passive radiators as tuned bass traps???

[RyanC]

Hello-

I was wondering about the possible effectiveness of using passive radiators (woofers with no motors) as a bass trap on a back wall corner in a mixing room. It seems as though this could also be done with cheap woofers.

It seems as though it might be an even more effective frequency specific trap if it was tuned to the fundamental resonance mode(s) of the room. I have a 20ft deep room that would yeild a ~30hz resonant freq- so If I had some passive radiators that had a 30hz resonant freq and were ported to that freq too it seems as though they would suck a significant amont of 30hz right out of the room. Any thoughts??

RC

Oh here is a link-
http://www.partsexpress.com/pe/showdetl ... er=295-492

[Ethan Winer]

Ryan,

> I was wondering about the possible effectiveness of using passive radiators <

Perhaps that could be made to work a little, but I have to believe it would be more trouble than it's worth. Especially compared to using more common and proven trap designs.

> I have a 20ft deep room that would yeild a ~30hz resonant freq <

Even if your room's fundamental resonance is at 30 Hz, that doesn't mean you need bass trapping there. First, very little music has any energy down there at all. So if there's no content at 30 Hz to excite the resonance, then the resonance will never be a problem. More to the point, all rooms need substantial bass trapping between around 60 and 300 Hz. That's the speaking range for bass instruments, and where muddiness caused by peaks and modal ringing are most damaging. So that's where you should focus your effort.

--Ethan

[lovecow]

RC,

There have been some recent advances in this technology. If you're into learning what's been happening and you don't mind a little (ahem) light technical reading, you might check out the AES Convention Preprint entitled "Electronic Bass Trap" by Kashani and Wischmeyer from last fall. Search for a PDF copy (not free, but not expensive) here. It would seem that the almost daily breakthroughs in DSP and composite technology are making things like this more realistic every day.

[Ethan Winer]

Jeff,

> entitled "Electronic Bass Trap" <

I find this idea fascinating too, even if it's not cost effective for the masses. But it seems to me you'd want a large panel rather than something the size of a woofer as Ryan asked about. And Ryan was asking about a passive design using woofers, rather than a real woofer with a driving power amp.

--Ethan

[drfrankencopter]

Too bad I don't have an AES membership....I'm interested in the concept. I figure you could use drive current into the speaker to effectively provide tons of damping at a broad range of frequencies.

There could be all kinds of weird side effects from this though...you'd definitely need some good filtering, and good understanding of the dispersion of your speaker. It might be an interesting concept for small rooms, where damping is badly needed, and space is at a premium. Who knows???

Cheers,

K

[Ethan Winer]

K,

> drive current into the speaker to effectively provide tons of damping at a broad range of frequencies. <

But "tons of damping" also requires tons of surface area. More conventional bass traps already achieve extremely high absorption at all but the lowest frequencies. So it seems to me that for an electronic trap to be truly effective it would still have to be very large and you'd still need a bunch of them. If a room has 800 square feet of surface area you need to cover some meaningful amount of that surface to make a big difference.

--Ethan

[lovecow]

There is a difference between active cancellation of sound and passive "absorption" by a loudspeaker-in-reverse type device. I thought RC was asking about the latter. And, if memory serves, that was the specific topic of the AES paper I referred to above. Perhaps I muddied the waters a little with my reference to DSP. A device like what I believe is described in the AES paper (only $10 to non-members, I think) is not an active device. It is a passive device that takes energy from the room and dissipates it into the "loudspeaker." Without getting more specific - I don't have immediate access to the paper myself right now - I believe the general advances in technology, particularly with "what's in the box" and "how the box is built" are getting this type of solution closer to being feasible. I have not heard any sort of device like this and I don't believe it could be more than a narrowband device. But narrowband might be all that's needed for controlling that pesky bottom octave. The only "off-the-shelf" passive devices I've seen designed specifically for controlling something like 30 Hz are very large, very heavy, and very expensive. Having something the size of a sub that would do the same thing sure would be attractive, IMO.

As RC said: "Who knows???"

[Ethan Winer]

Jeff,

> Having something the size of a sub that would do the same thing sure would be attractive, IMO. <

And in my HO too!

It seems to me the smaller a device is, the more danger you have of unwanted side effects. For example, in order for a small device to offer more bass absorption than it's surface area would imply, it would have to send more energy back into the room than it receives. So that means that you're now back to problems similar to using EQ. Where passive trapping improves things everywhere in a room, such active trapping risks making some places worse while it makes others (the "sweet spot") better. Not only frequency response, but probably ringing too.

What do you think? Does that seem reasonable?

--Ethan

[lovecow]

I think it's very simple, in theory: A subwoofer can faithfully transmit the lowest octave band or two into a room. So it can transmit, so it should also be able to receive. How efficient this process is is the bigger question. A loudspeaker is a notoriously inefficient electro-mechanical device. But a microphone can be used for a loudspeaker and vice versa. Of course, neither is designed for that express purpose. But that's not to say that a microphone-sized loudspeaker or a loudspeaker-sized microphone cannot work well for a given application. Roughly 20-30 years ago, acoustical refrigeration was considered a neat idea, but probably impractical. A little over 10 years ago, thermoacoustic technology found it's way onto the Space Shuttle. Since we are all daily beneficiaries of space-age technology, it's not a huge stretch to speculate that what is inefficient and impractical now could be improved upon and made more practical in the future.

On topic, I would say the jury is still out. Obviously, with no product on the market in the strict sense, one could assume that the technology is simply not there yet. But that's not to say that a passive absorber that looks and behaves like a loudspeaker-in-reverse will not be in future studios.

