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Understanding Mic Sensitivity Gain Structure

Discussion in 'Recording' started by bgavin, Jan 19, 2004.

  1. bgavin

    bgavin Guest

    Given: AT4040 has an open circuit sensitivity of -32dB/25.1mV in relation to 1 volt / 1 Pascal.

    Amplifying this signal, or raising it +32dB to 0dB produces a 1 volt output.

    Question: does this mean the amplifier must provide 32dB of gain to amplify the mic output to near line level?

    If so, then the mic sensivity specs are a good indication of the preamp gain requirements.

    Taking a worse-case example, the Beyer M201 dynamic full-range mic. It is rated at -58dB/1.2mV. Does this mean the preamp must have 60dB of gain to accomodate this mic?
  2. anonymous

    anonymous Guests

    Not really....what these specs give you is the relative sensitivity of the mic. In order to drive something to line level, you first need a source, which in turn generates your pressure changes (measured in Pascals). This is the big unkown factor...how loud is your source? That, combined with the mic sensitivity (and the headroom of the equipment downstream) will determine how much preamp gain is required.

    If you swap mics on the same source, then sensitivity can give you an idea of how much the gain has to change.

    Sensitivity is also usually a good indicator of noise level.


  3. realdynamix

    realdynamix Well-Known Member

    :d: Ok, you got me here. The 1 Pascal did it :d:

  4. AudioGaff

    AudioGaff Well-Known Member

    There is so much inconsistantcy in specs and measurements that you can't really believe or take them at face value. Many of the tests are done with test tones. I have never needed or wanted to record specs and test tones so they hold little value to me. Great sounding gear and great microphones have certain spec values associated with them but that is all after the fact and isn't really what makes them great. Class-A designs and tubes are all the rage again in pro audio gear now, but were being thrown away 20-years ago because of poor specs.

    Take a look at the specs of any of the cheap and/or crap sounding gear and they are almost always impressive but fail to deliver where it counts - in the sound...

    Mic preamp gain is not the problem it used to be and most modern mics now have as much as twice the output than years before. Any decent mic preamp will have enough gain for any decent microphone. Some ribbon mic's need extra gain but again, most decent preamps can handle them. And with some of real hot mics, you can bypass the preamp and go into a compressor or recorder but of course that doesn't mean that it will sound as you would expect. A lot of times the preamp is used more for it's color or tone than it is just for the gain.

    I know this doesn't answer the specific question, but that's because the source loudness is always an unknown variable and you can only get a specific answer using something like a test tone or tone burst and that doesn't have much useable value in the real world.
  5. bgavin

    bgavin Guest

    Sorry, I asked over your heads. I will rephrase.

    All mics have a generic sensitivity rating. With a volt meter across the open (not loaded) XLR terminals, it generates a specific voltage from a source of known loudness (pressure).

    Decibels are a relative measurement, and mic specs are relative to 1 volt. In this case, mics are referenced to a pressure of 1 Pascal which equates to a loudness of 94 SPL, 1 KHz, at the diaphragm. The mic produces X millivolts from this input. This is the sensitivity rating and is not subjective.

    The AT4040 output at 25mV is substantially higher than the Beyer M201 dynamic at only 1.2mV. Therefore, the AT4040 requires significantly less amplification factor (gain) in the preamp to produce a LINE level output of 1.23 volts.

    None of this has anything to do with variable source loudness. 3 different sensitivity mics in front of the same source, will produce 3 different output levels from that source. This is cut and dry engineering, and not focused on subjective qualities such as tone or sound quality.

    My question was entirely about matching the gain factor of a preamp to a given mic. The Beyer is a low output mic, and requires nearly 60dB of gain to produce a LINE output level. The preamp must be capable of 60dB gain, or it cannot deliver full output from the Beyer.

    Grace Design offers the 101 preamp with a High Gain option for use with low output ribbon mics. This option provides 20 ~ 70dB of gain, compared to 60dB gain for the standard model.
  6. teleharmonic

    teleharmonic Guest


    to what end do you want this information confirmed, explained, elaborated upon?

    i'm curious...

  7. anonymous

    anonymous Guests

    You can't say that this has nothing to do with source loudness when all the specs are referenced to 1 Pa which is equivalent to 94dB SPL @ 1khz.

    The problem I have is with this line:
    This is only true if your source is 94dB SPL...a common application of the Beyer 201 is snare drum, where you have about 120dB SPL (if not more considering the close mic technique and the cardiod polar pattern). This reduces your gain requirement considerably to get to line level.

    Also consider that mic sensitivity and max SPL will go hand-in-hand for this reason. A very sensitive mic will overload far easier than a less sensitive mic. Good luck using the AT4040 on a snare drum!

