This means the noise is given in units of dB SPL.
A noise spec might read 14 dB SPL equivalent, A-weighted, or shortened to just 14 dB-A (bad terminology, but common).
This is interpreted to mean that the inherent noise floor is equivalent to a sound source with a sound pressure level of 14 dB.
Problems arise trying to compare the mic’s noise rating of 14 dB SPL with a preamp’s equivalent input noise (EIN) rating of, say, -128 dBu. Talk about apples and oranges!
Luckily (again), tables come to the rescue. Table 4 provides an easy look-up conversion between a microphone’s output noise, expressed in equivalent dB SPL, and its sensitivity rating, in mV/Pa, into output noise expressed in dBu, A-weighted.
Using Table 4, a direct noise comparison between any microphone and any preamp is possible. The example shown by the blue column and row is for a mic with a noise floor of 14 dB SPL and a sensitivity rating of 20 mV/Pa, which translates into an output noise of -112 dBu, A-weighted.
Table 4: RMS noise summation for connected mic and preamp. (click to enlarge)
Now, time to return to Table 3. Unfiltered electronic noise, whether from a resistor, a coil, an IC, or a transistor is white noise consisting of all audible frequencies occurring randomly. Due to this randomness you don’t just add noise sources together, you must add them in an RMS (root mean square) fashion. Mathematically this means you must take the square root of the sum of the squares - which is why Table 3 is so handy - it does the RMS conversion for you.
Use Table 3 to convert a mic’s rated noise output into units of dBu. Find the difference in dB between the mic’s output noise and the preamp’s input noise. Find that difference in the left column of Table 3 and read what the preamp added noise will do to the mic’s noise in the right column.
For example, if the mic’s output noise translates into -120 dBu, and the preamp has an EIN of -127 dBu, then the difference between the mic and the preamp is -7 dB. That is, the preamp is 7 dB quieter than the microphone. Table 3, at the row marked -7 dB, tells you that this preamp will degrade the mic’s noise by only 0.8 dB. Looking at Table 4 tells us that after about a 10 dB difference, the noise added by the preamp becomes insignificant.
Similar to Table 1, you can use Table 4 to map out a preamp’s A-weighted noise to show the combinations that add insignificant noise. If you use a -10 dB difference figure as a guide, then the preamp’s noise amounts to less than 0.4 dB increase.
The red-shaded triangle area in Table 4 shows an example of this. The areas not shaded represent all possible combinations of mic sensitivity and noise specifications that can be used with Rane’s MS 1b Mic Stage, for instance, and add less than 0.4 dB of noise.
If you allow 1 dB net added noise, then even more combinations are possible. (The shaded area is figured by taking the EIN of the MS 1b at -128 dBu, reducing it to -133 dBu with the 5 dB factor for A weighting, and using the -10 dB difference found in Table 3 for 0.4 dB added noise, resulting in all combinations less than -123 dB being blocked out.)
The author would like to point out that this note was inspired by an article authored by Tomlinson Holman, published in September 2000 Surround Sound Professional magazine, titled “Capturing the Sound, Part 1: Dynamic Range.”
Dennis Bohn is a principal partner and vice president of research & development at Rane Corporation. He holds BSEE and MSEE degrees from the University of California at Berkeley. Prior to Rane, he worked as engineering manager for Phase Linear Corporation and as audio application engineer at National Semiconductor Corporation. Bohn is a Fellow of the AES, holds two U.S. patents, is listed in Who’s Who In America and authored the entry on “Equalizers” for the McGraw-Hill Encyclopedia of Science & Technology, 7th edition.