Suppose we want to build a tall, narrow enclosure that is 46" high by 8.5" wide by 4.5" deep inside dimensions. It will have a base so it doesn't fall over.
The illustration shows a Ported enclosure, but it could be a Closed Box.
Such a box will have a huge standing wave, very narrow band, (like a "crack"), at 280 Hz, which is the wavelength of 46".
In order to prevent this, we decide to build a divider that prevents the standing wave from hitting the speaker at 280 Hz. This divides the speaker volume into two chambers-The Small Chamber with the woofer, and The Large Chamber with the port, (if it's a Ported system).
The idea is keep the speaker in a smaller enclosure until the frequency goes below 280 Hz, then have The Large chamber open up for full volume around the bottom octave or so. This means that The Small Chamber would not have any standing waves that were not very short wavelength, therefore very easily absorbed by the inch or so of stuffing on the walls of The Small Chamber.
Let's say the tuning frequency will be 50 Hz.
A) Will this idea work without response irregularities?
B) If the idea does work, what would be a good frequency to tune the interior divider?
The illustration as shown shows The Small Chamber as being one fourth of the total box volume. The ratio is not set in stone. You can make the The Small Chamber one third the total box volume if you want. Just want a divider to prevent standing waves of low frequency from forming.
My guess is to make the vent of a size to tune The Large Chamber to 200 Hz, which is half an octave beneath the standing wave of 280 Hz. That should free up the total box volume when you need it.
With more and more tall, narrow enclosures being built for aesthetic and other reasons, I think this is a good way to cure the standing wave problem.
For the advanced course, we can discuss two or more dividers. However, for right now, let us deal with the guidelines to make just one work.
The illustration shows a Ported enclosure, but it could be a Closed Box.
Such a box will have a huge standing wave, very narrow band, (like a "crack"), at 280 Hz, which is the wavelength of 46".
In order to prevent this, we decide to build a divider that prevents the standing wave from hitting the speaker at 280 Hz. This divides the speaker volume into two chambers-The Small Chamber with the woofer, and The Large Chamber with the port, (if it's a Ported system).
The idea is keep the speaker in a smaller enclosure until the frequency goes below 280 Hz, then have The Large chamber open up for full volume around the bottom octave or so. This means that The Small Chamber would not have any standing waves that were not very short wavelength, therefore very easily absorbed by the inch or so of stuffing on the walls of The Small Chamber.
Let's say the tuning frequency will be 50 Hz.
A) Will this idea work without response irregularities?
B) If the idea does work, what would be a good frequency to tune the interior divider?
The illustration as shown shows The Small Chamber as being one fourth of the total box volume. The ratio is not set in stone. You can make the The Small Chamber one third the total box volume if you want. Just want a divider to prevent standing waves of low frequency from forming.
My guess is to make the vent of a size to tune The Large Chamber to 200 Hz, which is half an octave beneath the standing wave of 280 Hz. That should free up the total box volume when you need it.
With more and more tall, narrow enclosures being built for aesthetic and other reasons, I think this is a good way to cure the standing wave problem.
For the advanced course, we can discuss two or more dividers. However, for right now, let us deal with the guidelines to make just one work.
Attachments
Mr. Feedback:
Thank you for responding. Yes, there is a kind of aperiodic enclosure where a port to another chamber opens up that chamber at certain frequencies. There is also the double chamber reflex which had a thread devoted to it some time ago.
But what I am interested in here is to establish some guidelines as to how to tune a chamber so that the bass reflex or closed box acts identical to a non-divided reflex in the bass region, only the standing waves are broken up.
I realize there are alternative enclosures, such as the double chamber aperiodic, that have merit in and of themselves, but I want to put them aside momentarily and just concentrate on making the Reflex/Closed Box work normally with a divider.
With many builders no longer satisfied with the squat enclosure, and increasing attention being paid to tall enclosures, (not necessarily Transmission Lines), I thought that would be a good thing to establish for future reference.
The dividers can perform double duty both as braces and standing wave preventers. I just want to establish how big the vent in the dividers, (which vents might be no more than a hole in the dividers-no pipes), should be not to interfere with the bass action.
We know that in Ported enclosures, the vent action only extends about an octave above the tuning frequency. After that, it is essentially equivalent to a Closed Box.
Thank you for responding. Yes, there is a kind of aperiodic enclosure where a port to another chamber opens up that chamber at certain frequencies. There is also the double chamber reflex which had a thread devoted to it some time ago.
But what I am interested in here is to establish some guidelines as to how to tune a chamber so that the bass reflex or closed box acts identical to a non-divided reflex in the bass region, only the standing waves are broken up.
