gedlee said:
gedlee said:
The disadvantages of open baffles at low frequencies exist but are modest IMO. Even when you are a non-active speaker type (like #1, #2 shows)
Example #1
When I did this ultra compact open baffle N-shape subwoofer – placed below the front seats of my car
An externally hosted image should be here but it was not working when we last tested it.
- several years ago - it was mainly for a proof of concept.
Bottom line:
I got plenty of SPL to listen to music even in an noisy car environment at high way speed.
Measurements showed that I could get clean 100dB at ear height down to 30Hz !.
Remember this was from a simple 15W car-amp with a single 10" speaker.
Not much equalisation needed (except for a early cut of du to the peaking at 350Hz) as resonant frequency was around 17 Hz !
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
http://members.aon.at/kinotechnik/dipolsub/dipol-sub.html
Ok this might be somewhat OT to this thread.
Example #2
When I built the (active) Manzanita 12
http://www.diyaudio.com/forums/showthread.php?postid=1340084#post1340084
it proofed that bass equalisation can even be integrated into the XO giving a more than reasonable FR - or - with some dB's left if you *must* have flat FR down to 20Hz
(bass speaker at measurements around 70cm off the floor)
Example #3
my current 3/5way active OB system in progress with 2x15" (each side) intended for the range of 100-300Hz but until I have time for the subwoofers operating all the way down from 300Hz.
They already provide stunning bass almost without any membrane excursion.
Conclusion:
OB in the lower notes can be done without too much of an effort. The benefits are not only *no* box coloration but also a different presentation of the lower octaves. This I had to learn before I could accept - and enjoy.
Like basically all the discussion here about image perception boils down - to some extent – to how flexible we are in learning.
IMO *every* loudspeaker – as well as *any* recording / playback concept – has to be learned individually before the presentation makes any sense for anybody.
The easier it is to "learn" the language for a given speaker (recording / playback concept) the "better" it is.
This said, its of course also the question if the "language" is rich enough to transport sensation and emotion in a touching way.
gedlee said:
Earl, don't think this is correct in the sense that reverberation time can be lowered (as is the case with real absorbers).
Even if you use a multitude of speakers there is no effect of *absorbing* energy from the room despite at the resonance frequency of that speakers IMO.
Besides that -
- bass traps do work - but need lots of area - therefor the active absorbers (speaakers) are not that suitable.
markus76 said:
If you'd like to add reverberation in a relatively dead room I suggest putting some mics and (omni-) speakers to the walls.
Play back what your mics record through the wall speakers by adding some very modest reverberation. This gives your room a sonic appearance almost indistinguishable from a concert hall.
Neither speakers not mics have to be of highest level (except for modest noise level).
It's a beautiful way to "really" alter your room's sonics - subtile and powerfull.
greetings
Michael
markus76 said:
When researching the same topic I came across the work of Dr. Peter D’Antonio. He has a company called RPG diffusor systems but one of his inventions in particular caught my interest, he calls it s binary amplitude diffusor. It's just a piece of plastic or wood with holes drilled in it but unlike the pegboard some people have used in the past he put in the work to make sure the sound it reflected had a flat power response. So you get diffusion of frequencies above 1kHz and absorption below that. From my limited research it didn't seem that you would ever get absorption deep into the modal region of the bass with simple fiberglass treatments, but I haven't personally experimented with it yet.
Here's both the patent and the brochure for the binary amplitude diffusor in case anybody wants to experiment.
http://www.rpginc.com/download/Brochures/BADBrochure.pdf
http://www.google.com/patents?id=dVECAAAAEBAJ&printsec=abstract&zoom=4
gedlee said:
Very interesting stuff, e.g.
http://www.ibp.fhg.de/akustik/ra/vpr/index_e.html
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Basically it's foam with a glued on metal plate.
Best, Markus
Markus, is the Double Bass Array (DBA) plot you show for a system with the rear subs set in reverse phase (and with a time of flight delay) as in the systems that were being discussed about a year and a half ago on AVS? Just curious -- that's a system I've been very interested in hearing (or even hearing of).
Double Bass Array thread on AVS
markus76 said:
I'm not sure about metal plates (seems expensive - the German way to do things I suppose), but yes, plate absorbers can be very effective - if they are large. The best reference that I know of is Ted Shultz paper from BBN, which is superb at analyzing the designs. Basically my wall technique is a plate absorber where the whole wall is the plate. Works extremely well for both sound isolation and sound absorption. My room is also sound isolated by the way. Kids upstairs asleep, HT blasting away at just under 100 dB SPL, no problem.
Sounds interesting Markus but unfortunately I can't understand German. My comment about fiberglass absorbers not being effective to very low frequency was specific to something like a bag of fiberglass in a corner. More complex resonant designs seem to be needed to get down low.
The RPG website is somewhat out of date except for their monthly newsletter which shows their recent work:
http://www.rpginc.com/news/effusor.htm
He's also the co-author of a book on the same subject but I haven't read it because it's pricey and there's a second edition coming out after christmas:
http://www.amazon.com/Acoustic-Diff...=sr_1_2?ie=UTF8&s=books&qid=1223347332&sr=8-2
The RPG website is somewhat out of date except for their monthly newsletter which shows their recent work:
http://www.rpginc.com/news/effusor.htm
He's also the co-author of a book on the same subject but I haven't read it because it's pricey and there's a second edition coming out after christmas:
http://www.amazon.com/Acoustic-Diff...=sr_1_2?ie=UTF8&s=books&qid=1223347332&sr=8-2
gedlee said:
Earl, agree with you 100% -
- sorry for having made a translation mistake with my last posting.
