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Thread: Signal to Noise Ratio and Mic Self Noise - How does it work?

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    normington is offline Junior Member
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    Signal to Noise Ratio and Mic Self Noise - How does it work?

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    I'm struggling to get my head around the two different figures that microphone specifications seem to give.

    The first is Signal to Noise Ratio, given in dB. Is a high rating good here?

    The other is the self noise, given in dBA - what does dBA mean? And it seems that the reverse is true here, is a low rating good?

    Why do some specifications give a Signal to Noise Ratio, while others give a self noise rating? This makes it difficult to compare one mic to another, because I don't know how these two ratings link together?

    Thanks,
    Andy

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    tarnationsauce2's Avatar
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    Quote Originally Posted by normington View Post
    I'm struggling to get my head around the two different figures that microphone specifications seem to give.

    The first is Signal to Noise Ratio, given in dB. Is a high rating good here?
    Yes

    Quote Originally Posted by normington View Post
    The other is the self noise, given in dBA - what does dBA mean? And it seems that the reverse is true here, is a low rating good?
    Yes
    dBA is a noise measurement that takes into account the way the human ear perceives the audio frequency range. For example if a mic has 20dBA, all on it's own it will create a noise that is 20dB over the theoretical lower threshold of human hearing. Basically dBA is a useful way to measure noise because it discredits frequencies we don't hear as well (or at all). But using a different weighting it might be have more or less dB of noise.

    Think of the self noise as "hiss". The less hiss the better.

    Quote Originally Posted by normington View Post
    Why do some specifications give a Signal to Noise Ratio, while others give a self noise rating? This makes it difficult to compare one mic to another, because I don't know how these two ratings link together?
    I'm not sure why one manufacturer would give certain specifications and not others. It's lame. To confuse you even more there are dynamic range and max SPL too.

    Let me try and break it all down for you:

    • Max SPL = The point where the mic distorts, or clips the waveform. [More=better]
    • Self noise = The amount of noise the mic creates all on it's own. (i.e. hiss). [Less=better]
    • Dynamic range = The range between self noise and Max SPL. [More=better]
    • Signal to Noise ratio = The range between self noise and a reference signal. [More=better]


    One caveat is if you compare specifications of mic A to Mic B, be sure they use the same weighting, references, or THD.
    For example Mic A has Max SPL = 125dB @ 0.5% THD versus Mic B that has Max SPL 127dB @ 1.0% THD.
    They use different "THD" reference points. In actuality mic A could have a better Max SPL than mic B @ 1.0% THD.

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    normington is offline Junior Member
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    Thanks, that's helpful.

    Is there a way to convert a signal to noise ratio into a self noise rating, or is this impossible because of the different nature of the two readings? If not, is there at least a way you can estimate the value of one from the other?

    What would you say is a 'average' and 'good' figure for each of the ratings to be?

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    Quote Originally Posted by normington View Post
    Thanks, that's helpful.

    Is there a way to convert a signal to noise ratio into a self noise rating, or is this impossible because of the different nature of the two readings? If not, is there at least a way you can estimate the value of one from the other?

    What would you say is a 'average' and 'good' figure for each of the ratings to be?
    SNR = 94 - self-noise. Both measurements are usually A-weighted, but not always, and some manufacturers state a few different noise weighting measurements.

    Average self-noise ratings depend on capsule size, which is the primary factor. 1/2" mics are generally 12dBA - 16dBA self-noise; 1" mics, 6dBA - 10dBA. Very small diaphragm mics (1/4" / 6mm or less) will be more like 20dBA - 24dBA. Tube mics may have higher ratings; very old designs tend to be higher as well.

    Any rating that is too much higher than those is an indication of a suspect design--you will see a lot of cheap mics advertised as LDCs with self-noise ratings like 17dBA - 20dBA. There should be no reason for that, if those noise ratings (and capsule size) are accurate.