I realize there are alternative enclosures, such as the double chamber aperiodic, that have merit in and of themselves, but I want to put them aside momentarily and just concentrate on making the Reflex/Closed Box work normally with a divider.
With many builders no longer satisfied with the squat enclosure, and increasing attention being paid to tall enclosures, (not necessarily Transmission Lines), I thought that would be a good thing to establish for future reference.
The dividers can perform double duty both as braces and standing wave preventers. I just want to establish how big the vent in the dividers, (which vents might be no more than a hole in the dividers-no pipes), should be not to interfere with the bass action.
We know that in Ported enclosures, the vent action only extends about an octave above the tuning frequency. After that, it is essentially equivalent to a Closed Box.
Before moving on to the low-pass filter concept I'd be interested to know how much this 280 Hz standing wave could be mitigated by having the bottom at an angle - for instance putting a wedge shaped piece of MDF 8.5" x 4.5" at the bottom. This wedge could go from 2" thick at the thickest down to nothing.
In theory, this should give an even spread of the standing wave frequencies from, say, 280 Hz to 293 Hz.
What do you think?
Steve
In theory, this should give an even spread of the standing wave frequencies from, say, 280 Hz to 293 Hz.
What do you think?
Steve
7V:
My initial reaction would be that such a piece would not give enough spread to mitigate the standing wave.
On the other hand, I saw a response curve in a test where Audio magazine tested a Pioneer speaker, 4 feet tall, that had this problem. The "crack"-a large dive upward followed by a large spike downward-was very narrow band. So such a small difference in the bottom might spread the phenomenon across enough frequencies to mitigate it with a decent amount of stuffing.
So, offhand, I would say that something like your approach has a chance for success.
My initial reaction would be that such a piece would not give enough spread to mitigate the standing wave.
On the other hand, I saw a response curve in a test where Audio magazine tested a Pioneer speaker, 4 feet tall, that had this problem. The "crack"-a large dive upward followed by a large spike downward-was very narrow band. So such a small difference in the bottom might spread the phenomenon across enough frequencies to mitigate it with a decent amount of stuffing.
So, offhand, I would say that something like your approach has a chance for success.
What I am trying to say is, if we are going to build speakers that look the illustration below-and a lot of us are headed in that direction-we might as well use the dividers to break up the standing waves. At least one of the dividers, anyway.
Even speakers less extreme, but built along similar lines, would benefit from these guidelines.
By the way, the illustration below is from a speaker project by Dan Cramer. The entire site is very instructive and worth checking out. Give the page a minute or two to load.
LOL, those random blue spots are not on the original picture!
http://www.hilberink.nl/codehans/tannoy7.htm
Even speakers less extreme, but built along similar lines, would benefit from these guidelines.
By the way, the illustration below is from a speaker project by Dan Cramer. The entire site is very instructive and worth checking out. Give the page a minute or two to load.
LOL, those random blue spots are not on the original picture!
http://www.hilberink.nl/codehans/tannoy7.htm
Attachments
So, having (possibly) added a wedge at the bottom, my next question would be: what if we then had a ledge (at an angle of course!) with a gap in it for the port, rather than a tube? The ledge could be 1/2" or 1" thick (which would be the equivalent port length). Is there a gap size that would be right for a suitable low-pass filter?
I suppose that, given that it's inside the cabinet, a tube wouldn't cause any audible problems through air-flow but still, let's do without it if we can.
Steve
I suppose that, given that it's inside the cabinet, a tube wouldn't cause any audible problems through air-flow but still, let's do without it if we can.
Steve
(having just seen your last post with the photo) I know where you're coming from and I think you're absolutely on track with this approach.7V:
I put the divider with the port in it just to illustrate the concept. By all means, if we can make the port just a hole in the divider, let us do so. After all, there are plenty of speaker enclosures, especially PA enclosures, where the port is just a hole cut in the wall of the enclosure.
I mentioned the idea of a slanted divider earlier. While I think that is definitely the best way to go sonically, for ease of construction purposes, I would like to investigate the straight divider as well. Still, let's go with the slanted divider for now.
Okay, then we have 2 questions.
A) Where the should the divider go-that is, how big is The Large Chamber compared to The Small Chamber-or should both chambers be the same?
B) In our example-a 46" high enclosure with a standing wave of 280 Hz-how big a hole in the divider should we make to break up the standing wave while leaving the port action unchanged from an undivided enclosure?
I put the divider with the port in it just to illustrate the concept. By all means, if we can make the port just a hole in the divider, let us do so. After all, there are plenty of speaker enclosures, especially PA enclosures, where the port is just a hole cut in the wall of the enclosure.
