Member
Joined 2004
AGGEMAM said:
I suggest you do not leave out the tar as that is the reactive component of the sandwich described and it doesn't work without it or another kind of non-setting glue, but I found tar to be the best.
The sandwich works when it has a layer of vicious material (in this case the tar) between two layers of high mass, one being extremely rigid (the marble or granite, the later is better) and the other being (at least somewhat) flexible (in this case the HDF).
The absolute best results I had was when I used a 6 mm (1/4") copper plate instead of the HDF. But that is certainly out of reach for most DIY'ers to even consider when making a whole speaker.
Btw, I didn't even study engineering of any kind, I studied english but I hung around the lab a lot.
Those are fantastic inputs AGGEMAM!
I'll definitely try them out when I make my 15ft^3 sub.
Thanks for the suggestions and keep them coming!
There are two issues of damping and stiffness here. If you make the enclosure stiff enough to resist the drivers newtonian reaction and internal air pressure fluctuations, you shouldn't need excessive damping.
The application of enclosure wall damping just adds another resonant mass which while lossy, I suspect works at some frequencies with attendant hysteresis and not at others at all. IMHO damping it won't cure enclosure resonance problems it will just make 'em different!
I prefer going for ultra stiff constuction which doesn't obscure microdynamics and bloat bass.
Tufnol is great for this, it's a resin/paper composite, it's about as stiff and dense as corian but much easier to use. You can machine it with an ordinary router really well if you're careful. If you dado the joints and bond them with epoxy/ microsphere mix you'll wind up with something far superior to anything you can do with wood.
Wilson use this stuff extensively, it seems to have worked quite well for them!
The application of enclosure wall damping just adds another resonant mass which while lossy, I suspect works at some frequencies with attendant hysteresis and not at others at all. IMHO damping it won't cure enclosure resonance problems it will just make 'em different!
I prefer going for ultra stiff constuction which doesn't obscure microdynamics and bloat bass.
Tufnol is great for this, it's a resin/paper composite, it's about as stiff and dense as corian but much easier to use. You can machine it with an ordinary router really well if you're careful. If you dado the joints and bond them with epoxy/ microsphere mix you'll wind up with something far superior to anything you can do with wood.
Wilson use this stuff extensively, it seems to have worked quite well for them!
Member
Joined 2004
There have been some good points raised in this discussion, but I see a lot of "talking around the fundamentals" so to speak.
First, let's look at a sealed system. There was a very good summary made earlier which, in so many words, stated that for high frequencies mass loading is the easiest/best approach, mid-band frequencies damping is most effective, and low frequencies stiffness is most effective. I would agree, and add that a full analysis should go a bit further. For high frequencies, mass loading lowers both the resonant frequency (hopefully well below the driver operating bandwidth) and decreases the amplitude of forced excitation. Stick with that approach for high frequencies and you'll be ahead of the game. Low frequencies benefit most from a stiffness that places the resonant frequency above the operating bandwidth, but that is just a first order answer. We are concerned about not only the resonant frequency of the enclosure, but also the amplitude of that resonance. Further, if the enclosure is simply infinitely rigid then the backwave pressure must be dissipated elsewhere... which means back through the driver cone.
Those two issues need more examination. There are many ways to achieve a given panel resonant frequency. You can increase stiffness either through modulus or geometry, and can add mass or remove it to lower or raise the Fn, respectively. In general, you can polarize the various approaches as being "high mass high stiffness" or "low mass low stiffness." Either approach could give a resonant frequency of, say, 500Hz, but the amplitude of resonance and amplitude of response to forced vibrations in each case will be quite different. High mass and high stiffness will not only avoid resonances, but also reduce cabinet vibrations in non-resonant frequency reanges as well. In general, that's a good thing, as we don't want our cabinets transmitting significant levels of acoustic energy into our rooms.
Regarding the transmission of sound through the driver cone, this is a complex issue as the backwave resonant energy is a major contributor to the total system Q. Completely damp the backwave, and the effective enclosure size increases. That may or may not be desired depending on the target Q of the system, and the constraints under which the system was designed. IMO, damping in sub enclosures will rarely lead to negative results. I would rather increase electrical requirements and decrease resonant loading, turning the increased amplifier power around resonance into heat in the cabinet, and obtain better response of the driver, at least from a theoretical viewpoint.
