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Choosing a microphone for a new auditorium

Started 5 months ago

Hello everyone! A question was posted in Xchange's main Q&A that I wanted to bring to this room for assistance. 

@Anand Munuswamy Sachudanand has asked: 

"This question is from CTS-D AVIXA Activity which is from Design 1: ENV - Microphone. Microphone Pre-Amp Gain Required You need to choose a microphone for a new auditorium. Your sound source is a presenter located 2 feet (609.6 mm) away from the microphone, with a measured SPL of 72 dB. In order to route and process that signal, you need to amplify the microphone level signal to line (0 dBu). Most microphone preamplifiers will provide around 60 dB of amplification. You have a choice between two microphones: SM58: A Dynamic Microphone Equivalent Voltage Specification: -54.5 dBV/Pa (1.85 mV) MX418 A Condenser Microphone Equivalent Voltage Specification: -35.0 dBV/Pa (17.8 mV) 1 Pa = 94 dB SPL Answers: Pre-Amp Gain Required SM58: -74.29 dBu MX418/C: -54.79 dBu Till this it's OK and have understood. But given on same page is: Assuming you have a 60 dB gain in your microphone pre-amp, the SM58 is not sensitive enough for this application. The closer you can get to 0 dBu, the better the microphone will be for the application. You need 83.5 dB, but you only have 74.29 dB gain, leaving you 9.21 dB short. I have doubt on how this 83.5 dB has come. Could someone help me, please?"

Please share your expertise!

I think I'm a little confused at the issue.  In the CTS-D Exam Guide, they walk you through the calculations for figuring out Pre-amp gain required for microphones. It essentially walks through a dynamic microphone (like an SM58) and a condenser mic (like the MX418. Because you are trying to get these to 0dBu, and you only have 60 db of gain on most pre-amps, you won't be able to get the SM58 microphone to the 0 dBu. You will be about 14 db short.  Yes, many DSP's can have digital gain to make this up, but the signal will get rather noisy by also increasing the noise floor. This is why the condenser microphone will work, since you need less than the 60 db of gain that the pre-amp offers to get the signal to 0 dBu. 

The main issue is where the microphone is being used (2) feet distance, is a room oval, square uneven? And what are the furniture walls of the place made of, how many doors, how many windows, how many people?
Observing the environment, listening to one's own words can certainly happen in that case

Hi Anand, - you are right, the last sentence in the para highlighted is not in context to the data provided, unless i am missing something. So for dynamic microphone option, you need 74.29 dB gain but you only have 60 dB leaving you 14.29 dB short. Whereas for Condenser microphone option, you need 54.79 dB gain and with 60dB available in preamp, it would be able to meet the need to bringing it to line level. Hope this helps.

I am happy and satisfied with the answer, with 60 dB gain in pre-amp and 14.29 dB short. It created lots of confusions. This has to be reviewed and modified, if required. If there is something which we are missing, then it needs to be clarified in the material. Thank you Gopal and Aaron.