Imaginary Equivalent Tuning

[linear]

According to the “Project K2” literature, “Imaginary Equivalent Tuning” (IET) was a new concept developed by Greg Timbers for the S5500 loudspeaker. By using two LF drivers, each in its own ported enclosure with different volumes and tunings, he claimed to provide a “better balance between the high speed of Bessel tuning and the flat frequency response qualities of Butterworth tuning”. Subsequently, this technique was also used in the M9500 speaker.

So, is “IET” really a “better mousetrap? What do you S5500 and M9500 owners think? Did Greg Timbers ever publish a “white paper” on the idea? Has anyone ever built a custom speaker with “IET”? Did “Project May” consider using “IET? I would love to see some more discussion and technical information on this idea.

It looks like the S5500 and M9500 may be the only JBL speakers that ever used “IET”. (The current JBL S5800 seems to have its two LF drivers in a common enclosure, since the exploded view in the Technical Manual shows only one set of ports at the bottom of the cabinet.) However, I seem to vaguely remember some older JBL speakers that used something called “stagger tuning”. Could “IET” have been used previously under another name? Comments?

[Earl K]

- There's merit in discussing the different sorts of asymmetrical tunings .
- For Instance; I use an asymmetrical tuning that combines sealed with vented , all in an MTM setup .

:p

[CONVERGENCE]

This concept was also made for Japanese recording studios in the ealy 80's using a 416b ALTECLANSING WITH A 6048H DUPLEX ALTEC AND A SUPER TWEETER .They were 4 way monitors. Here is a view.

http://img2.uploadimages.net/14309003-15264-09712.jpg

[gtimbers]

The staggered tuning and different enclosure volumes for the S5500 and M9500 came from necessity, the IET came from Marketing "necessity". The whole point of the thing was to get the low midpoint of a MTM system up higher by having more volume in the lower box. This plus a kick base put the horn closer to ear level, at least for the Japanese for whom the systems were developed. At that time, I felt that by having different volumes and different tunings we could spread the various enclosure and tuning resonance frequencies over a range making them less of a problem. It seemed like a good idea at the time, and it certainly works fine in those two systems. I'm not sure it actually buys anything over two enclosures of the same combined volume that are split equally other than moving the center point.

The original K2.S9500 had two equal volume enclosures. The lower one had a partition to make it the same volume as the top box. I have always felt that this was a waste of volume which is a very important factor in woofer performance so we haven't done it again. The bessel tuning was thought up by another Engineer here at the time. The thought was to get the best possible transient response from the woofer section. The bessel tuning requires a rather small volume with an extremely low tuning. I believe those boxes were 2 cu ft tuned to 26 Hz. Although fast, the lack of output around the knee, made them expremely bass shy, subjectively.

[Titanium Dome]

the IET came from Marketing "necessity".

By this I assume you mean marketing to the Japanese high end consumer? or perhpas it was simply the continuing drive for differentiation in a crowded marketplace?

[louped garouv]

welcome Mr. Timbers.....

Thanks for the background.... :)

[Zilch]

How gratifiying it must be to know that we're still building B380's and repairing BX63A's to run 'em here, 20+ years later.

http://www.lansingheritage.org/html/jbl/reference/technical/1983-subs.htm

Sorry, off topic, but I couldn't help myself.... ;)

[Well, it IS about bass, kinda.... ]

[scott fitlin]

Welcome Mr. Timbers.

[Titanium Dome]

welcome Mr. Timbers.....

Thanks for the background.... :)


Yeah, look at me, not so much as a weclome or howdy-do! :o:

Welcome to the forum. It's a nice place to be. Lots of JBL fans here, as I'm sure you know.

[John]

Yes Greg welcome from the Great White north, We love JBL up here as well:applaud:

[DRG]

The staggered tuning and different enclosure volumes for the S5500 and M9500 came from necessity, the IET came from Marketing "necessity". The whole point of the thing was to get the low midpoint of a MTM system up higher by having more volume in the lower box. This plus a kick base put the horn closer to ear level, at least for the Japanese for whom the systems were developed. At that time, I felt that by having different volumes and different tunings we could spread the various enclosure and tuning resonance frequencies over a range making them less of a problem. It seemed like a good idea at the time, and it certainly works fine in those two systems. I'm not sure it actually buys anything over two enclosures of the same combined volume that are split equally other than moving the center point.

