jcr159

what makes a sub "sound good"

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It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

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I agree RIcci, is it because it is super difficult to figure out except by trial and error placement and tuning?

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It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

It is very effective, but requires knowledge and equipment to set up.

 

Also, I think many does not like the idea of a sub unit used only to fix a problem, it doesn't contribute to making a lot of rumbling bass. If you have heard the difference you quickly realize that this was actually a very good idea, as the overall sound quality improves. Capacity can always be fixed.

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You are comparing the distortion of sealed subwoofers to a ported subwoofer, and that is an apples-to-oranges comparison. Sealed subwooofers will always have much more distortion at the lower frequencies. Another thing is you are using SVS subs to compare them against, and SVS is very strict with their limiters. Most manufacturers are more permissive of distortion quantities than SVS. If you placed a Ultimax 18 or SI18 driver in a ported box, you would very likely end up with less distortion for the same output level than SVS, so long as the ported enclosure is semi-competently designed and built. About the SI24 driver, in order to achieve that low distortion level, Josh used a very large cabinet: 43"x36"x23". If you can not handle a cabinet that large, you will have more low-end distortion. That said, the distortion measurements for the SI24 are really terrific, and, in that enclosure, do resemble that of a ported subwoofer. 

 

To be fair the 24 that Josh tested had a motor that was 1/2 as strong as the current driver. The current 24 hits the same Qtc in 9 ft^3 that the previous 24 hit in over 20 ft^3. 

 

And as almost everyone else has said, what makes a woofer sound good is up to you. I personally enjoy proper full-bandwidth systems (sealed). But I also enjoy listening to other peoples ported systems too even though they lack the ultra low end that sealed is capable of...but you also need to have the program material to dig that deep which is few between. 

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To be fair the 24 that Josh tested had a motor that was 1/2 as strong as the current driver. The current 24 hits the same Qtc in 9 ft^3 that the previous 24 hit in over 20 ft^3. 

 

And as almost everyone else has said, what makes a woofer sound good is up to you. I personally enjoy proper full-bandwidth systems (sealed). But I also enjoy listening to other peoples ported systems too even though they lack the ultra low end that sealed is capable of...but you also need to have the program material to dig that deep which is few between. 

 

If other aspects of the driver didn't change much, then theoretically, the lower Qtc will probably improve distortion for a given box size for frequencies well below resonance, based on analysis I did some months ago.  Of course, I'd want to see the measurements before assuming anything.  :)

 

It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

 

I believe active cancellation can be very effective.  The reason it's not done more?  I believe it's because good tools for doing so aren't widely available.

 

The double-bass array concept could be considered a type of active bass cancellation, albeit a kind of crude, non-optimal implementation.  However, it is relatively easy to implement if one has the space and installs enough drivers.

 

At the other extreme, multiple independent sub EQ can be thought of as a more general technique.  Provided that the filters can alter phase and magnitude response independently, active cancellation is but one of many possibilities.  I will be investigating this intently probably after I get my 4 X behind-the-sofa MBMs built.  I'll have 6 high headroom bass sources along with my front stage mains and maybe surrounds (once they are upgraded) to effectively use as active devices.

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B

 

It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

Bag End still sells their Active Bass Trap although I haven't seen it reviewed in years.

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It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

 

 

B

 

Bag End still sells their Active Bass Trap although I haven't seen it reviewed in years.

 

My room 23' long and my front row of seating is about 8-9' from the back wall. I have horrible cancellation centered around 30hz on the front row, and room sims predict it perfectly so I assume it's the reflection from the back wall which also happens to be solid concrete then earth.
 
Someone else suggested I build a few empty ported boxes tuned to 30hz (with no driver or driver cutout) and place them near the rear wall to absorb that energy. Would that help the front row, and if so, would it have any adverse effects near 30hz for the rear seating?

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I use a VBA on my nearfield sub for this purpose - http://www.avsforum.com/forum/155-diy-speakers-subs/1939713-active-bass-trapping-using-spare-subwoofer.html

 

it's equivalent to the signal a real sub would produce so basically the rear woofer has inverted polarity, a delay offset and a reduction in level (in order to get it to "meet" the main wave). A purely passive device wouldn't give you that level of control so I'm not sure how well that will work.

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I use a VBA on my nearfield sub for this purpose - http://www.avsforum.com/forum/155-diy-speakers-subs/1939713-active-bass-trapping-using-spare-subwoofer.html

 

it's equivalent to the signal a real sub would produce so basically the rear woofer has inverted polarity, a delay offset and a reduction in level (in order to get it to "meet" the main wave). A purely passive device wouldn't give you that level of control so I'm not sure how well that will work.

 

I'm considering the passive option because of the limited space I have behind the rear seating.  Those seats are almost touching the back wall, so any depth requirement for an actual driver isn't an option.  However, a shorter box that's only ~4-5" deep but 48" wide would fit.

 

I suppose it'd only be $15-$20 of material, so if it doesn't work very well no biggie.   

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I'm considering the passive option because of the limited space I have behind the rear seating.  Those seats are almost touching the back wall, so any depth requirement for an actual driver isn't an option.  However, a shorter box that's only ~4-5" deep but 48" wide would fit.

