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17th January 2012, 07:49 AM  #1381 
diyAudio Member
Join Date: Jun 2007

Hi Djim,
At this stage I am just trying to understand the significance of a 1W/1m sensitivity figure calculated at the frequency of the lowest impedance within a given "passband", and how this information can then be applied in practice. It seems to be a rather arbitrary measure in that the frequency at which sensitivity is calculated, varies from speaker to speaker. How would the required upper and lower boundary frequencies for the "passband" be determined for a given loudspeaker? Kind regards, David
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17th January 2012, 03:14 PM  #1382  
diyAudio Member
Join Date: Dec 2009
Location: Les PaysBas

Hi David,
Quote:
Its not I'm suggesting an average number of its sensitivity throughout the bandpass, but instead of the usual SPL plot based on voltage, I am suggesting an SPL plot based on 1Watt/1m with one click of a button. Usually for horns the lowest impedance can be found between their 1/4 wavelength and the 1/3 wavelength of the system. Also usually this is the lowest impedance throughout the entire frequency range. Last edited by Djim; 17th January 2012 at 03:17 PM. 

18th January 2012, 06:30 AM  #1383  
diyAudio Member
Join Date: Jun 2007

Quote:
Now you've really got me confused :). As mentioned in an earlier post, the Maximum SPL tool can generate an SPL plot based on 1Watt/1m. I thought however, that what you wanted was a single sensitivity figure  calculated at the frequency of the lowest impedance within a specified passband, similar to the attached screenprint. Assuming that was the case, I was trying to understand why the sensitivity figure was required at the frequency of lowest impedance, rather than at say, the highest impedance within the passband. Incidentally, the attached screenprint shows the result for the default Hornresp record with resonances masked. Does this value seem right to you? Kind regards, David
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18th January 2012, 02:30 PM  #1384 
diyAudio Member
Join Date: Dec 2009
Location: Les PaysBas

Hi David,
Sorry David if my explanation became somewhat fussy, but it’s just the SPL plot but with a fixed 1w/1m readout. About the question if the value seems right, is difficult to say without any info of the driver model you are using ;) I made an example of how it could look like to prevent miscommunication. It would be terrific if it could be made in such way that when you turn to "Loudspeaker Wizard" that whatever change you make, it always shows the 1W/1m response in the Wizard (if chosen for Sf function) . I'm not sure if that is possible but it would be a very handy feature. Hopefully this explains everything :) Last edited by Djim; 18th January 2012 at 02:47 PM. 
18th January 2012, 10:35 PM  #1385 
diyAudio Member
Join Date: Feb 2004

I think what might be causing confusion is when we talk about thing in terms of constant power, we are making a useful abstraction. But in a sense, we are defining a state that doesn't really exist, since we almost always drive speakers with constant voltage, not constant power. A computer simulation can easily calculate using power as a constant, but when driven with realworld amplifiers, voltage is the constant.
As Djim said, when we make voltage the constant, then low impedance speakers get more drive current, and are therefore, louder. That's why many of us like to compare SPL with power being constant, not voltage. The problem then becomes, what frequency do we choose to measure impedance at, to know what voltage drive level to run when making our tests? Since impedance changes with frequency, what impedance do we choose? Zmax? Zmin? Average impedance? Or just use the manufacturer's advertised impedance and call it good? Where I have run across this dilemma is when comparing hornsubs. It is much like the problem of distance  Too far away and signaltonoise drops and/or drive level must be excessive. Too close and you're magnifying the ambiguity of the acoustic center. So how far away do we place the microphone and what drive level do we choose? I personally like measuring at 10M/100W for powerful hornsubs, and I like using Zmin to calculate drive voltage. When testing 2000 watt subs, a 100 watt test power level isn't large enough to shift the operating point too much. It allows measurement distance to be far enough that the acoustic center can be plus/minus a few feet and it won't matter, the ratio is small enough that a few feet of wiggle room is less than a decibel. Likewise, using Zmin is measurable and concrete, and there is less than a decibel difference between a drive voltage that produces a power level at Zmin and the voltage required to get the same power level at Zavg. Zavg would take some relatively complex calculation  You would need to take the impedance curve, bring it into a spreadsheet, define the passband, find the area under the curve between the low and high frequency points and average them to get Zavg. Zmin is much easier to use, you can see it at a glance, and the decibel difference is usually like a quarter decibel. Seems less ambiguous to me. So anyway, all that to say I think I understand Djim's dilemma. I think he is trying to use Zmin to define a "standard" drive level for comparison. In the Hornresp model, it isn't needed, since the calculations can be done with constant power. But in the real world, one will probably be driving the speakers with constant voltage, so an understanding of impedance is required. 
18th January 2012, 11:53 PM  #1386 
diyAudio Member
Join Date: Aug 2009

Re Zmin @ 1w/1m
I believe it depends on the f3 of the cab. Most if not All cabs will be used with a HP filter, which along with the box fb, defines the f3. We are talking about subs here afterar all
Therefore i suggest that taking Zmin as whichever is the lowest from between f3 & the next impedance minima, could be preferable. Without taking the HPF into consideration, the Actual real world performance might not be attained from simulation. 
19th January 2012, 01:19 AM  #1387 
diyAudio Member
Join Date: Feb 2004

Yes, definitely. I should have been more clear. When I said "Zmin", I meant minimum impedance above cutoff, not DC resistance.

19th January 2012, 02:06 AM  #1388 
diyAudio Member
Join Date: Dec 2009
Location: Les PaysBas

Thanks Wayne, for explaining my 'dilemma', although typing in English still seems to be my biggest dilemma :)

19th January 2012, 03:42 AM  #1389 
diyAudio Member
Join Date: Apr 2009

That is how I look at it as well. Voltage that supplies 1w into the minimum impedance in the useful pass band. This is where current is highest and the load on the amplifier is usually heaviest. With a higher minimum impedance an amplifier can usually provide more voltage before clipping at that point which can negate the apparent extra voltage sensitivity on the lower impedance cab.

19th January 2012, 07:23 AM  #1390  
diyAudio Member
Join Date: Jun 2007

Hi Djim,
In that case, as far as predicting sensitivity is concerned, I still don't understand why you just can't use the Maximum SPL tool with Pmax = 1 and Xmax = 99. Voltage Eg is automatically adjusted by Hornresp to maintain a constant 1 watt input, even though the load impedance changes with frequency. Quote:
Kind regards, David
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