Power compression question

Ok, I have a Blueprint 1803 subwoofer driver. It has a 3" voice coil and is supposed to handle 1kW constant.

What I would like to know, is the power compression of the driver? At what point do I start loosing the 3dB per doubling due not to BL/excursion, but heat buildup?

First, I need to understand exactly what power compression is. I do know it is heat related. But, say the driver has been sitting for 2hrs not used, and then I apply a 2kW sine to it for a fraction of a second, high enough frequency so that the BL nonlinearities are not as much an issue. Lets say that the power compression at 1kW is 3dB. So while the driver is 92dB at 1w/1m efficent it should produce 122dB. BUT, with power compression, it will only produce 119dB. If I want 122dB, I will need 2kW, correct?

Is power compression related to the long term heating of the VC and motor, or does it apply to even a short fraction of a second burst?

RC, when you put 20kW into some of those woofers to test them, what kind of power compression do you typically get?

Thanks! [img]smile.gif[/img]

[Audiolover]

Thats a difficult question becuase it depends on the driver.
Power compression is a result of VC heat (depends on time and given power). Also, it's the resut of non-linearities of the suspension and motor (BL). A very short burp isn't supposed to be affected by the firt issue, but the motor will behave bad, depending on the power/frequency.

Audiolover

[ October 19, 2002, 04:17 PM: Message edited by: Audiolover ]

That's a great question Jerry...I've been wondering myself about this. My hunch is that power compression does not affect short bursts (excluding BL nonlinearity, as you say).

excellent question that has been forgotten.everyone asks about xmax and power handling but forgets this very important issue!!!

[img]graemlins/thumb.gif[/img]

power compression is different at each power level

its a relatively simple question and I think you already know the answer..

power compression due to heat, becomes an issue as the coil becomes close to being damaged - if you are feeding the woofer 1kw continuously, then you will loose a bit of output through the power compression once the coil becomes hot

if you give it a quick 2kw burst when the coil is cold, then since the coil won't become very hot, power compression due to heat won't be an issue..

if you fed it much more power, then the coil will heat up fairly quickly and it could be an issue

Now, as long as the speaker has not suffered a significant drop in BL over the amount of excursion that the 2kw burst (at whatever frequency) requires, then you won't really have to worry about compression

Any comments on the power compression when you put 20kW into a driver, RC? Even for a fraction of a second do the coils heat up enough?

Power compression comes from heating the voice coil. As you heat metals, the resistance of the metal increases. Heat copper by 200 deg C, and you double the resistance of the copper. That means you just doubled the DCR of your woofer, which will cause a 3 dB loss in output.

BL compression comes from a loss in motor force as the driver moves. When the motor force drops by 30%, you've lost 3 dB in output (this is why DUMAX and Klippel set the Xmag limit to 70% of rest BL).

Of the two, power compression is a slower, more gradual thing. A quick burst of power will heat the voice coil quickly, but it will take a while to cool back down. In normal operation, the voice coil heats up and down over the range of 10-30 seconds, so that while you get compression, it's not a really "fast" problem.

BL compression is inherent in the motor, as the driver moves. No way around it, and it will happen on each and every stroke. And since it varies with excursion, it also typically gets worse as excursion increases (that's why motors like our XBL2 are good - keep the BL flat means reduce BL compression).

In typical usage, BL compression is by far the dominant source of compression. In SPL competitions or other high SPL/power situations BL compression starts of as dominant but is quickly swamped by thermal compression.

Note that thermal compression is constant across the entire range of motion of the driver! It's as if you added a series resistance to the driver. In this regard it's not a major source of dynamic (meaning changing-with-time) distortion (BL compression is always a source of dynamic distortion). However thermal compression can screw up your alignment overall - doubling the DCR will double the Qes as well!

Dan Wiggins
Adire Audio

[Richard Clark]

guys-----as dan has so well explained there are really a couple major sources of "high level nonlinearity" in speakers-------a very short 20K burst into a speaker with a high mass coil is not gonna generate much heat rise------BUT it is likely to tax the linear limits of the suspension---------Dan -------send me an e-mail-----the guys just left that came with the Brahma for the test-----we hit a maximum of 32K before audible stress------but i learned something that had escaped me till now ( concerning the DUMAX tests) and i think you might be interested in thinking it over before i post the entire results..........RC

HOLY CRAP - and RC, will you let us know what you're talking about here once Dan has thought it over - i'm very interrested, mostly because i can't know ------- and will you comment on which you prefer - the w7 or brahma?

[ October 21, 2002, 10:47 PM: Message edited by: Justin H Goodman ]

[thoraudio1]

Don't ya just hate teasers like that.... [img]smile.gif[/img]

I wish I could have been there, it was probably fun....

I'm guessing that because the Brahma is almost a linear force (the force at the limits of xmag is still substantial) driver, and the xmech of the venuzalun (sp?) basket is just above Xsus, that it does some funky things to the suspension when hit with 32kw.

but what do I know...

[ October 21, 2002, 11:11 PM: Message edited by: thoraudio1 ]