Rolling off woofer with HF absorbent material in front of it.

It just occured to me the other night when I was throwing together a truly awful shed speaker for the upcoming circlotron doomsday amplifier with N-channel hexfets, that instead of using a conventional crossover inductor, perhaps you could put a strategically placed wad of sound absorbent stuff in the hollow region of a woofer cone (not touching it) to roll off the high frequencies. That way there would be no dc resistance of the inductor in series with the voice coil and you may get better damping. Might also be cheaper. Anyone done this?

GP.

P.S. I am not a speaker designer. Does it show? :D
 
I have thought about this a long time ago but never followed through. I can imagine many variants on this idea; down firing woofers on a carpeted floor being an obvious one.

I've wondered how much the various possibilities are going to shift the phase, an important issue for the overall crossover design.

I can imagine a distributed acoustic filter on a mid-range driver that would allow the off axes responses to be the same as the on axis response.

I can also imagine a 3 way system that used acoustical low pass filters and electrical hi pass filters.

This idea is worthy, IMHO, of some serious investigation.
 
Now we're thinking! As far as horns are concerned, I could imagine with a large LF one lining the complete inside surface of the flare. Any unwanted hf's reflecting around inside it would be stopped. Not that I have played with horns, but if you clapped your hands deep inside one it would have a characteristic sound. If you could reduce or elimate that with an absorbent lining it ought to improve things.

GP.
 
JoeBob said:
Well a big problem it seems to me is getting the right frequency. If it's a two or three way (opposed to a sub) you want the high pass to be relatively simmilar to the low pass filter -3dB points. That just seems like a big problem to me.

Oh well, at least on a system with a conventional crossover it might be ok for shooshing up any unwanted minor mechanical noises coming from the driver. What about placing a wad of stuff maybe 6 inches in front of the tuning port to reduce any chuffing noises?

Even if it is no good as a crossover, for a LF horn, covering the surface might be worth a try as per the clap experiment. Would probably be especially effective in a folded horn because HF's bounce of shiny surfaces but would be almost completely absorbed by the treated surfaced seeing there is no direct line of sight path from the driver to the outside world.

GP.
 

woody

Member
Paid Member
2002-01-15 12:57 am
Tyrone Ga. U.S.A.
I read somewhere that the designer John Curl ofton flips a wool sock over his speaker's tweeter for overly harsh CD's. I believe the Deckware ?sp copany had a speaker designe using Dynaudio drivers where the woffer was run full range with some sound asorbing material in front of it.

Just my 2 cent's worth

Bob12345678
 
Yamaha Improvements

Hello Dave and All,
I used to have a pair of NS-20 - 8" 2way big brother to the NS-10 and using the same tweeter.
With factory crossover, both these cabinets are HF ear bleeders !.

My soloution was to ditch the factory crossover, and wire the 8" full range, with RC network across the voicecoil.
I did not bother to compensate the resonance impedence hump.
The tweeter was fed via 1.1111 uF, and it too had an RC network across the voice coil.
This cabinet then measured flat impedence to out past 40 kHz.

The network across this metal dome tweeter both smoothed its response and extended response past hearing range.
Highs were then rather good, and no earbleeding.
This setup transformed these cabinets into really fast detailed and efficient loudspeakers.

With this arrangement, phase linearity is excellent, however absoloute polarity of the source music is perfectly revealed, and for complete satisfaction, speaker polarity needs to be reversed accordingly.

It ends up being a PITA having to swap polarity just about every second track, but the result IS worth the 5 seconds of effort.
It is disarming to learn that there really is no standard polarity in the recording industry.
I found several albums where the feature/hit track was recorded in one polarity, and the rest of the tracks reversed.

Regards, Eric.
 
If pressed, I could probably dig up references showing that absolute polarity is not audible.

But back to Circlotron and his original and follow up thoughts.

I did a quick check with the Master Handbook of Acoustics, Chapter 10 and am left with the opinion that pass through absorption for low frequencies is problematical, at best, if one is thinking of replacing the crossover networks for woofers. The slope is not fast enough and the required thickness of material would be prohibitive.

