G’Day David,
This is an interesting problem… given the pressure differential the Boom measures is set up internally to the Boom i.e. across the highpass filter in one of the two tubes connecting the signal port to the sensor. The external tube you have connected to the shake should mean that the pressure changes inside the room shouldn’t matter i.e. the boom should not see them. I’d appreciate someone else either confirming or disputing my thinking on this.
This suggests to me that something else is not right. Possible problems that might cause this are:
a possible leak or disconnection of one of the tubes inside the Boom;
a leak or disconnection of the external tube close inside the room;
the tube is too flexible (doesn’t separate the inside and outside pressures effectively;
a blockage in the external tube such that pressure changes inside the room transfer through the tube wall into the tube to affect the signal. This might be due to condensation, kinking or squeezing of the tube close to the open end perhaps?
I’m not convinced the 20s mechanical filter is the solution you are looking for. Being a highpass filter, I would expect it to pass all frequencies above 20s period (i.e. 0.05Hz) and this won’t effect the pressure waves from the door.
Other than the external tube (which you’ve done) I’m not sure what else could be done to filter out the door pressure waves without losing a lot of valuable bandwidth in the Boom signal.
Al.