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Old 19th January 2012, 07:03 AM   #1391
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Quote:
Originally Posted by Wayne Parham View Post
I think he is trying to use Zmin to define a "standard" drive level for comparison.
Hi Wayne,

The real issue for me is identifying the upper and lower frequency limits defining the passband of interest. Once these two frequencies are known, it is then a simple matter to find the minimum impedance over that frequency range, and to calculate the voltage necessary to deliver 1 watt into that minimum impedance.

In the screenprint example included in my earlier post, I calculated the bandwidth using the same method as used to calculate corrected phase. That is, the upper and lower frequencies are those at which the SPL is 12 dB below the maximum value. However, from tests I have done, it is obvious that the frequency of the minimum impedance can change quite significantly depending upon the assumptions made regarding the specification of the bandwidth, so I am not sure how valid this whole process really is.

Am I correct in assuming that when you do your power tests, you set the amplifier to a constant voltage to deliver a certain number of watts into a notional impedance (Zmin), knowing that as the actual impedance varies with frequency, the input power will also vary somewhat.

I guess the ideal way to do the tests would be to sample at a number of selected frequencies, and adjust the input voltage each time based on the known impedance at that particular frequency, to maintain the desired constant power.

Kind regards,

David
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Old 19th January 2012, 10:05 AM   #1392
AndrewT is offline AndrewT  Scotland
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How do we carry out a sensitivity measurement of a real speaker?

Do we supply a band limited pink noise, or white noise, or some other defined noise, to a speaker and measure the voltage and the current at the speaker terminals and the SPL at the test distance?
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Old 19th January 2012, 02:36 PM   #1393
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Quote:
Originally Posted by David McBean View Post
The real issue for me is identifying the upper and lower frequency limits defining the passband of interest. Once these two frequencies are known, it is then a simple matter to find the minimum impedance over that frequency range, and to calculate the voltage necessary to deliver 1 watt into that minimum impedance.
Yes, that's why I use Zmin, minimum impedance above cutoff. Lower cutoff is easy to define, being the 1/4λ frequency of a horn or pipe, or the Helmholtz frequency of a reflex box. This method doesn't require you to define upper cutoff or bandwidth. You can consider the upper limit to be infinity, or an arbitrarily high frequency.

Quote:
Originally Posted by AndrewT View Post
How do we carry out a sensitivity measurement of a real speaker?

Do we supply a band limited pink noise, or white noise, or some other defined noise, to a speaker and measure the voltage and the current at the speaker terminals and the SPL at the test distance?
This is the way I do it:
Essentially, I take the speaker outdoors and use a swept sine to measure impedance. Then I examine the chart for Zmin, minimum impedance above cutoff. Next I calculate the drive voltage required to attain a specific reference power level. Then I set the (RMS AC) voltage drive to that reference value, e.g. 28.3v for 100 watts if Zmin is 8Ω. Then I position the microphone and run a sweep to get a response chart that shows SPL as a function of frequency. Note that 100 watts at 10 meters give the same figure as 1W/1M, i.e. 98dB@100w/10M is equivalent to 98dB@1W/1M. This is because 100w is 20dB greater than 1W, and 10M is 20dB less than 1M, so the two offset one another.

The Zmin method reduces the ambiguity of impedance, and the 100W/10M method reduces the ambiguity of the position of the acoustic center.
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Old 19th January 2012, 03:41 PM   #1394
Djim is offline Djim  Netherlands
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Since they are WL systems the 'bandpass' is between 1/4WL and full wavelength I would think.
Hi Wayne,

You need an upper frequency since there are speakers with impedance compensation build in. I agree for the theoretical model that isn't necessary but this excludes those rare occasions and seems to be theoretically correct.

Last edited by Djim; 19th January 2012 at 03:47 PM.
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Old 19th January 2012, 07:45 PM   #1395
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I would still use the same definition of Zmin, minimum impedance above cutoff. Seems useful for subs. You can define an upper cutoff point too, if you want, but I don't think it's necessary.

All we're trying to do is to find a consistent and reliable single-point impedance metric.

I wouldn't expect a subwoofer to have an impedance drop above the passband. Voice coil inductance makes it keep rising. Even if you hang a Zobel across it, this really just limits the rise. I can't imagine any subwoofer with a circuit that shorts the voice coil at HF. Sure would do a number on amplifiers.

