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Inactive speakers in the room...any effect?

A recent thread degenerated into a discussion about whether an inactive speaker in the listening room affects the sound of active speakers. I should have been more tactful, and called it a "hypothesis" instead of a "myth".

Now, a hypothesis can be proven by analytic means or by experiment. This particular hypothesis, from the analytic approach, is very unlikely to be true. So, we go on to an experiment.

A listening test was described where a group of listeners were unanimous in saying that sound quality was degraded by an inactive speaker. (By the way, I don't understand why, if there were any effect, it would have to be a degradation rather than an improvement).

However, to be acceptable as a proof, the experiment would need to be done as follows:
1. The inactive speaker should be introduced and removed from the room in such a way that the subjects, and the person conducting the experiment, cannot tell if it is in or out.
2. The listening test should be performed a number of times. A dozen might be sufficient. More would be better, but the subjects would get bored and results would be degraded.
3. The subjects should record their observations (Speaker IN/Speaker OUT) in such a manner that they do not know how the others are voting.
4. The results must be tabulated and analyzed in a statistically valid way.

I doubt that the reported experiment was done this way. It apparently convinced the subjects, but does not constitute a proof acceptable to an objective non-participant like me. Lacking a valid experiment, I must rely on the analytic approach, and find the hypothesis untrue.

Another game that would be fun would be to conduct the experiment in the manner that I suspect it was done, where everyone knew when the speaker was in or out, but use a speaker that, unknown to the subjects, has had its cones immobilized with glue and the vent (if any) closed off.
I think I know how that would come out if the subjects were believers. (Or, for that matter, if they were nonbelievers). This exercise would indicate how much confidence should be put in the experiment that was done.

Are we having fun yet?
eldartford
twl
3,251 posts
 
Nothing more to say on this subject. Take it or leave it as you please.
sean
6,231 posts
 
El: If you play music and the music excites specific resonances of items in the room, those resonances contribute to what you hear. If those resonances are of noticeable amplitude due to high level excitation, you will be able to localize where the sound is coming from. It is at this point that most people remove the item from the room or take steps to damp the resonance. Even if the output levels from such resonances aren't strong enough to stand out individually, they are part of the broad-band spl within the room and what you are hearing. Obviously, some items are easier to excite than others and speakers are some of those that are easier. This is especially true if they are of the vented variety. In some cases, you can easily measure this.

By attaching a sensitive voltmeter to the outputs of the unused speakers, you an actually see / measure the amount of energy being developed from the driver acting as a microphone. This becomes even more apparent if you have a raw driver that is large in surface area and makes use of a lightweight cone / loose suspension. Believe me, if there is enough energy to cause a woofer to act as a microphone, you can bet there is enough movement of the cone to alter the spl's / tonal balance within the room.

By the way, the subject at hand is commonly referred to as "sympathetic resonances". I made mention of this a few years back here on Agon. As to what the specific thread was or how it came up, i have no idea. Sean
>

PS... You folks have to remember that not all "audible differences" result in increases in amplitude or extension of bandwidth. Some changes can be subtractive. These are typically more subtle and take longer to recognize because the differences are not "in your face". While i'm not saying this is what takes place in this specific situation, i did want to make mention of it because cancellation can also take place when dealing with acoustics.
imin2u
392 posts
 
I think there's a very large effect. A very large negative effect to the WAF!
eldartford OP
4,262 posts
 
Sean...Your point about subtractive differences is very well-taken. The best known example of this is that a performance hall has very different acoustic characteristics when it is full of people vs when it is empty.

Of course speakers function as microphones, many (if not most) handheld communication devices make the speaker do double duty. Just for fun I will measure the output of a box speaker/microphone at some measured SPL. But what to do with this data? Perhaps I should express the signal in terms of the SPL which the speaker would produce if that voltage were applied. The idea is to assess whether the effect, which certainly exists, is of any practical significance.
subaruguru
1,376 posts
 
Rather than go through the hassle of repeated trials with a bunch of humans (audiophiles, even!), it'd be easier to just set up a RatShack mic on a tripod and run the S'phile or other 1/3 octave bass warble tracks and compare the curves with and without the added speaker "traps" (which is predominantly what they'll be, I suspect). Who knows, you may get smoother response after some experimentation!
After pondering Twl's telly-on-the-table trick, I wonder if it isn't the 1" diaphragm that's operative here, but the altered total reflective package in the upper mids...ESPECIALLY off a hard horizontal surface near the listener, that's changed their responses. I too doubt that it could be constructive/destructive waveform in the bass, but just a normal change in the diffraction mix when you start tossing stuff around the cofeetable type thing...especially hard-edged solids. Again, this could be measured, but is trickier as you have to be more careful to keep the effects of hand and head movements cloned in repeated trials in the uppermids. So I think Twl's customers heard something, but related to the phone's CASE, not its SPEAKER! Cheers. Ern
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