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Subwoofer Efficiency is Mostly Irrelevant
Subwoofer Efficiency is Mostly Irrelevant
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Old 2nd August 2020, 03:14 PM   #31
CharlieLaub is offline CharlieLaub  United States
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There is one other item of interest that the OP (Patrick) or other readers might want to consider, keep in mind, etc and that is true efficienicy vs midband efficiency. The true efficiency is more like the efficiency vs frequency (if I am recalling the terminology correctly) and is something that has come into the fore in recently times as very high BL, low Qts pro drivers become more commonly available.

When a driver has very low Qts the impedance peak around its resonance is quite broad. This means that the impedance magnitude (the AC impedance) does not fall back to Re for several octaves below resonance. Therefore the impedance at low (e.g. subwoofer) frequencies often remains several times higher than "nominal".

The higher impedance causes the driver to draw less current and it will do that in a relatively wide band BELOW the impedance peak. Because modern class-D amplifiers are available with very high voltage output you can bring much more voltage to bear when powering the driver, but without requiring so much current. This kind of operation should be less stressful for the output devices and will drain the power supply caps more slowly than when high current is required.

When you look at a box modeler for a low Qts (e.g. Qts=0.2 or less) driver, the situation looks hopeless and it often seems that a higher Qts driver would be better. But because the low Qts driver will not draw as much current as the high Qts driver, it is actually more efficient in a true Watts sense than the high Qts driver.

There was a thread about this topic in this forum (or at least on DIYaudio) about 6-12 months ago. Initially I argued that the high Qts driver would be better until I sat down and did some sims myself. I was surprised that when I calculated the true Watts (Volts*Amps, but not including phase angle) that the low Qts driver required LESS power to achieve the same SPL. That did come at the cost of having to apply a higher drive voltage, but that is indeed possible these days.

I immediately ran out and bought a Behringer NX6000 when I saw the results of my True Power modeling. This inexpensive amp can put out peak Watts of several kW into 4R but only for a brief period of time. The problem is that the PS is not large, so if you also need to pump lots of current you will not get good results at low frequencies because the peak of the signal waveform lasts longer than the PS has current reserve. But if we instead use a low Qts pro audio driver the current draw will be much reduced and the PS rails will be able to hold up even at low frequency. My plan is to use a large, low Qts driver in a large closed box and, with DSP, EQ the frequency response to flatten and extend the bass. The large box also helps to keep the true efficiency high, and keep distortion low while the CB should keep the time domain response crisp and clean. If you have the room for it, this seems like a winning combination to me.

So, for the low Qts versus high Qts case you can see that "subwoofer efficiency" can actually be quite relevant when viewed as SPL per "true" Watt of input power. This idea was touched on but not explicitly stated in the link that Marcello provided in his post, immediately prior to this one. I thought I would spell it out in a little more detail.
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Last edited by CharlieLaub; 2nd August 2020 at 03:20 PM.
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Old 2nd August 2020, 03:34 PM   #32
mark100 is offline mark100  United States
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Subwoofer Efficiency is Mostly Irrelevant
Very helpful, thanks Charlie
Well explained...builds nicely on the link Marcello provided.
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Old Today, 03:11 PM   #33
bolserst is offline bolserst  United States
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Quote:
Originally Posted by CharlieLaub View Post
... I was surprised that when I calculated the true Watts (Volts*Amps, but not including phase angle) that the low Qts driver required LESS power to achieve the same SPL. That did come at the cost of having to apply a higher drive voltage, but that is indeed possible these days…. So, for the low Qts versus high Qts case you can see that "subwoofer efficiency" can actually be quite relevant when viewed as SPL per "true" Watt of input power.
A nice easy to follow explanation , and thanks for sharing your road to discovery.
This was exactly what Bob Carver was getting at 25 years ago in his original Sunfire sub white paper. His use of analogy with electric motors, stall mode, and back EMF left people used to dealing in TS parameters shaking their heads. But, once you got past the eye-rolling hyperboles characteristic of Bob’s literature…many subwoofer designers slowly started to realize he was actually on to something.

The best analysis I’ve seen on the topic is the attached 2003 AES paper available on Keele’s website.
AES Papers -- Official website of D.B.Keele

"…To conclude, if your design can accommodate equalization before the power amplifier and the power amplifier can provide higher voltage swing, then raise your driver’s Bl product to the highest possible value consistent with material and economic constraints! This will result in the highest efficiency design."

I had previously posted the referenced Vanderkooy paper on the same topic here: How does Hornresp use BL?
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