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Happy Jack

Moving Air

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Guys, I'm crap at electrics and such, but pretty good at numbers.

I thought the amount of air a speaker could move was driven essentially by the area of the speaker, using the classic PI x r-squared formula. Since PI is a constant, the relative air-moving capability of different speakers should be:

10" = 5.0x5.0 = 25
12" = 6.0x6.0 = 36
15" = 7.5x7.5 = 56
18" = 9.0x9.0 = 81

Crudely, therefore, a 210 will move nearly as much air as a 115.

I was perfectly happy with this analysis, it all made sense to me, and it seemed to chime with my own experience.

Then I read this on the Ampeg site with reference to the SVT-410HLF:

[i]The SVT-410HLF actually moves 10% more air than two 18-inch speakers, and 25% more than three 15-inch speakers![/i]

What am I missing here?

:)

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[quote name='Happy Jack' post='138779' date='Feb 12 2008, 12:16 PM']What am I missing here?[/quote]

The third dimension! The amount of lows a speaker can produce is indeed dependant on the amount of air it can move but that depends not just on speaker area but also how far back and forth the speaker can move.

In speaker specs this is called Vd, which is the produce of Sd (surface area) and Xmax (max linear cone excursion). For every octave you descend the speaker has to move four times as far to produce the same SPL. The only exception to this is with ported cabs where the port assists with output as you approach the tuning frequency, which is why the only sealed bass cabs on the market tend to have lots of speaker area to achieve high LF SPL with a port helping out.

This then relates back to real world power handling - if speaker A can move twice as far as speaker B, then it can handle four times as much power and produce 6dB more output in the lows.

Alex

Edited by alexclaber

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Two possibilities that I can think of -

1. Speakers are not flat circles they're cones so its a volume calculation not an area calculation you'd need

2. The amount a speaker moves backwards and forwards will affect how much air is moved. More movement = more air moved.

You also get "coupling effects" whereby speakers positioned close together act on the air as a single large speaker. This is one of the principles of vertical arrays of speakers that are currently coming into fashion for large PAs.

It seems that air movement is not as simple as you would think. :)

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[quote name='Happy Jack' post='138779' date='Feb 12 2008, 12:16 PM']Guys, I'm crap at electrics and such, but pretty good at numbers.

I thought the amount of air a speaker could move was driven essentially by the area of the speaker, using the classic PI x r-squared formula. Since PI is a constant, the relative air-moving capability of different speakers should be:

10" = 5.0x5.0 = 25
12" = 6.0x6.0 = 36
15" = 7.5x7.5 = 56
18" = 9.0x9.0 = 81

Crudely, therefore, a 210 will move nearly as much air as a 115.

I was perfectly happy with this analysis, it all made sense to me, and it seemed to chime with my own experience.

Then I read this on the Ampeg site with reference to the SVT-410HLF:

[i]The SVT-410HLF actually moves 10% more air than two 18-inch speakers, and 25% more than three 15-inch speakers![/i]

What am I missing here?

:)[/quote]



The area of a typical 10" driver is 350cm2, 15" = 856cm2.

Therefore four tens =1400cm2 and three 15s = 2568cm2, nearly twice the area of the 4x10.

I seem to be missing something too...

Reminds me of how suspicious I was of Ampeg's claimed frequency response stats.

edit: of course, yes, it does depend also on Vmax but it's a bit sneaky of Ampeg to compare their 10" with a good Vmax with that of a Brand X 18" that just happens to have a tiddly Vmax

Edited by bremen

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[quote name='bass_ferret' post='138797' date='Feb 12 2008, 12:42 PM']Should have compared it to the old Cerwin Vega earthquake drivers that had 2 inches of XMax[/quote]
2 INCHES? Christ!

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[quote name='bass_ferret' post='138797' date='Feb 12 2008, 12:42 PM']Should have compared it to the old Cerwin Vega earthquake drivers that had 2 inches of XMax[/quote]

If that is correct I'll eat my hat. 0.2" of Xmax would be pretty damn good for an old 18" woofer.