I guess if you find an oddball case like that, you can deal with it as a specialized case. Probably want to limit the power from the amp at HF anyway.
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Old 19th January 2012, 07:57 PM   #1396
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by Wayne Parham View Post
I would still use the same definition of Zmin, minimum impedance above cutoff. Seems useful for subs. You can define an upper cutoff point too, if you want, but I don't think it's necessary.

All we're trying to do is to find a consistent and reliable single-point impedance metric.

I wouldn't expect a subwoofer to have an impedance drop above the passband. Voice coil inductance makes it keep rising. Even if you hang a Zobel across it, this really just limits the rise. I can't imagine any subwoofer with a circuit that shorts the voice coil at HF. Sure would do a number on amplifiers.

I guess if you find an oddball case like that, you can deal with it as a specialized case. Probably want to limit the power from the amp at HF anyway.
Hi Wayne,

I agree, accept that HornResp is not only used for designing subs. Therefore I do understand why David wants an upper limit.
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Old 19th January 2012, 09:34 PM   #1397
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Well, I was talking about measurements. You are talking about mathematical models. And really, even then, Hornresp is a horn simulator, not a loudspeaker simulator. So we're sort of talking apples, oranges and cherries here. Maybe we've gone down an unproductive path.

I was just suggesting the use of Zmin for hornsub measurements. Simulations don't really need this distinction since they can calculate under an assumption of constant power. But they can also be calculated for constant voltage too. I thought maybe that's what you were looking for when you asked for calculations using the impedance minimum. Then again, maybe not.
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Old 19th January 2012, 11:46 PM   #1398
Djim is offline Djim  Netherlands
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Hi Wayne,

Sorry for the confusion but to me it was all about HornResp in the first place. I know you can already use the "Maximum SPL" function in the "Tools" menu, for finding its 1w/1m response but it can become time consuming when you are going to compare or change between different modelled designs/drivers, as I tried to explain in my post#1378.

I also tried to explain how HornResp seems to be using some sort of steps in finding its lowest impedance point for its calculation and David explained to me in post#1379 it is limited by the graphic resolution of screen pixels.

Therefore I had the idea to implement it in a fixed function in "Input Parameters" so it can be used constant with the exact lowest impedance figure (and not stepped), what I tried to visualize in post#1384. For that David needs an exact description/formula of how to find the lowest impedance. Not just for sub horns since it needs to be working uniform for all designs in HornResp.

That way you can compare with other designs and different drivers without switching between several menu's in HornResp. Now I have to use Photoshop to make all these comparisons visual in the same graph and than everybody can use it and compare their modelled designs or drivers with the "Compare Previous" function.

Last edited by Djim; 20th January 2012 at 12:02 AM.
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Old 19th January 2012, 11:56 PM   #1399
Zero D is offline Zero D  United Kingdom
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@ Wayne Parham

Quote:
Yes, definitely. I should have been more clear. When I said "Zmin", I meant minimum impedance above cutoff, not DC resistance.
Hi, i'm presuming you were responding to my Post # 1386 ?

I'm surprised nobody else considers this of importance in simulation, as the DC resistance has little or no bearing on real world listening etc.

Unfortunately there is no HPF in HornResponse, or plans to include one If there was, i believe it would make a big difference to the simulations versus real world conditions. Xmax & other large signal factors, would then be more accurately portrayed. As it stands, simulations can appear less favorable than actually acheivable.
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Old 20th January 2012, 12:15 AM   #1400
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Originally Posted by Zero D View Post
I'm surprised nobody else considers this of importance in simulation, as the DC resistance has little or no bearing on real world listening etc.

Unfortunately there is no HPF in HornResponse, or plans to include one If there was, i believe it would make a big difference to the simulations versus real world conditions. Xmax & other large signal factors, would then be more accurately portrayed. As it stands, simulations can appear less favorable than actually acheivable.
As far as tapped horns, the minimum impedance is usually quite close to the DC resistance, so is important when choosing an amp to drive the load.

A simulated HPF still would not exactly predict LF excursion.

When you see simulated LF excursion rising to and above Xmax (below the Fb/Fc impedance minima) placing the HPF at around that point is a good choice for starters.
Then measure excursion on the real box and see what voltage level does what, and how low the filter can be lowered- unfortunately real world filters do not all have the same response using different DSP units.

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