Any thoughts on the sound field used in this setup? With the mics that close to the AMT, this isn’t a free field or diffuse field condition. I suspect the size of the mic/capsule and the implementation of the diffuser around the capsule might have quite some influence on the results.
It is more complicated by the fact that the smaller driver exhibits a higher SPL in the nearfield than a larger driver! For a typical 6.5 inch midwoofer and a 1 inch dome tweeter having the same SPL in the farfield , the difference between both drivers is around 26dB !!! So measuring a large AMT that seems to be low on distortion gives a false impression of the distortion to be expected at the listening position ....
I'm not entirely convinced by the idea of varying mic distance from the source. For one thing, a flat baffle (or a large cone etc) would be needed to ensure that the tonal balance remains consistent.
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What about a mechanical source of waves, such as a set of tuning forks, or some other bells? Rich harmonics might obscure the noise floor, but then again maybe not. Metal blocks tend to have stretched harmonics that follow more of a square law rather than simple multiplication. If multiple sound sources could be brought together, while guaranteeing mechanical integrity (eg, metal blocks suspended on strings, or resting on felt, with interaction between the various parts kept to an absolute minimum), that could be a great natural reference source.
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What about a mechanical source of waves, such as a set of tuning forks, or some other bells? Rich harmonics might obscure the noise floor, but then again maybe not. Metal blocks tend to have stretched harmonics that follow more of a square law rather than simple multiplication. If multiple sound sources could be brought together, while guaranteeing mechanical integrity (eg, metal blocks suspended on strings, or resting on felt, with interaction between the various parts kept to an absolute minimum), that could be a great natural reference source.
When you have a look at the frequency response of the SM85 compared to the real measurement mic you know why it's NO measurement microphone.
You could at least use a M201 or an omni dynamic mic.
But as you show here - it's not needed if you simply use a proper measurement microphone.
You could at least use a M201 or an omni dynamic mic.
But as you show here - it's not needed if you simply use a proper measurement microphone.
Can you explain why you think this would alter the IMD performance of the mic? Or you don't think it's the mic that's the source of the IMD?
If comparisons against standard measurement mics show substantially lower IMD why wouldn't it be applicable to the one role of IMD measurement mic?
Note, that there is a trade-off between the capsule slef noise floor and the THD level. The higher the mics capacitance is, the lower its self noise is (and higher sensitivity), on the one hand. On the other hand, the higher capsule capacitance results in higher THD level.
Its simply the coupling transformers electro magnetic non linearities when being loaded to varying degrees on the secondary side. The more the transformer is saturated, THD and IMD will just go up. This happens at fairly low levels on an SM58 (and SM57 - its the same mic internally). At miniscule levels with light loading on the output, the transformer will be cleaner along with the lack of self noise, being its a passive component.
The SM57/58 capsule itself isn't bad and can handle North of 150 dB without significant distortion. The transformer sucks badly IMO though and is responsible for the most of the mics negative traits. Its not practically useful for measurement purposes due to the proximity effect it has as well as the capsule size being an issue.
I've gutted a few SM58s of their transformers and wired them direct. Its a completely different/cleaner sounding mic without it. Noise rejection can become a problem doing this and gain can suffer a little, but the sonic improvements are worth it alone, especially in the low end and upper midrange. That nasty treble peak also goes away, which is what made the SM58 so popular with live sound along with the feedback rejection and durability. The 58s design was based on the 545 and 55 back from the 50s. You can use it as a hammer in a pinch, dunk it in a pitcher of beer, yet it will still work afterwards. Otherwise its a somewhat mediocre dinosaur of a mic at today's standards.
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As well as the "sound signature" and "character" of a dynamic (moving coil) microphone being affected by (optional) transformer saturation, frequency response, proximity effect and polar response, there is also waveform distortion to consider .
Wired according to the Pin 2 positive standard, a condenser (capacitor) microphone's output electrical polarity and phase response is the same as the acoustical waveform, while a moving-coil (dynamic) microphone's electrical waveform lags by 90 degrees and distorts the amplitude of the initial transient as can be seen in the attachment above.
Back to transformer distortion, in the early 1980's Shure developed the transformerless SM78 and SM77, but the market didn't care for the sound, look or feel enough for them to continue production.
Art
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That's a differentiation to time, equivalent to a +20 dB/decade response, not the intermodulation distortion that this thread is about. It would be nice to know the context. I guess it relates to the far-field bass response of a directional microphone that compensates for proximity effect, because a response like that over the complete audio range would be unlistenable.
That word again. It's like there are two threads. In the one I'm reading a designer doing uncommon loudspeaker tests questioned the results of his tools, made a change, and discovered some widely adopted measurement microphones are seriously inadequate for IMD measurement. By changing a variable it was additionally discovered loudspeakers may have much better - near amplifier - IMD performance than was previously expected and went to the next step.
The other thread is a critique centred around demands for absolute measurement accuracy, 'improper' devices and lacking formalism without explaining how the interim step of using an SM58 invalidates the unexpected loudspeaker IMD discovery later confirmed and exceeded with the AKO.
Honestly don't see the point.
Obvious on the first point, as noted at least three sample microphones returned higher IMD than the loudspeaker. That alone argues against the second regarding IMD, unless you saw something in the protocol indicating improper use.
It's easy. Cheap "measurement microphones" have to much noise for this measurement. Have a look in the datasheet.
Dynamic mics have low self noise but an SM85 has serious frequency response issues. (and some others)
A good measurement mic can do both without problems. So use one if you want to perform these very special measurements.
Done.
Dynamic mics have low self noise but an SM85 has serious frequency response issues. (and some others)
A good measurement mic can do both without problems. So use one if you want to perform these very special measurements.
Done.