Note: I want to be clear that I'm not talking about active control, or EQ, or "room correction." That's an equine of a slightly different hue.

Also: Which were you asking about, RC? Passive absorption or active EQ/cancellation/correction? I would hate to have muddied the waters that much!!!

[RyanC]

Interesting, I will have to check out that AES paper. Are they active like you feed them your stereo signal or they are just powered? I actually was daydreaming about active stereo fed anti-phase bass soakers and realized how counterpruductive that would be. You end up needing 5kW to generate 90dB, and if a driver were to blow you got 140dB of 30hz and everybody starts puking, among a hoast of other problems.

anyway-

I'm working on putting a new control room together and one of my design goals is to have a really big sweet spot not the "yea but it's good right here" type of room so I could not image any type of system like that.


I do a fair amount of music that does have 30hz, and I was thinking, and correct my if I am wrong here, but I was thinking that if you had an imaginary trap that killed 30hz, would it not kill it's overtones as well. The more I think about that it's probably not correct (unless the trap was more broad band), but that is what I was thinking.

The thing that made me think of this in the first place was that if I play a kick drum acousticallyn now my 12" sub drivers create enough power to light the 1st LED's on my amp - EG in reverse the 12" are dynamic mics. So I was just goin with the 1st law of thermodynamics for a second-

My thought though is how different is a helmholtz resonator from a bass-reflex enclosure? It just seems to me that if the cone moved at all and the box still acted as a helmholtz res then by the basic laws of physics it would be more effective than a helmholtz without a woofer. But I do understand that a woofer in an H res would chage the "air spring" quite a bit. Anyway thanks for your input, just wanted to bounce that one off of you guys-

RC

[RyanC]

Oh-

I was talking about passive, or if powered, not anti phase speaker system, meaning using DC current to hold the diaphrams in place would not be out of line of what I was thinking. But my thought is that wouldn't it be better to have high compliance surrounds so that the diaphrams would move more easily at all frequencies. After all a speaker in a box is a broad band device (not the whole audio band though), so in reverse it should do/be the same. However I would imagine that the more the cones move, the more effective the system, DC would only "hold" the cone in place and or reduce the overall effectiveness, in my mind at least.

However if you were hitting xmax then it would be necessary. I would probably go ahead and wire all the drivers just to try both ways if I decide to experiment with this.

As for surface area, there are plenty of woofers on ebay for .99c- If you were to make a back wall using 20-30 woofers it should provide a large amount of surface area to move with. For $30 I might give it a shot some time. I think that even on an open baffle you could find out rather or not it would work. Then you could wire them up and put a DC PS to and see if that does anything. It doen't take much DC to make a driver not move much (a 9v battery will make most drivers hit xmax).

The other issue I see is, even from a passive system it could produce higher order distortion as their is diaphram motion noise etc from any driver, especially cheaper ones. But I think this could be overcome by using a "grill cloth" that was absorbative to higher frequencies, but not to lower ones. This might need to be as thick as a couple packing blankets.

Here is a DIY construction idea too- you could buy a bookshelf and mount it to the wall, then use MDF as a baffle and mount it to the front of the bookshelf, with speaker holes cut. also the port tubes with variable length would allow you to tune them based on what you need. With a bass reflex enclosure you can create alomst any resonance freq (given long enough/short enough porting vs volume). Running bass box or Winisd would give you an idea of the range you could absorb. Also as the ported resonance goes up (in freq) beyond the free air resonance of the driver you get highly resonant "peaks" on playback, wich in reverse should translate to highly resonant tuned absorbtion no??

Also note that a bass reflex tuned box requires very little power to achieve xmax below the port frequency, inverse sqaure or better as freq goes down. This means that in reverse a high tuning frequency should allow the driver to be extreemly compliant with acoustic power in the room. This could provide a solution where the tuning frequency is a room mode and below that it should be extreemly efective as a broad band absorber. In this scenario you might need the DC to keep them from moving too much.

As for the effencency of the speaker driver, I don't think that matters, I'm not looking to create a good amount of power from the drivers. Most drivers are between .5 and 3% effecient electrically, but if all that in reverse turns into heat in the voice coil that wouldn't bother me one bit. I am not using them as transducers in this scenario, just thermodynamic absorbers.

On paper - or blog paper rather, this seems pretty interesting- I have plenty of drivers around so I might start experimenting with how to achive the most broad band compliance. Conceptually I can't see how this could possibly be less effective than the airspring created by a helmholtz, as it would have a true acoustic spring, and an air spring working together. Anyway it all means nothing until I have some waterfall plots!! Thanks guys-

Ryan

[Ethan Winer]

Jeff,

> A subwoofer can faithfully transmit ... so it should also be able to receive. <

Sure, that's a given. My point was that no passive device can absorb over an area larger than it's physical size. So even if the entire 15 inch cone surface absorbs 100 percent over the full bass range, it's still limited to some number of Sabins of absorption. And to increase the Sabins further requires driving the speaker cone actively. As far as I know the limited Sabins versus surface area is a physical factor, and not subject to future improvements in efficiency. If this is wrong, please explain.

--Ethan

[lovecow]

RC,

I will be able to pull up a copy of the paper on Monday to help me respond to your ideas better. This area of audio/acoustics is not my forté - the mechanisms are not as committed to memory as others are.

In general, my thought is that the efficiency is key since the heat loss does not translate to a heat gain when you reverse the system. In my mind, this will limit the effectiveness. But I could be way off base. (Pun intended. )

Anyway, I hesitate to discuss further because I'm not wont to delve into speculation! More Monday...

[RyanC]

Cool-

I think at this point it is more an interesting concept then anything. I like hearing all your input. thanks guys-

RC