With all the talk about thin wall/heavy damping cabinets to reduce resonance, I got to thinking.... wouldn't this approach increase the loss (Ql)? I found the paper where Small talks about lumping all the losses into a single factor, and later, he talks about building a special low mass driver to which weight may be added, but that's a bit beyond DIY, I think.
So I'm wondering, how would you measure leakage loss?
So I'm wondering, how would you measure leakage loss?
I'm talking about the losses incurred by the cabinet walls resonating and absorbing energy which straight calculations would assume might go out the bass port. Small identifies three forms of losses, but Ql refers to "leakage" losses of the box. In simple terms, if you built a bass reflex cabinet out of poster board with a toilet paper tube port, it wouldn't measure anywhere close to what software would predict because the sound energy would go through the cardboard. This is a silly "worst case" example, of course.
There are papers done by the BBC extolling the effectiveness of lightweight panels with heavy damping - the idea being that the lighter the panel, the more affect the damping will have in reducing vibration. While this makes sense, the fact is the panel will vibrate, and the damping will prevent ringing, but the panel and the damping together must absorb more energy than a heavy, inert panel, even though the "inert" panel is likely to vibrate longer and smear the sound. So while the lightweight, heavily damped panel may sound better, my thought is that it might also incur losses that need to be accommodated in the design equations.
Maybe I'm getting ahead of myself and should have first asked if anyone has tried the light panel approach, and if so, did they find the measured response to be off from the predicted response.
There are papers done by the BBC extolling the effectiveness of lightweight panels with heavy damping - the idea being that the lighter the panel, the more affect the damping will have in reducing vibration. While this makes sense, the fact is the panel will vibrate, and the damping will prevent ringing, but the panel and the damping together must absorb more energy than a heavy, inert panel, even though the "inert" panel is likely to vibrate longer and smear the sound. So while the lightweight, heavily damped panel may sound better, my thought is that it might also incur losses that need to be accommodated in the design equations.
Maybe I'm getting ahead of myself and should have first asked if anyone has tried the light panel approach, and if so, did they find the measured response to be off from the predicted response.
Line It,Yes! Stuff It, No!
Lining the interior walls of bass-reflex enclosure with an acoustical resistance is intended to reduce reflection of acoustic energy whose wavelength is much smaller than enclosure dimensions. We do not want this energy to be reflected and then pass out the port or through the driver diaphragm. System [Q], on the other hand, is addressing the amount of damping accruing at system resonance. Here, wall reflectance is not at issue as the cabinet is pressure loaded; so here, wall stiffness is at issue, not wall surface conditioning. As lining thickness is much smaller than the wavelengths at or near [Fs], it has a negligible effect on system [Q]. Here stuffing the enclosure volume with acoustical resistance material will have such an effect as well as others. For a bass-reflex enclosure such practice is contraindicated. Wall resonances should be raised well above the operation frequency band of the enclosure through the use of bracing and other measures.
Lining the interior walls of bass-reflex enclosure with an acoustical resistance is intended to reduce reflection of acoustic energy whose wavelength is much smaller than enclosure dimensions. We do not want this energy to be reflected and then pass out the port or through the driver diaphragm. System [Q], on the other hand, is addressing the amount of damping accruing at system resonance. Here, wall reflectance is not at issue as the cabinet is pressure loaded; so here, wall stiffness is at issue, not wall surface conditioning. As lining thickness is much smaller than the wavelengths at or near [Fs], it has a negligible effect on system [Q]. Here stuffing the enclosure volume with acoustical resistance material will have such an effect as well as others. For a bass-reflex enclosure such practice is contraindicated. Wall resonances should be raised well above the operation frequency band of the enclosure through the use of bracing and other measures.
Have you seen this? (Discussion and experimental results of the above topic.)
Volume filling a reflex box
Some box simulation programs such as Unibox allow the simulation of filling the box with a representative density material.
1/Qb = 1/Ql + 1/Qa +1/Qp
where
Qb = total losses of vented enclosure
Ql = leakage loss
Qa = loss of absorption from damping material
Qp = vent losses
Thiele researched any positive results from using a lot of damping material in a bass-reflex system and decided that there weren't any.
Dickason says that "in practice Qa and Qp tend to be so low that they are not significant." Of course if you use heavy damping then Qa is going to be significant. Dickason provides a method for measuring Qb = Ql (in most cases) of a finished box. As far as I know, it isn't possible to accurately predict losses before the box is built.
Regards,
Pete
I don't think the Small paper refers to energy going through the walls so much as frictional loss at resonance. Once the equivalent circuit of the vented box is creeated you see that there are numerous places where loss resistors can be added and those are the various Q's he describes. Each position has a slightly different effect around resonance, as he shows in the paper.
