The chart only gives single number damping. I would be surprised if the damping had a FR associated with it, and we must keep in mind that damping is only 1 property of many in a sheet material used for building loudspeakers.
dave
dave
I used to use chip board for my mobile disco speakers.
They got a bit wet when taking them in to a gig one night.
Next day they swelled up quite a bit and lumps started flaking off.
Since then I have always used cheap 18mm ply wood.
They got a bit wet when taking them in to a gig one night.
Next day they swelled up quite a bit and lumps started flaking off.
Since then I have always used cheap 18mm ply wood.
Bear in mind that a cabinet will have a complex mix of resonant frequencies/modes. Low damping will allow many of these to get excited by the music, and to slowly release that energy over time, blurring your detail. More damping should reveal better detail.
It may possess slightly more internal damping than other similar materials but the amount is still well short of what is required to do something effective about the lowest frequency resonances. Effective damping is rarely provided by the load carrying layer but a damping layer that is strained effectively by the load carrying layer. The lowest frequency modes tend to be the least well damped because most forms of damping becomes more effective at higher frequencies. The lowest frequency modes also tend to carry the most energy making them the ones you need to do something about to improve matters.
It is not difficult to design a reasonably inert speaker cabinet but in order to do it the designer needs to understand what is going on. The understanding also needs to be quantitative to a reasonable extent and not just qualitative so that a balanced amount of material is assigned to providing stiffness, damping and mass for the frequency ranges where these are controlling the cabinet motion.
andy19191 - the very lowest frequency mode, the one where all panels move in the same direction, also called the breathing mode, will tend to have the highest amount of effective sound radiation. This one is very hard to tame, but doable. As the frequency goes up the modes tend to become less and less effective at sound radiation (think monopole, dipole, quadrapole, each becoming less and less effective at sound radiation) and the damping become more effective.
I have tried to actually measure sound radiation from an enclosure, it is almost impossible. It is so low in level and gets swamped by the direct radiation of the driver. I don't give much credence to "box sound" once the enclosure is designed to control the first few resonances. But clearly I don't want any of them - the idea that they are positive things seems absurd to me.
jplesset - I totally agree with your point of view.
I have tried to actually measure sound radiation from an enclosure, it is almost impossible. It is so low in level and gets swamped by the direct radiation of the driver. I don't give much credence to "box sound" once the enclosure is designed to control the first few resonances. But clearly I don't want any of them - the idea that they are positive things seems absurd to me.
jplesset - I totally agree with your point of view.
If you put the driver in the side the direct sound at the listening position will be zero from this mode (the side of a dipole) leaving only the indirect sound. This is not the most common driver configuration but it demonstrates the need to address the modes of the specific configuration and to be wary about generalising until the modal density becomes sufficient.
The way I have seen it done is by mapping the outside of the speaker enclosure with a laser vibrometer and then using the normal velocities as boundary conditions for a BEM-type calculation. Not sure how it could be done with microphones. An acoustic camera will not register sources 30dB below a louder source.
Well I guess you are not in the target market sector for very expensive speakers made of solid wood designed to resonate like the body of a musical instrument.
Hi Andy
Yes, you can do what you suggest. We did this many decades ago for engines - they have the same problems with complex vibrations and sound radiation.
Yea, I am not in the market for a highly colored and yet very expensive loudspeaker - even if it does "look great!"
Yes, you can do what you suggest. We did this many decades ago for engines - they have the same problems with complex vibrations and sound radiation.
Yea, I am not in the market for a highly colored and yet very expensive loudspeaker - even if it does "look great!"
Although I likely know even less than you guys about this topic, I would like to debate a recent point: "Excessive use of damping deadens or kills the sound." Leaving aside the ultimate subjective nature of the sound ("Do I like it?"), isn't the ideal of the standard loudspeaker to radiate sound to the front, ideally reproducing correctly the frequencies at the correct phase and amplitude per the driving signal?
Not that this disproves the above assertion. In fact, it may be a valid (general) statement if by "excessive damping" it is meant that so much damping is used that the driver does not correctly reproduce the driving signal?
