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Unused Speakers (subwoofers) In Listening Room...

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As Chris points out, the larger the cone, the more the impact on the sound.  They will vibrate but perhaps not with sympathy for the original sound.  Depending on the enclosure type, whether there are electronics hooked to the speaker and such, the other speakers can also be effective traps that perhaps your room does not need.  Less of a problem with higher frequencies and just the reflective surfaces will be at issue which is a different problem as the cone movement.

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Has anyone ever tried to measure it?  Perhaps with big, heavy, single cone, sealed subwoofer that can be pushed fully against a wall with no gap (no base board).  One could then run an REW curve with the sub facing the wall, and another curve with the cone facing out into the room, in exactly the same position, and overlay the two curves.  Naturally, the sub would be turned off.  My guess would be that the two curves would be totally the same.

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Remove the extra speakers from the room or hook an amp up to them and turn them on with the volume down (keep the coil still).  Cover the port or turn the speaker face down is another choice. 

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Unused woofers returning energy to the room, delayed a few milliseconds does "sound" reasonable and believable.

 

You could short the input terminals of unpowered (sub)woofers and dramatically reduce the effect.  Surely the amp of a powered sub would would have low enough impedance to do a similar thing.  

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Unused woofers and subwoofers will soak up energy (output) from the used ones.

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I suppose a person could hook a good AC voltmeter to the unused speaker's terminals. Read the voltage then calculate the SPL using the reading as if it were the voltage from an amp. I have never done this but I suspect the SPL would be pretty low.

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Unused subs act pretty much like a bass trap and Helmholtz resonators.

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2 hours ago, jason str said:

Unused subs act pretty much like a bass trap and Helmholtz resonators.

The concept of a "plate absorber" in acoustic treatment uses the same general principle as large amounts of un-driven woofer area that are connected to resistive electrical circuits.  If the electrical circuit connected to the terminals of the non-driven woofers have no resistance in the loop, you'll get a simple delay return pulse (the -40 dB "echo" that I referred to above) whose characteristic delay time is proportional to the mass and stiffness of the woofer cone/surround. 

 

Horns amplify the effect, both ways...and quite effectively.  This is the principle of impedance matching, also used in reflection seismology (i.e., the so-called "row-not c" [ρ0 x c] matching of adjacent horizontal geological layers are invisible on processed stacked seismograms). 

 

Ports: well they couple not so much due to the losses in coupling going in the reverse direction (i.e., through the port the wrong way back into the vented box).  Ports don't couple very well with the incident acoustic energy coming from the room in terms of wide-band response, but this is the principle of the Helmholtz resonator (also called a "tuned bass trap").

 

Chris

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