Alpha15a distortion measurements log

[cuibono]

A year or two ago I built the H-frame open baffle 'enclosures' using the Eminence Alpha15a drivers, as designed by MJK. More info here. I've been very happy with them, but as 'progress' marches on, I've been examining other drivers, enclosures, etc. There were a few other options that really impressed me, like the XLS drivers in the Orion, or some of the Acoustic Energy drivers. Monte Kay has published a nice set of data, particularly woofers.

So I've become interested in taking a more in depth look at the Alpha15, and though I would share some of the findings, and perhaps get some feedback. First off, I know that people dispute the audibility of non-linear distortion. So lets not go there - we'll just talk about measurements here.

And in simulation, a lot of things look pretty good about this 'lowly' $60 woofer. In simulations, it seems to be more efficient than most other drivers. In an H frame, it has a particularly nice frequency response, requiring little to no EQ boost in the low end, if you can tolerate a f-3 of about 30Hz.

My only qualm with it is its smallish xmax - 3.8mm peak to peak (I think). It only takes about 8W to get there around 35Hz - but that isn't actually too bad, considering that you get about 96dB SPL out (at one meter), and minimal thermal problems compared to pumping 250W through some of the 'big boys'. But you can add 6dB for another driver (the other channel); and I just purchased another set (so I'll have two per side), so I should be easily able to hit 108dB SPL before running into big linearity problems.

Speaking of linearity, I had assumed that because I was already getting a lot out of a cheap driver, non-linear distortion would not be all that great. So I started doing some measurement today, and thought I would share. I use Soundeasy and a calibrated, DIY Linkwitz-style WM61 mic into a EMU 0404 USB. The mic was place at the center of the plane made by the opening of the H frame.



Well, things actually look pretty good. First, a look at the basic, unequalized frequency response and test conditions:

Not bad, rolls off a little higher than predicted (28Hz), that may be due to age or being set on carpet.

So I did a set of single tone distortion tests, at 30, 40, 50, 60, 70, 100, 150 and 250 Hz, and a warble tone test at 60Hz, just to see how they compare. All were done with a 10Vrms (or 14Vpp on the scope) signal at the driver terminals. All graphs were normalized to 0dB, so just read the legend on the left for the relative levels of the harmonics.

First - 30 Hz - not spectacular, not terrible. A good bit of 3rd:


40Hz - getting better, still some 3rd:


At 50Hz, things start to clean up their act:


And they just keep getting better:












Now, I would like to draw you attention to the data Monte Kay took from the Peerless 12" XLS, particularly F4 and F5 - they aren't terribly different.


Above 70Hz, the Eminence is its equal:


This may not be a fair comparison for a number of reasons, the first probably being that Monte does not use any baffle when he tests his drivers (there is no write up on that, but there is a picture of it on one of his pages). But both of our data should be valid, I would say a general comparison should be allowed - my conclusion is that the Alpha15 has surprisingly low non-linear distortion, particularly given its price and the number of other perks it has. Also consider how much less power it takes to drive the Alpha15 or and how much more volume displacement it has for a given voice coil displacement. And with 2 per side, they should be 'loafing'. I'd say thats not bad at all.

Now, how loud was I testing them at? Well, my mic/preamp has been calibrated, and is supposed to put out 199.2mV/Pa. Playing a 60Hz tone, the scope showed 7.3Vpp, or 5.16Vrms. I calculated this as being 122dB SPL (!). Remember this is right at the opening of the H frame, with no back wave cancellation - at a meter or two (where I listen) this would be a bit less. I hope someone checks my calculations. There are some very handy calculators here and here.

Next up, I'll be measuring the distortion levels at different power levels, and see were things crap out. I'll try and measure the in room response and SPL at the listening position. I would like to encourage people to share their perspective on this, as I'd like some more ideas of what to examine. In a recent post by Lynn Olson, he was saying that very low bass frequencies will cause substantial IM distortion of upper bass in OB configurations - so my thought was to high pass the OB somewhere around 40Hz and use a sub. I think the data above suggests this also.

[cuibono]

PS - I'd like to say thanks to MJK for publishing this design and all his work elsewhere, along with all the other people who share their work here and elsewhere on the net.

[MisterTwister]

cuibono, do you have impedance measurements of Alpha15a?

[cuibono]

Here you go - not as low as I thought...



The bump at 300 may be related to the H-frame resonance - or maybe its a cabinet resonance?

[cuibono]

Okay, here is some more data - harmonic distortion sweeps at different power levels. Each graph has 5dB higher drive signal, with the first being about 4.25Vpp, the second being 8Vpp, then 10Vrms, and then 17Vrms.

This first one is encouraging - low distortion, especially tall order. Its the 'first watt' that counts, right.



Distortion then gets progressively higher (ignore the two tall narrow spikes - the computer glitched here).



This graph should match the data in the first post - they have the same drive levels.



Distortions getting high here with a 24Vpp (17Vrms) drive signal. But read the notes about this below.

[cuibono]

I measured/estimated/calculated the dB SPL of that last signal at my listening position about three meters from the drivers. In the 100's I was getting about 105dB - louder than I thought! But look at this - yes, the ugly truth:



This is the frequency response of my listening position. Looks horrible huh? This was done with an ungated analog sweep, drive level 24Vpp. That broad dip from 50-100Hz concerns me, as does the overall large level drop below 100Hz. Where is my f3 of 30Hz? Who knows. I was getting about 95dB SPL through 60-100Hz. I only did a quick estimate while the signal was sweeping - I could measure individual tones, but right now I'm happy with an estimate. Although the frequency response is pretty bad, the SPL level is quite high, and as you can see in the last graph of the post above, tall order harmonic distortion was -60dB or better above 60Hz, which I think is okay. Add the speaker from the other side (+6dB), then the two more I'll add later (+6dB), and things should improve a bit - plenty of SPL capability, and lower distortion. I guess I should look for a better listening position though.

I also measured the harmonic distortion at the listening position with the same drive level, just to see what happens. In general the levels haven't changed, but just gotten a bit rougher.

[cuibono]

Here is another interesting tidbit. I wanted to see what happened to the drivers distortion profile when tested without a baffle - which is how Monte Kay and Siegfried Linkwitz do it.

The two graphs are for 10Vrms drive signal, and then 5dB lower, 8Vpp (whatever that is rms, I didn't calculate it).





The main thing here is that distortion is considerably higher - about +15dB for the taller orders. What this implies is that the XLS driver has 25dB less tall order distortion than the Alpha15 (MK's graphs show its 10dB down from my woofers, which are in enclosures; then, when my woofers are taken out of the enclosures, they gain about 15dB distortion - so a total (estimated) difference of 25dB). I would guess this is a significant difference.

The other interesting thing is how much smoother the individual harmonics are as they sweep up when unbaffled. 3rd and 5th show increasing levels at higher frequencies, which the driver doesn't show when baffled.

[Ang]

Very nice analysis of a commonly used woofer. Thanks!

[HiFiNutNut]

Thanks for sharing your knowledge.

Looking at the picture and guessing at the depth of the H frame I am not surprised that the SPL is much lower below 100Hz.

I have found John K's ABC Dipole to be very accurate when used to calcuate driver response when mounted on U / H frame. I did on 3 different U frames with 3 different sets of drivers and my measurements corresponded to the simulations very well.

With my latest U-frame I only targeted -3dB at 55-65Hz, and will use the Linkwitz sub below that.

Regards,
Bill

[MisterTwister]

thanks for info cuibono.
also, 300hz could be a spider resonance, or paper cone, or all together
I wouldn't cross it above 200-300 hz 4th order anyway.