 

I suppose it'd only be $15-$20 of material, so if it doesn't work very well no biggie.   

 

This will be very difficult to achieve with the space you have in practice, especially because of the absurd SPL capabilities of your system.  The last thing you want is port compression on your passive absorbers doing weird non-linear stuff to your in-room response at high SPL output.  Your absorbers will have to be able to displace a lot of air.

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So to keep this thread going...  and to continue my education...  :)

 

the typical recommendation from AVRant, and one i agree with overall is to go with 2 or 4 subs in opposing corners, or midpoints of the walls.  You are then calibrating to set level and delay (phase) relative to the primary, or group of primary listening positions....

 

This is basically the Toole recommendation based on his research, and generally agrees with Geddes as well (though Geddes prefers 3 or more and locates within the 3d space, so you are dealing with the height modes as well...)

 

All that said, are these techniques achieving any sort of "Active trapping", or is it more of a create a "uniform wavefront" that is helping?  or something else?  If the multi sub approach is just presenting improvements to room nodes with placement and uniform wave front generation, i'd expect better seat to seat average response by looking at a frequency response graph.  However, if it isn't providing any sort of "trapping" behavior, i wouldn't expect a better result (probably just "different") in a waterfall decay measurement.  Where adding "trapping" behavior I'd expect better waterfal measurements, less ringing, etc...

 

hoping i'm making sense, lol.  straighten me out if not!   :P

 

-j

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It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

 

 

B

 

Bag End still sells their Active Bass Trap although I haven't seen it reviewed in years.

 

The bag end device certainly works.  I had long ago come across this and talked to one of the Bag End engineers about it's effectiveness at a SynAudCon event many years ago.  I recommended this as an option to a friend with fairly exotic audio gear in his home demo room who wanted to have effective treatment for playing LP's without re-digitizing and something he could do for customers with the same preference.

 

In playing with the Bag End solution in 3 different rooms and on a few occasions in the first room, I came to a few conclusions...

  1. The controls are way to coarse and low precision. Nudging and bumping a dial to get center frequencies to bump up a little higher or lower sucks.
  2. Similar to what we know about conventional bass EQ, you need to get the Fc and Q very accurate to the original problem to get the best damping results, and much like EQ you do not want to over-do it.
  3. It was hard to do "enough" where the audible benefit was dramatic without hearing some unwanted effects in the room.  When set too aggressively you could hear the devices "speak back" while just talking in the room.
  4. They can be tricky to set and dial in.  It's easier to make a modest improvement if you have 1-2 dominant  and well spaced modes, so expectations should set be appropriately.

While the active trap is effective, we are trying to correct the reproduced sound, where the microphone and filtering has poor selectivity.  I would much prefer a input-based system that creates a cancelling signal in the subwoofer.  A microphone does have the ability to better track the real variances and decay, but we are limited with what sort of problems and how many we attack, as they can even influence each other when using a microphone.  We can also better measure and quantify the results from the specific stimulus of the speakers being used vs the listener, where the microphone-active systems will react to any sound produced in the room.

 

When done digitally up front, we also add the possibility of simultaneously using a device as both a producer of desired output and as an active cancellation device.  It seems a waste to have a good subwoofer location unavailable due to a device, or a cancellation location unavailable due to it being a worthwhile subwoofer location.  

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...

 

All that said, are these techniques achieving any sort of "Active trapping", or is it more of a create a "uniform wavefront" that is helping?  or something else?  If the multi sub approach is just presenting improvements to room nodes with placement and uniform wave front generation, i'd expect better seat to seat average response by looking at a frequency response graph.  However, if it isn't providing any sort of "trapping" behavior, i wouldn't expect a better result (probably just "different") in a waterfall decay measurement.  Where adding "trapping" behavior I'd expect better waterfal measurements, less ringing, etc...

 

...

 

In a sense, yes, but only for some frequencies.  A more "uniform wavefront" is not the right term.  A more "uniform sound field" is a better choice.

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It occurred to me why aren't active bass cancellation techniques employed more? A driver or 2 in the right spots tuned to target and absorb or cancel excess energy could be very effective.

Extremely late answer,I know, but I actually did this in my room to counter a 27hz mode and it did wonders. Tightened up the bass in the deep and resulted in the feeling of more bass in the sweetspot, while the vibrations from seats were lessened.

 

In my case I used my tower up to 40hz. It was annoyingly hard to hit it right though, due to the lpf making a phase shift, thus rendering the timing of some frequencies off. With a FIR filter it would have been much easier to integrate, as well as work longer up in the frequency band.

 

Here is a couple of measurements showing the impact of this bass cancellation project in the waterfall response.

 

post-5388-0-83413200-1490581091_thumb.jpg

 

post-5388-0-29084500-1490581110_thumb.jpg

 

post-5388-0-96115600-1490581118_thumb.jpg

 

post-5388-0-33686900-1490581127_thumb.jpg

 

 

Fwiw i tested the bag end device here at home, and it didn't work that well, it just didn't have the output capability to handle it. This tower was so much better for me in my room.

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