Mechanical isolation of driver from cabinet

[LineArray]

Hi there,

while experimenting i fell over a method useful for mechanical
driver - cabinet isolation. This is not necessarily restricted to FR drivers.

The driver is not directly attached to the baffle anymore, there is
only some soft material as a gasket between the drivers basket
and the baffle.

The driver is magnet mounted using a spring (e.g. damped metal beam),
which is connected at one or both ends rigidly (optional with some damping)
attached to the cabinet. (even a spiral spring with a damper would be
possible or an air spring).

The resonant frequency fd of the driver and the suspension is adjusted
well below the usable range of the driver/cabinet combination.
(e.g. one octave below fs for a closed cabinet).

Above fd the excursion of the driver basket suspended in
its "Pendulum" is "mass inhibited". At fs things looks pretty much like
the driver was mounted conventionally ...

The absence of a rigid connection between driver chassis and cabinet
reduces vibration of the cabinet and thereby structure born sound
radiated unwanted from the cabinet walls, which typically
intoxicates the middle to upper bass region ...

I do not know by now if this is "new" entirely, since magnet mounting is
known practice. But in many cases structures supporting the magnet
are so stiff (and underdamped), that fd is within the usable range of
the speaker, which is counterproductive.

Defined "fd" and "Qd" using a well designed mass spring configuration
and a damping mechanism would IMO improve matters.

Opinions ?


Kind Regards

[kevinkr]

I'm completely out of my element here, but I had always understood it was desirable to tightly couple the driver to the cabinet in order to minimize the energy stored in the driver components.

A lot of current full range designs feature heavily braced cabinets and drivers that are tightly coupled to the cabinet via notched braces in tight contact with the magnet assembly. (Like the HCC I recently built)

How do you damp vibration in the basket, flange and other areas of the driver? Based on your comments I assume the driver support/suspension compliance is tuned to some frequency way below the driver/box resonance. Does the compliance of the suspension damp the driver structure in its operating range effectively?

I'm just curious - I don't have sufficient knowledge or experience to know whether this is advisable or not, and hence have no sense of the merit of your idea.

[MJL21193]

I did some experimentation on this idea HERE and I think the results were good.
I'll need to be a bit more thorough in my testing to determine if it is worth the effort.

Sigfried Linkwitz recommends isolating the driver frame from the baffle while rigidly mounting the driver magnet.

[planet10]

The FujiTEN Eclipse uses a sysyem where the driver is connected to a mass-loaded base (blue & black) and the shell is floated on that with no rigid contact. Something like the attached sketch.

http://www.eclipse-td.net/

dave

[pinobot]

I've also thought about this for quite a long time. Manufacturers are trying to decouple the drivers since forever. One of my first real hifi loudspeakers was a Van Medevoort CD 2.5 (around 1985), it had the drivers glued in place using silicon without any screws. Just recenty i saw a picture of some monitor that had a midtone compartment embedded in foam in a hollow space inside the loudspeaker box.
My own experience is that everything must be as rigid as possible, i find that the speaker sounds less muddy that way. Ofcource it's possible to decouple and retain detailed sound but at a certain point it's just no longer economical or plain silly. You could think of increasing the mass of the driver by adding weight to the magnet but this would put additional stress on the frame, to avoid this you can make a heavy metal ring to attach to the rim of the driver but having that custom made costs a fortune. Another way to dampen vibrations is to use a damper, like the ones on an archery bow, i think a car manufacturer used isomething similar to dampen the vibration of a car engine.
The thing is if you decouple the drive from the cabinet then the cabinet becomes lighter so you have to make the cabinet heavier otherwise the soundpressure will just push away the cabinet. Cost, material and total weight will spiral out of control.

[speaker dave]

I was an engineer at KEF in the 80s when we were having good success decoupling drivers from the cabinets.

