Understanding - and exploiting the impedance curve? - diyAudio
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Old 19th October 2014, 04:42 PM   #1
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Default Understanding - and exploiting the impedance curve?

Hi there.

Trying to wrap my head around some of the data I've been logging for the past couple years... I compete in a car audio format that limits the amount of power (and cone area) we can use in a given class - so basically trying to build the most efficient setup possible for reproducing a single tone (typically between 58-60hz in my hatchback).

The metering equipment I have available are a TermLab, Fluke multimeter, and Fluke ammeter. I also have a Dayton Audio DATS that I typically take a impedance sweep with as well.

The current ported enclosure(s) I test with are pretty undersized (less than 2 cubes for three 10" woofers) to get the enclosure 'wanting' to peak around the desired 58-60hz. Depending on which enclosure I'm playing with, it has either a pair of 4" aeroports, or a single 6" aeroport. I then test - and systematically extend the port and take all the reading over and over, till I hit the point of diminishing return. This usually results in the port being decently long (~18"). When I look at the sweet in DATS, the point of highest efficiency occurs on the 2nd peak - but not necessarily at the tip, usually on the rising side (just a bit to the left of the 'peak').

Basically - what I've been noticing is the longer I extend the port(s), the higher the impedance peak - and more efficient the SPL to power ratio - (to an extent...), but at some point, I've altered the tuning severely enough that the score deteriorates.

So here's what I'm trying to understand... is the left peak the woofer/enclosure resonance, and the right the port resonance - or do I have that backwards? A smaller enclosure would raise the peak(s)? Less port area would also raise the peaks (to a degree)?

Last edited by n8skow; 19th October 2014 at 04:53 PM.
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Old 20th October 2014, 08:06 AM   #2
Sonce is offline Sonce  Macedonia
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Ported enclosure has two impedance peaks and a dip between. That dip is both the resonant frequency of the woofer (minimum movement) AND the port (maximum output) - it is the tuning/resonant frequency of the ported box. Optimal resonant frequency depends on Thiele-Small parameters and the size of the box and the tube. For maximum efficiency choose more port area.
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Old 20th October 2014, 12:25 PM   #3
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I'd definitely go for more port area.

Get the impedance dip around 60Hz, and you should be able to really make some noise.

Consider a 4th order bandpass, too. Play around in WinISD to get a feel for what does what.

Chris
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Old 20th October 2014, 01:29 PM   #4
wg_ski is offline wg_ski  United States
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This may be a crazy idea, and maybe not even legal. But what about tuning the second impedance peak to the drive frequency, and use a transformer to bring it down to 1 ohm or whatever? That would give you one hell of a boost on the same amplifier -- for that ONE tone. You could not run it at the dip at all - the amp really wouldn't like it. But it would be loud as hell at the anti-resonance, where the box really *is* efficient.

You'd need a driver with as high a Qms as you could find and the box would have to be braced VERY well to keep mechanical losses down (that influences the peak Z).
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Old 20th October 2014, 01:47 PM   #5
Sonce is offline Sonce  Macedonia
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It has been done - google for Philips BaryBass.
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Old 20th October 2014, 04:37 PM   #6
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Thank you for the explanation.
I'm using (3) 10" Kicker L7's in this setup - which get quite peaky in a box this small (good thing for burps).

I've currently got a pair of 4" aeroports - so you think adding a third would be an increase in spl? What's that going to do to my power though? Won't more port area lower the apparent impedance at the amp?


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Originally Posted by Sonce View Post
Ported enclosure has two impedance peaks and a dip between. That dip is both the resonant frequency of the woofer (minimum movement) AND the port (maximum output) - it is the tuning/resonant frequency of the ported box. Optimal resonant frequency depends on Thiele-Small parameters and the size of the box and the tube. For maximum efficiency choose more port area.
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Old 20th October 2014, 04:39 PM   #7
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Will try.
My two concerns with more port area are
1) won't the apparent impedance at the amp drop - and thus the amp want to put out more power?
2) The port length is getting fairly long - going to start running into physical limitations in the car going too much longer (which I would have to do when adding more port area).
Thoughts?

Quote:
Originally Posted by chris661 View Post
I'd definitely go for more port area.

Get the impedance dip around 60Hz, and you should be able to really make some noise.

Consider a 4th order bandpass, too. Play around in WinISD to get a feel for what does what.

Chris
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Old 20th October 2014, 04:41 PM   #8
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Unfortunately these woofers (L7's) are not very suited for a 4th (high VAS), probably better performance in a 6th, though I'm a bit limited in space for something like that...

Quote:
Originally Posted by chris661 View Post
I'd definitely go for more port area.

Get the impedance dip around 60Hz, and you should be able to really make some noise.

Consider a 4th order bandpass, too. Play around in WinISD to get a feel for what does what.

Chris
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Old 20th October 2014, 05:00 PM   #9
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by n8skow View Post
Thank you for the explanation.
I'm using (3) 10" Kicker L7's in this setup - which get quite peaky in a box this small (good thing for burps).
I've currently got a pair of 4" aeroports - so you think adding a third would be an increase in spl? What's that going to do to my power though? Won't more port area lower the apparent impedance at the amp?
Three 4" ports are better than two ports, but increase in the SPL is marginal, more important is a good flow of ear without turbulence. As you increase port area (three ports) you must increase the tube length - check if there is enough space inside the box.
One 6" has more port area than two 4" ports.
More port area will not lower the impedance.
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Old 20th October 2014, 05:09 PM   #10
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Can you simulate a series-tuned 6th order bandpass?

Those are the ultimate in one-note boom.

Hope your drivers are decent when it comes to heat dissipation - cone movement is minimal at the tuning frequency.

Chris
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