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Old 2nd June 2003, 10:31 AM   #21
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Does anybody have the URL for the original homepage for the second order gradient stuff?

Thanks,

Eric
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Old 3rd June 2003, 05:59 PM   #22
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Hi Bill,

Sorry for the late reply. It sounds to me like you've got the idea pretty much nailed.


Quote:
Originally posted by Bill F.
Hi John,

Thanks for starting this thread.

I think I'm beginning to understand this concept.

Let me run the following idea by you for a 2OG to cover 50-300Hz:

I'm picturing 4 15" woofers per channel (2 front, 2 back) in twin vertical frames (~15"x34&quot separated by 15".
If you want meaningful output down to 50Hz, it would be better to mount the drivers on 15" by 30" boards and lay them horizontally-one driver beside the other. The two drivers together will then act acoustically just like one 30" driver. The spacing between the front and back dipoles should then be 30". I know aesthetically it's not as nice as the vertical arrangement, but you'll get an extra octave of output this way. At 50 Hz, the phase cancellation will be about 6dB--so you'll get about the same on-axis output as one 15" sealed woofer. Conversely, with the vertical arrangement, you can get about the same output as a single 15" at 100Hz.

[QUOTE]
Instead of perfed sheet, how about stuffing between the drivers?


That's not going to work so well. The idea is to form a RC low pass filter on the back side of the diaphram only. The perforated sheet is the acoustic resistance and the air chamber between the back side of the diaphragm and the perfed sheet is the acoustic capacitance. The advantage of the perfed sheet is that it is two dimensional. This means that regardless of which direction the partical velocity takes, it will always face the same acoustic resistance as the wave front moves through the perfed sheet. If you use a bulk (3 dimensional) resister different parts of the wave front will see different acoustic resistances--in particular, the region near the edges of the driver will see very little resistance, whereas the region neer the cente of the driver will see a high resistance. The net result is the low resistance at the edges of the driver will be pretty much a short-circuit and you won't have a low-pass filter. An alternative is to use cloth or screen door material. Just put on a bunch of layers untill you get the desired amount of roll-off at the top end. This is easy to check by doing a frequency response measurement on axis and 180 degrees off axis. It should be easy to identify the knee of your acoustic low-pass filter by comparing the two--it's how I did it.

Quote:
For the EQ, I'm picturing a Behringer 8024 per channel feeding the two signals to the front and back halves, the back delayed ~1ms. Each channel out of the 8024 would go into a dual integrator with the upper knee setting the preferred lowpass point. Each integrator would then feed an amp powering its respective half. If a phase reinforcement peak is in the passband, you could use the digital PEQ on the 8024 to notch it out.

Does this setup make sense? Using 4 15s per channel, could I get decent SPL down to 50 Hz?

Sounds good. You'll want to add an additional 1st order low pass on the rear dipole with the knee around point where the delay equals a half cycle. You'll have to play around to optimize that parameter. If you do the acoustic low pass filtering correctly, you shouldn't have any phase reinforcement/cancellation peaks/valleys at all. It should be nice and clean to well above your 300Hz crossover point (assuming your 15" drivers are up to the task).

I was using pretty much the same setup except I was using four 10" drivers. I absolutely loved it, but now that I'm part way through building a 7 channel system (eventually hoping to evolve into an 11 channel system), it was just too much. I always said I was aiming for the ultimate in my system: cost be damned! But let me tell you, when you start building 7 channels of ultimate quality, the dollars start to add up. Dipole woofers will have to do for now. But don't let me talk you out of it. It really takes your room out of the equation like nothing else and for stereo the cost isn't bad.

Have fun experimenting, John
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Old 3rd June 2003, 06:34 PM   #23
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Quote:
Originally posted by capslock
Does anybody have the URL for the original homepage for the second order gradient stuff?

Thanks,

Eric
I don't know if you're talking about my web page, which Bill referred to or not. My "web page" was just a tutorial on doing finite element modeling of acoustical problems with an example for a second order gradient (run well above the point where it is supposed to be run.) The address is:

http://mypage.bluewin.ch/audio_experiments/FEM.htm

I don't know how long this will be available as I changed service providers about 3 months ago and was surprised to find I could still access it.

John
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Old 3rd June 2003, 06:38 PM   #24
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Quote:
Originally posted by Elias
Hello!

There have been at least one second-order gradient construction that I'm avare of. It was published in "Hifi", a hifi magazine here in Finland in the early 90's and also in the book called "Rakenna Hifikaiuttimet" by Pekka Tuomela, 2nd corrected print, Tecnopress 1993, ISBN:951-832-034-9.

