Baffled by the Speed of Sound measurements from my last recording, I decided to go back to Avebury early on New Year's Day and record again from positions 5 and 6. Arriving at 8 am I found the site almost deserted, with less traffic than I've ever experienced there before. The temperature was higher than last time ( I discovered later, about 6 deg C) and again the fog was thick, with 99% humidity. Standing as close as I could to my original Position 5, I struck the Claves and heard, as before, a returning echo from the same stone. But this time, due to the lack of ambient traffic noise, it was followed by a second echo from the left - softer, and more reverb-like. It was clearly coming from the clump of mature Beech trees in the southern corner of the SE Quadrant, just by the road. They can be seen to the left of the photo (below). I was aware that clumps of trees can cause confusing echoes, but it's a big issue and I had intended to raise it at a later date...
From recording
position 1
I then moved on to Position 6 - The Obelisk (right).
From recording
position 6
At Position
6 by the Obelisk the abnormally quiet conditions revealed more echoes
from trees. Striking the Claves produced, as before, a number of early
reflections coming from the right - the Z stones. This was followed
by an echo from the large stone immediately in front of me - the centre
one of the group of three large stones of the South Circle. Then followed
two echoes from trees - one from the same group of Beeches as at Position
5 (centre of the panorama above) and a second from the trees to the left
of the panorama.
The distance from the recording position to the large centre stone was
measured as before, with the odometer. This time it measured 50.6 m.
After the recordings were transferred to my DAW the echo delays were measured.
From Position 5, the stone echoed at 0.191 s and the trees
at 0.524 s.
In Position 6, the large centre stone echoed at 0.290 s
and the two groups of trees at 0.63 s and 0.806 s.
The Speed
of Sound formula was applied and the results were even worse than before!
SOS at Position 5 came out as being 358 m/s and at Position
6 it was 348 m/s.
Something was clearly wrong - it had to be the odometer, my lack of measuring
skills, or both.
Copyright Steve Marshall 2007