# Calculating the voltage generated by an 40KHZ Ultrasonic Tranceiver

In particular, this device: http://www.hexamite.com/hx40str.htm

Electrical Specifications HX40STR
----------------------------------------------
Parameters              Values      Units
Operating Frequency     40          kHz
Input Voltage           42 (max)    Volts (p-p)
Output (SPL) @20V       112         db
Impedance               300         ohm
Beam Angle              ±12         degrees
Bandwidth               1           kHz
Settling Time           5           mS
Temperature             -40 to 100  °C


Assuming the scenario that two trancievers are used, T1 as transmitter, T2 as receiver, 125mm apart. (Assuming no attenuation over distance.)

Output SPL at 20V => 112 db.

Converting pressure to uBar (because sensitivity is in uBar):
P = pressure in uBar

112 = 20 log (P/0.2e-3)

P = 0.2e-3 * 10 ^ (112/20)

P = 79.621 uBar

Input Sensitivity => -60 db V/uBar

S = Sensitivity in V/uBar

-60 = 20 log (S/1)

S = 10 ^ (-60/20)

S = 1e-3 V/uBar

Final Voltage V = P * S

V = 79.621 uBar * 1e-3 V/uBar V = 0.0796721 V

However, looking at the graph "G3, Amplitude vs Excitation" "Above: Continuous exitation voltage oscillating at 40Khz is applied to T1. The T2 resulting amplitude is plotted above."

We see that the output voltage at 20V is 0.6V. This also excludes the attenuation over the space of 125mm, which I've calculated to decrease the output V to 0.079 V to ~0.075 V.

What am I missing in my generated voltage calculation? Why is there a discrepancy?