Review on Portable Pressure Measurement Measuring Momenturn by Colin Watkins

I'm using it for testing as I want to soundproof a car. It's directional so it helps pinpoint hot spots. (The rear of the car is 2 or 3 times louder than the front.) This gives an idea of how the noise will change once the project is complete. It is not accurate for use as a dB meter. On a perfectly quiet night, it registers 35 dB before and after I get in the car. It looks like its zero dB is somewhere between 25 and 30 dB. It can be calibrated, but the tool for this is 5 times more expensive than a counter (and may not be accurate). There's no calibration to set it to match accurate dB meters. This is not uncommon for cheap dB meters, but I was hoping for more accuracy.