Hello. I'd like to take non-linear distortion tests for several different subs, all varying in cone diameter. In the farfield, it seems that baffle effects would give me a varying spl that interferes with making a valid comparison. In the nearfield, I have the problem that I cannot get an accurate reference spl between subs with different cone diameters.
I am wondering how everyone else is doing these tests, or if there is anything I'm missing or any useful suggestions that can be offered.
I am wondering how everyone else is doing these tests, or if there is anything I'm missing or any useful suggestions that can be offered.
Hmm, how can there be baffle effects at sub-frequencies? The wavelength should be far longer than the size of the subwoofer.
If you know the T/S parameters of the subs you could calculate the voltage sensivity of the subs, and from there calculate which voltage to feed them with to get the same SPL.
If you know the T/S parameters of the subs you could calculate the voltage sensivity of the subs, and from there calculate which voltage to feed them with to get the same SPL.
Or you could measure the SPL inside the box and recalculate it to the outside free space level at 1 m, if you know the box volume. This is particularly easy if you have BR boxes.
It would seem to me that the easiest thing is simply to set the SPL in farfield or use a SPL meter to do so, and compare nearfield measurements. You can simply scale the data to match the same reference level, since you know the absolute levels match.
Maybe I can clarify this a bit... when I set the nearfield spl of a 10" and a 15" driver to the same reference point, and then measure again in the farfield the spl numbers are completely different. It's also quite audible to the ear.
I thought about calculating the efficiency from the t/s parameters, but it seems as though that doesn't take into account the sub's low end sensitivity.
I've also thought about doing what Tiroth suggested, but above around 200hz the rising response is quite different depending on the driver being tested, and I'm unsure how to figure that in. Also, there's a large hump in the frequency response below 100hz in the nearfield which disappears in the farfield. Is it a measurement artifact or inductance?
I peeked at Linkwitz's site and he says "The drivers under test must be operated at the same volume displacement for meaningful comparison of distortion produced. With the microphone very near to the cone the close-up sound pressure must be adjusted in proportion to the cone diameter so that the far-field SPL will be the same for different size drivers." Unfortunately, he never gives the adjustment.
I thought about calculating the efficiency from the t/s parameters, but it seems as though that doesn't take into account the sub's low end sensitivity.
I've also thought about doing what Tiroth suggested, but above around 200hz the rising response is quite different depending on the driver being tested, and I'm unsure how to figure that in. Also, there's a large hump in the frequency response below 100hz in the nearfield which disappears in the farfield. Is it a measurement artifact or inductance?
I peeked at Linkwitz's site and he says "The drivers under test must be operated at the same volume displacement for meaningful comparison of distortion produced. With the microphone very near to the cone the close-up sound pressure must be adjusted in proportion to the cone diameter so that the far-field SPL will be the same for different size drivers." Unfortunately, he never gives the adjustment.
What microphone are you using? It should be omni, otherwise you are into trouble. Just checking.
If the room is making the trouble, you could try outdoors, depending on the climate and environmental noise.
If the room is making the trouble, you could try outdoors, depending on the climate and environmental noise.
What is the purpose of the comparison?
In the Linkwitz example, he calibrated them as they were all being considered for the same speaker, thus it was relevant to compare them all for a given output level.
but ....
if you are comparing drivers of different size, are they to be used for the same application? You may compare a 10" vs a 15" sub, but would you use them for the same application? If not then why be concerned with calibrating them to see how they perform at the same output level? Perhaps you could consider comparing 12" drivers to each other, 15" etc ...
In the Linkwitz example, he calibrated them as they were all being considered for the same speaker, thus it was relevant to compare them all for a given output level.
but ....
if you are comparing drivers of different size, are they to be used for the same application? You may compare a 10" vs a 15" sub, but would you use them for the same application? If not then why be concerned with calibrating them to see how they perform at the same output level? Perhaps you could consider comparing 12" drivers to each other, 15" etc ...
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