I don't think I have ever seen evidence of bass level drop from loss through the walls. In theory you should loose something but it seems to be quite small. The Harwwod/BBC proposal is not to have light walls so much as to shift their mass into damping material (at least a greater proportion). This shifts towards a limp mass ideal that all walls (speaker or room) roughly follow. Note that a limp mass boundary has a rising 6dB per Octave
Most of the curves of cabinet wall output that I have seen show a fairly low level of broadband transmission. It is the relatively tall output at each resonance that causes the problem (that appropriate damping fixes).
David S
I don't think I have ever seen evidence of bass level drop from loss through the walls. In theory you should loose something but it seems to be quite small. The Harwwod/BBC proposal is not to have light walls so much as to shift their mass into damping material (at least a greater proportion). This shifts towards a limp mass ideal that all walls (speaker or room) roughly follow. Note that a limp mass boundary has a rising 6dB per Octave
Most of the curves of cabinet wall output that I have seen show a fairly low level of broadband transmission. It is the relatively tall output at each resonance that causes the problem (that appropriate damping fixes).
David S
Thanks, everyone, but I haven't applied any damping yet. At this point, the Qb is so low, damping does seem like the right solution. And, as Dave pointed out, I should have referred to "low mass" construction.
Yeh, I should have published back when I still had a brain and could do the math. But an engineer friend looking over my shoulder said, "Keri, real engineers don't use 6th order equations!" (this, of course, was before everyone had a computer in their pocket). And, I suppose he was right, the math of a thermodynamic analysis wouldn't be as handy as electrical equivalents. But I still tend to think of things in terms of heat transfer rather than resistors hanging off of filter circuits.
I'm seeing about -12dB less coming from the port than Augspurger predicts, using Keele's close mic technique. Signal level is about +98dB, and the cabinet sides are 3/4" solid pine.
I haven't applied any damping yet, but that's my quandary - I can't see damping decreasing the losses, I'd expect it to increase. Maybe I should apply it to the outside of the cabinet?
There is definitely a broadband transmission occurring, in fact, that's the point of a pine cabinet. I was just surprised at the inefficacy of the ported/TL design. I guess I was thinking if it was air-tight, it would be okay. The resonance of the panels seems to be above 300Hz, so I wasn't expecting problems in the bass region.
When you say 12dB less than Augspurger predicts, are you referring to his TL paper? In that case port output is highly dependent on line stuffing. If it is a standard vented cabinet then you really should look at the output shape of the vent (and also the nearfield woofer). This can tell you about box Q and vent Q in that the depth of the woofer excursion null and the width of the port null are all determined by Q.
A prediction of these curves will not be accurate unless you know for sure the effective Q of your cabinet. That is, the Small paper and all its curves start with an assumtion of Ql of 7. If your effective Q is way off of that then your curves will look different.
Damping inside and outside the box should be the same. We are talking about loss to reduce the vibrational modes of the cabinet walls. Damping pads tned to reflect as much sound as the cabinet walls would. You still need conventional stuffing to reduce the internal acoustical modes.
Can you explain the pine box comment? Are you specifically looking for more loss through the side walls? (and why?) Even with an especially lossy box I think the mid band (assymptotic) efficiency will be the same, i.e. most of the losses will be at the corner frequency and below.
David S.
A prediction of these curves will not be accurate unless you know for sure the effective Q of your cabinet. That is, the Small paper and all its curves start with an assumtion of Ql of 7. If your effective Q is way off of that then your curves will look different.
Damping inside and outside the box should be the same. We are talking about loss to reduce the vibrational modes of the cabinet walls. Damping pads tned to reflect as much sound as the cabinet walls would. You still need conventional stuffing to reduce the internal acoustical modes.
Can you explain the pine box comment? Are you specifically looking for more loss through the side walls? (and why?) Even with an especially lossy box I think the mid band (assymptotic) efficiency will be the same, i.e. most of the losses will be at the corner frequency and below.
David S.
Remember, when Small dis his paper, everyone was still digesting Theil's paper and trying to figure out why it did not seem to work. Small was getting us close enough to build predictable results and he was addressing the OEM market, not DIY.
I have had good success with fill material, but it may have more to do with more complete reflection control. I have plans to experiment with that by stepping up the quality of the liners and the shape.
I have had good success with fill material, but it may have more to do with more complete reflection control. I have plans to experiment with that by stepping up the quality of the liners and the shape.
I'm using TLwrx with a formula for the stuffing that approximates the results of WinISD when using a Ql of 7.
The output shape of the vent looks right, it's just quite a bit lower than expected. Since the box has no damping, I was really expecting a higher peak.
Some guitarists favor pine boxes because of their resonant nature. Not those who like deep, solid bass however. I thought perhaps I might be able to get both qualities in one cabinet, but it's looking like that's not going to happen unless I figure out how to tune the box while making allowances for the losses.
