Unused subwoofers effect overall response? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 10 Old 06-26-2019, 08:31 AM - Thread Starter
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Question Unused subwoofers effect overall response?

Do unused subwoofers in close proximity of other subwoofers in use affect the response of the ones being used? For example, if I want to assess two different subwoofers and set them beside each other for A/B comparison, will the one not being tested have any impact on the response and output of the one being tested? In my application I am using passive subs but the question applies to powered subs as well. Any thoughts? Thanks.
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post #2 of 10 Old 06-26-2019, 01:24 PM
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Curious the answer to this one. I never thought of that possibility, hope not- I am using a pair that are not on, as speaker stands.
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post #3 of 10 Old 06-26-2019, 03:17 PM
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If you're performing this exercise in room, you're looking at a response created by room interaction with sub output, which in turn is dependent on sub placement. To get the best A-B comparison you would want to swap sub positions so the drivers being tested occupy approximately the same placement in room. Having the subs side by side and just swapping connections won't likely give you a good comparison.


If you follow the process above, whatever effect an inert driver in room would have on response would be the same in both cases assuming the subs are similar, so I can't see a comparative analysis as being negatively impacted for that reason. But if you're just generally curious I would measure with the unused sub in room and then again with it outside. My guess would be the difference would be minimal to not measurable, especially with a larger room, or smaller drivers.
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post #4 of 10 Old 06-26-2019, 04:01 PM
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Yes, they will act as a bass trap absorbing Fb (the natural resonant frequency of the enclosure with the driver in it). How significant the impact will be in the room I'm not entirely sure. I suppose it depends on their placement in the room. Short the un-powered passive sub's terminals together to minimize the effect.
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post #5 of 10 Old 06-26-2019, 04:19 PM
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Quote:
Originally Posted by DavidMountney View Post
Do unused subwoofers in close proximity of other subwoofers in use affect the response of the ones being used? For example, if I want to assess two different subwoofers and set them beside each other for A/B comparison, will the one not being tested have any impact on the response and output of the one being tested? In my application I am using passive subs but the question applies to powered subs as well. Any thoughts? Thanks.
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Originally Posted by Stereodude View Post
Yes, they will act as a bass trap absorbing Fb (the natural resonant frequency of the enclosure with the driver in it). How significant the impact will be in the room I'm not entirely sure. I suppose it depends on their placement in the room. Short the un-powered passive sub's terminals together to minimize the effect.
Short answer is that Stereodude is right here, where other subwoofers in the room will have some interaction. How much impact depends on both where the inactive subs are located and the subwoofer design. Ported subs have admittance at the port combined with the behavior/characteristics of the woofer, and sealed woofers have the natural Fb in box and the combined stiffness of the suspension + box size that determine the significance. As mentioned, shorting the terminals helps damp woofer motion, as can leaving an amplifier connected and powered on with no signal input. In some cases the effect will be very minor, with big boxes it can be more significant.
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post #6 of 10 Old 06-26-2019, 04:44 PM - Thread Starter
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Thanks for replies. For clarity, I’m talking about large PA subs in a very large room. This question came up with another audio tech while doing some testing and he suggested the inactive drivers would add to the overall SPL in the room while I figured they would act as absorbers, reducing the overall SPL. We have since conducted several tests and tried both shorting and not shorting the inactive boxes. The measured differences were small enough to be within a reasonable margin of error due to mix placement, etc.

In general, it seems the consensus backs up my suspicion that they would dampen the sound. This makes logical sense given my understanding of speaker mechanics but it also seems clear that for the difference to be noticeable the whole system would have to be scaled up immensely.

Thanks again for everyone’s thoughts.
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post #7 of 10 Old 06-26-2019, 06:27 PM
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Quote:
Originally Posted by DavidMountney View Post
Do unused subwoofers in close proximity of other subwoofers in use affect the response of the ones being used? For example, if I want to assess two different subwoofers and set them beside each other for A/B comparison, will the one not being tested have any impact on the response and output of the one being tested? In my application I am using passive subs but the question applies to powered subs as well. Any thoughts? Thanks.
That's been a controversial topic in audio for decades.

Back in the 1970's Ivor Tiefenbrun, owner of Linn Products, required any dealer selling his products to have a separate, one speaker demo/listening room. The logic was that any un-powered speaker(s) VC/cone assembly in the same room would vibrate/resonate sympathetically with the powered unit and color the sound of the unit your were listening to.

It's not rocket science. You only have to lightly touch the cone surface of the un-powered speaker with your finger tips to feel the cone moving. If it's vibrating it's moving air and making noise. But then, so is anything else in the room that is vibrating. So, is it enough to make a difference? If a tree falls in a forest and no one is around to hear it, does it make a sound?

The difference can be measured with the right equipment. Most high end/expensive audio product brick and mortar stores that are still in business have a demo room set aside for demoing a single set of speakers. This compared to a place like Best Buy/Magnolia with multiple speakers lining all 4 walls of a demo room.

And then there is the fact that final frequency response range and a speaker's sensitivity value is measured and verified with the unit standing alone, by itself, in an anechoic chamber.
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post #8 of 10 Old 06-26-2019, 07:21 PM
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Yes they are a huge problem. The easiest way to correct for this, is to private me, and get my shipping address. Ship off the unused subwoofers and be done with it, along with any other belongings in the room that obviously will interact with your active subs.
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post #9 of 10 Old 06-26-2019, 10:38 PM
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Quote:
Originally Posted by johnplayerson View Post
Yes they are a huge problem. The easiest way to correct for this, is to private me, and get my shipping address. Ship off the unused subwoofers and be done with it, along with any other belongings in the room that obviously will interact with your active subs.
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post #10 of 10 Old 06-26-2019, 10:57 PM
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Any effects of the drivers in the unpowered boxes can be minimized by shorting the terminals -- not a bad idea when transporting speakers. You will find it difficult to push in the cones with the terminals shorted, just like regenerative brakes on an electric vehicle.

If the boxes are ported, you may get a bump or dip at the port resonance frequency -- especially in the room corners -- which will be slightly different than with an active driver.
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