Drivers coupled with the baffle or not?

Janne

While cruising the internet to get inspiration for another tweak I came across this guy Albert who is making the Von Schweikert speakers. He was discussing the design of low resonance speaker cabinets. http://www.audiocircle.com/index.php?topic=70291.0

"VIBRATION CONTROL THROUGH ISOLATION
It is known that the cabinet walls are set into resonance by two different means: A) the physical vibration of the driver, transmitted to the front baffle, in addition to: the acoustic energy transfer from the rear of the driver into the cabinet cavity. In order to reduce this unwanted transfer of energy, we have devised a method to isolate the driver from the front baffle, in addition to isolating the front baffle from the cabinet proper in the UniField Series of speakers.

A) In order to effectively isolate the vibration of the driver from the cabinet, we have employed the use of a 6mm thick, visco-elastic polymer clay gasket material, originally designed to damp the hull of nuclear submarines. The damping property of this proprietary gasket is extraordinary and serves to nearly eliminate the transfer of energy from the driver frame to the front baffle. In essence, our drivers are floating on the surface of the baffle and hence cannot transfer vibration into the rest of the enclosure. This technique alone drops the resonances down by more than 12dB compared to conventional driver mounting methods used by our competitors. This is a tremendous reduction of distortion!

The drive units radiate energy acoustically into the interior of the cabinet, which can result in a highly audible cavity resonance. These reflections from the internal walls of the cabinet create a pressure wave on the rear of the cone, resulting in dips and peaks in the frequency response related to the wave lengths involved with the dimensions of the internal cavity. Our use of the hard felt inner damping layers, combined with our proprietary Gradient Density Damping,TM, reduces the internal cavity resonances below audibility. By contrast, the examination of our competitor's cabinets will reveal that they use hollow cavity design to "enhance" resonances to achieve their desired voicing."

When people are making the Moretite tweak it seems important to connect the driver to the baffle. In his theories it seems important to disconnect the driver from the baffle to reduce resonance transfer. So he is using a 6 mm gasket.

Is this something that is specific for the Polk design? Somehow he seems to make sense.

Bobsama

If I'm not mistaken, the Mortite mods are focused around acoustically deadening the effect of the driver cages against the cabinet while also providing an improved seal for the passive radiator. Remember; passive radiators are a bit different than bass reflex designs, mainly in the fact that internal noise is nearly completely isolated. Especially compared to the old foam gaskets, the additional material is preventing driver cage vibrations from passing as effectively. I doubt we'd get the same 12dB difference as this guy, but we've also got different speaker designs than him.

geoff727

Refer to pages 389 and 398 in Martin Colloms' High Performance Loudspeakers.

Also, there is information on S. Linkwitz's website reagrding the merits of de-coupled driver mounting. I believe it's somewehere in this section, but I'm not sure. http://www.linkwitzlab.com/frontiers.htm
If you don't find it there, just go exploring around the website.

Janne

geoff727 wrote: »

Refer to pages 389 and 398 in Martin Colloms' High Performance Loudspeakers.

Also, there is information on S. Linkwitz's website reagrding the merits of de-coupled driver mounting. I believe it's somewehere in this section, but I'm not sure. http://www.linkwitzlab.com/frontiers.htm
If you don't find it there, just go exploring around the website.

I found it. Decoupling seems to be the way to go. Now need to figure out a way to do it with the 1.2TL

geoff727

Janne wrote: »

I found it. Decoupling seems to be the way to go. Now need to figure out a way to do it with the 1.2TL

Keep in mind that you are talking about mid-bass drivers with a significant amount of excursion and inherent vibration. There are quite a few designers who advocate the de-coupled approach of baffle mounting, but only in certain applications, such as the midrange and tweeter, where excursions are low, and where a de-coupling from the baffle may be simply to protect the sonic integrity of those 2 drivers through isolation of their magnet structures from unwanted vibrations from the bass drivers. For examply, the Linkwitz Orion which I pointed you to, does have some inherent vibration of the open baffle top structure from those lower woofers. Protecting the critical midrange's basket from this vibration is certainly desirable.

In many cases, it has been found that a very rigid coupling of the woofer (a high-excursion/high vibration driver) to the baffle results in the required bass tightness and definition, and a de-coupling in this case can sometimes lead to subjectively wooly bass, as the basket frame moves slightly in response to the large cone excursions. This is, of course, dependent on the 'deadness' of the rest of the enclosure, and especially the front baffle. This is why you see some prominent designers using things like thick, very rigid aluminum (or other strong, inert material) for the front baffle and a very rigid coupling of driver-to-baffle.

How much vibration isolation from the baffle is provided by a one-bead Mortite seal? Who knows. Get out the MLSSA and the accelerometer and test it. Does the basket actually move more with the less rigid coupling to the baffle with one bead of Mortite? Who knows. Get out the MLSSA and accelerometer and test it. Does the impulse response change? Who knows. Test it.

One thing to be careful of with the Mortite is that **** down the driver too fast results in distortion of the basket rim shape. This has to be a very slow process if that rim shape is to be kept perfect.

Kep researching before you make any permanent changes.
Good luck.