The panoramic
photograph above shows the Southern Inner Circle as seen from the Obelisk
and facing SSW. In the foreground are the short stones and concrete markers
of the Z feature (also known as the D feature); behind are
the five remaining standing stones of the South Circle - a group of three
near the centre of the picture and a further two to the right; a concrete
marker between them.
Recording 6 was made with my back to the concrete pillar that marks the
position of the Obelisk and facing directly onto the centre stone of the
group of three - a distance that was measured as exactly 50m using the
odometer.
Note that on the map the circle appears to continue eastwards. In fact
there is absolutely nothing to see - not even concrete markers. The marks
on the map indicate parch marks in the grass as seen from aerial photos.
As to the proportions of the original circle, estimates vary. It seems
that the diameter was somewhere between 98 and 104 metres and that it
contained 29 stones.
Clicking on the small map (right) will open a large map in a new window.
From recording
position 6
mp3
A06
This would account for a discrepancy between the distance measured to the centre stone in the picture and the time delay of its recorded echo. Consequently, the speed of sound was re-calculated for this position as being 337 m/s. For a further explanation see below right.
NOTE: As with the previous page, these measurements, temperature and Speed of Sound were wrongly recorded and/or calculated (though by a fairly small amount). See later pages for corrections.
The graphic
display of the Soundscape DAW (left) shows one single hit of the claves
as recorded from recording position 6, and the resultant echoes. The Left
channel of the stereo recording is shown in red, the Right channel in
yellow.
Two position
markers, L & R, can be moved around the display and the distance between
them automatically calculated as time. The L marker is set at the initial
click of the Claves and the R marker at an echo that sounded 0.296 secs
later - assumed to be the large stone in the centre of the photo. If speed
of sound c = 346 m/s then it has reflected off a surface 51.21 m distant.
Yet the distance to the centre large stone was measured to be exactly
50 m. It is likely that the air was less damp in this position, so c was
assumed to be slower: 338 m/s.
RESULTS:
A single hit of the Claves in this position produced six returning echoes.
As can be seen from the DAW display (above) most of them came from the
right, as shown on the yellow channel. The first three echoes are bunched
close together at 0.0914 s, 0.096 s and 0.1099 s. If speed of sound c=338
m/s then the echoes are originating from surfaces at distances 15.4 m,
16.2 m and 18.6 m respectively. These distances indicate that they are
reflections from the southernmost standing Z stones.
The fourth reflection was quieter and doen not show up on the graphic
display. At 0.149 s it is the reflection from a surface 25.2 m distant
- as yet unknown.
Reflection
five comes at 0.1992 s - from a surface 33.7 m distant. As it only appears
on the right channel this can be assumed to be coming from the rear wall
of the old Chapel (now the Avebury tourist centre).
The sixth and final echo has a delay time of 0.296 s and as it is equally
loud on both of the stereo channels - so it originates from directly in
front of the mics. At 50 m away, it is certainly reflecting from the centre
one of the three large stones and possibly from all three, as they are
arranged on an arc and therefore equidistant from the sound source. A
repeat visit to the site confirmed that from position 6, all five standing
stones of the Southern Circle will reflect sound equally well. Because
they are all equidistant, it is sometimes impossible to determine which
of two adjacent stones is reflecting sound: they seem to reflect together
as one continuous surface.
Copyright Steve Marshall 2007