David, you are like a godsend for people like me who are just getting into this hobby. There is a lot of crazy stuff to read on the internet, but most (or at least I) are unsure about what will actually improve the sound. So, thank you for showing us the correct path forward and saving years of wasted efforts.
There is much to learn, but it helps when you have a good teacher!
There is much to learn, but it helps when you have a good teacher!
Something I've been thinking about, is what is the mechanism by which internal air space standing waves actually become audible ?
Do they just pass through the cone (somewhat attenuated by cone thickness) and enter the room that way ? Or does the air space resonance more strongly excite vibration in the cabinet walls at those frequencies, thus increase panel radiation at frequencies where the air space resonances occur ? (Resulting in a particularly bad situation if air space and panel resonances co-incide at the same frequency ?)
When lining the inside of exterior cabinet walls, is there any advantage in having an additional layer of material that is dense and provides isolation instead of absorption ?
If it turns out that a large part of the transmission of air space resonances to the listener is due to increased panel excitation and radiation, is it equally beneficial to the end result to provide a soft but dense isolation layer to minimise the transmission of said standing waves from the air space to interior cabinet walls, even though it may not do much to damp the standing waves within the cainet ?
Could the benefits of fibreglass etc come largely from simply isolating by attenuation the internal waves from the cabinet walls, with the reduction in air space standing waves being a nice but not essential side effect ?
The reason I ask is for bass reflex boxes I've generally only ever used about a 10mm thick layer of wool based carpet under-felt, and whilst I suspect it's sound isolation properties are quite good in reducing transmission of internal pressure waves to the cabinet walls, thus reducing panel radiation, I'm sure it's too thin and too dense to do much about the internal standing waves themselves, yet the end results are usually very good, and certainly far better than even an inch or more of polyester. (which is not saying much to be fair!)
It's got me wondering whether for a midrange enclosure that a combination of a layer of wool underfelt attached to the walls for isolation and fibreglass stuffing for standing wave attenuation might be better than the same thickness of either material on its own - at least for minimising colouration at the listening position, not necessarily internal standing wave amplitude.
Or is fibreglass on its own still going to be better for a given total thickness ?
Last edited:
I've always thought that both of these were true, you can hear what the cabinet does directly via the outside and sorta indirectly by sound passing through the cone / surround of the driver, and even perhaps those resonances adding to the driver movement itself and adding to the driver's output by moving it.
Yes definitely, I overlooked the port in my post. Just stick a microphone at the entrance to a port in a 2 way/full range system and there is a lot of high frequency crude coming out of it... putting a port on the front means you have a big leaky (to high frequencies) hole from the interior pointed right at the listener...
One reason I prefer rear ports, as the baffle step of the cabinet provides a large reduction in spurious port midrange reaching the listener but without adding lots of extra damping that might kill the bass performance.
Most of the accelerometer tests that I have seen seemed to just reveal panel resonances due to the usual wall thickness/mass/speed of sound type effects. My assumption is that the internal standing waves will come out the cone (and probably the port). I think I quoted a curve in Collums' book the other day that showed 15 to 20 dB reduction in sound transmitting through a cone (this would be entirely dependent on cone mass), so that is how the resonances get out. Next time you have a woofer removed from a cabinet, stick your head up to the opening and sing! If the stuffing is marginal you'll get a good idea of what sort of resonant energy is floating around.
As to a cabinet lining issolating standing waves from the walls, if there are standing waves then clearly the walls are reflecting (the cause of the standing waves) and the lining isn't preventing anything. Also note that a lot of cabinet resonance is cause by direct vibrational excitation. At KEF when we used vibration mounts we could see cabinet energy drop 15dB or so. It don't fall completly because there would be air pressure excitation even if the vibration mounts did their job perfectly. That gives you a good sense of what percentage comes from vibration and what comes from internal SPL.
Dense layers can rincrease TL (transmission loss) as measured by architectural acousticians. Most building wall constructions going for high TL (say in the range of 60 dB) take on a multi layer approach. Most successfull are mass/air gap/mass approaches. These are essentially LCL filters and the air gap reduces the energy coupled between the masses.
