In my conversion project I will have an LF chamber of about 10L (working below 200 or 300 Hz) and an MF chamber of about 3L (working about 300Hz-3kHz).
The walls will be 12mm MDF + 8mm valchromat MDF.
What should I use to line each of the chambers internally to reduce reflections and vibration? Which chambers should have wadding fill and what is the best material/parameters for that?
The LF chamber will have bracing added. Sealed box.
Should I add some reflective structure inside the MF chamber to improve absorption and reduce back wave to the driver?
Project thread here: https://www.diyaudio.com/community/threads/3-way-to-active-hypex-fa253-learning-project.421061/
The walls will be 12mm MDF + 8mm valchromat MDF.
What should I use to line each of the chambers internally to reduce reflections and vibration? Which chambers should have wadding fill and what is the best material/parameters for that?
- foam eggcrate panels?
- wool/felt sheets?
- butyl sheets?
- synthetic or wool wadding?
The LF chamber will have bracing added. Sealed box.
Should I add some reflective structure inside the MF chamber to improve absorption and reduce back wave to the driver?
Project thread here: https://www.diyaudio.com/community/threads/3-way-to-active-hypex-fa253-learning-project.421061/
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A damping material is attached to the wall of an enclosure to reduce its amplitude of vibration. Self adhesive loudspeaker damping sheets would be applicable for that purpose:
https://willys-hifi.com/collections...products/bitumen-damping-panels-self-adhesive
https://willys-hifi.com/collections...ts/bituminous-felt-sheets-2-8mm-self-adhesive
An absorbent material is inserted into the body of an enclosure to absorb sound reflections from the walls. Wool is an ideal material for that purpose:
https://www.woollyshepherd.co.uk/loudspeaker-wool/
https://willys-hifi.com/collections...products/bitumen-damping-panels-self-adhesive
https://willys-hifi.com/collections...ts/bituminous-felt-sheets-2-8mm-self-adhesive
An absorbent material is inserted into the body of an enclosure to absorb sound reflections from the walls. Wool is an ideal material for that purpose:
https://www.woollyshepherd.co.uk/loudspeaker-wool/
My understanding:
Dense sheets on walls add mass (butyl, roof felt or similar) and are used to lower the resonance frequency of the chamber - to shift it away from the chamber working frequency range. So, how do I estimate the resonance before/after application? I assume heavy sheets are supposed to work for the MF chamber (in my case) and will not be useful in the LF chamber. Right?
Wadding (wool or synthetic) is used to change the response of the bass drivers (equivalent to increase of volume). This may or may not be needed in my case.
Wool sheets or egg-crate foam (or similar) are used to absorb internal reflections of certain frequency range. In my case that would work inside the MF chamber (in addition to mass sheets) to reduce back wave. Right?
If the above is correct, I should:
For LF: use wadding if needed to tune bass response. Use bracing. Do not use heavy sheets or absorption sheets.
For MF: use heavy sheets and absorption sheets. Extra bracing not needed as the walls areas are small.
Right?
Dense sheets on walls add mass (butyl, roof felt or similar) and are used to lower the resonance frequency of the chamber - to shift it away from the chamber working frequency range. So, how do I estimate the resonance before/after application? I assume heavy sheets are supposed to work for the MF chamber (in my case) and will not be useful in the LF chamber. Right?
Wadding (wool or synthetic) is used to change the response of the bass drivers (equivalent to increase of volume). This may or may not be needed in my case.
Wool sheets or egg-crate foam (or similar) are used to absorb internal reflections of certain frequency range. In my case that would work inside the MF chamber (in addition to mass sheets) to reduce back wave. Right?
If the above is correct, I should:
For LF: use wadding if needed to tune bass response. Use bracing. Do not use heavy sheets or absorption sheets.
For MF: use heavy sheets and absorption sheets. Extra bracing not needed as the walls areas are small.
Right?
That sounds right according to the additional factors you have introduced.
I'm reminded of B&W's Prism technology:
"The back wall of the speaker enclosure is entirely covered with long plastic wedges. These break up standing wave patterns between the front and the back of the box, patterns that otherwise would emanate through the woofer cone and result in interference with the direct sound. Or in severe colourations, if you want. The Prism wedges are meant to be most effective in the critical range centered around 1-3 kHz, where the human ear is most sensitive."
I wasn't sure about the above statement regarding the lowering of the resonance frequency.