Things become much more complicated when considering midrange enclosures. Here, it is very difficult to drive the resonance out of the operating bandwidth, and it may not be desirable to do so due to the inevitable transmission of rear-wave energy back through the cone (keeping resonance below the operating bandwidth with a high stiffness and very high mass construction is optimal from a theoretical viewpoint, but not very shipper or user friendly). To combat cabinet transmission issues, a high mass high stiffness approach to the outer construction layers should be followed. However, as has been pointed out, internal damping is critical to removing wide bandwidth rear wave transmission through the cone. Ideally, we would like to absorb all rear wave energy and convert it to heat. Within the constraints of reason, that leads to a very stiff outer enclosure (and heavy - if it is light, it may still have significant amplitude response to forced excitation and/or resonance), with a flexible inner enclosure separated from the outer enclosure using materials with a high hysteresis damping property. Many have been pointed out so far, and most of these have wide bandwidth damping properties.
The bottom line is that the energy required to drive a cabinet panel to a given amplitude is just as critical a consideration as the resonant frequency of that panel (and more critical in certain frequency ranges). Additionally, you must give careful thought to what happens to rear wave energy that is not converted into heat or transmitted to the air by the cabinet panels, and what you wish/need to do with that rear wave energy (do you need it to boost low frequency response, do you wish to damp it entirely?).
Ported designs typically remove the desire to damp low frequency enclosures, as you need the resonant energy for reinforcement. Wideband or midrange enclosures typically benefit from heavy damping.
Open baffled designs have no rear wave energy to deal with, but are still driven by the forcing function of the driver mass being accelerated. As with box enclosures, it is preferrable to reduce amplitude response with high mass high stiffness construction. It is unlikely that high mass and high stiffness would drive the resonant frequency above the operating bandwidth, therefore damping would still be useful in preventing ringing of the panel. However, with no rear wave the required level of damping would be much less for a given level of cabinet response, so long as the panel is massive enough to have a low amplitude response to the driver forcing function.
First, let's look at a sealed system. There was a very good summary made earlier which, in so many words, stated that for high frequencies mass loading is the easiest/best approach, mid-band frequencies damping is most effective, and low frequencies stiffness is most effective. I would agree, and add that a full analysis should go a bit further. For high frequencies, mass loading lowers both the resonant frequency (hopefully well below the driver operating bandwidth) and decreases the amplitude of forced excitation. Stick with that approach for high frequencies and you'll be ahead of the game. Low frequencies benefit most from a stiffness that places the resonant frequency above the operating bandwidth, but that is just a first order answer. We are concerned about not only the resonant frequency of the enclosure, but also the amplitude of that resonance. Further, if the enclosure is simply infinitely rigid then the backwave pressure must be dissipated elsewhere... which means back through the driver cone.
Those two issues need more examination. There are many ways to achieve a given panel resonant frequency. You can increase stiffness either through modulus or geometry, and can add mass or remove it to lower or raise the Fn, respectively. In general, you can polarize the various approaches as being "high mass high stiffness" or "low mass low stiffness." Either approach could give a resonant frequency of, say, 500Hz, but the amplitude of resonance and amplitude of response to forced vibrations in each case will be quite different. High mass and high stiffness will not only avoid resonances, but also reduce cabinet vibrations in non-resonant frequency reanges as well. In general, that's a good thing, as we don't want our cabinets transmitting significant levels of acoustic energy into our rooms.
Regarding the transmission of sound through the driver cone, this is a complex issue as the backwave resonant energy is a major contributor to the total system Q. Completely damp the backwave, and the effective enclosure size increases. That may or may not be desired depending on the target Q of the system, and the constraints under which the system was designed. IMO, damping in sub enclosures will rarely lead to negative results. I would rather increase electrical requirements and decrease resonant loading, turning the increased amplifier power around resonance into heat in the cabinet, and obtain better response of the driver, at least from a theoretical viewpoint.
Things become much more complicated when considering midrange enclosures. Here, it is very difficult to drive the resonance out of the operating bandwidth, and it may not be desirable to do so due to the inevitable transmission of rear-wave energy back through the cone (keeping resonance below the operating bandwidth with a high stiffness and very high mass construction is optimal from a theoretical viewpoint, but not very shipper or user friendly). To combat cabinet transmission issues, a high mass high stiffness approach to the outer construction layers should be followed. However, as has been pointed out, internal damping is critical to removing wide bandwidth rear wave transmission through the cone. Ideally, we would like to absorb all rear wave energy and convert it to heat. Within the constraints of reason, that leads to a very stiff outer enclosure (and heavy - if it is light, it may still have significant amplitude response to forced excitation and/or resonance), with a flexible inner enclosure separated from the outer enclosure using materials with a high hysteresis damping property. Many have been pointed out so far, and most of these have wide bandwidth damping properties.