For cabinet damping of vented boxes, look at the Elliot sound page linked to above. He is showing curves that directly relate to that. He notes that you have to compromise between damping perfection and reduction of bass from lower port Q and Qm. The curves show the interchange between the two factors.
David S.
I've tried this before, in one set of boxes which have some wool felt lining and not much else if I hum at the opening with the driver removed at the fundamental length resonance of the box (around 260Hz) there is indeed a strong resonance like you might hear in a shower... and yet when I do a near-field sweep of the woofer in the box there is barely a blip in the frequency response at that frequency.
If the resonance was escaping from the box through the cone I would have expected to see a significant hump in the response in the near-field measurement, yet I don't.
That's why I'm wondering if pass-through radiation of internal standing waves through the cone is as much of a problem as thought, and whether significant standing waves in the box necessarily lead to audible effects at the listening distance.
Radiation through a front facing port would seem to be more of an issue, but even that only seems to be a major problem at the half wave length resonance of the port itself.
If a lining provides a dense but decoupled reflective surface it won't do much to decrease internal standing waves, no, but if it adds significant transmission losses between the cavity and the cabinet walls, that would seem to be beneficial from the point of view of coupling less of the internal energy to the cabinet, thus making the cabinet walls quieter.
As you say driver decoupling is a major factor in reducing cabinet vibration but once that is dealt with isolation between the internal volume and the walls with a dense material with high transmission losses may be quite beneficial in further reducing it and quietening down cabinet radiation ?
Yes I've seen that article before, its very good. As I've said before radiation through a front facing port can be a major source of midrange coloration (I'm sure you remember my measurements from another thread) and extra damping is always going to be a compromise between cleaning up the midrange and obtaining a useful bass response in this situation...the simple act of putting the ports on the back allows the cabinet baffle to act as a low pass filter that for all intents and purposes eliminates the port 1/2 wave resonance as a concern with no tradeoff in the bass performance.
If isolation (like the wool felt) can reduce transmission of internal vibration to the panel walls all that leaves is radiation through the cone.
It would certainly be interesting to measure the relative contribution of cone-passthrough, port leakage and panel vibration at the listening position at the internal standing wave frequencies.
Here's some measurements to add to the conversation.
The box is about 63cm long by 39cm wide by 30cm deep with an 8" driver. The lining is only around 10-15mm thick wool under-felt with nothing else. The longest internal dimension is about 57cm after panel thickness etc is taken into account, that's a half wavelength resonance of about 300Hz. (Not 260Hz as I guessed in the previous post)
The first image is a near-field measurement of the driver. Despite a huge internal resonance around 300Hz when singing into the box with the driver removed, the fluctuations in response at least at the driver cone are a mere +/- 0.25dB between 140-400Hz. (Much above that frequency the near-field measurement becomes invalid due to the size of the driver...) In a frequency range where floor/wall reflections introduce huge peaks and dips in the response, I struggle to see how this minimal ripple in the response from the driver cone would even be audible.
What's more, the worst ripple in the response is at 140Hz, which is too low in frequency to be a air space standing wave resonance - in fact its a panel resonance which shows up in the impedance curve, and one that is actually just audible. (The higher frequency ripples around 200-400Hz may also in fact be panel resonances and their harmonics which are leaking into the measurement through microphone proximity to the panel...)
So even though we're not near-field measuring the panel, this panel resonance as measured near the driver cone is greater than the leakage of internal resonances through the cone...measured at a distance I would expect the panel resonance at 140Hz to completely dwarf any 300Hz peak coming through the cone.
The second image is a near-field measurement of the rear port. I presume the peaks/dips in the 200-500Hz range are internal standing wave resonances that are leaking out, possibly mixed with some panel vibrations as well, since the port diameter is a lot smaller than the cone thus the mic is closer to the panel. They're only 20dB down so potentially audible.
Meanwhile the gigantic peak at 700Hz is the half wave resonance of the port tube - that folks, is why you don't put ports on the front on 2 way / full range systems
That peak in output is even higher than the maximum bass output of the port, and easily enough to interfere constructively and destructively with the direct signal from the driver if the port is on the front...