The original K2.S9500 had two equal volume enclosures. The lower one had a partition to make it the same volume as the top box. I have always felt that this was a waste of volume which is a very important factor in woofer performance so we haven't done it again. The bessel tuning was thought up by another Engineer here at the time. The thought was to get the best possible transient response from the woofer section. The bessel tuning requires a rather small volume with an extremely low tuning. I believe those boxes were 2 cu ft tuned to 26 Hz. Although fast, the lack of output around the knee, made them expremely bass shy, subjectively.Thank you. :)

[Earl K]

Yes,

- Thanks for the insight on IET.

:)

[stevem]

According to the “Project K2” literature, “Imaginary Equivalent Tuning” (IET) was a new concept developed by Greg Timbers for the S5500 loudspeaker. By using two LF drivers, each in its own ported enclosure with different volumes and tunings, he claimed to provide a “better balance between the high speed of Bessel tuning and the flat frequency response qualities of Butterworth tuning”. Subsequently, this technique was also used in the M9500 speaker.

So, is “IET” really a “better mousetrap? What do you S5500 and M9500 owners think? Did Greg Timbers ever publish a “white paper” on the idea? Has anyone ever built a custom speaker with “IET”? Did “Project May” consider using “IET? I would love to see some more discussion and technical information on this idea.

It looks like the S5500 and M9500 may be the only JBL speakers that ever used “IET”. (The current JBL S5800 seems to have its two LF drivers in a common enclosure, since the exploded view in the Technical Manual shows only one set of ports at the bottom of the cabinet.) However, I seem to vaguely remember some older JBL speakers that used something called “stagger tuning”. Could “IET” have been used previously under another name? Comments?

I have built enclosures with IET tuning using a pair of 1401Nd drivers with the same tuning used for the M9500 monitor, which is 4.1 cu feet and 28Hz for the bottom box, and 2.8 cu feet and 35 Hz for the top box. I decided not to use the MTM design because I have incorporated a midrange in my design as well. One woofer is about 5 inches off the floor, and the second woofer is almost touching the first. The midrange (a 2012H) sits above this in it's own box. On top I use a 2450SL with a 2332 horn. It sounds pretty good, but I have been recently toying with the idea of dropping the mid and going with an MTM design, which would make the system very similar to the DMS-1. That monitor used a single enclosure for both woofers, which I don't think is as good as having each driver in it's own compartment. Greg's post is interesting because I can see how the larger volume raises the center of the horn to where I want it, at ear level. I think I will go with the split tuning again for this reason, and because it keeps the overall system size down.



Hey Greg, how about convincing the powers that be at JBL to make more of their newer hi-tech drivers available to us DIYers? Who knows, maybe we might come up with something you can market!:D

[Tom Loizeaux]

Greg,
It's an honor to have you post in this Forum. It's also gratifying to know that you read this Forum from time to time. I also imagine you get some satisfaction from seeing how devoted, knowledgable and appreciative so many of us are for your efforts and the great products from JBL over the years.

Tom

[JBLGUY]

M Timbers

Welcome Sir and always room for more JBL devotee's

:)

[Ian Mackenzie]

Greg,

I have often wondered how the IET came about.

Thankyou,

Ian

Ps How many JBL tie days have you had this year?

[Robh3606]

Hello Greg

Thanks for dropping in and welcome. It's a pleasure to have you post here.

Rob:applaud:

[Don McRitchie]

Hi Greg

As you can see, there is a growing welcoming committee that thanks you for posting here. Your insights and expertise are of great value to us. While I know your time is at a premium, I hope you continue to pop in on occaision. We certainly appreciate it.

[linear]

Thanks very much, Mr. Timbers, for your reply to my IET questions. It was interesting (but not surprising, I guess) to hear that "marketing issues" were mainly responsible for the development of IET. Be that as it may, the M9500 is certainly a fine loudspeaker, and I think that I read (on this forum) that it is your personal favorite of the K2 series. If so, I share your opinion!

Earl K, could you comment further on your asymmetrical tuned (closed/vented) MTM system? What acoustical benefits do you think this arrangement provides?

Stevem, and Dennis, could you also comment on on any preceived benefits of the IET tunings of your M9500 systems?

Thanks to all for an interesting discussion.

[Mr. Widget]

Hi Greg

As you can see, there is a growing welcoming committee that thanks you for posting here. Your insights and expertise are of great value to us. While I know your time is at a premium, I hope you continue to pop in on occaision. We certainly appreciate it.


Ditto!



Widget

[JuniorJBL]

We are all benefiting from your awesome designs and thank you very much.
:applaud:
Shane

[speakerdave]

It seemed like a good idea at the time. . . .

