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-   -   Keystone Sub Using 18, 15, & 12 Inch Speakers (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers.html)

NEO Dan 13th April 2011 04:47 PM

Hi Art,
Is the 300" basically what the "normal mouth" would be at the end of the path, or did you choose to go smaller?

applehorn 13th April 2011 05:06 PM

Art, Did you try a Karlson type opening. Iain.

weltersys 13th April 2011 06:34 PM

Quote:

Originally Posted by NEO Dan (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers-post2537296.html#post2537296)
Hi Art,
Is the 300" basically what the "normal mouth" would be at the end of the path, or did you choose to go smaller?

IIRC, the end of the horn path is 312 square inches.
I experimented with much larger and smaller mouths.

weltersys 13th April 2011 06:49 PM

1 Attachment(s)
Quote:

Originally Posted by applehorn (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers-post2537306.html#post2537306)
Art, Did you try a Karlson type opening. Iain.

Iain,

Test P is similar in shape to a Karlson.
The narrow top and wider bottom resulted in less LF and a bit more peaky upper response than the Keystone.

Art

Josh Ricci 13th April 2011 07:25 PM

Good stuff Art.

I am surprised at how much of a difference the shape made in some of the tests. Obviously some of it is changing the driver relationship in distance to the mouth and the mouth area can also have a profound affect but still...Filling in by as much as 7db in some areas and at the knee some major differences in extension and hence the low frequency output changed by as much as 8db below 35hz.

Why do you think the keystone pattern was the best compromise? I mean how is the shape / location affecting the performance in such a positive way as compared with a regular rectangular or square exit which is more common? Did you try a rectangular mouth of the same area which also started in the same spot in the path and ended in the same spot without the expansion?

weltersys 13th April 2011 08:33 PM

3 Attachment(s)
Quote:

Originally Posted by Josh Ricci (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers-post2537462.html#post2537462)
Good stuff Art.

I am surprised at how much of a difference the shape made in some of the tests. Obviously some of it is changing the driver relationship in distance to the mouth and the mouth area can also have a profound affect but still...Filling in by as much as 7db in some areas and at the knee some major differences in extension and hence the low frequency output changed by as much as 8db below 35hz.

Why do you think the keystone pattern was the best compromise? I mean how is the shape / location affecting the performance in such a positive way as compared with a regular rectangular or square exit which is more common? Did you try a rectangular mouth of the same area which also started in the same spot in the path and ended in the same spot without the expansion?

Changing the mouth on a TH is like changing the port on a BR cabinet.
As you know, changing the shape of a port also can affect tuning quite a bit.
The comparison BR cabinet used a "top hat" kind of port, the addition of the "top hat" made the port tune way lower than I would have thought, same as what happened with your "25 Fb should have been 30" box.

I did try a rectangular mouth of the same area which started in the same spot in the path and ended in the same spot without the expansion, test "R".

The Keystone shape seems to be the best compromise for this particular cabinet, I don't know how to explain it other than the way Goldilocks did her testing- too hot, too cold, to hard, too soft, just right.

I'm sure if I had extensively studied laminar flow patterns and such I could give a more technical explanation.

I found the sound output of the Keystone can throw up large rooster tails of sawdust in the shop.
It would be interesting to use a strobe and smoke and watch the air movement patterns around the exit.

Although LF sound behaves in a cabinet more like laminar flow than ray tracing (light waves), if one visualizes the last portion of the horn, a "keystone" correction needs to be applied for the horn exit to appear rectangular from inside the horn. A regular rectangle or square is actually "too big" at the top of the exit. The overhead photos show this effect, when the keystone shaped exit appears more square, the cabinet itself looks keystone shaped.

It is possible another TH with different dimensions (but similar fold pattern) may work better with a slot or "A" shape, but I am fairly sure none would benefit from a "V" shape, as tests "E", and "N" resulted in worse response curves than a standard opening of the same area.
Those results support my "keystone correction" analogy.

Test "F" was poor too, it seems the sound waves are disturbed with side openings rather than center openings.

Mouth shape, size and positioning all have interrelated functions, it took a lot of tests to work them out.

Art

Josh Ricci 14th April 2011 01:47 AM

Fair enough... I like the way that you planned to have a removable panel at the mouth so that you could experiment with different configurations. That is good thinking and planning ahead.

weltersys 14th April 2011 03:29 PM

2 Attachment(s)
Quote:

Originally Posted by Josh Ricci (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers-post2537784.html#post2537784)
Fair enough... I like the way that you planned to have a removable panel at the mouth so that you could experiment with different configurations. That is good thinking and planning ahead.

The entire front of the cabinet was left off for most of the testing, as I was also experimenting with corner ramps too.

I'm glad to be done with the Keystone, lugging out the cabinet, all those clamps and bits of wood started to get old after a few days. Or weeks.

Art Welter

tb46 14th April 2011 03:44 PM

tapped horn / bandpass ?
 
Hi weltersys,

This is all very interesting. It looks like your keystone mouth converts the driver-to-mouth horn section into a tuned chamber, as you would find it in a bandpass. Did you take any measurements w/ the mouth at very end of the horn (i.e.: the bottom of the enclosure)?

Regards,

Djim 14th April 2011 03:55 PM

Quote:

Originally Posted by tb46 (https://www.diyaudio.com/forums/subwoofers/185588-keystone-sub-using-18-15-12-inch-speakers-post2538323.html#post2538323)
Hi weltersys,

This is all very interesting. It looks like your keystone mouth converts the driver-to-mouth horn section into a tuned chamber, as you would find it in a bandpass. Did you take any measurements w/ the mouth at very end of the horn (i.e.: the bottom of the enclosure)?

Regards,

Post #26 and Post#29 but maybe if enough people ask then Art might....perhaps...


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