The bottom line is that the energy required to drive a cabinet panel to a given amplitude is just as critical a consideration as the resonant frequency of that panel (and more critical in certain frequency ranges). Additionally, you must give careful thought to what happens to rear wave energy that is not converted into heat or transmitted to the air by the cabinet panels, and what you wish/need to do with that rear wave energy (do you need it to boost low frequency response, do you wish to damp it entirely?).
Ported designs typically remove the desire to damp low frequency enclosures, as you need the resonant energy for reinforcement. Wideband or midrange enclosures typically benefit from heavy damping.
Open baffled designs have no rear wave energy to deal with, but are still driven by the forcing function of the driver mass being accelerated. As with box enclosures, it is preferrable to reduce amplitude response with high mass high stiffness construction. It is unlikely that high mass and high stiffness would drive the resonant frequency above the operating bandwidth, therefore damping would still be useful in preventing ringing of the panel. However, with no rear wave the required level of damping would be much less for a given level of cabinet response, so long as the panel is massive enough to have a low amplitude response to the driver forcing function.
The earlier post about heatsink enclosures?
http://www.magico.net/
Corian ? I ordered some samples and hit this material
with a hammer ----- PING !!!! a very loud annoying noise,
how you can even consider this material ? It makes noise
like a bell.......
I'm trying to figure out what composite I'm going to
do for my speaker box to make the cabinet walls 'dead'.
Damping will come later with exotic materials, right now
I'm focusing on the mass.
I've tried plywood, particle board, and mdf composites
where you glue three 3/4" thick panels together of the
various woods to see if there is any 'magic' combination,
then using the famous 'hammer' trick to compare the different
sounds.
Obviously, three layers of 3/4" MDF wins over any other combination of wood composites. Six layers of 3/4" MDF is pretty
awesome but not practical. Well, considering that many people
may use concrete, granite, metal, etc., maybe a 3" - 4" wall of MDF
isn't so bad after wall as it may weigh the same as the other
exotic methods.
Add some heavy bracing to the 2.25" thick MDF composite,
it gets better.
Another variable that is very important and neglected is how
much energy is your speaker system delivering ? I have midrange
drivers that when combined with a 600 watt amplifier like to
reasonate boxes with ease at high power, but if you are designing a simple system that won't play at high spl levels,
there is no need to over engineer the box.
http://www.magico.net/
Corian ? I ordered some samples and hit this material
with a hammer ----- PING !!!! a very loud annoying noise,
how you can even consider this material ? It makes noise
like a bell.......
I'm trying to figure out what composite I'm going to
do for my speaker box to make the cabinet walls 'dead'.
Damping will come later with exotic materials, right now
I'm focusing on the mass.
I've tried plywood, particle board, and mdf composites
where you glue three 3/4" thick panels together of the
various woods to see if there is any 'magic' combination,
then using the famous 'hammer' trick to compare the different
sounds.
Obviously, three layers of 3/4" MDF wins over any other combination of wood composites. Six layers of 3/4" MDF is pretty
awesome but not practical. Well, considering that many people
may use concrete, granite, metal, etc., maybe a 3" - 4" wall of MDF
isn't so bad after wall as it may weigh the same as the other
exotic methods.
Add some heavy bracing to the 2.25" thick MDF composite,
it gets better.
Another variable that is very important and neglected is how
much energy is your speaker system delivering ? I have midrange
drivers that when combined with a 600 watt amplifier like to
reasonate boxes with ease at high power, but if you are designing a simple system that won't play at high spl levels,
there is no need to over engineer the box.
brilliat, some of these homepages have resources that i've researched everywhere, and the explanations are very instructive.
Laser measurements definitely sounds like a good idea for cabinets. Although my main testing method here would be generating harmonic profiles and examining the difference in between the digital and measured frequency graphs.
Laser measurements definitely sounds like a good idea for cabinets. Although my main testing method here would be generating harmonic profiles and examining the difference in between the digital and measured frequency graphs.
Member
Joined 2004
On a whim, I sat on a $500 commercial subwoofer while it reproduced the depth charges in the movie U-571 quite loudly. I am a large guy, but still the amount of vibration into my body was nearly painfull, and I had to clench my jaw to keep my teeth from chattering. According to my friend, the amount of bass reaching his listening position was reduced about 10 dB. This 'test' made me appreciate the importance of mass loading (or bolting the cabinet to bedrock) and cabinet construction. Everything one can do to hold the cabinet still and keep the outside walls from vibrating has to be a good thing. This was a smallish, cheapish sub. Imagine the vibrational horsepower of a large relatively heavy driver.
markp: I don't think you (or probably anyone) can make a cabinet stiff enough to push all (any?) panel vibrations over 20kHz.