I mentioned the idea of a slanted divider earlier. While I think that is definitely the best way to go sonically, for ease of construction purposes, I would like to investigate the straight divider as well. Still, let's go with the slanted divider for now.
Okay, then we have 2 questions.
A) Where the should the divider go-that is, how big is The Large Chamber compared to The Small Chamber-or should both chambers be the same?
B) In our example-a 46" high enclosure with a standing wave of 280 Hz-how big a hole in the divider should we make to break up the standing wave while leaving the port action unchanged from an undivided enclosure?
7V:
Thanks for the compliment. It is always a great thing to have a professional loudspeaker manufacturer on hand when trying to investigate something like this.
http://www.margolisweb.co.uk/7thveil.htm
Thanks for the compliment. It is always a great thing to have a professional loudspeaker manufacturer on hand when trying to investigate something like this.
http://www.margolisweb.co.uk/7thveil.htm
Blast, I was hoping you'd do all the mathematics.
I found this site on acoustic high-pass, low-pass and band-stop filters. The expansion chamber concept is interesting. Of course what we have is a "compression chamber" or whatever the opposite of expansion is. I suppose one of the programs for calculating ported boxes would give an answer but I'm not too familiar with them, having always avoided ported boxes.
I wonder if two dividers with holes separated by a normal wide section (ie. no dividers) would do the trick by giving us an expansion chamber.
Also on this site is a "band-stop" filter which might also be interesting. Can we put in some sort of a small box which resonates at 280 Hz, which would suck out this frequency?
This last approach may be best because what we are ending up with is a cabinet that looks like it's large as far as the low frequencies are concerned (small chamber plus large chamber) and looks small for the higher frequencies. Wouldn't we then have exactly the same problem for the higher frequencies in the small chamber, albeit with a standing wave at a now higher frequency? Bunging in a suck-out box could solve this potential problem.
Steve
I found this site on acoustic high-pass, low-pass and band-stop filters. The expansion chamber concept is interesting. Of course what we have is a "compression chamber" or whatever the opposite of expansion is. I suppose one of the programs for calculating ported boxes would give an answer but I'm not too familiar with them, having always avoided ported boxes.
I wonder if two dividers with holes separated by a normal wide section (ie. no dividers) would do the trick by giving us an expansion chamber.
Also on this site is a "band-stop" filter which might also be interesting. Can we put in some sort of a small box which resonates at 280 Hz, which would suck out this frequency?
This last approach may be best because what we are ending up with is a cabinet that looks like it's large as far as the low frequencies are concerned (small chamber plus large chamber) and looks small for the higher frequencies. Wouldn't we then have exactly the same problem for the higher frequencies in the small chamber, albeit with a standing wave at a now higher frequency? Bunging in a suck-out box could solve this potential problem.
Steve
This Works For Me
?,
I have a pair of (now reasonably old) Infinity Reference 30 cabinets that are 8" 2way, in a tall narrow box, with curved edges on the front baffle.
When I got them I was not happy with the bass character, so I went to a store and bought some 2" thick Polyester stuffing off the roll.
These cabinets are about 10" square in section, so I cut the stuffing so that I could make a tube of stuffing that fits the full height of the inside of the box (minus a hole cut for the back of the woofer), and a square pad for top and bottom.
This gives a very pleasing bass characteristic now, and is devoid of any bad bass resonance artifacts.
The stuffing is deliberately not stuffed hard into the inside corners, and the woofer back radiates into an effectively lossy tube inside the cabinet.
This was a cheap, quick and dirty Saturday afternoon fix, and I have not felt the need to alter it since.
Eric.
?,
I have a pair of (now reasonably old) Infinity Reference 30 cabinets that are 8" 2way, in a tall narrow box, with curved edges on the front baffle.
When I got them I was not happy with the bass character, so I went to a store and bought some 2" thick Polyester stuffing off the roll.
These cabinets are about 10" square in section, so I cut the stuffing so that I could make a tube of stuffing that fits the full height of the inside of the box (minus a hole cut for the back of the woofer), and a square pad for top and bottom.
This gives a very pleasing bass characteristic now, and is devoid of any bad bass resonance artifacts.
The stuffing is deliberately not stuffed hard into the inside corners, and the woofer back radiates into an effectively lossy tube inside the cabinet.
This was a cheap, quick and dirty Saturday afternoon fix, and I have not felt the need to alter it since.
Eric.
kelticwizard said:
In my case being a professional loudspeaker manufacturer simply means that I became so obsessed with design that I thought I'd better do it for a living or starve.