I can see filtering possibilities for the vent chuffing that would at least remove some of the high frequency components of chuffing but to reduce the low frequency components would require a solution, I believe, that results in a resistive vent.

I found myself most exited about the possibility of a fabric hemi-sphere placed over a mid-range and designed in such a way that the off axis response is the same as on axis. You might call it a fabric based acoustic lens with absorption characteristics. Alas, at present, I don't have time to make some preliminary tests let alone come to a solution.

Regarding a sock over the tweeter, John Curl could have come to a more elegant solution.

No, wait, I have an idea! "Tice Clock", "Tice Sock" - what a concept and he's just the guy to do it. Make mine an argyle!
 

Brett

Member
2002-01-07 6:02 pm
<b>If pressed, I could probably dig up references showing that absolute polarity is not audible.</b>

I disagree. And Clark Johnsen of Positive Feedback (who I personally think is an A grade prat) wrote a book on it called the Wood Effect, named after a researcher who did quite a bit of study on the subject.

Here's a link to his latest PF column where he mentions it again.
http://www.positive-feedback.com/Issue1/cjwoodeffect.htm

<b>circlotron sez

I could imagine with a large LF one lining the complete inside surface of the flare. Any unwanted hf's reflecting around inside it would be stopped. Not that I have played with horns, but if you clapped your hands deep inside one it would have a characteristic sound. If you could reduce or elimate that with an absorbent lining it ought to improve things.</b>

You're looking at it the wrong way. In a correctly designed horn, used within it's passband, there should be no reflection within the cavity of the flare. Depending upon the shape of the flare, and the frequency, there can be an abrupt discontinuity in impedance as the wavefront reaches the end of the mouth, and therefore some energy could be returned to the throat. Peavey used to have a white paper on their site by Charles E. Hughes called the Quadratic Throat Waveguide, which explained the benefits of designing into the mouth, shaped foam elements to eliminate some of these problems. I you can get it from my site <a href="http://www.members.optusnet.com.au/~xx308/Peavey.pdf">here</a> if you're interested. It's a 0.5 meg PDF.

A horn is only able to control it's dispersion up to a certain point, and then it becomes more directional with increasing frequency. If you use a large tractrix (talking domestic here), above about 25x it Fc, it will be beaming badly. Some experimenters have reported inprovements with small amounts of felt glued inside the flare near the throat. PA horns often use CD flares to overcome the beaming, but many sound pretty average in a home system. Yamamura use a type of felt lining in their Dionysio 27 rear horn, but I doubt they want any frequencies above a couple of hundred Hz coming out of the horn at all. In a front horn, any material in the flare will cause abrupt transitions in the flare shape, and possibly the cure will be worse than the symptom. Simply stuffing a wad of absorbent material in there will also create a fairly substantial and non-linear acoustic impedence, and mess up the working of the horn. Designers even compensate for the acoustic resistance of the bug-screens in comp drivers.

Much of the annoying 'resonant' effects in horns, especially metallic and die cast plastic/fibreglass is that the flares themselves ring if not adequately reinforced and/or supported. At higher levels there can also be some pretty severe distorion due to the high level of compression in the throat and the non-linearity of air.

circlotron, I hope that makes sense. I've been trying to do several things at once whilst typing.

Cheers
 
Get Polarised

Hi Bill, I reckon you've got the wrong end of the stick here....
If pressed, I could probably dig up references showing that absolute polarity is not audible.
From Wood Effect article - http://www.positive-feedback.com/Issue1/cjwoodeffect.htm
"To those of us who hear polarity, and we are legion, this whole state of affairs seems ludicrous. Over a low-phase-distortion audio system, no one ever misses the right call. Never! For example, everyone who crosses my own threshold gets it by the second try."

This is well confirmed by my long experience.
Note that in describing my Yamahas, I said "With this arrangement, phase linearity is excellent,...".
This is the clincher - On multiway systems with lousy crossovers, the result is a confused sound where the polarity is not discerned clearly, and is never totally correct when wired in either polarity.

