'exotic' techniques for absorbing the mid backwave (and bass)

[jeroen_d]

The Concave concept was inspired by the Sphere Horn which was a project from one of the Dutch DIY Audio Forum members. See the link that the TS posted in his first mail.
That project was a result of B&W's white papers on the Sphere Horn.

🙂 And here we meet again!!!

The sphere/horn works wonderful. A very nice invention of B&W. Before I build the final enclosure, I have tested the concept. To show my findings, I will post a couple of measurements. My way of identifying standing waves inside the enclosure is to measure with the microphone at a very short distance (0.5") from the driver. The standing waves leak through the driver cone, they are audible and can be measured.

I will start with the sphere with the back of the sphere open. You can see standing wave such as predicted in the white paper of B&W (900 and 1400Hz in my case are the most clearly visible). In addition, you can see the 1/4 lambda standing wave caused by the open back at 240Hz.

 

[jeroen_d]

When the back of the sphere is closed, the standing wave at 240Hz disappears.

 

[jeroen_d]

When instead, the sphere is not closed but connected to a tapered tube of about 30cm, the standing wave at 240Hz still disappears and the standing wave at higher frequencies are smeared.

 

[jeroen_d]

When the tapered tube is filled with about 10g wool, loose at the large opening and tight at the small end, all standing wave are gone. Even when the sphere is completely without stuffing! What remains is the respons of the driver, in this case the Peerless HDS134.

 

[jeroen_d]

The final enclosure was build with another driver, the Peerless HDS152excl. This driver provides a ruler flat respons in such an enclosure.

 

[jeroen_d]

By the way, working with straws is not a good idea. I have tested such things, it just doesn't work. If you want to keep it simple, build a small heavy box and stuff it traditionally to obtain a clean and uncolored midrange.

 

[Brett]

(i'm assumming you are talking about drinking straws in the port as i didn't see any drinking straws in that photo gallery)

Brett, I'm surprised you haven't run into that before, the idea has been around for a long time. Mission even did an injection moulded port that commercialized the idea. This is one of the concepts that aided in coming up with the porting on the Fonkens (which was a huge success)

dave

I had Proac Tablettes for years which used them in the port.

However, I was curious about them used behind the driver.

By the way, working with straws is not a good idea. I have tested such things, it just doesn't work. If you want to keep it simple, build a small heavy box and stuff it traditionally to obtain a clean and uncolored midrange.

Thanks.

 

[asterduc]

Simplyfied drawing to show the difference between a 'normal' enclosure and the Concave Enclosure.

An externally hosted image should be here but it was not working when we last tested it.

http://home.scarlet.be/~collier6/Concave theorie.jpg

 

[PigletsDad]

By the way, working with straws is not a good idea. I have tested such things, it just doesn't work.

I am interested in your experiences; what happened? What flow resistance did you use?

 

[jeroen_d]

I've removed the data. But it also doesn't make sense. In a port, it provides controlled damping in exactly the amount you like. In an enclosure, you have to damp reflections from all directions and damp them well! Stuffing is easy and works adequate, no reason why straws would do better. Just plug some stuffing into your port and see what happens...for the most effect, traditional stuffing is still the way to go.

If you want more advanced stuffing, try TWARON Angel Hair. I have tested it and for 40gr wool you will only need 10gr Twaron. It is 'springy', it sets itself within the enclosure. Expensive, but very effective for a mid or tweeter enclosure.

 

[terry j]

Thanks HenkJan, I'm flattered being called 'a friend'!

That project was a result of B&W's white papers on the Sphere Horn.

Taking this theory in my own project brought me to the Concave mid enclosure. This enclosure is a combination of the Phere Horn, an infinite Transmission line and what I call, projection technology.

Ed.

Hi Ed, thank you for the excellent build pics. Take enough pics, and you don't need language eh? (tho your english is not too shabby!)

They must be a heavy box, and I did like your bass guitars too.

Is there a link to these B&W white papers?

Subjectively, how would you describe the sound differences between a cavity like this for the mids vs a bog standard cavity?

Having a cad router certainly helps, but at the end of the day do you feel the effort of something like this is justified?

And in the mid chamber, the only stuffing you have is that little layer of black stuff? No wool or anything right at the final sharp end??

I think you used a scan mid, for scaling purposes what is the cutout size? What you have shown is certainly a starting point, but what I am curious about is the relation of the size of the chamber to the driver. I mean is the volume hence shape dictated by the driver used, two different drivers will have the same basic shape yet the volume different because of different parameters? Dunno, but say if a certain driver works in 20l best, another in 5l best, is that reflected in the size of this chamber of yours?

[BIf you want to keep it simple, build a small heavy box and stuff it traditionally to obtain a clean and uncolored midrange. [/B]

See my last question to Ed, given the results you measured, do you too feel the effort is justified?, or is keep it simple yet well done enough.

Simplyfied drawing to show the difference between a 'normal' enclosure and the Concave Enclosure.

That little chamber at the back, serves no purpose?

 

[asterduc]

hey Terry,

was it justified? "defenitely not", my wife would say!
To me it was an interesting learning process being happy with the final result.

