How does a odd expansion rate affect a quarter wave pipe? - diyAudio
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Old 7th February 2013, 07:38 PM   #1
DrBoar is offline DrBoar  Sweden
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Default How does a odd expansion rate affect a quarter wave pipe?

I am planning to build the Decca Corner horn. Really a Voigt pipe not a horn

My main problem is that the expansion is all over the place especially the constriction around the driver with a negative taper.

My second one is that the closed end of the pipe is 150% of the cone area. The other ones I have seen have all been in the 0-75% range.

By filling out the first section I could reduce the current
297-428-336 to something like 150-250-336. But then the first part is also the chamber for the driver so reducing the volume will increase the frequency and Q of the driver/box resonance.

Finaly that slot in carpet loading has to go, it will use a proper vent on the backside.

I am planning to either use the Philips 9710M or a Philips 10" full range driver.

The original design use 9mm plywood ( 3/8") and no damping what so ever...


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Old 7th February 2013, 07:46 PM   #2
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I'd review the design properly on my own first. Do some simulating before you build it. I'd recommend hornresp or TL design something that you can find on diyaudio
I'd just question how much though has been put in this design if it is supposed to be made out of 9mm plywood :s
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Old 7th February 2013, 08:15 PM   #3
xrk971 is offline xrk971  United States
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The Decca horn was probably designed long before there were simulation tools. I would expect that it is probably far from ideal if one were to compare it to a modern design that aided by simulations like HR or MJK. One thing that comes to mind in looking at it is to add angled surfaces internally to avoid any flat-flat surfaces that can act as resonators that may amplify undesirable modes. I see the top lid, the bottom panel (957) and the panel forming the closed end (297/0) as being potential flat-flats that should be angled. Certainly, thick damping could be applied but easier to angle first. With proper additional bracing internally, there is no reason why 3/8 in plywood can't work. I think 3/4 in thick plywood and MDF are often over-used and many times, not necessary when bracing can work fine and reduce weight of the cabinet. There is a semicircular cutout in the internal divider - perhaps that was to reduce the effect of the constriction due to the driver motor assembly?
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Old 8th February 2013, 06:02 AM   #4
DrBoar is offline DrBoar  Sweden
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The Decca Corner Horns is 70-60 years old, a time when the number of computers on this planet was in the single digit. The idea was that the flexing in the cabinet would absorb resonances, there is a separate DCH thread.

Spendor BC1 used 10 and 12mm plywood with 12mm bitumen loaded light fiber board lamination. The idea is to damp resonances and pull them down from the critical midrange. Harbeths current models is built along similar lines.

The cutout is due to the fact that the alnico magnets of the day sticks out more than current flat ceramic magnets. I would also add corner reflectors at both ends of the front, that is the pipe oposite of the driver.
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Old 8th February 2013, 12:05 PM   #5
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Quote:
Originally Posted by DrBoar View Post
I would also add corner reflectors at both ends of the front, that is the pipe oposite of the driver.
Why?

Bob
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Old 8th February 2013, 12:53 PM   #6
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the Kapellmeisters do the same in TL design, although many years later i believe the concensus is that its generally a bad idea.
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Old 9th February 2013, 12:28 PM   #7
DrBoar is offline DrBoar  Sweden
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BB
If I am lucky it reduces standning waves between parallell surfaces, otherwise it is just a corner brace or do you envision a scenario where it will do harm?

The front section is sloping gently with a 50% increase in area over 0.6m length then it flares 50% in way less than that perhaps 0.3 or less meters. I might try to increase the angle of the divider and have it going into the narrowing section of the box, but then there are some compund angles to tangle with.
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Old 9th February 2013, 06:24 PM   #8
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Originally Posted by DrBoar View Post
BB
If I am lucky it reduces standning waves between parallell surfaces, otherwise it is just a corner brace or do you envision a scenario where it will do harm?
Square corners tend to act as low-pass filters. Rounded corners can pass higher frequency sound to pass to the port. Since you will most likely have to stuff that particular area rather heavily, standing waves should be a non-issue.

Bob
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Old 9th February 2013, 09:18 PM   #9
sreten is online now sreten  United Kingdom
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Hi.

I had a single example of the decca speaker in my youth.
The details are somewhat different to the DIY version.
(More complicated, but a more consistent taper.)

Very driver specific. Nothing worked remotely as well
as the original 8" which I managed to fry with a tiny
Tripletone mono valve amplifier.

I really wouldn't build it on a wing and prayer hoping
it will suit your chosen driver, it very likely will not.

IMO there are much better and simpler build
BLH designs available for use in a corner, and
the basic driver matching can be simulated.

rgds, sreten.
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Old 9th February 2013, 11:21 PM   #10
DrBoar is offline DrBoar  Sweden
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OK, I skip the corner reflectors, low pass is good.
I do not have a rational reason to build the Decca but good irrational one: it is a hobby!

Philips 9710 have been used a lot for omnidirectional speakers, and I want to use this vintage classic driver for an other vintage design, the decca. The 9710 has a huge lift above 2 kHz so having it facing forward is not an option up or backwards is.

The only images of a comercial Deccas I have seen is Decca Corner Horn and they look similar to the kit.

I intend to build them along the findings in dämmung-und-versteifen and those BBC studies that was the foundation for the Spendor BC1. So the case will be lined with bitumen loaded softboard and then 10mm felt on top of that on the first half of the pipe

BB
Any hints on how to massload that Decca contraption?
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