The issue came up with the original KEF 105 (not the 105mKII) that was found to have a fairly nasty cabinet resonance in the middle hundreds. Cabinet stiffening moved the resonance up but never got rid of it so rubber mounting the driver was tried. We found that we could get a strong reduction in the amount of "drive" to the cabinet resonances via decoupling. It was easy to measure and you could hear the difference, especially on test impulses or impulsive sound.

The trick is to get the decoupling resonance low enough. Soft glues, silicon, etc. don't really do it. The 105 II used rubber motor mounts and soft foam tape. We even went to 3 mounting bolts to get the resonance a little lower. You had to be careful that mounting bolts didn't touch the cabinet and "short circuit" the effect. If I remember right the accelerometer measurements showed about a 20dB reduction in resonance levels with decoupling. Even with perfect decoupling their will be an acoustical drive to the cabinet, only the direct vibration coupling is reduced. The later 105 MkII used two woofers in opposition (plus decoupling) for even greater vibration reduction. I think people have even played with dummy motor structures with cone mass substitues that vibrate out of phase and cancel vibration (read about antivibration countershafts in 4 cylinder engines).

Audiophiles will tell you that dirvers must be rigidly mounted because this is a well ingrained part of accepted dogma. Our tests showed that this wasn't the case.

David S

[graaf]

Originally Posted by planet10

The FujiTEN Eclipse uses a sysyem where the driver is connected to a mass-loaded base (blue & black) and the shell is floated on that with no rigid contact. Something like the attached sketch.

speaker | ECLIPSE TD series speaker

dave

designed/patented by Yoshii Hiroyuki
decoupling seems to be technically much easier to achieve with upward firing speakers
TIMEDOMAIN �F�S‚̃I�[ƒfƒBƒI �`Ž©‘R‚ȉ¹‚̃Xƒs�[ƒJ�[
Speaker apparatus equipped with means for producing complicated waveform of low frequency with higher improved fidelity - Patent 6796401

[stevieg]

Naim and Mordaunt - Short (440?) both gave decoupled cabinets/speakers a lash. IIRC, the MS has a rigid skeleton to which the drive units were affixed, and the cabinet floated around this, sealed/supported with rubber gaskets. Roksan also decoupled tweeters. No doubt Paul Voigt/ Emil Berliner/ Gilbert Briggs/ your own name here suggested a similar system.

[graaf]

Originally Posted by stevieg

No doubt Paul Voigt/ Emil Berliner/ Gilbert Briggs/ your own name here suggested a similar system.

oh yes, because decoupling is good, it's just good engineering

on the other hand it raises some issues of DFM - additional costs and manufacturing complications

fortunately in DIY where we are free from DFM considerations decoupling is quite easy to achieve, and in case of upward firing speaker it becomes easy as pie

[LineArray]

Originally Posted by speaker dave

...
If I remember right the accelerometer measurements showed about a 20dB reduction in resonance levels with decoupling. Even with perfect decoupling their will be an acoustical drive to the cabinet, only the direct vibration coupling is reduced.
...

David S

First of all gentlemen i thank you very much for the detailed
input and the examples of constructions which had already
been realized.

@ David: If the reduction of cabinet vibration was that good,
one could ask how much effort would have been necessary in
improving cabinet damping to achieve the same effect without
decoupling ...

I am convinced, that it may be an effective and economically
reasonable method to implement high quality cabinets.

If you have got a set of components that work well in effectivity,
durability, manufacturing effort, ...

you can easily customize them to meet the parameters needed
for different speaker/cabinet situations.

Since in audiophilia there have been invented
(and sold) very much gadgets to compensate minor effects, i
wonder why this has not found a wider application ...

Cabinet vibration - to me - is NOT a minor effect. It is one of
the nastiest hurdles for real musical enjoyment.

A speaker meant to be "high end" which in an appropriate listening
environment can be convicted to exhibit disturbing cabinet
resonances is IMO disqualified.

edit: For shure, direct excitation from the drivers motor is
not the only source of unwanted cabinet vibration.
But in many cases that mechanism is underestimated IMO.


Kind Regards