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Author Topic: New Taper here !  (Read 1553 times)

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Offline Gutbucket

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Re: New Taper here !
« Reply #30 on: September 17, 2020, 06:09:54 PM »
BSM-7 specs:
Frequency response: 20- 20,000 Hz
Signal to noise ratio: 62dB, 1khz at 1pa
Open circuit sensitivity : -35dB (5.6 mv) re 1v at 1pa
Dynamic range: 95dB, 1kHz
Maximum Input Sound Level: 105dB SPL, 1kHz at 1%

Self-noise level is not indicated above, but can be inferred from the highlighted specs.  I'll do my best to decipher that for you, but the quantification of noise is complicated. If DSatz happens upon this thread, I humbly ask him to correct me anywhere I've gone astray..

A microphone's self noise is expressed in terms of equivalent noise level, most commonly as an RMS A-weighted measurement, less commonly as a quasi-peak CIRR / ITU-weighted measurement, sometimes both. The two measures give different numbers.  RMS is essentially the average level over a short time interval (good for characterizing consistent noise) and quasi-peak is the peak level within the audible range (better for capturing transient noise spikes which peak above the general average). A-weighting verses CIRR or ITU noise weighting refers to different filtering curves that attempt to provide a meaningful match with the human perception of noise at low levels.  Due to the differences in curve weighting and time-interval, a CIRR or ITU quasi-peak self-noise measurement value will be significantly higher than a A-weighted RMS measurement.  It's a more accurate estimate of actual human perception of the noise, but is less frequently reported because it doesn't look as good on a specification sheet. Okay, with that out of the way..

The Signal to Noise Ratio of a microphone is a measure of the interval between the level of self-noise (RMS, A-weighted) and a 94dB (1 pa) reference SPL.  The signal to noise ratio of BSM-7 is listed above as: 62dB, 1khz at 1pa. 94-62 = 32, so we might assume an (optimistic) RMS A-weighted self-noise level of 32dB at 1kHz for the BSM-7.   

We can't determine the more realistic CIRR or ITU noise level rating from the specifications provided, but consider as an example the miniature low-voltage DPA 4060 omnis I use, which are small low-voltage powered microphones like the MM's, yet more costly.  They perform very well and are popular with music tapers.  Their self noise level is specified as:

Equivalent noise level, A-weighted: Typ. 23 dB(A) re. 20 µPa (max. 26 dB(A))
Equivalent noise level, ITU-R BS.468-4: Typ. 35 dB (max. 38 dB)

^
As you can can see, in this case the ITU noise spec is 12dB higher than A-weighted.

Comparing self-noise A-weightings, the 4060 is significantly quieter than BSM-7 (nominally by 10dB but could easily be more), yet 4060 is not the quietest microphone available by far.  It is generally considered quiet enough for professional use on actors in stage-productions and for close instrument mic'ing for live classical concert use, yet not quiet enough for professional classical recording with the mics further away in quiet performance conditions without an audience.  In my experience it's self noise is not low enough for very quiet close-mic'd nature and environmental recordings that will be amplified and used at levels considerably higher than that of the original sound.  In such a case the microphone's self-noise is amplified enough that it does become apparent.  However, their self noise is low enough for all of my music recording needs including live classical music performed in specially built quiet halls with near-silent audiences.  In that case, the acoustic noise floor of the hall is higher than the self-noise of the microphone so the noise of the hall determines the noise floor of the recording.  The same would remain true if I substituted a quieter microphone. [edit- I'm not proselytizing for DPA 4060 here, only using it as comparative example, both in terms of specs and in my personal experience with its self-noise level]


A microphone's dynamic range is the difference between it's self-noise level (RMS A-weighted) and its maximum SPL level.   Since max SPL is always higher than 94dB, the mic's dynamic range will always be higher than its SNR.

BSM-7 has a specified dynamic range of 95dB @ 1kHz, and a max input  level of 105dB SPL @ 1kHz, before distortion exceeds 1%.  But 105-95= 10dB, not 32dB as figured by the SNR spec above, so what gives?  I think that has to do with the 1% distortion constraint.  The microphone can accept higher SPLs before eventually clipping, but only with increasing levels of distortion  Doing the math the other way, a 32dB A-weighted noise floor and a 95dB dynamic range suggests outright clipping at an SPL of 127dB, which is reasonable for that type of microphone.

Whew! Apologies for the long post, but thanks for the mental exercise.  I hope that sheds some light on mic self-noise.
« Last Edit: September 17, 2020, 06:17:27 PM by Gutbucket »
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Offline Schr

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Re: New Taper here !
« Reply #31 on: September 17, 2020, 07:47:29 PM »
BSM-7 specs:
Frequency response: 20- 20,000 Hz
Signal to noise ratio: 62dB, 1khz at 1pa
Open circuit sensitivity : -35dB (5.6 mv) re 1v at 1pa
Dynamic range: 95dB, 1kHz
Maximum Input Sound Level: 105dB SPL, 1kHz at 1%

Self-noise level is not indicated above, but can be inferred from the highlighted specs.  I'll do my best to decipher that for you, but the quantification of noise is complicated. If DSatz happens upon this thread, I humbly ask him to correct me anywhere I've gone astray..

....

Whew! Apologies for the long post, but thanks for the mental exercise.  I hope that sheds some light on mic self-noise.

Wowza!! V interesting, go you. Is there a way that I can test this maths, by making a recording in a quiet room and looking at it in pro tools for example?

