1. Register NOW and become part of this fantastic knowledge base forum! This message will go away once you have registered.

Quick question regarding mic SPL and input dynamic range

Discussion in 'Pro Audio Equipment' started by digdug, Apr 25, 2005.

  1. digdug

    digdug Guest

    Just trying to see if I have this correct:

    If a mic has 130db max spl handling and you plug this into a mic pre input that only has 86db of dynamic range and recorded something above 86db it would clip the input?

    Placing 3 -20db pads would bring the mics max input down lower than the mic pres input and you wont have any clipping problems?

    Am I on the right page here?
    ~Thank you for your time.
     
  2. digdug

    digdug Guest

    Well... In the other case finding a new mic pre that has more than 130db dynamic range or a more common dynamic range of 110db and just using one pad so you dont bring the mic closer to its noise floor with THREE -20db pads...

    Am I speaking completely out of context here? 27 views and no responses.
     
  3. karbomusic

    karbomusic Active Member

    Well, I was going to reply earlier, but its been too long since I've done the math to respond with authority. But since you asked again I will tell you what I think and hopefully others will correct my mistakes here:

    I don't think that total dynamic range and the mic's maximum SPL are going to have a 1:1 relationship. I believe you have to subtract the noise floor (of the mic) from the max spl to get the TDR. Since max spl is an indication of the maximum sound pressure the mic can take before distorting and and also losses while translating that maximum into a signal I don't think your going to end up with 130dbm or dbv (told you its been awhile) at the input. But again, I could be very wrong on this.

    So in a nutshell, the mic probably isn't going to produce 130db at the input. But as I said in the beginning... too long since I did this so don't take it as gospel...

    Example:

    U87 has 117 max spl minus a 23db noise floor which equates to 94 db of total dynamic range; again I think so....

    It would help if you posted what mic it is and also the pre your going into....

    Best regards-

    Karbo
     
  4. dpd

    dpd Active Member

    ^^^ pretty much gets it.

    It's hard to calculate when the mic / preamp combination will clip without knowing the sensitivity of the mic (volts out for SPL in) the gain of the preamp and the maximum peak-peak output voltage of the preamp.

    High SPL signals? You'll want low gain, and probably the pad on the mic. I wouldn't put pads after the mic - that reduces the signal and requires the next gain stage to have even lower noise or the whole signal chain will get noisier.
     
  5. digdug

    digdug Guest

    As an example I was using a 130db spl mic such as the Sennheiser MKH416 going into a Marantz PMD670 Digital Field Recorder which on has an 86db dynamic range.

    Since posting I forgot I had a book that has a nice section on mics and preamp matching.

    In the book they say for an example:

    If a mic (in this case a Senn 416T) has a max undistorted output voltage of 630mV, and the preamp which happens to be on a Nagra has a max undistorted input of 43mv. The connection is only capable of capturing 101db SPL w/o clipping.

    Oviously they say that you have to bring down the mic max output to the preamps max input by padding it down 23db. But keeping the mis noise 10db higher than the preamp noise they say use a compromise of 15db...

    Question 1: What equation did they use to get 101db spl from 640mV and 43mV.?

    Question 2: What equation did they use to get 23db of padding?
    Im guessing 630 minus 43 = the number in millivolts equal to DB they needed to reduce, if so...

    Question 3: What is the equation of "mV = db"? This would probably end alot of the confusion for me...

    The mic mentioned has 110db of DR and the Preamp mentioned has 105db of DR.
     
  6. digdug

    digdug Guest

    For mV to db:

    When I subtract 630 minus 43 = 587 and use 0.587 log in my calc I get -0.23136189875238552393644070140329

    Am I getting anywhere on that one?
     
  7. karbomusic

    karbomusic Active Member

    See if this link has any info that will help...

    http://www.microphone-data.com/pdfs/mw mic specs.pdf

    Best regards-

    Karbo
     
  8. dpd

    dpd Active Member

    1) those two numbers alone will never result in 101 dB. something is missing

    2) 20*lgt(630/43) = 23.3 dB so that's how they got the 23 dB.

    dB is just a ratio of numbers in logarithmic space

    3) what you need is the conversion gain of the mic (volts out for SPL in). Probably in the mic spec sheet
     
  9. digdug

    digdug Guest

    1. Yeah the section was pretty vague in this book, Im really not sure.

    2. Completely understand that now.

    3. Yeah I actully just printed out some of the sheets on the pdf karbomusic posted and used an AT825 as an example to work out all the info.

    So far I have:

    Max SPL: 80 Pa = 126dB SPL, 352mV, _____ dBu
    Output Sensitivity: 1 Pa = 94db SPL >>> 4.4mV/Pa => _____ dBu
    Self Noise: 8750Pa = 24dB >>> Some Huge Number-MuV = _____ dBu
    Total Dynamic Range = 102

    I'm assuming the output dBu would be the same number as the difference between 94 db SPL and 126 dB SPL?

    As for the rest still working on it.

    I thank yall for the help and resources so far, I'm learning quite a bit from this. :)
     

Share This Page