Cab vibration - speaker or air load?

[Keriwena]

This was mentioned a while back in a thread I can't find.

Does the speaker itself contribute more to vibrations in cabinet panels through mechanical conduction, or is it the sound in the cabinet itself?

Off the top of my head, it would seem that speakers are so inefficient in moving air, air moving cab walls would suffer the same losses. It also strikes me that the sound is going to be on both sides of the cab at once, possibly cancelling out.

But the top of my head isn't flat, so sometimes my thoughts are slightly askew.

 

[Cal Weldon]

I think you'll find that due to the driver suspension effectively decoupling the moving parts of the motor from the basket that it is the air that is moved by the cone that is creating the pressure in the box which in turn creates the vibration.

Further, when the box flexes outward or inward on all sides at one same time it does not cancel, it sums.

I think.

 

[Keriwena]

(the server saved my post due to nightly maintenance, but seems to have lost some.)

Thanks, Cal.

I think you'll find that due to the driver suspension effectively decoupling the moving parts of the motor from the basket that it is the air that is moved by the cone that is creating the pressure in the box which in turn creates the vibration.

I think the idea is there's an "equal and opposite reaction" in the parts of the motor that don't move. Dickason mentions the whole box moving (a tiny bit) and Linkwitz goes on at length about decoupling the driver from the baffle, but he doesn't offer anything quantitative.

Further, when the box flexes outward or inward on all sides at one same time it does not cancel, it sums.

Yeh, I got confuzed on that one. I was thinking of equal pressure inside and outside the box (or baffle) being applied to both sides of a panel. Obviously, they'll be opposite polarity and sum. My bad.

 

[jcarr]

AFAIR, mechanical conduction accounts for 60~70% of speaker vibrations, and acoustic pressure accounts for the remaining 30~40%. But my memory could be faulty (smile).

 

[chris661]

I think you'll find that due to the driver suspension effectively decoupling the moving parts of the motor from the basket that it is the air that is moved by the cone that is creating the pressure in the box which in turn creates the vibration.

Further, when the box flexes outward or inward on all sides at one same time it does not cancel, it sums.

I think.

Yeah that'll contribute, but...

If you pick up something heavy (like a crate of beer), and wave it around, your body moves too, because F=ma. You're heavier than the beer, so you won't move as much.
To take that back to the speaker, if you're waving a mass around with a mechanical system, the other end of which is connected to the cabinet, the cabinet has to move around too. A heavier cone would give more cabinet vibration than a lighter cone.
To top it off, the baffle of an OB speaker vibrates too, so it's hard to imagine it's just the air pressure causing cabinet vibrations.

Chris

 

[Cal Weldon]

I believe you are correct. I was comparing the moving mass to the fixed mass and when Keriwena was asking which contributed more to panel vibration, I believe the cone movement is the real factor. I would also suggest that the mechanical movement of the motor and basket would be restricted primarily to the baffle unless there is a brace(s) on the motor which transfers the vibration to other panels.

 

[benb]

I'm strongly suspecting that how much cabinet vibration comes from which effect would be frequency dependent, in several ways. One is that each side has its own resonant frequency and is most easily excited at that frequency. Here's another way it would be frequency dependent:

Yeah that'll contribute, but...

If you pick up something heavy (like a crate of beer), and wave it around, your body moves too, because F=ma. You're heavier than the beer, so you won't move as much.
To take that back to the speaker, if you're waving a mass around with a mechanical system, the other end of which is connected to the cabinet, the cabinet has to move around too. A heavier cone would give more cabinet vibration than a lighter cone.
To top it off, the baffle of an OB speaker vibrates too, so it's hard to imagine it's just the air pressure causing cabinet vibrations.

Chris

Below speaker and cabinet resonance, and below the "cutoff" frequency of an open-back cabinet, the mass of the voice coil and cone would certainly be the main mass that would cause the equal-and-opposite reaction of the magnet, basket and thus the baffle. This mass is quite low, but at higher (more audible than below-cutoff) frequencies the cone also sees the air as a substantial mass added to it, and the magnet/basket would have a greater force moving it. I suspect that at "operating" frequency a cone acts as if it has SUBSTANTIALLY more mass than at a frequency below cutoff.

Then again, The pressure variation in the air (especially the differential between the inside and outside of the cabinet) also becomes greater as the frequency goes through and above the "lower cutoff" frequency of the speaker-cabinet combination (such as the Helmholtz frequency in a ported cabinet), so the pressure from the acoustic signal from the cone onto the cabinet walls also becomes greater.

It's surely possible to simulate a driver and cabinet to determine how much contribution each effect has at different frequencies, but it's beyond my ability, and surely many others here. An experimental method would be attaching sensors on all sides and test vibration amplitude in free air vs. in a vacuum, but still that would only measure the front-panel vibrations through the magnet and basket caused by a cone "unloaded" by air.

 

[planet10]

I think you'll find that due to the driver suspension effectively decoupling the moving parts of the motor from the basket that it is the air that is moved by the cone that is creating the pressure in the box which in turn creates the vibration.

Further, when the box flexes outward or inward on all sides at one same time it does not cancel, it sums.

I think.

Really only a consideration for woofers. Even so, the majority of the energy feed to the box comes from the mechanical connection to the box. This is pretty easy to illustrate.

The energy (momentum) produced by the cone is equal to that imparted into the frame. Half of that produced by the cone is transmitted directly to the room (and we listen to it). All of the energy imparted onto the frame is directly fed into the cabinet. Further the cone energy that goes into the box is decreased by damping and the large impedance mismatch between the cone & the air.

Decoupling a driver from the baffle means that all the energy from the basket stays in the driver (reducing DDR) or whatever supports it. Take a look at the fujitsuTEN enclosures to see how to do it "right".

As to the large mass of the box making transfer of momentum into the box not important, one has to keep in mind that the box panels can have considerably more radiating area than the cone and further act as a filter only radiating at frequencies where there is a resonance (and when the panel starts resonating it effectively becomes transparent.

Back to woofers: by taking advantage of push-push driver pairs one can actively cancel most of the energy mechanically imparted, leaving only (mostly) the air tranmitted LF pressure pulses to feed energy into the box. It is pretty easy to make a box stiff enuff with resonant modes high enuff that this sshould not be an issue (for one use only light stiff box materials)

dave