VMPS Super Towers/R For Sale

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John Casler

Re: VMPS Super Towers/R For Sale
« Reply #60 on: 13 May 2014, 09:22 pm »
For those interested, the "putty" mass damping is to adjust the MASS of the Passive Radiator and its response to the internal cabinet pressures.

A greater mass slows response, and a lighter mass allows for a faster response.

If it is lighter then the response frequency will rise, if it is heavier the response is slightly delayed and the frequency is lower.

The pinching or adding is a process that "synchronizes" the PR with the active woofers so they respond at a lower frequency, but in a phase relationship that enhances the lower bass.  When the mass is "just right" that synchronization will produce the lowest distortion and the system (all the woofers and the PR) works much like a single driver.

If the mass is too light, the system will sound "thin" and not very deep or rich.  If the mass is too great then the bass will get flabby, wooly, sluggish, and rubbery.

When perfect the drivers are also in a synchronized phase relationship.

There you have the physics of Brian's PR system, and why it works to offer the cleanest, lowest distortion, harmonically accurate bass of most any multiple driver bass system.

James Romeyn

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Re: VMPS Super Towers/R For Sale
« Reply #61 on: 13 May 2014, 11:03 pm »
Comparing phase relationships perceived from any box speaker vs. phase/modal effects perceived from room modes, the latter are about 100-200x more audible.  When persons describe bass response in a room, their perception depends many times more on infinitely variable relationships secondary in nature to the box than it does on the box and its contents.  The reason is that bass wavelengths exceed the dimension of any two walls in a domestic sound room.  Bass waves must "bounce" between two walls before they are audible.  By definition, this means the walls make a permanent indelible "finger print" prior to anyone perceiving the bass note.  You can't undo this finger print no matter how much you spend on EQ, acoustic treatment, and no matter how many hours anyone spends fiddling with PR mass.  It's a permanent indelible artifact.  This is why I said the modal effect is much more highly perceived than any gobbledygook Brian or anyone else promoted.  Speaker tuning can work for or against any one modal effect, but it can't undo any of them.       

Brian talked a good game and built some good speakers.  The net effect of changing PR damping is identical to simply altering port volume.  This is not to say PR and port loading are identical, but rather saying the net effect of altering PR mass is the same as altering port volume, no more, no less.

Again: modal effects present over 100% THD in the bass range.  Considering this context, one might recommend caution regarding distortion claims for certain loudspeakers.  Persons have a hard time understanding that THD is the least of their problems regarding bass.  Modes synthesize their own bass notes directly unrelated in pitch to the original bass notes, and these notes resonate long after the original note stops.  Imagine two bass players, one playing with his bass mis-tuned 1/4 tone low or high, plus he plays out of time.  This is so much more audible than THD it can hardly be described. 

The problem is persons who have not listened to a room with modal effects solved don't have a reference.  They are not necessarily great authorities on the subject, having no experience with the issues described (modal effects) solved.