I'm always amazed at the creativity and enthusiasm that many "amateurs" display and I bet that, in these days of the Internet and forums such as this, the standard of many DIY loudspeakers exceeds that of many "professionals". Often success is the enemy of innovation.
Steve
PS: I have to get on with some other work now so you carry on and I'll catch you later.
Re: This Works For Me
You're right Eric and we could end up doing the same thing. I'm just having some fun playing with alternatives right now and, well, you never know!
One potential problem with stuffing, particularly with the 2" Bandor full-range units that I've been using, is that it can cause a sluggishness or slowing in the sound. Perhaps we can come up with something that's cheap and easy but better overall.
Steve
PS: I really do have to get on with some other work now.
mrfeedback said:
You're right Eric and we could end up doing the same thing. I'm just having some fun playing with alternatives right now and, well, you never know!
One potential problem with stuffing, particularly with the 2" Bandor full-range units that I've been using, is that it can cause a sluggishness or slowing in the sound. Perhaps we can come up with something that's cheap and easy but better overall.
Steve
PS: I really do have to get on with some other work now.
Kelticwizard,
I have played a lot with a 1m (39") tall enclosure to eliminate the standing waves with variable results. The best solution seemed to be similar to the TL designs - placing 8-10 sheets horizontally evenly distributed starting at around 2/3rd height of the cabinet.
In my case the standing wave effect appeared at 180Hz and in your case it is 140Hz (half wave) not 280Hz. Some tapering also helps.
I have also tried the two chamber setup, but this way you actually create another Helmholtz resonator which sucks energy from the other one. This was clearly seen from the nearfield measurements.
All I can tell, it is not easy to get rid off the standing waves in this kind of cabinets, but good luck whatever way you go !
Zoltan
I have played a lot with a 1m (39") tall enclosure to eliminate the standing waves with variable results. The best solution seemed to be similar to the TL designs - placing 8-10 sheets horizontally evenly distributed starting at around 2/3rd height of the cabinet.
In my case the standing wave effect appeared at 180Hz and in your case it is 140Hz (half wave) not 280Hz. Some tapering also helps.
I have also tried the two chamber setup, but this way you actually create another Helmholtz resonator which sucks energy from the other one. This was clearly seen from the nearfield measurements.
All I can tell, it is not easy to get rid off the standing waves in this kind of cabinets, but good luck whatever way you go !
Zoltan
The difficulty with stuffing the enclosure to kill the standing wave is that you also kill the effectiveness of the venting!
The best technique if using stuffing is to re-locate the vent away from the bottom of the enclosure towards the top, preferably on the back behind the driver, then put a layer of absorbant at half the height of the enclosure. Use for example the stiff, self supporting,fibreglass insulation about one inch thick. The standing wave will have maximum air velocity at this position, so is most effectively damped without needlessly overstuffing the cabinet. The vent is in the same part of the enclosure as the driver, for best vent Q.
This works very effectively.
The alternative of using a tuned resonator also works very very well, but is extremely difficult to tune. Pioneer regularly use the technique in speakers made for the japanese market. Their technique uses an internal half-wavelength pipe of carefully calculated diameter and stuffing.
happy tuning
Andrew
The best technique if using stuffing is to re-locate the vent away from the bottom of the enclosure towards the top, preferably on the back behind the driver, then put a layer of absorbant at half the height of the enclosure. Use for example the stiff, self supporting,fibreglass insulation about one inch thick. The standing wave will have maximum air velocity at this position, so is most effectively damped without needlessly overstuffing the cabinet. The vent is in the same part of the enclosure as the driver, for best vent Q.
This works very effectively.
The alternative of using a tuned resonator also works very very well, but is extremely difficult to tune. Pioneer regularly use the technique in speakers made for the japanese market. Their technique uses an internal half-wavelength pipe of carefully calculated diameter and stuffing.
happy tuning
Andrew
Zozo said:
Most of what I know comes from a review of a 4 foot tall Pioneer reflex that appeared in Audio magazine perhaps a decade ago. It seemed to me that the narrow-band spike that appeared was at 280 Hz-but I cannot say for sure that it was not 140 Hz. I do vaguely remember the article mentioning half wavelengths.
If the spike is indeed at 140 Hz, that makes matters worse, since a port tuned to 50 Hz will still have action slightly above 100 Hz. Not much room for the action of a second resonator to be totally separate.
AndrewJ said:
If worse comes to worse and we cannot eliminate the standing waves by creating wooden or MDF dividers, at least we know the most effective way to use the stuffing. thanks.
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.
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