Offline fireonshakedwnstreet

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Re: New Taper here !
« Reply #32 on: September 17, 2020, 11:31:33 PM »
Welcome! This is a great place, as you can see!
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Offline Gutbucket

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Re: New Taper here !
« Reply #33 on: September 18, 2020, 10:00:34 AM »
Is there a way that I can test this maths, by making a recording in a quiet room and looking at it in pro tools for example?

You can set things up in such a way that you are pretty sure all other sources of noise are lower than the microphone's self noise - both those in recording environment and the signal chain, make a recording, then analyse the resulting file to determine its noise floor level.  That may mean finding the quietest part of your home, burying the microphone in a sealed box under a bunch of heavy cushions or something, turning off the air-conditioner and any noisy appliances and doing it late at night. Also need to know the gain through the recording chain, which you subtract from the whatever you determine the noise floor to be when looking at the waveform.

TS member Jerryfreak recently started a new thread dedicated to the measuring performance of audio interfaces and has expressed the intent to extend the discussion to DIY testing of recording gear in general. He's done a good bit of DIY microphone and recorder testing recently and posted about it in other threads here, and is better versed than I in setting up the correct testing regimen.  I'm interested in seeing a thread dedicated to DIY testing of microphones, and would love to see a new subforum for collecting threads on testing methodologies and results.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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Offline Scooter123

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Re: New Taper here !
« Reply #34 on: September 19, 2020, 03:19:37 PM »
Welcome

The CM3s are one of the best microphones in the sub $500 range.  Very flat.  A little large for my purposes. 
Regards,

Scooter123

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Offline jerryfreak

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Re: New Taper here !
« Reply #35 on: September 19, 2020, 03:43:07 PM »
BSM-7 has a specified dynamic range of 95dB @ 1kHz, and a max input  level of 105dB SPL @ 1kHz, before distortion exceeds 1%.  But 105-95= 10dB, not 32dB as figured by the SNR spec above, so what gives?  I think that has to do with the 1% distortion constraint.  The microphone can accept higher SPLs before eventually clipping, but only with increasing levels of distortion  Doing the math the other way, a 32dB A-weighted noise floor and a 95dB dynamic range suggests outright clipping at an SPL of 127dB, which is reasonable for that type of microphone.

sadly, statement of this spec is not standardized. some manufacturers like schoeps list max spl at 0.5%. others like DPA use 1%

worth noting that, as peaks go, 1% distortion on transients is probably still not very audible
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Offline wforwumbo

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Re: New Taper here !
« Reply #36 on: September 22, 2020, 12:04:30 PM »
Greetings, Schr! Welcome to this weird little world. I myself come from a studio background - glad to know I’m not alone ‘round these parts anymore. We should nerd out about compressors sometime in another thread that won’t derail the topic at hand,

To add to what’s been said so far re: your old noisy preamp... how old is it, and when’s the last time it was serviced? You might need a few recaps, especially in the power section. I’ve got an eventide rack harmonizer here that is emitting a weird whiny hum, and my debugging is pointing to the filter caps needing replacement.

Those CM3s you have should be okay even if doing field recording. I don’t think their self noise is high enough to be evident even in high dynamic range scenarios, I wouldn’t think they’re the problem.

On the whole though, welcome. Ask lots of questions, take everything with a grain of salt (especially if it comes from me, and especially if it’s me telling you to take things with a grain of salt), and enjoy what this wonderful community has to offer.
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Offline DSatz

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Re: New Taper here !
« Reply #37 on: September 22, 2020, 02:35:33 PM »
schr, since I can't send you these PDFs via private message (to my knowledge), here they are for you to download. One is the sales brochure for the Sound Devices MixPre from when it was still identical to the Shure FP24, down to the coax connector used for powering the preamp. The other is the owner's manual from a little later; by that time SD had changed over to a 4-pin Hirose power connector, but as far as I'm aware, that was the only substantive revision in the product.

--best regards
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Offline Schr

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Re: New Taper here !
« Reply #38 on: September 22, 2020, 05:36:18 PM »
Greetings, Schr! Welcome to this weird little world. I myself come from a studio background - glad to know I’m not alone ‘round these parts anymore. We should nerd out about compressors sometime in another thread that won’t derail the topic at hand,

To add to what’s been said so far re: your old noisy preamp... how old is it, and when’s the last time it was serviced? You might need a few recaps, especially in the power section. I’ve got an eventide rack harmonizer here that is emitting a weird whiny hum, and my debugging is pointing to the filter caps needing replacement.

Those CM3s you have should be okay even if doing field recording. I don’t think their self noise is high enough to be evident even in high dynamic range scenarios, I wouldn’t think they’re the problem.

On the whole though, welcome. Ask lots of questions, take everything with a grain of salt (especially if it comes from me, and especially if it’s me telling you to take things with a grain of salt), and enjoy what this wonderful community has to offer.

Thanks! Much appreciated! I’ll have to be honest in that I’ve been focusing so hard at the job at hand for the past 5 years, I’ve really moved away from all my music production technical knowledge, so I wouldn’t have much nerdy stuff to say! Also hence why I’ve joined this forum, trying to soak up some of this technical knowledge!

I was mistaken and I just have a 9v battery thing, not a preamp (shows what I know, heh!). I’ve solely used it for my stereo bsm-7s and I note that it sounds noisier than it used to. Something to address in the future me thinks.

For now I’ve got DSatz Shure FP24 on the way and I’m looking forward to doing some mono FX recording with that, my CM3 and either m10 or Tascam DR100mk2   

Thanks for your help everyone !!

 

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