WKLB Spectrograms - initial email discussion between Steve Marshall & Victor Reijs

 

SM: I've found the best setting on Audacity is: spectrum, hanning window, size 16384, axis log. I've been selecting a 5 or 6 sec section to analyse - a full bull roarer cycle.
WAV(2) is a recording of silence, but appears to contain infrasound. It was windy at the barrow and, unusually, the wind was blowing from the N, across the open end of the entrance. I think it was resonating the passage!
Note also, how just a human voice seems to trigger infrasound.
I've idented the recordings - if not clear, please ask.
I've got some good results from analysing the passage resonance - seems to be 9 Hz, rather than the 8 Hz I calculated by measurement.

I think the N wind blowing across the open end of the monument is resonating the passage at 9 Hz (not 8 Hz, as I initially calculated from its length).

See below- from WKLB Sample 01.WAV



SM: There are no even harmonics in a closed pipe - only odd harmonics.
In this sample there are peaks at 27, 47, 65, 81, 96, 107, 113, 130, 142, 158 etc Hz. These correspond closely to the odd harmonics of 9 Hz - 3f, 5f, 7f etc.
Also - there are troughs at 34, 54, 61, 72, 100, 128, 149 etc Hz. These correspond to the missing even harmonics!
What do you think?

Below are a couple of recordings of me overtone singing in a stone chamber. See how clear the harmonics are!

WHill Overtones 1.WAV

WHill Overtones 2.WAV

WHill Overtones 3.WAV

SM: Below is a panpipe I just recorded - a closed pipe, which is what the WKLB's central passage is (in theory...).
If you move Audacity's cursor along the plot it will snap to the peaks and show their freq.
As expected there are only odd harmonics present, but they're not exact multiples of the fundamental.

Panpipe.WAV

VR:
Indeed only odd numbers:
Peak [Hz] fx/f1 fx/int(fx/f1)) [Hz] f1 565 1.00 565.0 f2 1755 3.11 585.0 f3 2882 5.10 576.4 f4 4034 7.14 576.3 f5 5220 9.24 580.0 f6 6373 11.28 579.4 Average 577.0 [Hz] StDev (1 sigma) 6.7 [Hz]

It is though all quite close enough, IMHO. The peaks also have a broadness, so that can also influence this.

By the way, is for low frequencies is WKLB an open pipe? I thought there was a large standing stone one or two meters from the start of the passage. So for low frequencies >>2m (so <<170Hz) that gap might be seen as a closed pipe...
I gave the same reasoning around Newgrange (although there is half of the passage opened always).

SM: That's a very good point! There may be two systems working simultaneously - I'll investigate.
It definitely does work as a closed pipe though, as we can clearly see the harmonics.

from the closed end of the passage - the W chamber - to the huge blocking stone no 45 is about 15 m.

Steve Marshall 08-03-2016