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Can someone explain microphone self-noise and signal-to-noise in mic specs to me? Utility Plugins
Old 1st July 2011
  #1
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Can someone explain microphone self-noise and signal-to-noise in mic specs to me?

With a "signal/noise ratio", should the number be as high as possible?

What's the difference between a figure quoted in dB and a figure quoted as "dB-A"?

For example, here are some noise specs for the AKG C2000B. I'm deliberately quoting this mic, as I've always found it to be slightly noisy. In fact I'm surprised to read that it's got an SNR ratio of 74. That's perfectly respectable, no? But in practice, that mic has audibly much more self-noise than an AKG 414B, or any of my modest collection of ribbons and dynamics.

Sensitivity 20 mV/Pa (-34 dBV)
Max. SPL 140/150 dB (k=0,5%)
Equivalent noise level (IEC 60268-4) 30 dB
Equivalent noise level 20 dB-A
Signal/noise ratio (A-weighted) 74 dB
Old 1st July 2011
  #2
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http://www.lindos.co.uk/cgi-bin/Flex...cles&VIEW=full

... they use A-weighting because it used to be considered valid practice for noise measurements. It is now not really, and ITU-R 468 (Neumann quote CCIR 468-3, which is an outdated term for the same thing iirc) weighting should be used for noise measurements ... the use of A-weighting instead actually allows manufacturers to quote noise spec that is as much as ~12dB lower than what it should be quoted as if they had used ITU-R 468 ...

The audibility of the noise depends on its spectrum, which is why a weighting filter is required in the first place. However, it also depends on how our hearing responds to dynamic changes in the noise. ITU-R 468 specifies quasi-peak detection which mimics how our hearing responds to changes in loudness, while A-weighting has no such thing, and is generally measured with rms detection (mathematically sound, but not related closely enough to how auditory perception works).

There are some good articles around about what to watch out for when reading audio equipment spec. Here's one: Audio Specifications
Old 1st July 2011
  #3
Reference levels are what these specs are all about.

One company uses dbu = .775 volts. AKG uses dbV = 1 volt.

Without common reference levels, specs mean little. dbV buys you some extra 'spec' over dbu. A weighting noise also buys you a few more db's of s/n ratio.

Basically, you need to know the math or be an EE to translate all these various specs into a comparative table.

Organizations like AES used to do stuff like that before they were taken over by the code writers.
Old 1st July 2011
  #4
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Quote:
Originally Posted by Jim Williams View Post
Organizations like AES used to do stuff like that before they were taken over by the code writers.
I'm not gonna name any names, but there are some manufacturers out there who would get a *****-slap from the AES if I had my way heh
Old 1st July 2011
  #5
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Greg Curtis's Avatar
 

That is a noisy mic, according to the specs.

But, yours could need repair, also.

Compare it to a very quiet mic, like the TLM 103. TLM 103

Dig into the data, and you will learn how these specs work on your own.
Old 1st July 2011
  #6
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Yes, I know from past experience that it is indeed a noisy mic. I've never actually owned one - I just picked that one off the top of my head as being a "noisy mic I have known".

The "A-weighted" distinction looks like what to watch out for, more than anything. Looking at the TM103 specs, as you suggested, makes that very clear:

"Signal-to-noise ratio, CCIR1) (rel. 94 dB SPL) 76.5 dB
Signal-to-noise ratio, A-weighted1) (rel. 94 dB SPL) 87 dB"
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