The box is about 63cm long by 39cm wide by 30cm deep with an 8" driver. The lining is only around 10-15mm thick wool under-felt with nothing else. The longest internal dimension is about 57cm after panel thickness etc is taken into account, that's a half wavelength resonance of about 300Hz. (Not 260Hz as I guessed in the previous post)
The first image is a near-field measurement of the driver. Despite a huge internal resonance around 300Hz when singing into the box with the driver removed, the fluctuations in response at least at the driver cone are a mere +/- 0.25dB between 140-400Hz. (Much above that frequency the near-field measurement becomes invalid due to the size of the driver...) In a frequency range where floor/wall reflections introduce huge peaks and dips in the response, I struggle to see how this minimal ripple in the response from the driver cone would even be audible.
What's more, the worst ripple in the response is at 140Hz, which is too low in frequency to be a air space standing wave resonance - in fact its a panel resonance which shows up in the impedance curve, and one that is actually just audible. (The higher frequency ripples around 200-400Hz may also in fact be panel resonances and their harmonics which are leaking into the measurement through microphone proximity to the panel...)
So even though we're not near-field measuring the panel, this panel resonance as measured near the driver cone is greater than the leakage of internal resonances through the cone...measured at a distance I would expect the panel resonance at 140Hz to completely dwarf any 300Hz peak coming through the cone.
The second image is a near-field measurement of the rear port. I presume the peaks/dips in the 200-500Hz range are internal standing wave resonances that are leaking out, possibly mixed with some panel vibrations as well, since the port diameter is a lot smaller than the cone thus the mic is closer to the panel. They're only 20dB down so potentially audible.
Meanwhile the gigantic peak at 700Hz is the half wave resonance of the port tube - that folks, is why you don't put ports on the front on 2 way / full range systems
That peak in output is even higher than the maximum bass output of the port, and easily enough to interfere constructively and destructively with the direct signal from the driver if the port is on the front...Attachments
Last edited:
Barely a blip is all I would expect to see. Remember that submerged resonances will not always be strongly evident but can be audible due to their high Q. Harwood was hearing panel resonances that were close to 30 dB below the direct sound level.
I agree that port resonances are a similar problem. They too should be considered as only blips in the total response. It is only because you can take a near field measurement of them in issolation that they seem even worse.
A heavy internal layer wouldn't really work as you suggest. Again I would refer you to architectural studies. A mass layer does an excellent job but only if it is a complete layer with no penetrations. This splits the wall into a mass, air space, mass assembly. Adding strips of mass layer in front of a single wall would do no good because the sound would get around the strips. Same with a speaker cabinet. Without a complete and unperforated inner layer that is detached from the outer layer (and separated by a significant air space) you aren't increasing the TL. Now, if you adhere your mass layer to the outer layer then at least you are adding limp mass and reducing the Q of the outer wall.
Agreed. Damping the internal acoustical drive to the cabinet walls is always a good thing. I just think the measurements indicate that mechanical drive (magnet reaction force) is the greater issue.
Good measurements in your follow up post, thanks. But the Elliot curves show a nice reduction in the port resonance strength with internal damping, no? The only design challenge is to damp resonances without killing off the one resonance we do want (the primary vent resonance). In that regard we are playing the same game as the TL folks, trying to get useful line output without allowing too much upper range garbage out. Wouldn't a middle of box partition (woofer above and port below) kill off a lot of the mid energy to the port without killing port Q too badly?
David
standing waves
Dave would you say that non parallel enclosure walls are irrelevant? Something as simple as the NHT box will have four panels of different dimensions, thus a dispersed spectrum of resonances, while the standard box has three pairs, though the front baffle is not identical to the back obviously. Say your ideal, or optimum, 1/4 wavelength fg damping allows half the energy to be imparted to the wall before some fraction is reflected back into the box. Then what have we? My ear up against any two way box I own hears plenty of junk, not to mention what a port, not my taste, is serving up.
By the way I agree with ra7. You elucidate the way we got here better than anyone on the forum. The blocks you haven't been around are mostly, but not entirely, to be avoided. OK I'm a quarter waver forgive me if you can.
