Step Two: Evaluating Noise Performance
Dynamic Mics
1. Find the impedance specification on the data sheet (use “actual” instead of “rated” if given the choice).
2. Use Table 2 to find the Output Noise in dBu, A-weighted, by finding the closest impedance listed.
3. Find the EIN (equivalent input noise) in dBu rating on the preamplifier’s data sheet.
4. Reduce the preamp’s EIN by 5 dB to approximate A-weighting.
5. Compare the two to see if the proposed preamplifier degrades the mic noise appreciably.
Condenser Mics
1. Find the Noise rating on the microphone date sheet (this is stated as Equivalent Noise Level, Self-Noise, Equivalent Noise SPL, or Noise Floor), expressed in dB SPL, A-weighted.
2. Locate the microphone’s Sensitivity rating on the data sheet.
3. Using Table 3, find the microphone Sensitivity rating down the left side.
4. Find the Noise rating in dB SPL, A-weighted along the top of Table 3.
5. Move along the Sensitivity rating row and move down the Noise column until they intersect and note the number—this is the output noise converted to dBu, A-weighted.
6. Find the EIN (equivalent input noise) in dBu rating on the preamplifier’s data sheet.
7. Reduce the preamp’s EIN by 5 dB to approximate A-weighting.
8. Compare the two to see if the proposed preamplifier degrades the mic noise appreciably.
Condenser Microphone Example Using Sample Data Sheets
1. Microphone’s Equivalent Noise Level is 14 dB SPL, A-weighted.
2. Microphone’s Sensitivity rating is 20 mV/Pa.
3. Table 3 shows the Sensitivity row marked 20 mV/Pa shaded.
4. Table 3 shows the Noise column for 14 dB SPL, A-weighted shaded.
5. The intersection point is at -112 dBu, A-weighted: this is the output noise of the microphone.
6. The MS 1S Mic Stage data sheet lists the Equivalent Input Noise as -128 dBu (no weighting).
7. Reducing this by 5 dB yields a preamp EIN of -133 dBu, A-weighted.
8. The difference between the microphone’s output noise of –112 dBu A-weighted and the preamp’s EIN of –133 dBu A-weighted is –21 dB so the preamp’s noise will not degrade the performance of the mic.
This article would not exist had it not been for an article authored by Tomlinson Holman, published in the September 2000 Surround Sound Professional magazine, titled “Capturing the Sound, Part 1: Dynamic Range.”
In that article Tom deftly demonstrated the difficulty in properly matching microphones and preamplifiers. His article motivated me to do this expanded and generalized article.
Read and download a PDF of the original article here.
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.