Vented Divider To Cure Standing Waves In A Tall, Narrow Enclosure?

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In the German mag "Hobby HiFi" they sometimes use something that is different to your idea but not that much:

They use an internal, well damped Helmholtz resonator as a trap for the frequency of the dominant standing wave. It has to be placed at either the upper or lower end and the woofer is whithin the SAME compartment as the port.
If you don't understand what I'm talking about I can make a simple drawing.

Regards

Charles
 
I am trying to attach here the nearfield response of the 39" tall cabinet. A big spike can be seen at around 180Hz which is a clear indication of the standing waves. The cabinet was completely unstuffed.
I think with the 46" length the base harmonics will be at 145Hz and the second harmonics -which could also be present with much less amplitude- will be at 290Hz.
I forgot to mention an interesting design from the German Visaton. However it is a TML design not a BR, it incorporates two Helmoltz chambers to reduce response peeks (look at the coloumn speakers and select VIB 130 TL construction plan):

http://www.visaton.de/english/homehifi.htm

Zoltan
 
"A stuffed resonator? Sounds like a variation of Mr. Feedback's idea of making the enclosure "aperiodic". Although this version seems to use wavelength to resonate instead of volume/port."

Not quite. The Q of the resonator is critical in order to correctly compensate for the Q of the standing wave. To optimise this, the open area of the resonant pipe is adjusted, and a very light fill of absorbant can be introduced to fine tune the results.

regards

Andrew
 
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phase_accurate said:
They use an internal, well damped Helmholtz resonator as a trap for the frequency of the dominant standing wave. It has to be placed at either the upper or lower end and the woofer is whithin the SAME compartment as the port.
If you don't understand what I'm talking about I can make a simple drawing.

A drawing would be nifty. Thanks. :)
 
So, it seems that one way to eliminate the standing wave would be to put stuffing in the cabinet at the point where the velocity of the wave is the highest - half way up the cabinet (thank you Andrew).

Concentrating the stuffing here would increase its effectiveness and minimize the problems that could be caused by the stuffing interfering with the port loading (in a vented enclosure) and slowing down the drivers making them sound too dead.

A wedge-shaped segment at the top or bottom of the cabinet could also help by spreading the frequencies of the standing wave.

Another method of eliminating the standing wave is to use a Helmholtz resonator as a "band-stop" filter, tuned to the standing wave frequency. In this case, a wedge should probably not be used. This is because the Helmholtz resonator could, in theory, be tuned to the precise frequency of the standing wave.

Apparently this technique has been used by Pioneer and other manufacturers who have employed a tube of the correct length and diameter. Andrew, do you know whether the tube is closed at one end?

My limited research (so far) into Helmholtz resonators has led me to bottle-shaped devices where the resonant frequency depends on the length and diameter of the neck and the volume of the body of the bottle. Alternatively, a chamber with a hole can be used in which case the frequency decreases as the chamber size increases or as the hole size decreases.

Zozo pointed out that Kelticwizard's original concept of the partition with a tube doesn't work as presented because it itself acts like a Helmholtz resonator. However, phase_accurate pointed out that these resonators, when used, should be placed at the top or bottom of the cabinet.

In other words, kelticwizard's original concept could work if it the partition was (a) tuned to the correct resonant frequency, (b) had the correct Q to match the Q of the standing wave (Andrew) and (c) was placed at the top or bottom of the cabinet. If you visualize a partition at the bottom of the enclosure with a hole or port in it, we can see that this resembles a bottle or "chamber with a hole" - the ideal Helmholtz resonator.

The problem that I have is that, in my research so far, I have found 3 different formulas for calculating the resonant frequency of the resonator and none of them mentions the Q. Going back to the chamber with a hole, I can only guess that two chambers, one medium sized with a medium hole and the other larger with a smaller hole, could have the same resonant frequency but a different Q. Can anyone help here? :confused:

I will try to set up the various equations on a spreadsheet when I get a few minutes and see if anything emerges. My sense is that this is still an interesting approach to the problem and I would like to explore further.

Is there anyone who can help with calculating the parameters of the Helmholtz resonator? One of the difficulties is that acoustic "circuits" like this are generally translated into electrical equivalent circuits and the equations that emerge are quite complex by nature.

Steve
PS: I believe that these sorts of devices are also used to mitigate the effects of standing waves in rooms and this may be another fruitful area of research.
 
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Zozo said:
I am trying to attach here the nearfield response of the 39" tall cabinet. A big spike can be seen at around 180Hz which is a clear indication of the standing waves. The cabinet was completely unstuffed...

....I forgot to mention an interesting design from the German Visaton. However it is a TML design not a BR, it incorporates two Helmoltz chambers to reduce response peeks (look at the coloumn speakers and select VIB 130 TL construction plan):

http://www.visaton.de/english/homehifi.htm


ZoZo:

A) Which Transmission Line project at that site were you referring to?

