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Alpha 10 modeling [message #53435]

Fri, 17 April 2009 16:09

solomty
Messages: 9
Registered: May 2009

Esquire

Hy Wayne. Just model Alpha 10 with WinISD and the resulting graph is a very impressive. 29.4Hz @ - 3db in a 10 cf box a very big box for sure. Could tell me how compare your two Pi tower with 4.7cf against the 10cf box? Thanks

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Re: Alpha 10 modeling [message #53436 is a reply to message #53435]

Fri, 17 April 2009 16:37

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

In the tower two π loudspeaker, f3 is about 40Hz and f10 is about 30Hz. You can safely run the full 100 watts throughout the passband.

You could use the Alpha 10 in a larger box with an EBS alignment to get deeper extension. But excursion will limit your bass output in a large box, so be sure to limit the power below 100Hz. Excursion will become excessive in a 10ft³ cabinet at about half the power of a 5ft³ box.

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Re: Alpha 10 modeling [message #70519 is a reply to message #53435]

Tue, 20 December 2011 09:48

rvsixer
Messages: 58
Registered: February 2011

Baron

Sorry for bringing back an old thread (and hope the OP does not mind). This just seemed a relevant place for my question without starting a new one.

I'm a winisd/modeling newbie, for the 2pi towers I am seeing xmax at only 2.2W input, while its posted here we can send 100W through the passband Rolling Eyes

. What am I doing wrong?

Inputs:
Alignment: tried all of them (what is the correct one, EBS?)
Volume: 129 liters
Port dia: 11.43 cm
Port length: 1.59 cm

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Re: Alpha 10 modeling [message #70520 is a reply to message #70519]

Tue, 20 December 2011 13:51

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Make sure you are band-limiting when you check the numbers. If you look at the mechanical limits below 30Hz or so, they're very small because the cone is unloaded below the Helmholtz frequency. Also check the easy (but easy to miss) stuff like metric verses imperial and make sure you aren't off by a decimal point.

What you should see is thermal limits exceeding mechanical limits except below the Helmholtz frequency. There's a range just above Helmholtz where the mechanical limits dip, but they never dip below 100 watts anywhere in the passband (above 35Hz). Below the passband, the limit is mechanical and can be reached with little input because the cone is unloaded. From 35Hz to 50Hz, excursion is greatly reduced and so the limit is thermal. In between about 50Hz and 60Hz, the two limits (mechanical and thermal) are very close to the same. Above 70Hz the limit is thermal.

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Re: Alpha 10 modeling [message #70522 is a reply to message #53435]

Tue, 20 December 2011 16:17

rvsixer
Messages: 58
Registered: February 2011

Baron

Thanks for the thermal/mechanical explanation. My winisd modeling follows the expected responses you outlined, except that xmax appears to be exceeded. Or is xlim the value we are trying to stay under?

Not doubting, just trying to learn modeling vs. real world. I've checked and rechecked the input numbers and just can't get it straight. Even more confusing is I have modeled several other woofer/subwoofer designs, and all of them were pretty much spot on modeled vs. real world (using xmax as the mechanical limit, I assume to avoid high distortion values).

Please take a look at the attachments, maybe you can see where I am going wrong. Thanks!

Attachment: 2pi_tower_spl.jpg
(Size: 401.26KB, Downloaded 3980 times)
Attachment: 2pi_tower_port.jpg
(Size: 349.94KB, Downloaded 3863 times)
Attachment: 2pi_tower_4w_xmax.jpg
(Size: 307.48KB, Downloaded 3968 times)
Attachment: 2pi_tower_100w_xmax.jpg
(Size: 312.41KB, Downloaded 3982 times)

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Re: Alpha 10 modeling [message #70523 is a reply to message #70522]

Tue, 20 December 2011 17:00

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

It looks like your red line is around 1.25mm, yes? But xmax of the Alpha 10 is 3.2mm and xlim is 9.1mm.

You can check this with distortion measurements, if you like. It stays low in the passband, and rises below it. All through the passband, distortion stays low at power levels under 100 watts. As power is increased to 100 watts and above, distortion rises around ~60Hz, as excursion begins to become excessive in that region. So the models track well with measurements.

One other thing: The Helmholtz frequency is 40Hz, not 50Hz. If you make this change in your computer model, it will be more representative of the behavior of the physical device. Your response curve will change from being slightly underdamped (humped, as shown in your simulation) to slightly overdamped, (flat, no hump) and a bit gentler rolloff than max-flat. Just an FYI.

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Re: Alpha 10 modeling [message #70524 is a reply to message #70523]

Tue, 20 December 2011 18:03

rvsixer
Messages: 58
Registered: February 2011

Baron

The graph is in imperial units, xmax = 0.126in ~ 3.2mm.

I have played with the default tuning winisd calculates after providing box size/port dia/port length to 40Hz. This change shows 8.86mm cone excursion @ 60Hz (still way over xmax).

I am going to give unibox a whirl and see what happens.

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Re: Alpha 10 modeling [message #70525 is a reply to message #70524]

Tue, 20 December 2011 18:09

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Now that you've changed fb (the Helmholtz frequency), I think you probably have a pretty good model, although I think you're interpreting the charts overly cautiously. I'm tempering my interpretation of the simulations with what I know from the distortion measurements. Excursion does go over xmax around 60Hz (but nowhere near xlim), and distortion at that point is just breaking double digits. It doesn't get anywhere close to damaging the driver at 100 watts, and while distortion is higher between 50-60Hz, it doesn't become severe at those power levels.

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Re: Alpha 10 modeling [message #70526 is a reply to message #53435]

Tue, 20 December 2011 20:14

rvsixer
Messages: 58
Registered: February 2011

Baron

That's good to know on the distortion.

WinISD was installed in imperial mode (note all other graphs are in imperial units). Unibox shows the same cone excursions (very near xlim @ 60Hz).

Attachment: VB Excursion Eminence Alpha-10a.gif
(Size: 30.10KB, Downloaded 3640 times)

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Re: Alpha 10 modeling [message #70527 is a reply to message #70526]

Tue, 20 December 2011 20:28

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Like I said, I think your model is probably pretty good at this point. While I wouldn't generally suggest designing a speaker to operate near xlim full time, I would say that exceeding xmax in this speaker isn't terribly objectionable. This speaker doesn't sound bad when the ~60Hz region is between xmax and xlim at high volume levels. And as I said earlier, you'll reach the thermal limit before the mechanical limit with these speakers, unless you lean on them below the Helmholtz frequency. They're safe at that level, all day long.

What does the response curve look like with your current model?

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