Please, don't get me started on Thiele. In recent interviews, he claims he was a "speaker designer" in the '50s, and makes no mention of Novak's work. That certainly wasn't the story going around when he was introduced.
Please, don't get me started on Thiele. In recent interviews, he claims he was a "speaker designer" in the '50s, and makes no mention of Novak's work. That certainly wasn't the story going around when he was introduced.
The story I heard, was he was a prof at the university and someone said something like "your're the EE, you design a PA system" and it kind of went on from there. There is no question about the long list of great work that went before him. I credit Theile for recognizing the filter equations in gas match the filter equations in electricity. I credit Small with the loss factors that made it all work. Many came before them, many after. I credit Marty Gerstin for reading the work and making the, or close to the first ( Ohm C2) commercial success using the math, yet you never heard of Hascal Scott who was doing the same in Boulder. We need to credit White and Bulloc for simplifying the math for us heathen DIY types. Like all history, I am sure what I "know" was filtered.
I still have my copy of Speaker Builder with their original tables. They sure made my life easier as I was trying to work out a couple general rules myself. I just about knew how to build a 2 cubic foot box out of 5/8 PB when they published. I was so happy to throw away all the pages of measurements and working backwards.
The beauty of the Thiele paper was that it gave us an easy cookbook approach and simplified things to Vas, Qt and fs, etc. Before that Beranek and Novak and others had worked out the math but it was with long equations and no computers to do the heavy lifting. Remember this is the time when popular thinking was to "put it in 6 cubic feet and tune to equalize the impedance peaks".
David S.
David S.
TVRgeek, the story I heard was Thiele was working for a broadcast company as a filter designer. He read Novak's article in Electronics World, and recognised Novak was describing a filter's response (If you've ever designed filters, a line like "the 3dB down point occurs at 0.7 times the closed cabinet speaker resonance"* is a giveaway). Thiele proceeded from there to produce a series of other "alignments" that would also produce flat response. Now, Novak stated that "Only one condition of cabinet tuning and damping will result in optimum transient response"*, and this is why I think he came first. I think he also invented the "standard volume box" protocol as he was a hands-on kinda guy.
Nevile seems nice enough, but I'm not sure he advanced speaker design as much as i'net pundits would have you believe. As you noted, some alignments didn't work very well, and that brings us to Small and Keele. Now, those guys....
Funny you mention Marty. I wonder if the C2 was his design or John's? (What's funny is John Strohbeen now owns Ohm. While he owned SCS (Tech Hifi) he designed a couple of Ohm models, such as the Ohm E. Ohm basically served as Tech Hifi's house brand.)
* James F Novak, Electronics World, Jan. 1966 (A guide for DIYers, including nomographs for common speaker sizes, based on his 1959 AES paper "Performance of Enclosures for Low Resonance High Compliance Loudspeakers")
Nevile seems nice enough, but I'm not sure he advanced speaker design as much as i'net pundits would have you believe. As you noted, some alignments didn't work very well, and that brings us to Small and Keele. Now, those guys....
Funny you mention Marty. I wonder if the C2 was his design or John's? (What's funny is John Strohbeen now owns Ohm. While he owned SCS (Tech Hifi) he designed a couple of Ohm models, such as the Ohm E. Ohm basically served as Tech Hifi's house brand.)
* James F Novak, Electronics World, Jan. 1966 (A guide for DIYers, including nomographs for common speaker sizes, based on his 1959 AES paper "Performance of Enclosures for Low Resonance High Compliance Loudspeakers")
More on BR Damping
I have not visited Rod Elliott’s excellent website (ESP) in several years. Thanks for the heads-up on the article published there. I note that major differences between lined (L) and lined and stuffed (L&S) alternatives are occurring at low frequencies and results in a diminution of output there for the (L&S) alternative. Currently I find that the introduction and maintenance of loose fiber tangle in a BR enclosure to present more problems than it solves. I may revise and extend my remarks here later after revisiting this issue in more and current detail.
Here is an interesting treatise [1] on BR Enclosure design that may be of interest to readers of this thread.
Regards,
WHG
[1] White Paper: KEF LS50, The Engineers Loudspeaker
http://www.kefamerica.com/july12/LS50 White Paper.pdf
I have not visited Rod Elliott’s excellent website (ESP) in several years. Thanks for the heads-up on the article published there. I note that major differences between lined (L) and lined and stuffed (L&S) alternatives are occurring at low frequencies and results in a diminution of output there for the (L&S) alternative. Currently I find that the introduction and maintenance of loose fiber tangle in a BR enclosure to present more problems than it solves. I may revise and extend my remarks here later after revisiting this issue in more and current detail.
Here is an interesting treatise [1] on BR Enclosure design that may be of interest to readers of this thread.
Regards,
WHG
[1] White Paper: KEF LS50, The Engineers Loudspeaker
http://www.kefamerica.com/july12/LS50 White Paper.pdf
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