Cool.

David

[Guido]

Welcome Greg!

A big hello from good old europe. Nice to have you here.
Hope you'll be back soon.

We still have to learn alot here.

[JBLnsince1959]

I too would like to thank gtrimbers for the post. Most refreshing and what a surprise!!... and I almost missed it as I was busy. Hopefully someday ( when you retire) you'll be able to write about your experiences ( with lots of pictures). It would be on the HL BEST SELLER LIST for sure!!! :bouncy: :bouncy: :applaud:

[alskinner]

It is always a pleasure to hear from one of the major forces in JBL speaker development. Please visit us again when you have time

AL

[johnaec]

It is always a pleasure to hear from one of the major forces in JBL speaker development. Please visit us again when you have time

ALMy sentiments exactly!

John

[soundmotor]

I felt that by having different volumes and different tunings we could spread the various enclosure and tuning resonance frequencies over a range making them less of a problem. It seemed like a good idea at the time, and it certainly works fine in those two systems. I'm not sure it actually buys anything over two enclosures of the same combined volume that are split equally other than moving the center point.

Wouldn't (2) different tuning points tend to average out as one acoustically?

[toddalin]

I belive that the center channel I designed incorporates IET at least to some extent. (Cyclotronguy asked me to post the cabinet diagrams, but they are too light to scan. Sorry guy.)

I came up with this idea when I decided to house my center channel and subwoofer in the same cabinet. I wanted to let the speakers share cabinet volume while reducing their interaction.

At the time (about 13 years ago), I read an article on dual chamber cabinets of unequal volume and their effect on smoothing the speaker's impedience curve. I decided to incorporate this principal, but carry it a step further.

To this end, my cabinet actually incorporates three chambers. The woofers are in the outter chambers and each of these chambers is 4 cu ft. These chambers each have two ports. The ports are 4" ID and 9.5" long. One port directs outward while the second port ducts into a center chamber. The center chamber is 2 cu ft, and also had a 4" x 9.5" tube to the exterior (as well as each of the outter chambers). It's intent is to serve as a "buffer" between the two woofers to reduce interaction, but not totally avoid it. It certainly does smooth out the frequency response the the impediance peak becomes two smaller peaks spread over some frequency distance.

There are no parallel walls on the interior. The center chamber walls are angled and the front baffleboard lays back (like the L200). This reduces internal standing waves.

The cabinet sounds good and makes good bass, (for 130As), as good as(maybe better) than the L200 cabinets. I'm sure it will come into it's own when the 2235s are installed.

(BTW, these woofers are now for sale.)

http://www.largescaleonline.com/eimages/lsolpics/Team_Member_Pics/toddalin/speaker4.jpg

[Earl K]

Earl K, could you comment further on your asymmetrical tuned (closed/vented) MTM system? What acoustical benefits do you think this arrangement provides?


- Quickly ( OK, not really :o: ) : I presently use some Lansing "Legacy" components in the following fashion ;

(i) I run two woofers together , biamped and crossing over somewhere around 700-750 hz. into an Altec 288-8K ( on a round horn ) .
(ii) The JBL woofers are arranged as an MTM setup with the compression driver in the middle. This channels' MTM setup is located in a corner of my living room , about 2 feet out in either direction ..
(iii) The "woofers" used are ; Le14H (or A) and Le10H (or A). ( The other channel is twin 15s MTMed with the same horn )
(iv) Right now the 14" is still in a 1.7 cu' test box tuned to 42 hz
&
- the 10" is in a .65 cu' test box that was tuned/vented to around 64 hz.
- This vent ( I forget its' dimensions right now ) is stuffed full of open cell , medium density "case" foam .
- This stuffing turns the box into an "aperiodic" / sealed type .
- The sealed box with the 10" installed has a Fb of about 70 hz with a Zmax of @ 50 ohms . I don't think I've ever measured the "Q" of that Zmax peak .
- Sure Enough, the "aperiodic" tuning of that port produces a single wide plateau of a "peak" that has a Zmax never much higher than a couple of ohms more than the speakers' own impedance value (for that frequency range) ..
(v) ( I need to dig up the response simulations to move the discussion forward. )
(vi) Suffice it to say ; when combining vented along with sealed , there is a frequency specific area of overlap where ;
- there is a differential in the predicted phase angles for the two woofers that play back the same frequencies . I use the word " predicted" because I've never successfully measured "real" acoustic phase angles .
- if the difference in "predicted" phase angles exceeds 120° , the sounds start to cancel / not add .
- IME , if the phase differential is effectively capped to be no more than 90° , one will hear an audible change in the character of the bass ( in the overlap region ) that is quite pleasant .
- This change in "character" can be described as a change in texture along with all sorts of some other hyperbole .
- As GT alluded to , whether or not this change is worth the effort is unclear .
- Since room modes will also change phase angles in these regions , it's quite debatable whether or not a person should conciously design into the playback system , a " summed phase-shifted/differential, bass-response " .

