Dynamic Microphones and SPL - John Dunn, Consultant, Ambertec, P.E., P.C.

Let us first consider some definitions regarding sound pressure level (SPL):

1 Microbar = 1 Dyne / cm². 

10 Microbars = 10 Dyne / cm² = 1 Pascal.

200 Microbars = 200 Dyne / cm² = 20 Pascals.

20 Micropascals = 0.0002 Dyne / cm² = Zero dB SPL

Zero dB SPL is often taken as the threshold of human hearing.

Thus: SPL of 28 Dynes / cm² --> 20 * log (28 / 0.0002) = 102.92 dB SPL  ~  103 dB SPL

which is the sound pressure level at which dynamic microphones are often specified for their output signal levels into their specified load impedances.

With all of this in mind, we now look at the Thevenin equivalent circuits of four real-world dynamic microphones, noting their different specified voltage generations at specific SPLs, their different source impedances and their different specified load impedances, all of which can get a bit confusing but which we can understand more clearly from the sketch below:     

 
That's not so bad anymore, is it.