    It's also possible to lower a noise rating by increasing polarization voltage, to some extent (you need to double voltage for a 6dB decrease). That also changes the capsule's response; in fact if you crank the voltage too high, you can suck the diaphragm right onto the backplate! But if you see a manufacturer with super-low ratings on their mics (there are a couple of suspects I am thinking of), that's probably why.

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    Quote Originally Posted by tarnationsauce2 View Post
    [*]Signal to Noise ratio = The range between self noise and a reference signal. [More=better][/LIST]
    Quote Originally Posted by mshilarious View Post
    SNR = 94 - self-noise.
    So the reference signal is 94 (and all the manufacturers use it (albeit with sometimes different weighting)?)?

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    Quote Originally Posted by antichef View Post
    So the reference signal is 94 (and all the manufacturers use it (albeit with sometimes different weighting)?)?
    Yes, the standard reference signal for microphones is 1kHz at 94dBSPL. The weighting factor for 1kHz is 0dB in every scale I'm familiar with--stated another way, all those weights are referenced to 1kHz. Such that if you have an A-weighted self-noise figure of say 14dBA, then your dynamic range is always 80dB (A, but often not stated as such).

    A-weighting is pretty close to a first-order bandpass filter (6dB/octave slope) with corner frequencies of 10kHz and 500Hz. The rolloff accelerates to higher orders below 100Hz, but basically that gives you an idea of the range being measured--you can't easily hear noise about 10kHz, and below 500Hz, it's usually drowned out by program material, and you don't notice it as much anyway, even though it is technically audible.

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    Quote Originally Posted by tarnationsauce2 View Post
    One caveat is if you compare specifications of mic A to Mic B, be sure they use the same weighting, references, or THD.
    For example Mic A has Max SPL = 125dB @ 0.5% THD versus Mic B that has Max SPL 127dB @ 1.0% THD.
    They use different "THD" reference points. In actuality mic A could have a better Max SPL than mic B @ 1.0% THD.
    In fact, it is pretty much certain that mic A has a better Max SPL. What those specs are saying are that at 125 dB of sound, mic A produces a signal that differs from the ideal signal by no more than 0.5%. (I'm being exceptionally sloppy here; it's actually a fairly complex set of mathematical sums to get that 0.5% number, but that's good enough for here.) Mic B produces nearly twice as much distortion at very nearly the same volume level.
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    fcabanski is offline Newbie
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    Question on this.

    The MXL 990 listed here: http://pro-audio.musiciansfriend.com...one?sku=271009

    shows S/N at 80dB but an equivalent noise level of 20dB. By the calculation described (S/N = 94 - self noise) the self noise would be 14dB rather than 20.

    Does this mic have more self noise? http://www.zzounds.com/a--2676837/item--AUTAT892CT4TH

    It shows S/N at 60, which by the formula (60=94 - self noise) self noise would be 34dB.

    And then this microphone: http://img3.musiciansfriend.com/dbas...pec/271593.pdf

    Lists self noise at 5dB, but by the formula (it's S/N is 88dB) it would be 6dB. Even though there's the difference between what they list and the calculation, it's about 2/3 quieter than the second one (the Shure)?

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    I think the numbers are fudged a bit between manufacturers. Since not all measure from the same distance from the mic and other factors. It's really only useful for comparing mics of the same manufacturer. If they use a consistent method across the board. And/or publish specs to start with.

    You must also consider the output level of a given mic. If it doesn't put out that strong of a signal you've got to boost the gain on your preamp which has it's own noise specs. So all things are not created equal. Yes it's a reference where there might otherwise be none. But trust your ears more than the specs. I just glance at SNR, then look at frequency response and max SPL. Since the last two are more apt to determine if you can even use the mic in first place.

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    fcabanski is offline Newbie
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    Of the three two are 20 - 20KHZ while the MXL is 30-20KHZ

    SPL:

    MMX is 103dB at .5%
    The NTI is 137 at 1%

    What's the difference between those specs - .5% as opposed to 1%.

    Still, it looks like the NTI has a slightly larger frequency range, less noise, and a better SPL. Yes or no?

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