[sjalloq]

Hi there,

my current speaker is a quick hack of a prototype using a B200 and an Alpha15a in a single sheet of MDF.  It got me hooked on the open baffle sound and I'm keen to experiment some more.  One of the things I thought I was lacking was some dynamics and so I've bought two pairs of the BetsyWOW to give an MTM design a try and I'm currently trying to decide on the tweeter.

I've read a lot about the TPL-150 and would like to give it a go but I'm stuck deciding on which version to go for - the horn version or standard.  The benefits of the horn seem to be the added output under 7KHz but I'm going to be driving them actively and don't need super high sensitivity.  I was also thinking that the c-t-c spacing of an MTM would be smaller with the non-horn version and I also personally think the non-horn version looks better.

Does anyone have any insight that may help me decide?

Thanks.

[JohnR]

As I recall the horn on the TPL gives it relatively constant 80 deg dispersion down to about 1200 Hz. I've never even seen one but I'd be inclined to think about it in those terms.

I'm curious why MTM? Also, why not first add a tweeter to the existing Alpha+b200?

[sjalloq]

My understanding is that one of the benefits of MTM is added dynamics and I wanted to try them as I've never done it before.  If I buy the TPL-150, then I may also attempt to add it to my current setup but I'm limited in tooling.  I would be having a local CNC house construct the new baffles.

[scorpion]

You perhaps already saw this: http://edsaudiopages.blogspot.se/2011/05/system-two-update.html and
have followed Stig-Erik Tangen's adventure in this thread: http://www.diyaudio.com/forums/multi-way/142015-my-open-baffle-dipole-beyma-tpl-150-a.html . 

/Erling

[sjalloq]

Thanks scorpion,

I've seen Stig-Erik's thread before but I always get lost in it as it's so long.    I'm also not sure if I have found anywhere where he has added any other benefits of the TPL-150H other than the higher sensitivity.

[scorpion]

Here is a link collection over TPL-150: http://www.audiovoice-acoustics.com/forum/showthread.php?t=1141
You might find something useful here.

/Erling

[JohnR]

Erling, great metalink I spent some time on it.

The TPL-150H seems to me like a fairly specialized tool that, frankly, would need some careful design work to make work really well in a hifi setting.

[sjalloq]

Hi again,

I decided to go with the non-horn version in the end.  I didn't particularly need the extended low frequency response so compromised on the slightly cheaper version.  I received them in the post last week and although they are reassuringly heavy, the metal front plate isn't the best made in the world.  One of the drivers seems to have small bubbles along one edge - either a casting manufacturing defect or something to do with the paint finish.  Anyone else seen this on their pair?

Anyway, I'm starting to think about baffles and thought I'd run a few tests first.  I have a MiniDSP microphone that I can use for measurement with RoomEQ and I was thinking about trying a number of different baffle widths.  I will have access to a laser cutter soon so plan on producing some simple plywood test baffles.  Does anyone have any suggestions on the widths I should try?  Perhaps from naked through to a 12" baffle in 2" steps?  Does anyone have any advice on measurement techniques as well or common issues I should try to avoid?

Thanks.

[JohnR]

A simulator like the Edge might give some insight. The MJK worksheets might also (not yet managed to have a go at those yet).

I suspect the main thing to look for (and I'm assuming you're talking about the tweeter only?) would be whether there's a notch in the tweeter passband.

Wish I had a laser cutter for my prototypes 

[sjalloq]

OK, thanks.  I haven't actually had a play with those tools/worksheets yet so will give them a go.  I'll also try and post lost of pics and possibly a video of the frickin' laser in action.   

[sjalloq]

It has taken a lot longer than I thought to get to starting this project but I now have some test baffles and access to a CNC router.

My first baffles have been made with the ability to add different sized cheeks so that I can increase the width of the baffle from 280mm to 520mm in 60mm steps.  I hope to now start taking some measurements and comparing with modeled data. If anyone has any pointers on how to get the best out of REW I'd appreciate it. I have the 2 channel mini-dsp.

I'll continue to post pics and my working thoughts.

[JohnR]

My first baffles have been made with the ability to add different sized cheeks so that I can increase the width of the baffle from 280mm to 520mm in 60mm steps.

That's a cool idea.

  I hope to now start taking some measurements and comparing with modeled data. If anyone has any pointers on how to get the best out of REW I'd appreciate it.

REW has a lot of functions, but at this point I'm assuming you'd be looking at gated measurements. If you take some measurements with the baffle as clear of reflective surfaces as possible, then I'd be happy to point you to the gating window and/or do some work on your mdat file.

[sjalloq]

Done my first pass at testing and have an mdat file with a series of measurements (can I upload it here?).  I can't make any sense from them as the SPL curve is crazy.  There's a 10db dip at 430Hz and then a fast fall off at 300Hz which seems to stabilize at 170-70Hz before falling again.

[sjalloq]

And here's the TPL with the widest baffles (not much difference to the thinnest baffle).

[JohnR]

What is the baffle width, and what distance to the mic?

[sjalloq]

Can't remember the exact baffle width but between 280mm and 520mm.  I've copied the links below to the mdat files on my Google Drive account. The Betsy measurements have all baffle widths.

Driver was 0.93m off the floor; mic was 1m from dust cap.

https://docs.google.com/file/d/0BzOBSkTWjdcaRkt2c3dKeVZfUzQ
https://docs.google.com/file/d/0BzOBSkTWjdcaUnNIaEVoVHhDdms
https://docs.google.com/file/d/0BzOBSkTWjdcaamN2NWxtTHFDLUE

[JohnR]

Your "outside" measurement is undoubtedly more reliable for the Betsy, although it looks as perhaps it had a narrower baffle?

I'll post some screenshots to show how to do the gated measurement on the tweeter, will have to be tomorrow (my time).

[studiotech]

Done my first pass at testing and have an mdat file with a series of measurements (can I upload it here?).  I can't make any sense from them as the SPL curve is crazy.  There's a 10db dip at 430Hz and then a fast fall off at 300Hz which seems to stabilize at 170-70Hz before falling again.

You probably do not want to test anywhere below about 500Hz with this driver.  At higher SPL, you might damage the pleated diaphragm.  And yes, their native response is not flat at all and takes some work to flatten out, but they are a good driver if high SPL and killer dynamics are needed.

EDIT:  Now I see this was not the TPL.  Sorry.

Greg

[JohnR]

And here's the TPL with the widest baffles (not much difference to the thinnest baffle).

Unsmooth the measurement, then click on 'IR Windows" and set the "Right Window" to 2 ms. Going back to the SPL window, this is the result:



Why 2 ms? Go to the Impulse Response window, set the dropdown selector to %FS and adjust the scales, you can see that the first reflection is a little after 2 ms:

[sjalloq]

Thanks for that.  I might have a go at setting up my mini-dsp this weekend.