Increasing the material stiffness is used to raise the panel resonance frequencies above the working frequency range of the driver, in order that they won't be excited by the driver.
https://www.audioholics.com/loudspeaker-design/detailed-look-proper-loudspeaker-cabinet-bracing
Quote: "The goal of increasing the stiffness of a speaker panel is two-fold, to decrease the overall deflection of the system under dynamic load and to increase the natural frequency away from the operating frequencies of the drivers. This task could be accomplished in one of two ways, either by increasing the overall panel thickness or by adding braces. Thickening the entire panel is a global approach to the problem, while adding discrete stiffeners is a local approach."
P.S. Hopefully, by generating additional posts, the speaker enclosure designers will chime in!
For the bass chamber, I would highly recommend forgetting about damping and just go mad with the bracing so it will only resonate well above the frequency of the bass driver.
For midbass/midrange enclosures, merino wool is by far the most natural sounding material.
If you want a very dry, inert sound, mineral wool works best.
The white dacron stuff isn't that great at absorbing a linear range of frequencies. Every time I use it, the LF enclosure tends to sound very midrange forward and not in a good way.
I'd rather use open cell urethane foam, which depending on thickness and density will absorb down to the mid 100s Hz.
If you want a very dry, inert sound, mineral wool works best.
The white dacron stuff isn't that great at absorbing a linear range of frequencies. Every time I use it, the LF enclosure tends to sound very midrange forward and not in a good way.
I'd rather use open cell urethane foam, which depending on thickness and density will absorb down to the mid 100s Hz.
You ask 2 questions. Damping the airspace and damping the box walls are very different tasks. I use cotton felt nd accoustastuff for airspace damping.
Using something like bituminm sheet of the LF walls will decreas etheir performance. In a LF enclosure you would like (potential) resonances to be above the woofer’s bandwidth. Given your XO frequency, that is should be fairly easy. Given you are using MDF (at least a higher quality MDF), you will need more bracin gthan if you had chosen quality plywood. You want stiff and light to push box resonances up.
For the mid you can take the opposite approach, pushing box wall resonances below its bandwidth.
SInce i typically am using the first technique forthe entire box and it works really well. For a box that size, i’d probably use 15mm ply.
dave
Using something like bituminm sheet of the LF walls will decreas etheir performance. In a LF enclosure you would like (potential) resonances to be above the woofer’s bandwidth. Given your XO frequency, that is should be fairly easy. Given you are using MDF (at least a higher quality MDF), you will need more bracin gthan if you had chosen quality plywood. You want stiff and light to push box resonances up.
For the mid you can take the opposite approach, pushing box wall resonances below its bandwidth.
SInce i typically am using the first technique forthe entire box and it works really well. For a box that size, i’d probably use 15mm ply.
dave
There are two kinds of damping we need to consider, and sometimes our terminology and wording is not clear (1) acoustical damping is meant to control the resonance of the air space in the cabinet. It suppresses the standing wave resonances. (2) structural damping is meant to control the vibration resonance of the cabinet walls.
Acoustical damping can be very effective, and it is not difficult to get it right. Many different materials will work. In a sealed box system, it is a straight forward, almost simple process: simply fill the volume loosely with an appropriate stuffing (wadding) material. With a vented box, it is a little more complicated because there has to be a clear air path between the woofer cone and the vent, and too much stuffing will cause a reduction in bass output. In a small vented box 2-way, it can be a challenge to get the balance just right.
As @profiguy says above, different materials can have a slightly different character. I use natural wool fibers for midrange enclosures and for sealed box woofer enclosures. The last bass reflex box I built used 2" and 3" thick urethane foam. Many people have had good performance with shredded denim insulation, rock wool, and melamine foam (Basotect).
Structural damping is a harder problem, and even the best methods do not fully dampen the structural resonances. I found that with the kind of well-braced cabinets that I build, a layer of 80 mil foil-backed-butyl-rubber-self-adhesive deadener works well on all interior surfaces.
I am skeptical that felt or carpet glued to the interior walls has much benefit. It may offer a small amount of structural damping, and it probably does provide some acoustical damping, it would not be as effective as a layer of butyl rubber combined with conventional stuffing (wadding).
Acoustical damping can be very effective, and it is not difficult to get it right. Many different materials will work. In a sealed box system, it is a straight forward, almost simple process: simply fill the volume loosely with an appropriate stuffing (wadding) material. With a vented box, it is a little more complicated because there has to be a clear air path between the woofer cone and the vent, and too much stuffing will cause a reduction in bass output. In a small vented box 2-way, it can be a challenge to get the balance just right.
As @profiguy says above, different materials can have a slightly different character. I use natural wool fibers for midrange enclosures and for sealed box woofer enclosures. The last bass reflex box I built used 2" and 3" thick urethane foam. Many people have had good performance with shredded denim insulation, rock wool, and melamine foam (Basotect).
