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.