With a really good system, and the appropriate recordings, you should have sounds coming from around you, from behind you, and from miles through the wall behind your speakers.

If you are not hearing it, I say it is because you are not listening, or your system is not phase accurate enough to be revealling
enough, or you have interconnects or speaker wires that are overly directional.

"put a sock in it!" - I usually do that to speaker ports - less bass but nicer bass usually IMO.

Brett thanks for your info, and Graham, Brett has been toying with horns a long time - he has good info.

Regards, Eric.
 
Well, I'm open minded and I don't believe everything I read. Frankly, I've never tried it myself. The concept just sounds like the 11th of the 10 biggest lies in audio as put forth in a recent issue of Audio Critic.

I'm rebuilding my electronic crossover and I'll just stick a switchable inverter on the input.

On the other hand, I read further and discover that you have Yamaha speakers that sound good. I become highly dubious of your rationality index. Any then you lay this "directional wire" trip on me. To that I can only say, "bull ****." Now I'm curious enough to do a search so I can discover what other nonsense you have embraced.
 
Hi Bill ....OUCH, no need to gnash, huh?.

" Well, I'm open minded and I don't believe everything I read."
I'm glad to say that we are like minded here.
A Lou Reed quote is " Never believe half of what you see, and none of what you hear !". - http://www.whom.co.uk/dora/reedyork.htm

"I'm rebuilding my electronic crossover and I'll just stick a switchable inverter on the input."
A quick and easy soloution is an audio isolation transformer, with reversing switches on the primaries - and other sonic benefits too.

"On the other hand, I read further and discover that you have Yamaha speakers that sound good. I become highly dubious of your rationality index."
Be polite. I had such pair of cabinets 8 years ago, and back then they were cooking hot - modified 150W/ch rack mount amp, modified CDP, non directional interconnects, AC mains power filtering, modified Yamaha loudspeakers with perfect mounting, positioniong and acoustical treatments, and good source material.
Bill please note that nothing was factory standard, and much trouble gone to make things electrically correct - RF and Power guys and will understand this statement.
The result was a pair of 8" 2 ways that on a natural sound, and other recordings gave 3D sound, and on some rock resonated the block of 6 apartments - only on Sundays when the neighbours were all out, of course.

Any then you lay this "directional wire" trip on me. To that I can only say, "bull ****."
No tripping - Bill, if you do some appropriate experimenting and A/B comparisons, and you are receptive and sensitive you are likely to discern these directional subtle effects.
Bretts link mentions directionality.
There are guys here who say that even digital cables are directional and also that they don't understand why, but they are observing the effect.

" Now I'm curious enough to do a search so I can discover what other nonsense you have embraced.
Please inform which that is.

Your site mentions a lot of experience - please don't be cynical.

Eric.
 
Brett said:
In a correctly designed horn, used within it's passband, there should be no reflection within the cavity of the flare

Unfortuneatly, due to practical reasons and a lack of a complete understanding of the horn (you could probably say this last bit about anything in HiFi), not many fall into this category.

Much of the annoying 'resonant' effects in horns, especially metallic and die cast plastic/fibreglass is that the flares themselves ring if not adequately reinforced and/or supported.

This is a place where ductseal (or other ductseal-like compounds) really shine. When i was doing proPA it was a staple.

dave
 
Re: Get Polarised

On multiway systems with lousy crossovers, the result is a confused sound where the polarity is not discerned clearly, and is never totally correct when wired in either polarity.

your system is not phase accurate enough to be revealling enough, or you have interconnects or speaker wires that are overly directional.

This is probably more likely the case. With a phase accurate speaker, i have not run into anyone who cannot, once shown what to listen for, discriminate absolute phase (it has to be on the recording as well -- a lot of albums have tracks with different AP mixed down togther causing total confusion.

"put a sock in it!" - I usually do that to speaker ports - less bass but nicer bass usually IMO.

I usually use foam. I too, often find that changing a relex system into an aperiodic system produces a better sounding bass.

dave