-Elias

I'd really love to see a copy of that article if anyone out there has access to that book. It's nice to know there are other people out there similarly disturbed.

John
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Old 3rd June 2003, 07:01 PM   #25
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Quote:
Originally posted by kelticwizard
March 1973. I can scan the whole article and Email it to anyone interested.
Can you please email me the article to:

ron.steinberg@sympatico.ca

I have a very large email box so it will all fit. I can also take a stab at turning it into a pdf, as I have Acrobat writer.

Thanks,
Ron
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Old 3rd June 2003, 07:15 PM   #26
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Quote:
Originally posted by capslock
John, could you provide a link to your homepage?

See my prior post for the address

Quote:
On a side note:

Wvier (http://www.wvier.de/download.htm) propagate Unipoles. Some of the articles are even in English. A Unipole is a dipole and a monopole used on top of each other. They say this is the best way to get an efficient excitation of all modes.

In my eyes, they - being PA driven - are striving for an efficient and uniform excitation of modes.

For the most natural sound reproduction (including temporal decay characteristics!), I think we should be striving for an efficient non-excitation of modes.

Regards,

Eric

Thanks for the reference, I'll take a look. After living in Zurich for 5 years, my German should really be much better than it is.

I do know that concert hall designers go to great lengths to get maximum excitement of lateral room modes. It is those lateral modes that provide the sense of envelopment. The medial modes at bass frequencies are thought to be counterproductive: they decrease intelligibility without adding to envelopment.

However, all that is for large concert halls, not for living rooms. SInce you mention their PA focus, I presume they are also dealing with large rooms. In large rooms, the Schroeder frequency is very low. For those who don't know...the Schroeder frequency is a somewhat arbitrarily defined frequency above which modes are tightly spaced and you get "true" reverberation from a room, ie no individual modes result in coloration. Below this frequency the modes are spaced far apart and individual resonances cause coloration problems. The Schroeder frequency for a particular room depends on the size of the room: the bigger the room, the higher the Schroeder frequency.

In a typical living room the Schroeder frequency is much higher, usually around 200-300Hz. Furthermore, the reverberation time of a small room is much shorter than that of a large room. It is the reverb beyond around 400ms that results in envelopment in the bass frequency*. The reverb in a typical living room will have decayed by maybe 40dB by the time you get to 400ms. Exciting room modes in a small room will just get you coloration without any sense of envelopment--you just get that boomy bass in your head feeling. In short, a small room will always sound like a small room: bad.

I agree with you, if you want big room sound in a small room, it's a much better strategy to do as much as you can to take the room out of the equation (by for instance using second order gradients or dipoles) and maximize the effectiveness of the reverberation in the recording by adding some surround speakers for early reflections and late reverb at the sides of the room. Believe me surround sound matrices are not a sound effects gimic. If you want the ultimate, you need more than two channels. I can't wait until I get my 5 to 11 channel matrix going...

John

*reference: David Griesinger
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Old 3rd June 2003, 09:09 PM   #27
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Default Re: Second Gradient Dipoles

Quote:
Originally posted by mefistofelez
John,

Also, what drivers did you settle on?

Thank you,

M

I was using four Scanspeak 8525-00's as described in an earlier post. I'm still using the Scanspeaks, but as dipoles now.

John
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Old 4th June 2003, 11:31 AM   #28
Elias is offline Elias  Finland
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Written by hancock:
Quote:
I agree with you, if you want big room sound in a small room, it's a much better strategy to do as much as you can to take the room out of the equation (by for instance using second order gradients or dipoles) and maximize the effectiveness of the reverberation in the recording by adding some surround speakers for early reflections and late reverb at the sides of the room. Believe me surround sound matrices are not a sound effects gimic. If you want the ultimate, you need more than two channels. I can't wait until I get my 5 to 11 channel matrix going...
This is very interesting. Can you tell more about the decoding matrix you are using for 11 channels. Is it a commercial device or your own design? How are the speakers located in the room? Are you using some Dolby method to get the initial 5 channels?

-Elias
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Old 4th June 2003, 11:51 AM   #29
Elias is offline Elias  Finland
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hancock:

It is great to hear that you experience on 2nd order gradients has been positive, also in practice.

Can you provide any pictures of your speakers? I didn't found link from this thread. I'm sure that would help a lot when figuring out the details. Especially "The perforated covering on the back of the driver" would need some more info, at least to me, how it is actually located? How big should this covering panel be, should it exceed beyond the borders of the element size?

Any other info about the construction is more than welcome!

-Elias
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Old 28th October 2009, 12:46 AM   #30
jamikl is offline jamikl  Australia
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Default Second Order Gradients

Did the PDFs of this article ever become available?
jamikl
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