PSA: Short Unused Speakers and Subs in the Listening Room


I think most people know this, but it’s important to electrically short speakers and/or subwoofers in the listening room that aren’t being used. This can be accomplished by connecting the terminals together with a spare speaker cable or jumpers.

I had a pair of passive, sealed subwoofers I didn’t have hooked up in the listening room. I could tap on the subwoofer cone and the room would energize with bass at the resonance frequency of the subwoofer. When I shorted the terminals with a copper wire and tapped on the cone, there was silence. It was quite remarkable actually.

Has anyone else experienced this? One question I have is if I should short a subwoofer that is not in any sort of box or enclosure. When I tap on it, it doesn’t seem to energize the room with sound.

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Speakers move when input voltage applied to the coil creates a magnetic field that attracts it towards, or repels it away from, the fixed magnet. Moving the cone with your finger does the reverse, creating a voltage in the output. This is exactly the way a microphone works.

Shorting the terminals creates a situation where the same voltage being generated by moving the cone with your finger, is simultaneously putting voltage right back into the coil resisting this same motion. As a result it moves hardly at all.

If the speaker is sealed design then you have pretty much turned it into a closed box. But if it is ported then you have created a speaker sized Helmholtz resonator.

 

Shorting the terminals creates a situation where the same voltage being generated by moving the cone with your finger, is simultaneously putting voltage right back into the coil resisting this same motion. As a result it moves hardly at all.

Is that why it works? Makes sense. My assumption was that with an open circuit, only voltage is created and with a closed circuit, current can flow and the internal resistance of the wire and speaker eats up the power in the circuit. I hadn’t put too much thought into it. I think I like your explanation better.


If the speaker is sealed design then you have pretty much turned it into a closed box. But if it is ported then you have created a speaker sized Helmholtz resonator.

That would be a good thing right? Or is it only a good thing if your room has a nasty resonance peak at the absorbing frequency of the shorted and ported subwoofer?

By the way, I think the low frequency performance of my system increased after shorting the unused subwoofers. I was only able to listen to one song (that wasn’t especially bass heavy) after I made the change but it seemed clearer in the bass frequency department.

Rooms are more similar than most think. Geddes paper, the gold standard study and paper on this, is linked below. The one thing I would add, we didn't know enough back then (this was more than 30 years ago) about the importance of isolating speakers in order to avoid them mechanically exciting room resonances. Without isolation, vibrations generated by speakers and subs flows right into the floor, walls and ceiling, with the result the whole room structure is mechanically set to vibrating. Walls in effect become radiators of sound, speakers, themselves. 

As with so many other things that happen so often we've probably never heard anything else, it is difficult to even know this is what's happening, and even harder to appreciate the extent to which this is going on- until it's gone!

Most typical solutions involve lots of monstrously large bass traps. The best room I've been in, faux walls were incorporated that hid bass traps the size of a large walk-in closet! Wonderful sound, but who has the room and construction budget for that? When my subs were isolated on Nobsound springs the improvement was greater than a large tube trap that had been used. When they were moved to Townshend Pods the improvement was profound! The extent of this was hard to appreciate until another visit to the best room, and this time when coming home was no longer disappointed in my awful bottom end. This experience was repeated at a friends, when we moved his Pendragons onto Townshend Podiums. He was shocked, it was like a whole different room!