AVEBURY ECHOES

Recording A - 8.30 am, Sunday December 23rd 2007 - Frost & Fog
www.stevemarshall.org.uk
Avebury's south-east quadrant is quite well restored - 11 of its 22 stones are standing and they come in a variety of shapes, from pillar to lozenge. None are particularly big. There are only a few small trees, all growing alongside the road and well spaced; they do not reflect sound significantly. At the northern end of the quadrant are some buildings (the Henge Shop and the Post Office) and as these reflect sound so much, I made the first recording some distance south of them. In all, five recordings were made in the SE quadrant. In each of them the recording position was exactly 5 metres from the fence that runs alongside the road and I stood with my back to the road, directly facing towards a different standing stone for each recording. On the map, standing stones have been highlighted in green - the small black crosses indicate the concrete blocks that mark missing stones. The panoramic picture above was taken facing south, with the road on the left. Recording 1 was made directly facing the square-ish standing stone seen to the right of the above picture. I then moved along south (towards the left of the picture).

Clicking on the small map (right) will open a large map in a new window.


From recording
position 1

mp3
A01


From recording
position 2

mp3
A02


From recording
position 3

mp3
A03
 

From recording
position 4

mp3
A04

From recording
position 5

mp3
A05
 From recording
position 5 - shows the stone to left that is reflecting sound (see yellow line on map).In this picture there is a figure standing just left of the stone.

Temperature was slightly below freezing and fog moderately thick.
The speed of sound at 0 degrees C is 330 m/s and at 20 degrees C it is 340 m/s - but that is only in dry air. Sound travels faster in moist air.
Recording position 5 produced an extremely clear echo to the left, which could only have come from one large stone - the southernmost one standing in the quadrant (marked with a yellow line on the map). There was no stone beyond it - only a concrete marker. As the echo could not have come from anywhere else, the distance to this stone was measured with an odometer and used to calculate the speed of sound for these conditions. This turned out to be 346 m/s - higher than expected, the fog compensating for the low temperature.

When the recordings were transferred to a DAW most of the echoes were loud enough to be visible on the waveform display and the delay time was measured for each. From this, it is possible to calculate the distance to whatever surface the echo is reflecting from. The large map is accurate enough to determine this by measurement (which can be done quickly by simply holding a ruler to the PC monitor and marking off from the scale).

NOTE: The figures quoted on this page turned out to be incorrect, due to faulty measurements. Hence the calculated Speed of Sound is wrong. See later pages for corrections. The temperature also was lower: it was probably about -4 degrees C.

  By following this procedure it becomes apparent that it is the stones themselves causing the echoes - excepting intrusion by buildings or trees. Each recording position was directly in front of a standing stone and the delay times confirm that it is that particular stone that was echoing, at least in positions 2, 3 &4.

Position 1 - echo from the right at 0.423 s - reflecting from 73.18 m distant. Appears to be a reflection from the back wall of the Henge Shop.

Position 2 - echo at 0.354 s - distance 61.25 m - the stone immediately in front.

Position 3 - echo at 0.345 s - distance 59.74 m - the stone immediately in front.

Position 4 - echo at 0.281 s - distance 48.71 m - the stone immediately in front.

Position 5 - echo at 0.206 s - distance 35.57 m - large stone to left. At position 5 the stone immediately in front was only half height and seems not to have produced a significant (audible) reflection.

After revision, these distances were adjusted (timings were unaffected). The main conclusion is correct though: the stones are reflecting sound and it is audible from over 60 m away. All of the distances were slightly shorter than quoted here, which seems to fit better with the map positions. See Appendix THE MATHS
Copyright Steve Marshall 2007