I tried and tried, but failed to get good readings. I was really interested to measure what was going on with the cabinet walls. None of my DIY pickups or any acoustic guitar pickups I tried gave me enough signal. I needed a better preamp.
An accelerometer might be a better choice for a probe.
George Nvantaras has a kit for an accelerometer that will cost you under 200 USD to put together. How is the availability of parts on your island, do the Mousers of this world provide free shipping?
Yeah, a piezo one, but we weren't allowed to either use at work or borrow the plant's B&K system, so made do with screwing a cheap 'FR' driver to a panel or in the box before any holes were cut and used our ears.
GM
non-parallel walls are over rated - the panels still bend, resonate, the internal volume still has standing wave modes - they're just more complicated, not necessarily smaller unless you have more damping
ADI has mems accelerometer chips that can see some things - I've used them in machine vibration measurement but the numbers look possible for some audio panel resonance
we had a more sensitive capacitive accelerometer to compare against, also a laser triangulation vibrometer with high speed CCD line sensor that could measure over to 10 kHz
ADI has mems accelerometer chips that can see some things - I've used them in machine vibration measurement but the numbers look possible for some audio panel resonance
we had a more sensitive capacitive accelerometer to compare against, also a laser triangulation vibrometer with high speed CCD line sensor that could measure over to 10 kHz
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Only a smallamount of the energy that gets to the walls comes from the air space inside the box.
Why?
dave
No, but isn't this the equivalent of a series RLC bandpass circuit where stiffness is the C, mass is the L, and damping is the R which is in series with the load of coupling to the air? And if so there are two opportunities..
Greater stiffness and greater mass each reduce the bandwidth, and hence the damping.
On the other hand the damping resistance creates a divider reducing the coupling to the air.
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Mass can prevent motion at low frequencies even in the absence of stiffness. It may not be the most satisfying approach, though. I believe the Harwood approach is the best approach for full range loudspeakers but I will concede that, in the case of subwoofers only, it can make more sense to shoot for extremely high stiffness and hope to push the resonances above your crossover frequency. On the other hand, we don't need to fear some loss of sound through the cabinet walls due to flexure. The real issue is resonances that make the walls transparent over a narrow band. That will always cause delayed narrow band ringing that can be audible. Non resonant wall flexure, on the other hand, can only cause the slightest of level drops. Not an audible problem.
I never said that thin rubber sheets would be adequate. Adequate mass might require a 3 or 4" thickness. I was embellishing to make a point. What is clear and is absolutely true is that we could all make better cabinets if we put more emphasis on damping and less on high stiffness. Stiffness does not reduce the amplitude of cabinet resonance. Stiffness pushes resonances upwards but seldom by enough to make them less audible (Harwood found the resonances were more audible higher in the band). Only damping can prevent wall transmission at frequencies of resonance. Stiffness makes any damping less effective in direct proportion to stiffness increase
David
It is a myth that non-parallel walls prevent standing waves. They just make them more complex. Anyhow, the issue is not internal standing waves that are easily damped with a bit of fiberglass. The problem is structural resonance as driven by reaction forces from the woofer.
And why is that?
Mass is poor at preventing motion at low frequencies because force = mass*acceleration and acceleration is small at low frequencies. At high frequencies acceleration is large and mass is good at preventing motion. For low frequencies it is stiffness that is good at preventing motion but, unfortunately, it is not much good at high frequencies where deflection is relatively small.
The claim that mass is the answer is just as wrong as the claim that stiffness is the answer when low order resonances are in the passband (forces due to mass and stiffness are of similar size). Perhaps it is a bit more wrong in practise because high frequencies tend to be easier to control than low frequencies.
It is not adequate mass but adequate stiffness that these excessively thick layers are probably trying to provide. In the work you cite why do you think Harwood opted for plywood which has a high stiffness rather than a limper material like chipboard? In order to extend the mass controlled region down to 20Hz the low order resonances would need to lie decades below this at probably below 1Hz which is not practical. In practical cabinets, including Harwood's, the lower frequency motion in the passband is stiffness controlled and so the cabinet requires a sufficiently high stiffness to do the controlling.
What is practical and sensible is to use is high(ish) stiffness (good for low frequencies), high damping (good for midrange) and to passively isolate the tweeter (good for high frequencies) although many cabinets may have sufficient mass anyway not to bother. Not surprisingly this seems to be roughly what the competent speaker manufacturers do in their more expensive speaker ranges.
Which is why I was careful not to state that. The answer is to increase damping and to thereby increase the ratio of damping mass to stiffness contributing mass.
For example, we know that wall stiffness goes up approximately by the cube of thickness increase. Double wall thickness and you will have 8 times the beam stiffness and 2 times the mass. Resonances will double in frequency and therefor any damping applied will be half as effective. There is no way to win at that game.
On the other hand if you cut wall thickness in half and make up the mass loss with damping material you will have much lower Q for your resonances, less wall transmission (at resonance) and resonances moved to a more beneficial frequency.
Whether you can take that to the extreme or not may be open to debate, but it is clearly in the right direction.
