# Discrepancy in my dB SPL calculations

Having read through this post I tried an experiment.

I connected a Extech sound level calibrator to a Type 4189 microphone. The output sensitivity of the microphone is

50 mV/Pa,-26 dB ± 1.5 dB re 1 V/Pa

This microphone is connected to a Type 2671 preamp. Using a BNC 50 ohm cable the preamp is connected to a signal conditioning amplifier through a 50 Ohm BNC-BNC cable. The output from the amplified is connected to a standard oscilloscope.

The calibrator generates a consistent 1 kHz, 94dB SPL sound. The calibrator was recently calibrated, so the output is assumed to be the standard.

The gain at the signal conditioning amplifier is 100mV/unit.

The Vpp measured at the oscilloscope was 16.2 Vpp. Converting that to Vrms,

Vrms = 16.2/2.828 = 5.72843

Calculating for dBrms = 20*log10(5.72843/0.05012) = 41.1605

Since the gain at the signal conditioner is 100 mV/unit. the dB value is -20 dB.

For dB SPL = 41.1606+94-26-20 = 89.1605.

Would I be wrong in expecting this to have amounted to 94 dB SPL instead? If so, why the discrepancy ? what am I missing?