Second Order Gradients

[capslock]

Does anybody have the URL for the original homepage for the second order gradient stuff?

Thanks,

Eric

 

[hancock]

Hi Bill,

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

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") 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.

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.

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

 

[hancock]

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

 

[hancock]

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

 

[transducer]

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

 

[hancock]

John, could you provide a link to your homepage?

See my prior post for the address

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

 

[hancock]

Re: Second Gradient Dipoles

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

 

[Elias]

Written by hancock:

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

 

[Elias]

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

 

[jamikl]

Did the PDFs of this article ever become available?
jamikl

 

[kelticwizard]

Let me check to see if I still have the article. Reasonably certain I still do. If I do, I should be able to get some jpg's of the pages over to you in a couple of days. I don't do pdf's, but getting jpg's of the pages is the equivalent of somebody using a copy machine to make a copy for you. Still interested?

 

[deathlord]

Hi

Very interesting concept!
I am tempted...

Has anyone else tried it out?
Any experience would be helpful. In particular I still don't really understand how the perforated covering should look like (material, geometry?).

Maybe a good starting point would be to build a subwoofer? It could be used only below the half wavelength point which would make things a lot easier. E.g. no perforated cover necessary.
It could be placed in front of the listening position, directed at the listener.
Or maybe two such subwoofers at the front speaker positions?

Oliver

 

[john k...]

2nd order gradient bass is not very useful in real terms. First of all, aside from the driver eq, a boost of 12dB per octave is necessary to achieve flat response in the gradient region. Second, the idea that there is directional control in an acoustically small room is incorrect. As always, in an acoustically small room low frequency response is controled by room modes. Third, driver excursion, once eaulaized to flat response, increases at 24dB/octave leading to excessive excursion if asked to opeate over any significant frequency range.

 

[Elias]

There was a second order cardioid DIY speaker published in Finland in early 1990's. It was second order cardioid in the freq range of 100-400 Hz, if I remember correctly. It had two groups of 4 pcs of 8" seas elements, and the back group was delayed in order to cancel the back wave.

The most right in the picture. However can only see the front group clearly from it.

 

[Elias]

HA ! I posted the same thing twice, last time more than 10 years ago ! Damn my memory is poor

This is the speaker:

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.

The book is in finnish, if that makes sense to you

The loudspeaker was called Hifi-C1. The second-order gradient principle was employed in the 100-400Hz range by using 4+4 Seas P21REX elements, 4 in front group and 4 in the rear. These elements were placed in square around midranges and tweeter.
Other elements were Philips RT8 ribbon tweeter >4kHz, 2* Seas MP14RCY 400Hz-4kHz placed below and above the tweeter, and cardioid bass < 100Hz using 2* Seas P21REX and 2* Seas P17REX. Filters were passive.

From the looks of the book it sure looks an impressive idea for a loudspeaker! It was said have been used as a reference speaker in the mentioned Hifi magasine tests. I have never heard the speaker, though.

Also, it is stated in the book that the author made also design of the second-order cardioid speaker for a commercial manufacturer, but I have no info about that.

Yes, it is a rare consept that I would like to see used more often. Actually, I've had the second-order cardioid concept on my mind when thinking what kind of speakers to build, but I've neglected the idea as too expensive to realise at home.


-Elias

 

[deathlord]

2nd order gradient bass is not very useful in real terms. First of all, aside from the driver eq, a boost of 12dB per octave is necessary to achieve flat response in the gradient region.

Hello John

Using an 18" driver the amplitude falls off at 12 dB/octave below 370 Hz. When we want to use it down to 20 Hz a boost of 48 dB is necessary. This is indeed heavy...

Second, the idea that there is directional control in an acoustically small room is incorrect. As always, in an acoustically small room low frequency response is controled by room modes.

A second order gradient can only excite room modes at velocity antinodes. And it can only excite them in one direction.
When it is placed in a velocity node corresponding to its orientation, no (axial) modes are excited at all meaning the behaviour is more like in free space. Apart from the increased power requirements I consider this an advantage.

Third, driver excursion, once eaulaized to flat response, increases at 24dB/octave leading to excessive excursion if asked to opeate over any significant frequency range.

This is a challenge. Still, large drivers have essentially zero excursion at higher frequencies. Multiple drivers could be used. Maybe 21" drivers?

Best,
Oliver

 

[deathlord]

Hi Elias

There was a second order cardioid DIY speaker published in Finland in early 1990's. It was second order cardioid in the freq range of 100-400 Hz, if I remember correctly. It had two groups of 4 pcs of 8" seas elements, and the back group was delayed in order to cancel the back wave.

The most right in the picture. However can only see the front group clearly from it.

Thanks for the picture!

The idea to use a different mechanism in the lowest frequency band is good. Maybe the driver to the front could simply be used as a dipole there. It only falls off at 6 dB/octave