Harwood clearly puts little importance in stiffness. I assume he uses plywood because it is more durable in the thinner dimensions that he desires (and lighter). His objective was to lower resonances to a reasonable degree, primarily so that the applied damping would be more effective. His own tests showed that lowered resonance frequencies were less audible (had a higher threshold of perception).
Nobody is really advocating a totally limp cabinet, but I would again repeat that inadequate stiffness can only lose response level in a non-resonant way. Not the worst result.
Disagree. Most manufacturers still worship at the altar of thicker walls, more stiffness, generally with no applied damping. They are fixated on the common, and wrong, belief that thicker/stiffer is always better.
Regards,
David
Why not box-in-box with sand poured in?
I'm sure this has been tried, along with water filled boxes, etc. Why not build what is effectively box inside a box separated by a layer of plain old sand? The speakers can weigh hundreds of pounds each (if you have a slab floor!) and you can install the boxes and fill with sand in place. And if they spring a leak, no problem, just sweep it up with a whisk broom and dust pan. Or just out the door if you live in a sandy area...?
Would a pair of thin-as-possible wood boxes with sand between them be satisfactory?
Of course I am just talking out my a--. My latest speaker is the "Marimba I" which "cabinet material" is 4" cardboard tubes and stuffing whatever rags were on hand
[toot.own.horn.mode = "ON"]
http://www.diyaudio.com/forums/multi-way/270544-line-array-varying-length-tl-pipes.html
[toot.own.horn.mode = "OFF"]
I'm sure this has been tried, along with water filled boxes, etc. Why not build what is effectively box inside a box separated by a layer of plain old sand? The speakers can weigh hundreds of pounds each (if you have a slab floor!) and you can install the boxes and fill with sand in place. And if they spring a leak, no problem, just sweep it up with a whisk broom and dust pan. Or just out the door if you live in a sandy area...?
Would a pair of thin-as-possible wood boxes with sand between them be satisfactory?
Of course I am just talking out my a--. My latest speaker is the "Marimba I" which "cabinet material" is 4" cardboard tubes and stuffing whatever rags were on hand
[toot.own.horn.mode = "ON"]
http://www.diyaudio.com/forums/multi-way/270544-line-array-varying-length-tl-pipes.html
[toot.own.horn.mode = "OFF"]
Wharfdale was doing it in the 50s with their SFB open baffle.
Designing Loudspeakers - Part 20 Open Baffle Room Responses
Designing Loudspeakers - Part 20 Open Baffle Room Responses
you have to try to stop the effect of Newton's third law, you either have to decouple the driver from the box so that there is no vibrational transmission to the enclosure(probably like Speaker Dave did at Kef) or tie the driver chassis into every panel in the box and "dampen" the box and hope for the best,
And we were doing it in Paris in the '80s. Boxes and horns. Makes for a heavy, difficult build. But it does work great.
Andy, never heard of Hardwood. Their recipe doesn't seem to lead to overwhelming success.
Where you go amiss in your reasoning is that you don't differentiate between forces on the speaker panels induced by reaction forces, and those by fluctuating pressure differentials. For the latter, you may be right, but these are by far not the main cause for panel vibrations. Reaction forces are. These require a different kind of logic as Dave, myself and others set out before, leading to different conclusions.
Where you go amiss in your reasoning is that you don't differentiate between forces on the speaker panels induced by reaction forces, and those by fluctuating pressure differentials. For the latter, you may be right, but these are by far not the main cause for panel vibrations. Reaction forces are. These require a different kind of logic as Dave, myself and others set out before, leading to different conclusions.
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Relating to Andy and Dave's discussions, information on frequency ranges and acoustic attenuation of building walls. This also applies to loudspeaker enclosure walls with careful considerations, as the frequencies for the different attenuation control ranges generally shift upward (stiffness controlled, resonance + damping controlled, mass controlled, coincidence + damping controlled.
Noise insulation case
Noise insulation case
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Good reference, thanks.
Gives an interesting quote on coincidence dip and damping:
"The depth and width of the coincidence dip are determined by losses of sound energy in the material and energy transfer to the supporting structure. The greater these losses, the shallower and broader the coincidence dip, and the less it affects the transmission loss. The most effective way to increase energy losses of the panel is to use a viscous interlayer, that has high dampening, such as soundproofing mat. It transforms panels bending waves into heat energy. Increased dampening of the panel therefore decreases coincidence dip and also resonance that is explained in next section."
Of course coincidence dips are peculiar to large wall surfaces (more or less a bulk property), but the lower frequency resonances also referred to are exactly analogous to the speaker cabinet case.
To be fair, the architectural acoustics crowd doesn't worry about wall stiffness because the wall dimensions are unknown and largely big enough that the fundamental resonance (that separates the stiffness control region from the mass control region) is too low for stiffness to aid in noise isolation.
David
Dave,
Yes, for a "moderately" stiff speaker enclosure, made of wood products with some bracing, primary wall resonances are typically in the 180 to 500 Hz range, with coincidence in the 1000 to 1200 Hz range. Am I missing something here about the coincidence range characteristics?
Yes, architectural walls have typical fundamental resonances at 20Hz and below. That's why that 'Greenglue' product for constrained layer damping of architectural walls is so soft, a semi-liquid paste adhesive that barely hardens.
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