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Inactive speakers...some measurements

A while ago we talked about the effect of as few as one inactive speaker in the listening room. It was suggested that I measure the voltage generated by the inactive speaker. OK. I did it. That's the easy part, and the fact that there is a voltage is not surprising. Let's think about what it means.

With steady loud music cranked up to 100 dB (RS meter), the output of an 8 ohm speaker sometimes hit 30 millivolts. If 30 millivolts were applied to this speaker it would amount to 0.0001125 watts.

2.82 volts applied to an 8 ohm speaker is one watt, and produces roughly 88 dB. In general…

0.03 volt……0.0001125 watt
2.82 volts…..1 watt………88dB
5.64 volts....3.97 watts....94 dB
11.28 volts...15.9 watts...100dB
22.5 volts....63 watts.....106 dB
45 volts......250 watts....112 dB

So, if the speaker were driven by the 30 mv (an absolute worst case assumption) its SPL relative to the SPL environment is:

20*LOG(0.0001125/15.9) = -103 dB

This number is referenced to 15.9 watts, not the full power customarily quoted for Hum and Noise. Unlike Hum and Noise the inactive speaker effect (if any) comes down with the SPL, and of course goes to zero when there is silence and when Hum and Noise at –100 dB (referenced to full power) is pretty much inaudible.

I do not believe that anyone's ears can detect a sound source that is 100 dB lower than a prevailing 100 dB SPL. And what would that sound source sound like anyway. Probably much like the 100 dB sound that induced it, making it even harder to detect.

I would still like to participate in a double-blind test for this effect, and I repeat my previous thought that a salesroom full of dozens of speakers may be a different story.

Until that happens…OK you believers…it’s 30 mv at 100dB. How would you analyze it so as to convince me?
eldartford
sogood51
2,157 posts
 
Eldartford, I agree with you now as I did before.

Dave
unsound
7,329 posts
 
I'm curious as to what inactive speaker you used, what the room was like and where was the inactive speaker relative to the room and active speakers. What conclusions, if any, does this have on the use of passive radiators?
sean
6,231 posts
 
You're missing the point. Not only are the non-driven speakers being excited, they are re-radiating the energy that they absorb out of phase. Depending on the suspension of the drivers and spl's in the room itself, the amount of excitation and re-radiation will vary at a non-linear rate. This is not to mention that some drivers have far looser suspension than others, making them more susceptible to excitation and re-radiation. Using a lower efficiency speaker of the sealed variety as a point of reference, all other speakers will produce greater variances into such a situation.

If you want to do this and do it simply, take a few frequency sweeps in the room at various repeatable volumes and chart the frequency response. Then do the same thing after introducing some good sized speakers that aren't going to be used into the room. These speakers should be relatively efficient and use either a vent or a passive radiator. Duplicate the previous test and then compare the results.

If you really want to do this and do it quickly, borrow some type of device that does digital room correction and run the above tests. It will give you all the variables in a matter of minutes. Sean
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the_smokester
174 posts
 
Okay, I'll bite:

Your measurement seems to make sense on the back-of-an-envelope.

0 dB of sound intensity is, by definition, 10^-12 W/m^2. This is "accepted" as the threshold of human hearing. If you have 100 dB of sound impinging on the face of your speaker that is 10^-2 W/m^2. The area of an 8" diameter speaker is about 0.032 m^2. So the sound power available to excite the speaker is no greater than 10^-2 x 0.032 = 3.2 x 10^-4 W or 320 microwatts.

If you were using an 8" speaker and measured 112.5 microwatts then the sound energy impinging on the speaker driver was converted to electrical energy with about 112/320 = 35% efficiency.

From these numbers it is hard to believe this could ever be a problem even for flea-powered systems.

But then, the human ear never ceases to amaze.

Thank you for making these measurements.
eldartford OP
4,262 posts
 
sean...It isn't easy to figure out what to make of the data, so what I did was to determine what kind of power level we are talking about. I assumed that this power was all radiated (or absorbed if you like). And my conclusion is that inphase or out of phase it's just too damn small to be heard.

The frequency sweep you suggest sounds like the obvious thing to do if you had equipment, but, in practice I doubt that any instrument would be sensitive enough to even get close to the power level in question. I believe that the ear is much more sensitive than any instrument, but when an effect is large enough for the instrument to detect the instrument gives a description that the sensitive ear cannot provide.

The speaker I used was an aperiodic (damped vented) enclosure containing two LF drivers (8" and 10") which is good to about 30 Hz when powered. It was placed about 2 ft in front of a MG1.6 + subwoofer source. The 100 dB environment was measured right at the inactive speaker.

Regarding passive radiators, I generally do not like speakers that use them, but I can't be sure it is the fault of the PR. Supposedly a PR is functionally similar to a vent but behaves in a more controlled manner and without wind noise. (I don't like vents either). One idea that sounds interesting is to use an actual (inactive) driver instead of a PR, and tune it by L/R/C loading of the voice coil, instead of by the usual way of mass loading the cone. This can be extended to actively driving the "inactive" driver with some sort of signal not the same as the primary signal going to the "active" driver. (In fact I have such an experiment in process).
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