Alpha 15 - It's that good!

[cuibono]

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.

[cuibono]

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.

[cuibono]

Finally, the manufactures IB response - which shows why things look the way they do above 1K.

[cuibono]

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

[cuibono]

And a little cherry on top: distortion is about -60dB. The green line is the second fundamental, and the red is THD. The red peak at about 240Hz is probably the cavity resonance. I cross these guys at 125Hz, 4th order LR.

[tinitus]

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 ?

[cuibono]

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.

[cuibono]

Originally posted by tinitus

About distortion being -60db ... at what Level SPL is that measured ?

Good question - we weren't tracking SPLs so I don't know exactly, but I would guess at about 1W1m (or 85dB SPL?)..

[Peter Menting]

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

[cuibono]

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.