Acoustic low-pass filter in front of subwoofer to limit distortion output?

[audiodog]

Acoustic low-pass filter in front of subwoofer to limit distortion output?
When I say acoustic low-pass filter, I mean something like thick matting (sort of like this but not so much damping material Top Image) in front of the driver or an acoustic filter like a band-bass subwoofer has. If tuned to an appropriate low pass frequency, those types of techniques should be able to decrease measured distortion while allowing the fundamental frequencies to pass through, right?

Or is it not worth it considering the additional size needed to implement it. Might as well use that volume to simply add another subwoofer?

What do you think?

 

[DVDdoug]

I doubt it does much. The 2nd or 3rd harmonic is still "low frequency" and woofers already don't put-out high frequencies unless they are rattling or buzzing, or something like that.

Most music contains harmonics anyway, especially in the bass range, so it takes quite a bit of harmonic distortion before you hear it.

If you are hearing distortion you should probably look for the root cause.

 

[Chris A]

Or is it not worth it considering the additional size needed to implement it. Might as well use that volume to simply add another subwoofer?

What do you think?

I know, since I've done it for many years now (15+).

I've measured the effect and have noticed that I cannot localize subwoofer output at higher frequencies due to harmonics and other "chuffing" events are effectively attenuated.

So the question is: how large is too large for you?

I run dual 14 Hz tapped horn subwoofers (DIY), one each behind my left-right 1st-Gen Jubilees, and use the space provided behind the bass bins, the side wall, and TH subwoofer cabinets as an extension of the 21' internal horn length, oriented vertically (seen as the red triangular area in the plan-view cross section figure below, the green box representing the TH subwoofer box sandwiched between the front wall and each Jubilee bass bin, with its horn mouth at floor level in the room corner, facing the Jubilee bass bin side. (The mouth position of this SPUD TH subwoofer design is selectable based on where you attach the output mouth cover.)

How much does this attenuate the unwanted harmonics and noise? My guess is on the order of 5-10 dBC--perhaps more, depending on frequency.

My in-room measurements show this configuration to have lower measurable harmonic distortion (up to ~105+ dB SPL in-room) than any other horn-loaded subwoofer that I've seen on the data-base website. It even significantly beats the flagship Klipsch Heritage Jubilee bass bin distortion down to 18 Hz in terms of clean output.

Chris

 

[DanielT]

First. Hello and welcome to ASR audiodog.

Without a sub-bass speaker driver that itself has low distortion, there is not much to do. Plus if it's a ported subwoofer to have a port that doesn't create a lot of resonance and huffing and puffing.

Several sub bass drivers that share the task increase the SPL and get lower distortion, at higher volumes, than with one driver. It might go without saying, but I'll mention it anyway. That's if we're talking about the same driver, which in itself should have low distortion.

As far as I know, this type of placement of two sub-bass drivers can reduce second order harmonic distortion:

Some nonlinearities are canceled in this way. Same thing with isobaric, for example saucer push pull configuration:


Note, take it with a pinch of salt. I'm not entirely sure if it works that way. Or much reduced distortion with that configuration.If I remember correctly, it is covered in this thread:

subwoofers with dual opposed drivers

Subwoofers with dual opposed drivers are increasingly popular, witness B&W, Kef and now also SVS. I have two subwoofers, one a B&W PV1d with dual opposed 8 inch drivers and the other a more recently prurchased and much cheaper KEF Kube8b with one 8 inch driver. Not surprisingly the PV1d goes...

audiosciencereview.com

Then there's the matter of when you hear sub-bass distortion. We are generally more insensitive to distortion in the lowest frequency ranges.
I don't know where you experiences the distortion level of that annoyance is. However, I suspect if you got used to really good low-distortion subwoofers, you won't want to go back to some that aren't so good. The same thing also applies, of course, to the speakers.

More subwoofers usually make it easier to deal with the troublesome room modes that are always there. These room modes create audible effects with fluctuations of 10-15 dB in FR.Fixing a better FR is then done via a combination of the placement of subwoofers and EQ. Fixing it with physical means, absorbents, acoustically is not the easiest thing to do. Or rather, a lot is required, which then takes up space in a combined listening room/living room. In a pure studio, it's another matter if the whole room is filled with lots of absorbents, but few people want it like that in a combined living room, listening room.

Finally, a tip.General interesting information about subwoofers by the master himself can be found in the thread below. Page 20, #381:

Totem Acoustics Rainmaker Speaker Review

Thread is locked temporarily to prep for major surgery to move off topic content to a new thread.

audiosciencereview.com

 

[DanielT]

I know, since I've done it for many years now (15+).

I've measured the effect and have noticed that I cannot localize subwoofer output at higher frequencies due to harmonics and other "chuffing" events are effectively attenuated.

So the question is: how large is too large for you?

I run dual 14 Hz tapped horn subwoofers (DIY), one each behind my left-right 1st-Gen Jubilees, and use the space provided behind the bass bins, the side wall, and TH subwoofer cabinets as an extension of the 21' internal horn length, oriented vertically (seen as the red triangular area in the plan-view cross section figure below, the green box representing the TH subwoofer box sandwiched between the front wall and each Jubilee bass bin, with its horn mouth at floor level in the room corner, facing the Jubilee bass bin side. (The mouth position of this SPUD TH subwoofer design is selectable based on where you attach the output mouth cover.)

View attachment 428938

How much does this attenuate the unwanted harmonics and noise? My guess is on the order of 5-10 dBC (perhaps more, depending on frequency.

My in-room measurements show this configuration to have lower measurable harmonic distortion (up to ~105+ dB SPL in-room) than any other horn-loaded subwoofer that I've seen on the data-base website. It even significantly beats the flagship Klipsch Heritage Jubilee bass bin distortion down to 18 Hz in terms of clean output.

Chris

That was a cool solution with good bass potential.

..how large is too large for you?
The eternal question with bass boxes/ subwoofers; how much space can they take up? Plus the factor of how subs should fit into the listening room's interior style and furniture.

And the usual:
How much $ is planned to be spent, DIY or not and SPL/listening volume.
In addition, good integration speaker-sub.

 

[Salt]

Some examples (recently seen child's room ):

18" closed:

12" TML:

 

[RickS]

Welcome to ASR!

When seeking potential alternative solutions, it is useful to state the problem. Please describe the current implementation in detail and why your proposed solution is more desirable than others you have considered.

 

[bmc0]

When I say acoustic low-pass filter, I mean something like thick matting

A likely better configuration is a 4th- or 6th-order band pass enclosure. Dr. Earl Geddes has recommended the use of 4th-order band pass subs in the past (not sure if his opinions have changed in the last 10-15 years).

 

[audiodog]

I just came across this image from Bose where they incorporated both:
1. Inverted dual opposed woofer arrangement to cancel motor BL and suspension compliance asymmetries as well as cone inertia cancellation.
2. Bandpass to reduce upper harmonic distortion output.
I know, I know, Bose. But frankly the 2 above are great concepts to put together.