the show, big or small, this room... [was] the finest thing I heard." - Thomas Portney, reader comment on Stereophile RMAF '07 blog

Duke
dealer/manufacturer

Rodman99999, I'll take you up on your bet. My house up against your farm. Here goes:

The speakers used were 92 dB efficient two-ways.

The woofer in the RMAF '06 room had a moving mass of 79 grams; Fs = 40 Hz; Qes = .35; Qms = 11.69, and Vas = 77 liters.

The speaker in the RMAF '07 room had two woofers, each with a moving mass of 41 grams [combined moving mass of 82 grams]; Fs = 31 Hz; Qes = .32; Qms = 4.35; and Vas = 121 liters [combined Vas of 242 liters]. This speaker is a bipolar, so the efficiency is 3 dB lower than the combined T/S parameters would indicate.

I am the speaker designer (thanks for asking!). The amplifier was an Atma-Sphere S-30.

So tell me.... just where is my new farm located??

;o)

Duke

Hi Rodman99999,

I don't blame you one bit for fighting to keep the family farm.

92 dB is higher than average efficiency, I'll admit. Whether or not it qualifies as true "high efficiency" is a judgment call - over on Audio Asylum they discussed at length where to draw the line, and if I recall correctly the consensus was 96 dB. You and I will probably have to hire lawyers and go to court to settle this issue. Or maybe we could have a cage-fight? We could sell tickets and split the proceeds, maybe both be able to buy some new gear (allow me to recommend a nice low-damping-factor amplifer...).

Sorry, couldn't resist.

Okay, back to the RMAF rooms. No autoformers were used. The only things that made the speakers "low damping-factor friendly" were a smooth impedance curve and a fairly low port tuning. The port tuning is actually user-adjustable, so the speakers can be adapted to different room acoustic situations, but this feature is also useful in amplifier matching. With a solid state amp, a higher tuning frequency would probably work better.

Duke

Rodman99999, I chose those woofers based on low thermal compression, suitable combination of bass extension and efficiency in a size box that I could live with, and sufficiently smooth response between 1 kHz and 2 kHz. I'd have liked higher voice coil DC resistance in both cases, but went with what was available. These woofers were not picked out as being uniquely "low damping factor friendly".

The heavier-cone woofer is a 12" with plus or minus 7 mm of linear excursion, and the lighter-cone one is a 10" with plus or minus 6 mm of linear excursion. Both are prosound drivers with unusually low efficiency (and consequently good bass extension) for prosound units of that size; most prosound 10" and 12" woofers are really midranges with efficiencies in the upper 90's.

Duke

Thanks, Kijanki.

One other thing I should mention: I don't intend to give the impression that a low damping factor amp will work with just any speaker. I'd want to see the impedance curve and possibly the frequency response curve before making that call.

That being said, a couple of weeks ago a friend of mine wanted to try a low damping factor amp on his 89 dB, 5-to-20 ohm two-way monitors. I was skeptical because of the impedance curve. He liked the sound, but there was a lower treble emphasis associated with an impedance peak in the crossover region (which I had predicted, having seen the curve). So I calculated and assembled a circuit that smoothed the impedance peak, and that solved the problem. Maybe that would work with other speakers as well. I'd like to adjust the bass tuning in his speakers also, but we haven't gotten to that point yet.

Duke