( I'll look around for those prediction studies and post them later )

Oh ;
- the benefits ? Well. the bass in that overlap region could be construed as being more transient or faster sounding. OTOH; that "benefit" might just be " Imaginary" on my part / though there is no doubt there is a change to the texture of the bass response ( within the overlap region )

- the 64 million dollar question ? why screw around with these additive phase angles, when one can count on the room to do it "for/to" you ?
I really can't answer that one at this time .

<> Cheers

[Mr. Widget]

Earl,

Please let me know if I read your post correctly.

Are you saying that for a stereo playback you have one channel that is an "MTM" with a 14" woofer and 10" woofer and the other channel is using a pair of 15" woofers?


Widget

[Earl K]

Earl,

Please let me know if I read your post correctly.

Are you saying that for a stereo playback you have one channel that is an "MTM" with a 14" woofer and 10" woofer and the other channel is using a pair of 15" woofers?

- Yes, indeedy, you've read that right . ( Both Channels are MTM / same horn & same compresson driver / different woofers )
- It's been this way for about 16 months .
- I had thought I was pretty clear about this in my posts within the Project May Forum . I guess not .
- The horn centers are also not at the same distance above the floor. There's about a 4" difference in height. This does "skew" imaging a bit .

- I listen in mono when I want to make comparisons to the bass textures .

- And , as stated before , I continue to listen to "horns only" for probably 8 hours a week ( accumulated time ) . This experience will definately drive a person to find ways to "sweeten" up the sound of a compression driver .
Typically, Mozart & Vivaldi sound quite fine this way :p

:)

[stevem]

Stevem, and Dennis, could you also comment on on any preceived benefits of the IET tunings of your M9500 systems?

I honestly can't tell you much. When I built my current enclosures, I used the same drivers, but changed too many other things for me to say what change in sound was directly attributable to the IET tuning. My previous enclosure was a single box that housed both woofers, and it was overall smaller in volume than the compartmentalized IET version. I can say I preferred the later enclosure by a fair margin. I just don't know which changes affected the sound in what way.



I am re-thinking my approach if I do go with a two-way MTM. I can use the same enclosure volume for each woofer if I use a slanted partition between them. This shouldn't be to hard to design and implement.

[Mr. Widget]

- I had thought I was pretty clear about this in my posts within the Project May Forum . I guess not .
Hell, I can't even remember what I was listening to 6 months ago much less what other forum members were using in their systems.

I do remember you telling us you don't use a tweeter though... I guess we listen very differently. To me having identical speakers that are truly Hi-Fi is essential for stereophonic playback.:D


Widget

[Earl K]

I guess we listen very differently.

- I think so . My present setup took on its' current form at the beginning of the official launch for Project May . So maybe it's actually more like 20 months not the 16 as first posted .
- As a practicing "live" sound mixer / I find all of this stuff more like R&D. Usually, not much stays static in my world .


To me having identical speakers that are truly Hi-Fi is essential for stereophonic playback.

- Well, the present "static" setup allows for many qualitative opinions to be formed . Obviously, some interpolation is involved .
- Plus, giving up true HiFi playback, through identical componentry, is my idea of a "sacrifice for the project". ;)

:p

[Mr. Widget]

- Well, the present "static" setup allows for many qualitative opinions to be formed . Obviously, some interpolation is involved.There certainly is no shortage of opinions is there...;)


Widget

[Earl K]

I do remember you telling us you don't use a tweeter though...

- Actually, I've been working on a 3 pole, UHF contour-filter to be used with a small-format compression driver ( masquerading as a tweeter ). This will eventually get included into the final build of my own MTM project . Right now the "horn" is a simple diffraction slot . It's a 2344 ( metal ) throat section pressed into service for that slot as well as the small bit of HF loading provided. Eventually, I may buildup a bonafide mini-Westlake/Smith horn ( though mini really does become "mini" when the Xover point is 10K .

- This passive 3-way approach will be switchable into & out-of the circuit / therefore toggling the system between 2-way & 3-way modes .

:)