People often confuse blast overpressure and noise. An over-simplistic comparison is that noise is what the human ear can hear. Blast overpressure, however, is often below the range of human hearing.
The typical range of human hearing is from 20 Hz to 20 KHz. This range can of course vary significantly between individuals. Blast overpressure may often be very low in frequency, 2 Hz or less. By definition, blast overpressure is the pressure generated by a blast that is over and above atmospheric pressure.
Although blast overpressure is often below the range of human hearing, it can cause structural response that is quite noticeable to those inside a structure. It is not possible to gauge the amount of blast overpressure by what is heard outside. How many times have you been outside near a blast, heard virtually nothing, and yet still received a complaint call that was overpressure related?
There are generally five sources of blast-generated overpressure (from the ISEE Blasters’ Handbook 18th Edition):
Air Pressure Pulse – Low frequency pressure caused by rock displacement at the face (piston-like movement or bulking of the rock mass). Gas Release Pulse – High frequency pressure caused by gases venting through the face. Stemming Release Pulse – High frequency pressure caused by gases venting through the stemming. Rock Pressure Pulse – Typically insignificant air pressure generated by the ground vibration. Noise – High frequency energy from detonating cord or surface delays.
Taken from White Seismology’s newsletter
Written by Randy Wheeler, President of White Industrial Seismology