Recently, following Martin J. Kings H-frame instructions for the Eminence Alpha 15a, I made myself a dipole woofer. Today, I visited a friend/speaker designer with a nice Praxis setup and did some measuring.
There has been a bit of interest in the Alpha 15 lately, and people seem to question if it is really possible for a single woofer to do what Martin's simulations suggest.
Anyway, here is part of today's "graph harvest". Enjoy:
First, Martin's simulated response for the Alpha 15 in a 16" by 16" H-frame.
There has been a bit of interest in the Alpha 15 lately, and people seem to question if it is really possible for a single woofer to do what Martin's simulations suggest.
Anyway, here is part of today's "graph harvest". Enjoy:
First, Martin's simulated response for the Alpha 15 in a 16" by 16" H-frame.
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Here is the measured response:
Take NOTE: we did not measure exactly at 1W input, just close. This measurement was taken in Praxis, using gating set up for full range frequency response, measured at about 1 meter - we did no finessing of FFT resolution (so the low end is a little chunky), no finessing of gating, or finessing of room placement (the H frame was set on the edge of a table several feet from any wall). The point is, these are basically first trial responses, and could be improved. But, I think you will see, things look good already. In fact, everything looks very close to as predicted.
Take NOTE: we did not measure exactly at 1W input, just close. This measurement was taken in Praxis, using gating set up for full range frequency response, measured at about 1 meter - we did no finessing of FFT resolution (so the low end is a little chunky), no finessing of gating, or finessing of room placement (the H frame was set on the edge of a table several feet from any wall). The point is, these are basically first trial responses, and could be improved. But, I think you will see, things look good already. In fact, everything looks very close to as predicted.
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The only point of disgression I see so far is that Martin's simulations predict a f3 of 28Hz for the H-frame, while the above measurement sets it at about 35Hz. In another measurement I did elsewhere I got a f3 of about 28Hz. I will do more measurements and report back, but none the less, things look quiet good to me.
And most importantly, they sound nice to me 😀
And most importantly, they sound nice to me 😀
That peak is almost 15db above response at 100hz ... at least they are honest about that 
About distortion being -60db ... at what Level SPL is that measured ?
About distortion being -60db ... at what Level SPL is that measured ?
Yeah, its a bit big (the 2k peak). But crossed LR4 at 125Hz drops it almost 50 dB. In my crossover calculator, it doesn't show up.
tinitus said:
Good question - we weren't tracking SPLs so I don't know exactly, but I would guess at about 1W1m (or 85dB SPL?)..
Can I ask what drivers you are using for the mid-hi end? I am currently building a similar unit using the FR-125 on top, and planned on a c/o at 250.
Pete
Pete
Hi - I'm using a Visaton B200 above 125, and using the computer to do crossover, frequency and phase response correction. I wrote up a little on it in another thread, but soon I'll write up a more complete thread. I'm very happy with how things are going. The B200 is reputed to have a very nice midrange, and modified, the highs have a lot of promise.
Thanks for your response--I will look forward to your comments on the complete project especially the use of the PC for shaping responses.
Regards
Pete
Regards
Pete
cuibono said:
If I change my simulation and raise the H frame 1 m above the floor to simulate it sitting on a table, the calculated response looks even closer to your measurements. If you place the H frame on the floor, I would expect the measured bass response to improve and produce more efficient output. Looks like things are working out well.
Martin - that nice to hear!
I spent today working with a totally different measurement system. I measured the H-frame on the floor near-field, and got just what we expected. This particular graph was done with a non-gated sweep, but I also measured white noise without gating and MLS with gating, and got exactly the same results.
Depending on how you look at it, f3 is at either 28Hz or 30Hz.
Enjoy!
PS - the levels are not absolute SPL! I think 80Hz is about 85dBSPL..
I spent today working with a totally different measurement system. I measured the H-frame on the floor near-field, and got just what we expected. This particular graph was done with a non-gated sweep, but I also measured white noise without gating and MLS with gating, and got exactly the same results.
Depending on how you look at it, f3 is at either 28Hz or 30Hz.
Enjoy!
PS - the levels are not absolute SPL! I think 80Hz is about 85dBSPL..
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My concern would be exceeding xmax with a driver like this. Any idea how much output this is capable of before exceeding xmax? Say, at 30-40 hz.?
mazeroth said:
pro-audio drivers are designed to exceed xmax. damage (xlim/xdamage) doesn't typically occur until multiples of xmax in such drivers. of course, once past xmax, force per watt goes down, so the driver is no longer completely linear in response to power. this further protects the driver from damage.
mazeroth said:
This is from the current "OB with AE IB15" thread, quoted from MJK:
"If you are listening to a pure sine wave with a frequency of 40 Hz and and an input power of 12.5 watts then I agree that you will exceed the 3.8 mm of Xmax (I'll take it on faith that you have done the math correctly) of the Alpha 15A. But I don't listen to pure sine waves, music is the summation of many sine waves of all different frequencies so I think your calculation is extremely pessimistic. I don't worry too much about Xmax when designing a speaker for music. If I was designing for HT maybe it is more important. I can tell you that the Alpha 15A mounted in an OB that gets it down to 40 Hz can play very loud and you have to touch it to prove to yourself that it is even moving, the motion is not easily visible."
My limited knowledge is that the H-frame reaches Xmax with about a 97dBSPL level with a steady state level - in reality, people don't listen to steady state levels, and Xmax can be exceeded, just with a decreas in linear displacement. For home music listening in standard sized rooms, I think the results are quite decent. There are of course other drivers/configurations that could reach higher levels in larger rooms, but your talking more money/complexity/space.
cuibono said:
Some good info here. This would make for a great speaker that you were sure didn't have much low-end (sub 40hz.) information. Every time you half frequency with a dipole it requires 8x the displacement. It could get scary with a 3.8mm xmax woofer if some 20hz. info was played through it.
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