    Like I said in my first post, the relative change in sensitvity from one mic to another will give you an idea of how much change in gain at the preamp is required given that the source loudness and equipment downstream remain the same, but it does not tell you how much gain is required from your preamp, as that is a function of the source loudness itself, and the reference 0dBFS calibration (if recording digital) of the converter downstream. That is to say, if you are recording a Vulcan Lute with the 4040 and your preamp gain is set at 15dB, you'll need to change it to about 40 dB or so if you swap to the Beyer 201. If you're recording cricket farts on the other hand you may need 60dB on the 4040, and 95dB gain on the M201. When recording saturn V rocket launches, you'll want -30dB on the 4040 (I hope it has a built in pad), and 5dB gain on the M201. Source loudness is a factor in determining gain requirement.


  8. bgavin

    bgavin Guest

    I agree, source loudness is the final arbiter in how much voltage the mic produces, up to the specified limit for THD. It would be nice if more mics had this spec published. As it is, most publish only the sensitivity. I suppose I can calculate voltage from the published max input SPL, relative to 94.

    My point in all this is learning to spot a mic with a sensitivity that is close to optimum for the job at hand. I don't wish to make a poor choice then have to crank the preamp to +70dB gain to compensate.

    I want to place the mic right in the sweet spot of the preamp (i.e. middle of gain range) for the job at hand. Yes, the AT4040 is a very poor choice for close micing a snare.

    I have better things to do than record cricket farts, so I will leave that to others.


    [ January 21, 2004, 01:26 PM: Message edited by: bgavin ]
  9. anonymous

    anonymous Guests

    Just a note....but sometimes the sweet spot of a preamp is actully full up! Some pre's just attenuate the amplifier output, and thus are pure-est at their full gain (Langevin is a good example of this).

    For things like Neve's I like to run them at full tilt too, and then pad the output back down...this seems to result in a 'fatter' sound. This is probably a result of transformer saturation...whatever it is, I seem to like it!


  10. AudioGaff

    AudioGaff Well-Known Member

    Dr. F states it well. It does have something to with the source loudness and tone. By moving or padding the mic I can compensate or change both the tone and sensitivity of how much gain is needed. You didn't go over my head, I'm a real electrical engineer. But your question and it's answer has very little practicle value to working in the real world of recording as long as you have enough gain in the preamp for the mic to do it's job. Recording cricket farts has way more to do with the use and application of a mic than it does doing db gain math. Also, it is subjective as to where the sweetspot of a mic preamp is. It is often not in middle gain posistion and the so called middle sweetspot may not meet the desired tone goal as much as another spot in's gain range.
    You do what you have to do to get what you need. Very few mics need 70db of gain and the number of times that you'll use a mic that needs 70db of gain is likely to be even fewer. And even if you don't have a preamp that can achiecve 70db of gain, you can almost always make do with less gain if necessary for that one mic.
  11. bgavin

    bgavin Guest

    Then you can understand learning for the sake of learning, regardless of real-world use.

    I've been intrigued by the Beyer M201 series dynamics, for their flat and full range response. The 1.2mV sensitivity is much lower than my Shures, so my question was aimed at a typical preamp being capable of driving one.

    My Aardvark Q10 pair has up to 75dB of gain, in 3 separate ranges. My Rane (Burr-Brown) pres offer a gain range of only 12~50 dB, so I was concerned about the low output mics. As demonstrated above, this is not a concern when used in a high SPL situation.

    A number of us here, including me, will never own high end preamps or mics. I'm a computer and subwoofer engineering type, not a recordist. The best I will do is on-site live performance recording, so I don't have access to expensive gear or the ability to swap it around. My purchases must be made carefully.
  12. AudioGaff

    AudioGaff Well-Known Member

    Your Aardvard may be 75db of gain capable but I doubt that it is 75db of clean, stable, linear gain. You sure won't be anywhere near that sweetspot in the gain range you were worried about before. Without having a great mic pre you lose out on one of the best reasons on what makes a preamp great. There are other mics that are as flat and full ranged as the Beyer your looking at that would be much better suited to the mic pres you have. Don't take for fact and face value how flat any mic is from a plot. It almost never works that way in the real world of recording.

    And for the record, just because someone like me has a lot of gear, that in now way means that I don't also need to make very wise and careful decisions in buying gear in every single case.
  13. anonymous

    anonymous Guests

    Hey AudioGaff I didn't know you were a "real" engineer....I'm a "real" mech engineer myself. Though, these days I spend more time messing around with electronics and software (DSP and control systems).

    Bgavin...if the M201 doesn't work out for ya, I'd be interested in buying it; I've always wanted one for snare use, and I've been a big fan of Beyer mics ever since I got my M-88 classic 5 years ago.

