My open baffle dipole with Beyma TPL-150

[Rudolf]

I did a simulation for a triangular baffle with 80 cm base and 120 cm to the top. All drivers are at their intended positions on the baffle with their true radiating areas. Individual TSP don't apply of course:



On a flat baffle the 21" driver would have its dipole peak at >300 Hz. You could double the effective baffle width to raise the bass efficiency and lower the dipole peak to your intended Xover of 150 Hz.

The mid has its dipole peak at ~700 Hz. Your intended xover of 1500 Hz is at the position of the first dipole null. Ideally the midrange driver should be smaller (and the baffle at the driver height too), but the W22 looks like a good enough compromise.

The TPL150 may be a great driver, but its baffle dimensions are those of a midrange driver - not a tweeter. This causes the dipole peak to be much below the desired frequency. The falling response at 1800-3800 Hz (on axis) will show up at rising frequencies for rising angles and can't be consistently equalised.

I have added a 20 mm dome tweeter in the top of the triangle - it would theoretically give a much better radiation pattern. For this application I would possibly prefer tweeters like the Visaton SC4 ND (or better ones of same small dimensions) mounted back to back in the top of the OB.

[JohnL]

Hello Erik,

I love your design, I was looking at something very similar when I first started desgining my dipole. I'm currently using the old 'Great Heil' AMT, but would love something I could mount to a baffle. It would certainly allow me some latitude in the design department. I've had my eye on those Beyma's since they came out.

I had originally looked at some 18's, then 21 inch drivers. Of the 21's I plugged into XLBaffle, I could never get one to model much better than a good 18. I haven't plugged the new Beyma's parameters into that spreadsheet, but do you expect mech better output than the 18's. Also noticed the inductance was a little high, how high up do you plan on running the 21's?

[FE3T]

Stig Erik?

Did you measure the TPL-150 before you removed the rear chamber?
I would really like to see if their graph are anything near the one Beyma publishes.

I would also like to know the price of a pair in Norway
I own a pair Morel H10.1 drivers i belive could work well together with the TPL-150 in a sealed 2 way setup

[tinitus]

Originally posted by FE3T
Stig Erik?

I would also like to know the price of a pair in Norway

Blue Aran. UK

[gainphile]

I like the triangle design. Have thought about it for quite sometimes for my next build, but can't really accept the wide bottom.

This is from about a year ago, the triangular bafle produced smoother sound but I did not understand why at the time.

[StigErik]

Originally posted by Rudolf
[B]I did a simulation for a triangular baffle with 80 cm base and 120 cm to the top. All drivers are at their intended positions on the baffle with their true radiating areas. Individual TSP don't apply of course:

On a flat baffle the 21" driver would have its dipole peak at >300 Hz. You could double the effective baffle width to raise the bass efficiency and lower the dipole peak to your intended Xover of 150 Hz.

The mid has its dipole peak at ~700 Hz. Your intended xover of 1500 Hz is at the position of the first dipole null. Ideally the midrange driver should be smaller (and the baffle at the driver height too), but the W22 looks like a good enough compromise.

The TPL150 may be a great driver, but its baffle dimensions are those of a midrange driver - not a tweeter. This causes the dipole peak to be much below the desired frequency. The falling response at 1800-3800 Hz (on axis) will show up at rising frequencies for rising angles and can't be consistently equalised.

Great, thanks a lot!
Our intended baffle is 80 cm side at the bottom. We will make wings (point rearwards) to increase the effective width, the wings can be up to 40 cm long, actually doubling the effective width.

I'm comfortable with the measured performance of the mid, so we'll keep it like that.

Regarding the tweeter... its radiation pattern is not spherical like a dome. How will this affect the simulation? Making the tweeter baffle narrower is almost impossible, as the drivers front plate is 120 mm wide.

[StigErik]

Originally posted by FE3T
Stig Erik?

Did you measure the TPL-150 before you removed the rear chamber?
I would really like to see if their graph are anything near the one Beyma publishes.

I would also like to know the price of a pair in Norway
I own a pair Morel H10.1 drivers i belive could work well together with the TPL-150 in a sealed 2 way setup

Yes, I have measured it with the chamber, both with a baffle and without baffle. The baffle affects the rear end (below 2-3 kHz) a lot. On a wide (40 cm or more) baffle, the TPL150 measures a lot like Beyma's own measurements.

You can buy these drivers from me for NOK 3000 each plus shipping.

[Telstar]

Originally posted by Overkill Audio
I will send you (and Telstar) a few photos which are too big file size to post (and way to ugly anyway!) they might be of interest.

Thanks

[Overkill Audio]

Hi Scott and StigErik,


Scott,

Thanks for the links to the old Adire article on driver speed, I read them with interest years ago after I met Dan at a CES or Denver show about 5 years ago.
I was sad to see the demise of Lambda and Adire.
Dan and John are indeed very clever driver designers and I have great respect for their new Acoustic Elegance drivers, I hope to have a chance to evaluate a pair of TD12M drivers soon.

Interesting that their new flagship drivers (TD 12 M & TD15M ) feature ultra low Mms to high Bl ratio's….!
50g over 17Bl = 2.94 for the TD12M one of the lowest Mms to Bl ratios of ANY Pro 12 inch driver.
Ditto the TD15M 70g over 17 Bl a great figure of 4.1, most 15 inch pro drivers are in the 5 or 6 bracket.
Maybe they have come around to my way of thinking….!
I first published my theory on this subject on my website in Jan 2005….
I am in full agreement with Dan and Nick that the lowest possible inductance is a valuable goal and will reduced stored energy therefore increase speed & lower distortion.
BUT, that is not the whole story, not even the most important chapter...! This is a very important subject, it was in my opinion THE most important issue in driver design until last week when MG sent me his Thermal Compression white paper...!

I don’t want to go off topic on this thread so I will start a new thread on driver design. I will put forward my views on the Mms to Bl ratio, the importance of low VC inductance, thermal compression (hopefully with the help of Michael Gerstgrasser!) aerodynamics, efficiency, power handling and materials.

That should be a good "cat among the pigeons"

StigErik,
The Beyma 18G50 (not the 18G550) is a very fast driver: Mms170g over Bl of 28.8 = 5.9. Great result for an 18 inch power house pro driver.
The 15 inch SM 115 K I am using = 5.1,(17% "better" ) but your 18 inch has 50% greater Sd therefore only has to travel 50% as far for the same Vd so your 18 G50 is actually 33% faster than the smaller and lighter SM115K!
The 18 G50 is a great open baffle driver and I would be Oh so interested to see how the measurements compare and more importantly how the sound quality compares to the 21 inch Beyma driver. Again Plleeeese post the measurements!

My point is that only in a "like for like" ie same Sd for same Sd, the lower the ratio the faster the driver. This assumes all other parameters being equal (Efficiency, Thermal compression, VC Le, etc) which they never are...!

All the best

Derek.

[StigErik]

It's interesting then that one of the slowest woofers I've heard, got a Mms/BL ratio of just 2,5 (Seas CA25FEY)....

I don't think that its just ONE parameter that affects the subjective impression of "bass speed". This most important in my opinion is the frequency response through the 100-500 Hz range, especially the transition between the woofer and midrange driver, which is the case of our dipole happens at 200 Hz. (we changed it from 150).

Best,
Stig Erik