B) Wouldn't mind seeing that response graph with the spike if you can manage to upload it. However, I do believe you when you say that the spike is at half the frequency of the wavelength. You don't need to upload the response graph to prove it. However, it would be nice to see the graph.
 
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7V:

One thing I guess I should have mentioned is that the "spike" goes down as well as up. It isn't just an upward movement followed by a swift return to the midpoint.

As I recall, the positive movement was slightly lower in frequency than the negative one. This is a decade since I saw the graph, mind, but I seem to remember it that way.

The following is an illustration of what the Pioneer graph looked like, as near as I can remember. Clumsily drawn, but what the heck. Again, very narrow band.

The importance of this I will explain tomorrow.
 

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OK, success :cool: ...not the best quality, but couldn't attach bigger picture. By the way it very similar to the Pioneer picture you attached.

Regarding the Visaton plan : click on the link, then select from the list "Column Speakers", "Summary", then look for the "VIB 130 TL". At the bottom of the VIB 130 TL project page you'll find the Construction Plan. I think this is the same project what Phase_accurate mentioned. Usually they take project over from the "Hobby HiFi" magazine.

Zoltan
 
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ZoZo:

Thank you for the link to the Transmission Line system. I will check that out shortly.

Thank you for uploading that graph. It contains two important pieces of information. One of them is discouraging, but it might as well come to light.

The first piece of info is that that the "spike" is precisely where you said it was-not at the frequency whose wavelength corresponds to the longest dimension, but at half that frequency. I believed you anyway, but nothing like seeing definitive proof.

The sad news is the second part. I was wrong in the way I drew my graph-the dip in the response is lower in frequency than the peak. This is going to present a problem to the idea of making a Helmholtz resonator to cancel the "spike", as the next post will show.
 
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LspCAD models Helmholtz resonators within enclosures-what we are talking about there. A Helmholtz resonator is simply a volume, (box), with an opening or port.

LspCAD calls this "Double Chamber Reflex" Type 1 and Type 2.

I will show the difference.

A demo version of LspCAD, using Adire Drivers, (Shiva, Tempest, etc.) is downloadable atthe Adire website, www.adireaudio.com.

Here are the Box Types.

PS: Alas, LspCAD does not model the most famous Double Chamber Reflex model of all-George Augspurger's style. Audio author David Weems did much with that type of Double Chamber.
 

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Well, maybe the news is not so discouraging after all.

I am playing around modelling the Shiva on LspCAD, free demo version.

Using Box Type 2, the news doesn't look good. That is because the "spike" takes the form of: lower frequency dip, followed by higher frequency peak. Exactly the same form of "spike" that we are trying to cancel. Alas.

However, using Box Type 1, the response seems to have the form of: low frequency peak, followed by higher frequency dip. This is the reverse of the form of standing wave, therefore might prove useful. I am having some difficulty getting it to behave just so, though. Still, Box Type 1-where the the port is on the other side of the divider from the speaker-seems to hold the most promise.

Some observations.

First, the larger the chamber without the speaker mounted in it is compared to the speaker chamber, the more pronounced the "spike". So we can not only tailor the frequency at which the "spike" occurs, by changing the ratio of the two chambers, we can vary the amplitude of it as well.

Second. LspCAD might indeed be able to model a Closed Box with this. Although a Closed Box with an internal Helmholtz resonator is not "on the menu" of LspCAD, with other programs I have found that a Closed Box can be simulated from a Ported Box program simply by putting the tuning of the Ported Box very, very low-like 5 Hz or lower. The lower the tuning, the more the Ported Box resembles a Closed Box in output, impedance, everything. Try adjusting the main port-the one that vents to the outside-so it tunes the box to 1 or 2 Hz. Then adjust the Helmholtz Resonator inside. I haven't done this for this program, but people interested in Closed Boxes should give it a go.
 
Hi all,

I read somewhere that the best way to eliminate the standing waves is to glue little pieces of wood on each sides and on the back (inside the cabinet). These pieces need to be of different shapes and different length & you need to glue them in a random order & location.

I can try to find the web pages where I read this....if you are interested to read it.

Regards,


Daniel
 
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Zozo said:
in your case it is 140Hz (half wave) not 280Hz

Why not take advantage of the resonance. Remove the top partition, make sure the port is at the bottom of the cabinet and you have the basic elements of a Mass Loaded Transmission Line (with the port at the "end" of the line it becomes a quarter-wave resonator (ie 280/4 = 140/2 = 70 Hz)

Zozo said:
OK, success :cool: ...not the best quality, but couldn't attach bigger picture.

Save pics like that as gifs, they will be smaller & higher quality.

dave
 
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