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?