Didn't realise that "bass traps" is limited basically to a bunch of fibre glass.
Only sort of "disadvantage" with plate absorbers is you have to tune them to your room – which can become a difficult task.
Best - straight forward - solution I have found is to place several different tuned plate absorbers according to what your room simulation says.
http://www.cara.de/GER/caraconsult-heimbereich.html
only in German available AFAIK– I'm afraid
Greetings
Michael
bwaslo , what I found is that a DBA might only work well in a perfectly rectangular room with very rigid walls, i.e. creating a plane wave makes sense at all. That's something not found very often. In a real room where calculated modes and real modes don't match very well the multisub approach seems performs better.
Multisub in DBA configuration:
DBA (delay and phase invertion of rear subs):
Best, Markus
Multisub in DBA configuration:
An externally hosted image should be here but it was not working when we last tested it.
DBA (delay and phase invertion of rear subs):
An externally hosted image should be here but it was not working when we last tested it.
Best, Markus
gedlee said:
Fuchs found that the missing restraint at the edges makes the plate resonant at all its modes. So it's not only a mass-spring-system like a plate absorber. The foam adds a low Q to the modes so the whole system works very broadband. They measure 1 m x 1.5 m x 0.1 m.
A company in Germany is building anechoic rooms with this new type of absorber:
http://www.faist.de/typo3/faistprodukte/faistakustik0/faistfreifeldraum0/faistfreifeldraum.html?L=1
Best, Markus
The bottom line to plate absorbers is that they must have resonant modes at the LFs that you want absorption at. This means that they still need to be somewhat large. They work because the high wave velocity in the plate causes them to resonate at a lower frequency than if they had been just air. These resonances are then damped mechanically. They can be made somewhat broad band above their lowest resonance and I suppose that the idea that you are talking about has to do with a free standing plate having a very low fundamental resonance in its Lumped Parameter vibration against its mounting suspension. But this too will begin to have diminishing effeciveness as the mass and compliance required to get a very low f0 will also dictate a high impedance, hence small displacement and hence low absorption.
Data suggests a very effective absorption. There's a paper by Zha and Fuchs that outlines how to measure absorption in the low frequency region. Sorry again only available in German (Rundfunktechnische Mitteilungen 40 '96).
But there's another paper "Relevance and Treatment of the Low Frequency Domain for Noise Control and Acoustic Comfort in Rooms" in Acta Acustica 91 '06 where Fuchs, Zha and Drotleff offer some details on how the Fraunhofer CPA (Compound Panel Absorber) works.
Best, Markus
But there's another paper "Relevance and Treatment of the Low Frequency Domain for Noise Control and Acoustic Comfort in Rooms" in Acta Acustica 91 '06 where Fuchs, Zha and Drotleff offer some details on how the Fraunhofer CPA (Compound Panel Absorber) works.
Best, Markus
Markus, is it this the plate absorber from Zah and Fuchs you were referring to ?
http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=DE19506511
Do you have the article from Rundfunktechnische Mitteilungen 40 '96 ? If so could you please PM me?
Greetings
Michael
http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=DE19506511
Do you have the article from Rundfunktechnische Mitteilungen 40 '96 ? If so could you please PM me?
Greetings
Michael
Michael, yes that's the thing I'm talking about. Have a look at http://www.casakustik.de/forum/index.php/topic,98.0.html
Best, Markus
Best, Markus
Marcus
Very nice absorption curve on the one. Wish my German was better. However, I do believe that such devices can easily be created in-situ, that there is no reason to have to purchase one. The Ted Shultz paper outlines the process and I have used this often. It would be cool if someone could find that paper and post it. Its got to be 30 years old by now.
The interest in LF absorpion is finally coming to light as a critical aspect of small room acoustics. I hope that my books and papers had something to do with this!
Very nice absorption curve on the one. Wish my German was better. However, I do believe that such devices can easily be created in-situ, that there is no reason to have to purchase one. The Ted Shultz paper outlines the process and I have used this often. It would be cool if someone could find that paper and post it. Its got to be 30 years old by now.
The interest in LF absorpion is finally coming to light as a critical aspect of small room acoustics. I hope that my books and papers had something to do with this!
Both my books have extensive discussions of the need for LF absorption in small rooms and in the HT book several aspects of how to do this are discussed in detail. The Audio Transducers book dates back to 2001, but I had fully developed the concepts by that point.
I suppose one could say that the need was identified in my PhD. thesis back in 1981, it was certainly discussed, but that work was aimed more at rooms for sound testing than for critical listening, however, the need and concepts are exactly the same. This thesis was used in the design of several sound studios including the ones for the satellitte radio companies studios.
Floyd had made several queries about my ideas and the techniques that I use so I hope there is some discussion of this in his book.
I suppose one could say that the need was identified in my PhD. thesis back in 1981, it was certainly discussed, but that work was aimed more at rooms for sound testing than for critical listening, however, the need and concepts are exactly the same. This thesis was used in the design of several sound studios including the ones for the satellitte radio companies studios.
Floyd had made several queries about my ideas and the techniques that I use so I hope there is some discussion of this in his book.
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