You could look into the topic on the Dutch DIY forum. This topic takes 86 pages and has over 2000 replies. Unfortunately for you guys, the topic is in Dutch. Maybe with Babelfish you can get something done. Otherwise there are is a lot of measured data together with a bunch of pictures that may explain our findings.
http://www.zelfbouwaudio.nl/forum/viewtopic.php?t=1102&start=0

On the Sphere Horn theory, Jeroen_D was my source of inspiration and mentor in this project. You may find the link to the white papers on his webside.

In brief:
The internal volume is based on an IB Q=0,5 for the specific driver (notice that this is different from the B&W whitre paper).
Then the pipe is calculated as closed transmission line, with a mouth area which is identical to the cone area of the driver (devided by 2 since there are 2 lines).
The shape of the internal is designed that way that most of the waves are guided into the dampened lines.
The goal here is to lower the standing waves and decrease the backwave to the cone.

What I could not avoid was the mechanical backwave from the cone to the rubber surround of the driver. I hope that the new Illuminator 15MU driver will show an improved behavior.

What does it do in practice?
More dynamics due to the missing dampening and a more energic sound without loosing the high-end touch!

Regards,
Ed.

 

[jeroen_d]

Was it justified? Difficult to answer. A simpler enclosure with triangular shape is easier to build and almost as good.

Here you have the link to the white paper, see appendix V for the sphere/tube enclosure. This is no B&W marketing talk but the result of some nice R&D! http://bwmedia.keycast.com/download/Libraries/3/800s_l2_w0_h0_2.pdf?dm=633108447502000000

The step towards going full range open baffle was a much bigger one for me in terms of sound quality. You loose some pin point imaging, but the image is still very stable. The sound of voices, reverberation, tonal balance, openess, dynamics in the midhighs, detailed bass, all favor the open baffle approach to me. This is however quite personal. At least, a baffle is not difficult to build. Crossover work is more difficult.

 

[terry j]

thanks for the replies. and the link.

trouble is, this is what I got from the link



~Úä£,Áêz}FÁøî]ŠÑº?mùüú'Wô{ǺF¿çêœ"EË&êYÛ=ðmð3ëõÍŸy)IÑ[\Gõ÷Œ}}åû¸Kv)VÇtDïïwe¿Äm©ãé'Vç”2 ­’8f·˜[·µ¸[ø´�G–kˆ�Œq�¯íêƒã†.·b]½Ýs¶ó#»üo:.اíÂî¼&ôf°–øžÚU÷4ÒQ:Ö°zµ«áÛHWbÜ¥Ùø²îàø˜$ãK!$‹l’…à¶Üy¯Dº~j\…ûrŒø.~}I‘¥ýñ{ì*±®æ¥èþÞ—¿W}çÂñl/ÃòªúLJI$âÌjq,›§Ø|zIÑrZ±Îrq<í&¹Û½—/—sZNFǶ5í|_Íßâoþžaï%µóñx.<)¦6ÇÝÓŽô¹-ÖôÑ¿t—\:Š-Ó°i9�³«sHNã}ž›’c5‹ßÜ61Eëû½(žÌ‰8J)]5A’ÇÈ»Ž©Ûè�ŹäQ5š”ÓT
˜aÏPTJÅ"{ÙûÍ —ãŽbà¦Ëßk±¸9替—<Ð'Õ¹§ÏçmG.¾>Ï×Y\TI\%¥±ËÖ~úNò¨þÜÂwå:›ëòW0ãY^Âg¯çpÑyÚ

Umm, obviously I have something wrong, does anyone know what it is that is up??

read a dutch forum with babelfish..ouch. Is there a translator that people find works better? (to save Ed the effort of translating it manually for us...which he has assured me by e-mail he is willing to do heh heh)

jeroen, might have not seen the missing link, but do you mean your project has gone to open baffle? Not an option for me, so simply back to finding out if there are obvious benefits. (just now had a look at your website, the main page must be the OB you talked of)

 

[planet10]

thanks for the replies. and the link.

trouble is, this is what I got from the link

I clicked the link and got a pdf of a glossy brochure that i have a shiny hardcopy of.

Try another browser.

dave

 

[terry j]

I use firefox (is that what you mean?), tried explorer and got the same thing.

oh well. I can follow the 'formula' above for starters.

another thing that is confusing me, why does the b&W use a tapered pipe on the tweeter?? most tweeters are enclosed, why do we need a TL for the backwave, is there one??

signed,

confused.

 

[planet10]

I use firefox (is that what you mean?), tried explorer and got the same thing.

I guess you are saddled with Windoz? Try right clicking and see if there is a "Save Link As..."

another thing that is confusing me, why does the b&W use a tapered pipe on the tweeter?? most tweeters are enclosed, why do we need a TL for the backwave, is there one??

All tweeters have a backside loaded into something. There are a number that use a TL instead of a sealed (or aperiodic) chamber behind.

dave

 

[Brett]

TJ, email sent.

 

[terry j]

thanks dave. I did wonder if they had a tweeter that was 'open backed'

thanks brett (and james), replied.

 

[zBuff]

Always wondered what effect progressive layering of light to denser damping materials would have? Anything useful?