Good dialog, Ivan, thanks. Let's continue to look at this.
Ivan Beaver wrote on Thu, 02 August 2007 12:15 |
The way you want to do it (using the minimum impedance) is out of industry standards, the ONLY thing you could even hope to compare would be between boxes measure at your shootout. Other boxes measured in different ways have no corrilation to your results-except maybe actual freq response, but with no absolute value assigned regarding sensitivity, becuase of the drive level you have choosen.
...
Yes I totally agree that the rated impedances are just arbritrary numbers, but at least there is some coorilation to the actual impedance graph, but you have to have some standards. Just ask how people feel about a 6 ohm loudspeaker and they will usually ask what is wrong with it so that it doesn't have a "standard" value. It is usually so ocnfusing for them to think outside of their little boxes of 4-8 16 ohms.
|
You have suggested that there is a "standard" for published advertised impedance. I do not believe there is one. Advertised impedance appears to be an arbitrary number to me.
Minimum impedance, on the other hand is not arbitrary. Minimum impedance is easy to measure, definite and useful for making these kinds of comparisons.
Let's look at some actual impedance charts, shall we? Then we can run the numbers and see what we come up with.
This is the impedance chart of a Fitzmaurice Tuba 36. Looks to me like Zmin is 9Ω. From the forumula E = √PZ, we find the drive voltage required to dissipate 100 watts in a 9Ω load is 30 volts.
What would you say average impedance is? Should it be 8Ω, because that's the closest multiple of 8? Should it be 12Ω, since that's the next higher multiple of 4? Or should it be closer to 10Ω, since averaging the area under the curve up to 150Hz puts it closer to that?
Let's run the numbers and see what each of the drive voltages would work out to:
28.3v into 8Ω dissipates 100 watts
30.0v into 9Ω dissipates 100 watts
31.6v into 10Ω dissipates 100 watts
34.6v into 12Ω dissipates 100 watts
Now let's calculate the difference between each of these in decibels:
The 28.3v (8Ω) level is 0.94x the 30.0v level, which is -0.25dB.
The 31.6v (10Ω) level is 1.05x the 30.0v level, which is +0.23dB.
The 34.6v (12Ω) level is 1.15x the 30.0v level, which is +0.62dB.
We're only talking about a half decibel max difference between Zmin and any sort of realistic average impedance. You actually could eye-ball an average value and be pretty close, like probably less than +/-0.25dB. But the minimum impedance method uses a tangible number that nobody has to argue about. It's what you see on the chart.
Honestly, Ivan, I think we could go with fixed voltage measurements and calculate impedance offsets, just like you've described. I would be comfortable with that, and might do it myself if I were viewing my own data.
But what we're talking about here is
confidence. This is an event where a lot of exhibitors will be bringing their best products to be compared with others. I think they would be more comfortable seeing what steps were taken to determine their SPL charts,
right there in front of them. They
see the impedance chart. They
see the power formula run. They
see the voltage set on the amplifier. And they then
see the SPL chart made during the sweep, and that's what they
see published later on the web.
No surprises.