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Re: are there, other problems here

Posted by tomservo (M) on January 25, 2006 at 18:23:34

In Reply to: There are other problems here posted by EGeddes on January 25, 2006 at 07:04:55:

Hi Earl

I don’t have a way to post anything unless its on another forum.
If you model it, you will see what I describe however.
When you connect to a horn along its length, on finds down low that the operation is like a normal horn, set by its size, governed by area, local expansion rate and so on.
As Mark mentioned, the small volume forward of the entry point, in the horn stub, is an extra throat compliance. The mid and low drivers couple in through small ports in the horn wall, this port mass combined with the front volume of each driver forms a low pass filter in addition to the effect of the throat compliance / horn mass.

As the frequency climbs, the space between the entrance holes and acoustic closed “end” of the horn begins to produce a delay / phase shift.
When the frequency reaches the point where the phase or returning delayed signal is approaching 180 degrees out of phase from the source, there small rise and then a deep cancellation notch at 180. Above that frequency, the combination of throat compliance and repeating cancellation notches and the low pass filter formed by the entrance holes and front volumes has a net effect of being a significant acoustic low pass filter for each driver set. This extra low pass filtering significantly reduces the harmonic distortion compared to normal horn loading.
To make all this work, one must limit the upper frequency of each range with the crossover so you don’t have the notch in its response.
The front to back spacing (as much as I can) off sets the time delays associate with each driver.

When everything is right, it really looks like one horn, one driver in radiation pattern and nearly resistive acoustic phase, hence the ability to reproduce a square wave over more than a decade. A curious property for multi way speakers in the market we are in, or in hifi for that matter.
While one can argue “if” it is audible or not, at least preserving waveshape is not a “bad thing” from a theoretical transducer point of view.
The only thing wrong with the SH50 for the home is its big, ugly, heavy and WAY too much cost in drivers and cabinetry for home sound levels.
Too bad too because they sound beautiful for stereo to me.

Actually it was in thinking about the destructive reflection in these horns, that the Tapped horn idea came about. I wondered, umm, what if I substituted a second source that was already 180 degrees out of phase for the reflection. That second source might as well be the “other” side of the driver. Now, both sides of the driver couple to the horn body with a section of horn continuing on to the outside world.
This results in a horn driver that in effect has an Sd that changes with frequency.
If one makes the horn mouth small enough, this change in Sd can be made to largely offset the change in acoustic loading on the driver, as it traverses from its quarter wave resonance, to the region above (where the big dip is on a normal bass horn that is much too small).

At the low cutoff, the two side of the driver are approaching 90 degrees apart within the horn.
At the low cutoff, being lets say exactly 90 degrees apart, only one side of the driver, the one at the end “feels” the acoustic load and its motor strength, compliance and Sd are suited to drive the velocity minimum condition of the quarter wave resonance.
As the frequency climbs, the two sides of the driver begin to add constructively as the phase shift increases. When the phase reaches 180 degrees, both front and rear radiation’s add fully, the driver radiating area has in effect become much larger, more suited (with the same motor) to driving the reduced load above the velocity minimum.
The result is for a given ripple magnitude, one can make the horn mouth and so the horn package much smaller than a normal bass horn.
The smaller it is compared to “ideal”, the stronger it seems to be compared to a “normal” horn of the same size.
Anyway, we will have to see if the patent office thinks its novel.
I should run, homework to check.
Best,

Tom

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