All of the cabinet noise is transmitted into the room and acts to raise the noise level thus hurting the signal to noise of the speaker. Anything you can do to the cabinet/structure to keep it quiet will improve the overall sound quality.
Any undamped structure will ring excessively and this extended noise will interfere with the signal coming from the driver. Damping may not reduce the first cycle of vibration or change the frequency, but it will dramatically lessen the time that it is sustained.
It's what you want.
All of the cabinet noise is transmitted into the room and acts to raise the noise level thus hurting the signal to noise of the speaker. Anything you can do to the cabinet/structure to keep it quiet will improve the overall sound quality.
Any undamped structure will ring excessively and this extended noise will interfere with the signal coming from the driver. Damping may not reduce the first cycle of vibration or change the frequency, but it will dramatically lessen the time that it is sustained.
It's what you want.
I did some PHL 2520 driver tests where I placed a cheap test box
on my test bench in the garage and used my 600 watt amp.
At full power and certain frequencies, I can feel small vibrations
between my shoe and concrete floor. It was a strange experience. High spl applications can probably just about
excite any material. /hehe
When I mentioned that I will use three layers of MDF, that
recipe was just for the midrange drivers ... If I was doing
monster subwoofers I would go thicker
on my test bench in the garage and used my 600 watt amp.
At full power and certain frequencies, I can feel small vibrations
between my shoe and concrete floor. It was a strange experience. High spl applications can probably just about
excite any material. /hehe
When I mentioned that I will use three layers of MDF, that
recipe was just for the midrange drivers ... If I was doing
monster subwoofers I would go thicker
The idea is to use the corian and damp the inside. No one would just use the corian without some form of viscous damping to get the best of both worlds. It is also very ,very hard to make an object of mass of a speaker cabinet resonate at even 10khz. The amount of acoustic energy at high freqs is tiny compared to the bass region and as such cant excite the panels. I've only been doing this for 25 years so I have grasp of the concept of cabinet induced distortions.MartinQ said:
That's one reason that I like to build my subwoofers as 'reaction canceling' types where two or more woofs on opposite ends of the cabinet neutralize most of the mechanical vibration resulting from the cone motion, plus that gives the option of harmonic cancelling & I think the spatial offset between more than one driver in a subwoofer may be a good thing as far as evening out room mode excitation goes.
Member
Joined 2004
Hi, Andrikos -
I built a set as push-pull a while back for my music only system to get the even harmonic neutralization, but the set I'm preparing to build now for my HT system will be push-push to simplify construction a bit and maximize cabinet volume per space used.
My personal preference for the best sound quality is for stereo subwoofers under or near each associated channel (I know some dismiss this, saying the lowest few octaves are 'nondirectional', but I'm thinking overall wavefronts from each channel), but for my HT setup, I'll build two subwoofer cabinets more to keep the endtable style cabinets from getting too large, since I plan to use 16 (8 per cabinet - 2 on each of 4 vertical faces) 12" long throw (31mm p-p) woofers all told for the HT subwoofer system.
I built a set as push-pull a while back for my music only system to get the even harmonic neutralization, but the set I'm preparing to build now for my HT system will be push-push to simplify construction a bit and maximize cabinet volume per space used.
My personal preference for the best sound quality is for stereo subwoofers under or near each associated channel (I know some dismiss this, saying the lowest few octaves are 'nondirectional', but I'm thinking overall wavefronts from each channel), but for my HT setup, I'll build two subwoofer cabinets more to keep the endtable style cabinets from getting too large, since I plan to use 16 (8 per cabinet - 2 on each of 4 vertical faces) 12" long throw (31mm p-p) woofers all told for the HT subwoofer system.
thoriated said:
Maximizing push-push:
An externally hosted image should be here but it was not working when we last tested it.
Downward dynamic range is also improved ... at subwoofer frequencies you'd have to have a pretty big box to have substantial spacial offset.
dave
I've come up with an idea that is a little different, it uses treated compound curves. very very light and seriously stiff which just about eliminates resonance. very simple, quick and cheap to build. nothing worse than a flat surface for amplifying resonance.
I'll be using this method in the next project
Stew.
I'll be using this method in the next project
Stew.
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