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Old 21st January 2012, 04:51 PM   #1421
tb46 is offline tb46  United States
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Hi Djim,

Post #1420: "In the industry they usually measure the current. This allows changes in impedance at different power stages."

Could you, please, expand on that a little, maybe with an example? I just dont' get what you mean.

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Old 21st January 2012, 05:02 PM   #1422
AndrewT is offline AndrewT  Scotland
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Djim,
thanks for an answer.
Wayne,
I read your reply "how I do it", but that is not how voltage sensitivity, nor power sensitivity are carried out on actual speakers.

I think and said so, back in my question that, just as Djim has confirmed, they measure Voltage and Current.
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Old 21st January 2012, 05:34 PM   #1423
Djim is offline Djim  Netherlands
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Hi Oliver and Andrew,

Using an impedance chart to calculate the impedance has its limits. These impedance charts are often measured with relative small signals compared to 'normal' powering.

By measuring the Current, Voltage and frequency, all in the same time you can calculate the impedance per frequency in relation to all powering stages. This gives a much higher accuracy especially for someone who wants to measure at 100W/10m. It also shows what happens to the load when a driver comes near its Xmax.

If you prefer to measure sensitivity based on complex signals (noise for instance), measuring the current is the only way to calculate the right power values in relation to impedance.

Edit: Most DIY don't have the possibility to measure the current since their volt meters are not RMS or suited for audio measurements. In that case they have to rely on impedance charts.

Last edited by Djim; 21st January 2012 at 05:43 PM.
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Old 21st January 2012, 05:49 PM   #1424
tb46 is offline tb46  United States
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Hi Djim,

Thanks for the explanation.

Regards,
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Old 21st January 2012, 07:18 PM   #1425
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There is no standard method of arriving at a sentitivity figure. Some use 2.83v, side-stepping power. Others use a calculated voltage, much as I've described. Even the measurement distance varies from manufacturer to manufacturer. Most scale back to 1W/1M or 2.83v/1M, but some don't even do that. The better ones at least describe their measurement process.
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Old 22nd January 2012, 01:35 AM   #1426
Zero D is offline Zero D  United Kingdom
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Interesting deviation from a HR simulation & a real world build & test here.

Attachment 261767

Quote:
Above is the impedance response of the stuffed system. The peaks and dips are a bit lower than the HornResp predictions (the green line) now, and the magnitudes are significantly different - not unexpected, as the damping would cause that.

The Subwoofer DIY Page v1.1 - Projects : A "Proof of Concept" tapped pipe** - introduction
The Revc was actually tested & was found to be 3.55 Ohms. HR shows very close agreement @ various Impedance minima throught the FR range, without any damping

I realise that the difference is due to damping being added, & that not all designs include it, but it does i believe highlight an area for further discussion etc.
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Old 22nd January 2012, 05:03 AM   #1427
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Quote:
Originally Posted by Wayne Parham View Post
I'm sure you could use either Re or Zmin and get the same results to several significant digits.
Hi Wayne,

From tests I have done, it would seem that the SPL will typically be about 1 dB less when Re rather than Zmin is used. One advantage of using Re is that the results are conservative, or worst case.

As a refinement, using 1.25 * Re gives results very close to those generated using Zmin.

Kind regards,

David
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Old 22nd January 2012, 05:07 AM   #1428
Djim is offline Djim  Netherlands
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Hi David,

You mean 1.25 x Zmin?
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Old 22nd January 2012, 05:39 AM   #1429
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Quote:
Originally Posted by Djim View Post
You mean 1.25 x Zmin?
Hi Djim,

No, I mean 1.25 x Re :-).

To illustrate - for the Hornresp default record, and using Wayne's suggested method to find Zmin:

Zmin = 7.52 ohms at 905 hertz (see attachment).

1.25 x Re = 1.25 x 6 = 7.50 ohms.

The SPL curve generated using 1.25 x Re = 7.50 ohms is effectively identical to the one generated using Zmin = 7.52 ohms.

Kind regards,

David
Attached Images
File Type: png Zmin.png (25.4 KB, 220 views)
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Last edited by David McBean; 22nd January 2012 at 05:43 AM.
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Old 22nd January 2012, 05:49 AM   #1430
Djim is offline Djim  Netherlands
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Hi David,

Sorry I meant Re, but to be honest there is no constant for the figure added to Re. Sub Horns have sometimes no more than 0.01 Ohm added.

But I'll give you a clue what I am looking for. There seems to be a relation between particle velocity (in the horn throat) and the figure added to Re (result of air resistance?)

The higher the particle velocity, the higher the figure added to Re.

Maybe that helps.

(but know I really need to look for my bed... :-)
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