    AudioGaff is bang on in terms of not judging the sound of a mic from a spec sheet...not matter how detailed the specs are, they will never tell you how the mic 'sounds', or what it's really good at.

    In my mind, a really flat and full sounding Beyer is the M160 or M180 dual ribbon mics...now these suckers need serious gain (1 mv/Pa sensitivity)!


  14. bgavin

    bgavin Guest

    I won't be needing that 75dB of gain, as I plan on using the right mic for the job at hand.

    The M201 is only 0.2mV more sensitive than the above mentioned ribbon mics. I don't own the M201, and there have been none for sale on eBay. I got my hands around one at a friend's studio a few days back. I want one for sure, probably two.

    The only other dynamic that comes close is the Sennheiser E609 (not E609 Silver).

    My Aardvark gear doesn't have the snob appeal of $1000/channel preamps, and I don't have the skills of a $1000/channel tracking engineer. What I've learned over many years is, middle to upper-middle gear will do a remarkably good job for the vast majority of us. I very much doubt there is 10x noticeable improvement from a preamp that is 10x the cost.

    I have 226 microphone entries in my spread sheet now, that I have researched. The M201 and E609 are clear winners in dynamics for snare/hat. I'd also like to have a pair of SP C4 for acoustic and SDC stuff, since they are in price range. I'd prefer a complete set of AT4049, AT4051, AT4953, but that ain't gonna happen.
  15. AudioGaff

    AudioGaff Well-Known Member

    Yep, I have years of experience as a semiconductor product/test/applications engineer for ASIC, DSP, mixed signal, microprocessors and microcontrollers.

    bgavin, Power to ya man. I still own and use mid level gear as well. But at the same time there is a difference in the very good and great stuff that is well worth while the price in providing a much better sound and perfomance over the mid level. It may or may not be as great as 10X, but even as low as 2X better can result in a very significant and easily noticed difference.
  16. BGavin,
    Perhaps if you tell us more specifically what kind of material you are recording, what types of places you will be recording in, the purpose of doing all this (for $$ or for pleasure), the end use of your recordings, the gear you currently have, and your budget for improving your setup, we can help you use your gear more effectively and guide you in spending your money wisely. Their is no replacement for going out and doing it yourself, but RO has a lot of people who have been doing this for years and can share knowledge with you. That you wish to have a technical understanding of you gear is a plus, but "what sounds good" and "what works" are the final standards. Mic choice, mic placement, eq, compressor settings, logistics, dealing with clients, these are all things we have experience with.
  17. bgavin

    bgavin Guest

    I do live rock/punk/blues performance recording, usually simultaneous with PA support. The client gets the raw tracks for their own use, and I get a small fee to pay for my habit. We also do some garage (literally) demo recording for my daughter's band.

    My mic locker is all dynamic except for a single AT4040. I'm reluctant to expose condensor mics to head-bangers on stage. The rest are SM57, SM58, ATM25, and a pair of ancient electret SDC for overheads. I want a second AT4040, a pair of SP C4, and replace the snare and hat SM57s with Beyer M201s.

    My pres are 16 channels of Aardvark Q10, Sonar XL 2.2, everything done in the workstation. This is hobby/demo level work only.

    Photographically, I've delivered excellent work for years with a Pentax 6x7. I don't need a Hasselblad and Zeiss optics to realize good results, so I'm applying the same logic to my recording.
  18. realdynamix

    realdynamix Well-Known Member

    :) bgavin, just going into this a bit more, I have a question. Since the dynamic range of the digital world has caused designers to minimize noise even more in the signal path.

    Where is the extra gain from the AT4040 comming from? Is it in the pre of the mic itself, or is this condensor that sensitive on it's own? If it is the later, I would love a mic like that for wide dynamic recording. It's noise floor must be way down there. What is it's self noise?

  19. bgavin

    bgavin Guest

    The AT4040 seems to be a well kept secret.

    145 SPL, 155 with pad
    82 SNR (A)
    12 Noise (A)
    133 DNR

    Transformerless, -6 and -11 rejection at 90/120 degrees. Flat to 5k, +4dB Hi peak at 6,500, rolls back below 0dB at 16 KHz. Single 25mm diaphragm, 2-micron.

    I'd characterize it as Bright/Neutral.

    Due to the high output, I don't think it is ideal for close micing. I've done it with Eden D410XLT bass cabinets, but switched over to ATM25 for this. No sense exposing the AT4040 to damage, and it won't make the Eden sound any better than the ATM25.

    I don't own a pair, so no report on use as overheads. I'm spoiled with my friends' TLM-103 for overheads... the AT4040 doesn't have the highs. I want a set of SP C4 for this.

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