Rescue Helicopter Signal 

The Westpac Rescue Helicopter sometimes passes right over our house on its way to the helipad near the hospital, 1.2 km away. If we're watching TV as we were in the evening of Sat 2-Sep, we can't hear a thing and the house seems to shake.

Did the helicopter make the house shake?

Here is the raw signal from the RaspberryShake. It shows an initial set of small vibrations as the chopper approached, followed by a set of more intense vibrations as it passed overhead.

These plots show the continuous wavelet spectrum of the two parts of the signal. It is a way of showing how the energy is distributed with the period of the oscillations (vertical axis) and time (horizontal axis). The contours vary from brown (high energy) to blue (low energy).

The energy scale of the two contour maps is different so as to show how the structure of the signal for the approaching helicopter varies from when it is directly overhead. The time scale is different also.

The period of the main oscillations in the two signals is just under 0.2 s, or 5 Hz, which corresponds to 300 revs per minute (rpm). For most helicopters, the blades rotate at 400 rpm, so maybe this helicopter was slowing as it approached the helipad.

The signals are a series of 1-s pulses.

Did the helicopter make the house shake?

Yes, it did. And the Raspberry Shake picked it up.