Thanks for the tutorial
Dave would you say that non parallel enclosure walls are irrelevant? Something as simple as the NHT box will have four panels of different dimensions, thus a dispersed spectrum of resonances, while the standard box has three pairs, though the front baffle is not identical to the back obviously. Say your ideal, or optimum, 1/4 wavelength fg damping allows half the energy to be imparted to the wall before some fraction is reflected back into the box. Then what have we? My ear up against any two way box I own hears plenty of junk, not to mention what a port, not my taste, is serving up.
By the way I agree with ra7. You elucidate the way we got here better than anyone on the forum. The blocks you haven't been around are mostly, but not entirely, to be avoided. OK I'm a quarter waver forgive me if you can.
Thanks for the tutorial
What about automotive heat exchanger filled with fiberglass or stone wadding?
You can even have it filled with coolant and connected to an external heat exchanger
Something like this:
It can have surfaces spaced a little bit further and filled with something.
Five panels can be made like this. Then an outer box...
But... here is another idea. There was a recent topic about driver Fs, where became clear that a suspension wit zero spring effect and pure resistive character would be one without resonant frequency as it absorbs averything and does not release anything but heat back... Then what about a membrane suspended on a friction shock absorber?
This can be made with an elastic liquid filled volume connected to a breathing reservoir through a restrictive pipe...
In theory the ideal friction suspended membrane will eat any air movement without returning it. And this should be the cure for long waves in small enclosures!
Wow, I thing this is a good one!
You can even have it filled with coolant and connected to an external heat exchanger
Something like this:
It can have surfaces spaced a little bit further and filled with something.
Five panels can be made like this. Then an outer box...
But... here is another idea. There was a recent topic about driver Fs, where became clear that a suspension wit zero spring effect and pure resistive character would be one without resonant frequency as it absorbs averything and does not release anything but heat back... Then what about a membrane suspended on a friction shock absorber?
This can be made with an elastic liquid filled volume connected to a breathing reservoir through a restrictive pipe...
In theory the ideal friction suspended membrane will eat any air movement without returning it. And this should be the cure for long waves in small enclosures!
Wow, I thing this is a good one!
We built a sound room with non-parallel walls at the first company that I worked for. It wasn't resonance free, with some standing waves occuring with one frequency on one side of the room and a slightly different frequency on the other side. The modes were probably not as strong but they didn't disappear. Damping will always be required although I won't go so far as to say that odd shapes will have no effect.
That depth of FG generally absorbs 90% or more. The only question is how well the lowest 1/4 wave mode is damped. All modes above that frequency will be very well damped.
David S
Thanks again Dave, for the sharing of your vast experience and knowledge.
I never meant to contradict or challenge you in any way, in fact I am agreeing with you - I am just saying that while FG does a great job it's a shame to enhance one aspect of life at the expense of another. While it is common knowledge that once FG is installed it is no longer a risk as long as it isn't disturbed, I can't help thinking how 'disturbing' high powered bass might be, especially with a vent in a vented box. So, I am not saying one shouldn't use FG, I'm just saying if it is to be used, one should take proper precautions, whatever those may be.
Fiberglass Fact Sheet
Thank you for your patience.
Last edited:
When I first started building speakers I had reservations against using fibreglass because I really do itch in reaction to it. I then read a beginners building book (Weems )which recommended using a layer of polyester on top of the glass and I have done so in most of my builds. The polyester probably has no or minimal effect on resonance or box size, but it does solve the issues of fibreglass migrating.
I used the generic bonded polyester used for insulating air-conditioning duct work. mainly because it was free, you see tradies throwing it away all the time. All I do is ask if I can take the scrap ducting. It has various thicknesses depending on the R value but mostly about 55mm thick.
It is quite open, stiff and coarse and very easy to cut to shape, not at all like the acoustic polyester used in small midrange boxes.
Locally we can also buy bonded acrylic fibre which has better acoustic properties but otherwise quite similar
It is quite open, stiff and coarse and very easy to cut to shape, not at all like the acoustic polyester used in small midrange boxes.
Locally we can also buy bonded acrylic fibre which has better acoustic properties but otherwise quite similar
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.