Structural damping is a harder problem, and even the best methods do not fully dampen the structural resonances. I found that with the kind of well-braced cabinets that I build, a layer of 80 mil foil-backed-butyl-rubber-self-adhesive deadener works well on all interior surfaces.
I am skeptical that felt or carpet glued to the interior walls has much benefit. It may offer a small amount of structural damping, and it probably does provide some acoustical damping, it would not be as effective as a layer of butyl rubber combined with conventional stuffing (wadding).
There will always be resonances. We can target were they go, and their Q. If you can get both high enuff there will not be sufficient continuous HF energy to excite them.
When yoiu do the knock test on the woofer box, it should clearly ring at high frequencies
dave
@planet10 It surprising that some cabinets which measure poorly with structural box resonances actually don't sound as bad in real life. Yes, if the resonance Q is fairly low and the decay times are only a few periods long, the whole speaker can sound pretty decent if the resonances lye in a benign frequency range, evenly distributed.
Hi,
few extra nuggets:
if a panel resonates with the 0,0 mode it has very great radiating area meaning it beams sound. Higher modes radiating area shrinks but could also beam, and average out to some directions at least. Large area also means great volume displacement which means great SPL even with seemingly small vibration. Although haven't measured the box walls could be louder than the driver at their resonance simply because panel radiating area is likely >> driver's.
So, audibility of panel resonances depends on many things. Good way to deal with this and other stuff is to make a prototype box, at least adjustable innards, to weed out audible issues by whatever means.
Without prototyping few quick thoughts how to reduce likelihood of audibility:
the stiffer the panel the higher the resonances in frequency, which means more beaming for given panel size (of all the modes) and possibly not coming to listening spot depending on positioning and toe in and stuff. Consequently, the lower the resonant frequency the more likely the sound radiates wider and possibly toward listening spot, or toward an angle a specular reflection makes it contribute to sound.
Small but important detail is that, in general, a resonance is excited by all vibration whose frequency is below the resonant frequency, and especially at the resonant frequency, and very little by higher frequencies if not at all. For this reason one could assume all panel vibrations are always excited by a woofer since it very likely has pass band below panel resonant frequency. Thus one want's to dampen the panels if they resonate audibly.
edit.
example, 1m tall, 30cm deep and 20cm wide floorstanding speaker box panels have total area of roughly 2* (20*30 + 20*100 + 30*100) ~= 11000cm2. An 8" woofer on the front panel has radiating area of about 220cm2, so on such speaker the potential box wall radiating area of panels is whopping 50x. So a speaker box could have tens of times more panel surface area than the transducer's, so needs tens of times less excursion than the transducers to emit equal amount of sound power into the room. To have lot less sound radiation than the transducer the panels most likely need to be damped.
few extra nuggets:
if a panel resonates with the 0,0 mode it has very great radiating area meaning it beams sound. Higher modes radiating area shrinks but could also beam, and average out to some directions at least. Large area also means great volume displacement which means great SPL even with seemingly small vibration. Although haven't measured the box walls could be louder than the driver at their resonance simply because panel radiating area is likely >> driver's.
So, audibility of panel resonances depends on many things. Good way to deal with this and other stuff is to make a prototype box, at least adjustable innards, to weed out audible issues by whatever means.
Without prototyping few quick thoughts how to reduce likelihood of audibility:
the stiffer the panel the higher the resonances in frequency, which means more beaming for given panel size (of all the modes) and possibly not coming to listening spot depending on positioning and toe in and stuff. Consequently, the lower the resonant frequency the more likely the sound radiates wider and possibly toward listening spot, or toward an angle a specular reflection makes it contribute to sound.
Small but important detail is that, in general, a resonance is excited by all vibration whose frequency is below the resonant frequency, and especially at the resonant frequency, and very little by higher frequencies if not at all. For this reason one could assume all panel vibrations are always excited by a woofer since it very likely has pass band below panel resonant frequency. Thus one want's to dampen the panels if they resonate audibly.
edit.
example, 1m tall, 30cm deep and 20cm wide floorstanding speaker box panels have total area of roughly 2* (20*30 + 20*100 + 30*100) ~= 11000cm2. An 8" woofer on the front panel has radiating area of about 220cm2, so on such speaker the potential box wall radiating area of panels is whopping 50x. So a speaker box could have tens of times more panel surface area than the transducer's, so needs tens of times less excursion than the transducers to emit equal amount of sound power into the room. To have lot less sound radiation than the transducer the panels most likely need to be damped.
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