Alex

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[quote name='BOD2' post='138789' date='Feb 12 2008, 12:29 PM']1. Speakers are not flat circles they're cones so its a volume calculation not an area calculation you'd need[/quote]

It's the swept area that matters, so it's still a circle. Though you do need to bear in mind that the 10", 12" or 15" diameter is nominal and the real diameter once frame and surround have been subtracted is significantly less.

Alex

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[quote name='alexclaber' post='138840' date='Feb 12 2008, 01:39 PM']It's the swept area that matters, so it's still a circle. Though you do need to bear in mind that the 10", 12" or 15" diameter is nominal and the real diameter once frame and surround have been subtracted is significantly less.

Alex[/quote]

Thanks for that correction. :)

It seems it's the cone movement that's the critical dimension in any case.

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[quote name='BOD2' post='138855' date='Feb 12 2008, 09:01 AM']Thanks for that correction. :)

It seems it's the cone movement that's the critical dimension in any case.[/quote]
The technical spec is Vd, the volume displacement of the cone at full excursion, and can be found on the manufacturer's driver spec sheet.

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[quote name='Bill Fitzmaurice' post='138867' date='Feb 12 2008, 02:12 PM']The technical spec is Vd, the volume displacement of the cone at full excursion, and can be found on the manufacturer's driver spec sheet.[/quote]


Ah now I think you're having me on ! You just want me to walk into a speaker shop and snigger while I ask for speakers with serious Vd :)

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For all of the above, many thanks.

I understand that the cone's bi-directional movement comes into the equation, but I'm guessing that the Vd effect is actually relatively small ... an 18" cone will be easier to vibrate than a 10" cone, but a 10" cone will weigh less and therefore respond faster. So Vd will certainly make a difference, but as a factor it will be massively outweighed by simple size.

Fair enough?

In which case, is that Ampeg blurb a load of complete bullshit? Or am I still missing something?

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[quote name='Happy Jack' post='138898' date='Feb 12 2008, 02:48 PM']I understand that the cone's bi-directional movement comes into the equation, but I'm guessing that the Vd effect is actually relatively small ... an 18" cone will be easier to vibrate than a 10" cone, but a 10" cone will weigh less and therefore respond faster. So Vd will certainly make a difference, but as a factor it will be massively outweighed by simple size.[/quote]

No, Vd matters more than anything when it comes to moving air at low frequencies. The best way to understand all this is to read this: [url="http://www.eminence.com/resources/data.asp"]http://www.eminence.com/resources/data.asp[/url]

Download this: [url="http://www.linearteam.dk/default.aspx?pageid=winisdpro"]http://www.linearteam.dk/default.aspx?pageid=winisdpro[/url]

And then start running simulations of different speakers - some typical bass cab choices are the Eminence Alpha 10 (for sealed cabs), Beta 10 (ported cabs), DeltaLite 2510-II (modern neo ported cabs).

When you're dealing with low frequencies even the heaviest cone can move fast enough. Once you get into the midrange a lack of acceleration can manifest itself as a reduction in sensitivity but by then you're often running into voice coil inductance reducing response as well.

Considering how most bass guitar 18" cabs are rather undersized and use relatively cheap (and thus low Xmax) woofers, I wouldn't be surprised if Ampeg are correct regarding their 410HLF.

Alex

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what you are missing is that it is an advertisement.

louder than which 15's? not all 10's are equal either. technically :) speaking, it's a trade off between volume, frequency response, and money. a loud ten will lose in the other two categories. a larger speaker will too, but not as much.

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It looks like Cerwin Vega dont sell their drivers and just do cabs now - serious cabs mind [url="http://www.cerwinvega.com/vegabass.php"]http://www.cerwinvega.com/vegabass.php[/url]

No specs for the drivers.

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[quote name='bass_ferret' post='139040' date='Feb 12 2008, 02:43 PM']It looks like Cerwin Vega dont sell their drivers and just do cabs now - serious cabs mind [url="http://www.cerwinvega.com/vegabass.php"]http://www.cerwinvega.com/vegabass.php[/url][/quote]
Obsolete IMO, too large a driver coupled with too small a horn. The minimum length for a horn to work to 40 Hz is seven feet, and when you load an 18 into a box that size there simply isn't room for a seven foot horn. For that matter I wouldn't use less than a twelve foot horn in a box that size.

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