Not that this disproves the above assertion. In fact, it may be a valid (general) statement if by "excessive damping" it is meant that so much damping is used that the driver does not correctly reproduce the driving signal?
Nothing to debate 😉, sufficiently damp the cab's resonances [or by-pass them by designing them to be outside the driver's intended BW], so we only hear the driver's output, yet not so much it over-damps it [introduces harmonic distortion] unless you want to quell excessive HF 'ringing' also.
GM
GM
Great point
Agreed
Okay, interesting, thanks.
Yes I guess that is another interesting point, although you hint that it might be rare, so not crucial to general conclusions. (Although I suppose your main point above is that there are no general conclusions -- although you seem to have made a general conclusion regarding light stiff cabinets for speakers, below.)
Which is why the argument in favour of mass arises by looking at F=ma. If one has chosen the drive units, then F can be treated as fixed, so more mass means less acceleration/motion, which, as you say above, *will* lower sound radiation.
"statistically"? I don't understand.
True if we assume unchanged stiffness and mass, but how often do we see damping changed without changing stiffness and mass? Which leads to... vvvv
I don't think that there is a clear connection. Often, the more massive material is more inherently self-damping, which is also not a rule but suggests your claim above is not useful. I do think that damping is less effective on stiffer objects.
The above statement is what moved me to type a reply. As I wrote above, damping is less effective on stiffer structures, so they will tend to have more motion.
Isn't the force fixed? (as I explained above)
If all else is the same, yes. But that's the issue: our practical choices seem to change other key factions like average motion as well as damping, so it is complex.
Stiffer yes, well damped yes, but not lighter: see F=ma above. Surely your statement should be "Heavier, stiffer and well damped materials seem to work best in lowering the net average motion".
And that is why there is a debate: our practical options always seem to trade away one of the three desirable traits.
Agree. And since this is a DIY forum, we lust for generalities that we can apply to our projects, but the subject is too complex.
I think it would helpful if you wrote the equation more fully:
F = m.a + c.u + k.d
where F is the force from the driver, c is the damping coefficient, u the velocity, k the stiffness coefficient and d the displacement. The velocity is the time derivative of the displacement and acceleration the second time derivative of the displacement. What does that tell you about how the relative size of the terms must vary with frequency while balancing the applied forces?
Since this is your major concern let me address it. We agree that stiffer and higher damping will lower the "statistical average motion" (I think we agree on this.) But you contend that more mass will decrease the motion based on a simple static Newton's equation. Except that this is not a statics situation it is dynamic and it is the resonances that are the issue since at each resonance the displacement goes up as high as the damping will allow. Hence higher damping lowers the "net displacement". But here is the problem: the more resonances there are in a given bandwidth the higher the net displacement for a given force. Increasing the stiffness lowers the modal density while increasing the mass raises the modal density. This means that the net statistical displacement will increase with mass and decrease with stiffness for a given amount of damping. Maximize the damping and stiffness and you will minimize the net displacement.
Andy - I think that you see the issue as well. It is not a simple statics problem. It is actually a classic problem in noise control - to wit a lighter stiffer car has less body vibration - mass is not the answer (although in the old days they did think this way and built the massive tanks that are now classics. Cars today are much quieter and much lighter, stiffer and well damped.)
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In the BL, does the L (more than one turn) or elswhere the size of the coil (ratio length/diameter) is important to minimize that ?
Or if we putt a higher B factor in relation to the higher Mms (so most of the time for given material and size = more cone steefness) it's just good enough to minimiz uncontrolled displacement (in particular when the cone comes back to be ready for the next "close signal" ?
To say it on a different way : could we have one of the factors above which is more dangerous to match the signals (e.g. a signal out of phase because the cones doesn't start from where it should start due to the electrical damping, loss, whatever ... between two close signals.
Should we have more data when buying a driver about the length of the coil in relation to the L of the wire alone (not just the BL factor), especially when microdynamic occurs, i.e. when climbing in the FR ! or Xmax would give enough data !
Why are metalic cones giving more détails at equal steefness in relation to the same ratio BL/Mms ?
Sorry if mixing the "concepts"
Or if we putt a higher B factor in relation to the higher Mms (so most of the time for given material and size = more cone steefness) it's just good enough to minimiz uncontrolled displacement (in particular when the cone comes back to be ready for the next "close signal" ?
To say it on a different way : could we have one of the factors above which is more dangerous to match the signals (e.g. a signal out of phase because the cones doesn't start from where it should start due to the electrical damping, loss, whatever ... between two close signals.
Should we have more data when buying a driver about the length of the coil in relation to the L of the wire alone (not just the BL factor), especially when microdynamic occurs, i.e. when climbing in the FR ! or Xmax would give enough data !
Why are metalic cones giving more détails at equal steefness in relation to the same ratio BL/Mms ?
Sorry if mixing the "concepts"
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"L" is the total length of the wire seen by the "B". The speaker doesn't know "B" from "L", only the combination of the two, so it doesn't really matter which is which or to optimize one or the other.
There are many things that one needs to know about a driver to determine its capabilities, but mostly, to me, it is how it handles the upper limits of its pass-band. All drivers have an upper limit, but not all of them do this gracefully. This is especially important if this upper limit is within the region of 700-7000 Hz. This is because our hearing is so acute in that region.
The coil length is important for MaxSPL, but if one is operating the speakers well below this excursion limit (as I always do) then this factor is basically irrelevant.
There are other parameters such as Q, Vas, F0, etc. that get thrown around all the time, but for me these are also irrelevant (in a woofer that is.)
There are many things that one needs to know about a driver to determine its capabilities, but mostly, to me, it is how it handles the upper limits of its pass-band. All drivers have an upper limit, but not all of them do this gracefully. This is especially important if this upper limit is within the region of 700-7000 Hz. This is because our hearing is so acute in that region.
The coil length is important for MaxSPL, but if one is operating the speakers well below this excursion limit (as I always do) then this factor is basically irrelevant.
There are other parameters such as Q, Vas, F0, etc. that get thrown around all the time, but for me these are also irrelevant (in a woofer that is.)
Thanks again. The bits in bold I need a little more help to understand:-
- I would really like to see some experimental evidence (or know that it exists) that general music playback is exciting the modes for long enough for the excitation to become damping-limited. The quote in bold suggests it is true for *every* resonance.
- I don't quite grasp the term modal density (is it the number of modes per bandwidth?), and if so, I need more help to understand how it goes up when mass goes up. I cannot see that intuitively.
- Plus minimize weight, IIUC?
Once I grasp and accept this finding, to me it is rather exciting, because the three key attributes are not so much in conflict. The Gedlee-type cabinet can deliver stiff, damped and light, whereas stiff, damped and heavy tend to be more in conflict IMHO.
cheers
Earl, this statement fascinates me. You don't use Thiele/Small parameters for woofer alignment?
Of course every musical note will not excite every resonance to steady state if the damping is extremely low, but that point isn't critical. As I said before, at a resonance the sound level will go up for every note no matter how short, even if it does not reach steady state, it still goes up. Is it damping limited? It doesn't really matter.
Modal density goes up as freq^2, ie. every octave has four times as many modes as the one below. Increased mass lowers resonance frequencies - all of them. So since the density increases with frequency, as the frequencies of them drop, the density in any given band will increase. This is exactly analogous to the modal density in rooms. As the room gets larger the density in any given band goes up.
Put a driver in a closed box, EQ it, what difference does it make what its Q, f0, or Vas is?
Earl, you can't be serious. You must be aware of the elegant work of Linkwitz and the spreadsheet this resulted in to calculate the transform that will deliver the required equalization. You need the TS parameters of the driver, as well as data on the enclosure, to perform the necessary calculations. Go into the theory a bit more (which you will have to reverse engineer because Linkwitz is sketchy), and you will understand why you need to know especially Q, Fs and Vas.
Or, you can just cobble up something, anything, stick a microphone in front of it and ride those slide pots in REW or MiniDSP till it sort of looks all right. But I am convinced that is not the way you work.
Or, you can just cobble up something, anything, stick a microphone in front of it and ride those slide pots in REW or MiniDSP till it sort of looks all right. But I am convinced that is not the way you work.