As we've already seen and heard, if a listener in the centre of Avebury's Southern Inner Circle were to make a sound, audible echoes would reflect from all of the stones of the circle after a delay of about 290 ms. At around 100 m diameter, this is about as big as stone circles ever got (Avebury's 'Great Circle' is far bigger, but is not actually circular). Only five of the circle's huge stones remain, but archaeologists believe there were originally 29 stones* plus a central 'phallic' pillar known as the Obelisk, standing over 4 m high. Strangely, just a few miles northwest of Avebury is the remains of an even bigger circle. At 107 m across, the now ruined Winterbourne Bassett circle was even bigger and also had a central pillar. Neither circle has been succesfully excavated or even surveyed underground.
* Or
was there an extra one? Of the stones that remain, the northernmost one
looks to me suspiciously like half a stone.
My first
thought on hearing the echo from the South Circle's stones was that it
may have produced what I called a 'dome' effect. This was based on my
experience of recording on the 'Golgumbaz' - a huge dome in India: GOLGUMBAZ
WIKIPEDIA
The Golgumbaz has a 'whispering gallery' at the top, similar to St Pauls
in London. The dome is 38 m across and sits atop a building that is 33.2
m high and 50 m square. Total height from the ground to the top of the
dome is 60 m. To a listener in the top gallery, a handclap produces repeat
echoes with a delay of about 0.34 seconds - here's part of my recording
of it: GOLGUMBAZ
MP3
A listener at ground level hears no repeats - just an incredibly long
and smooth reverb. Unable to find any information on echoes in domes,
I puzzled over this building for a long time - assuming that the delay
time was related only to the diameter of the dome, and unable to understand
why it is actually three times longer than it should be. I eventually
consulted Barry Blesser, who pointed out that the dome merely acts as
a sound mirror, focussing sound from below; the echo delay time actually
comes from the height dimension below the dome.
So what would happen in a 100 m stone circle like the South Circle at
Avebury? Making a sound at its centre produces reflections from all the
stones simultaneously - they act as one curved surface, just like the
bottom part of a dome - the 'whispering gallery'. But when those echoes
return from the stones, don't they carry on across the circle, to reflect
off the other side and back to the centre? Because if this did happen,
surely a repeating series of echoes would be heard, just as in the Golgumbaz.
The South Circle is more than twice the size though, and sound waves lose
intensity with distance, so maybe this is too much to ask for.
But if the audible echoes returning to the centre of the circle are all
of equal delay, what would happen if the listener moved away from centre?
The echoes from one side would get longer and those from the other side
shorter. Intrigued by this idea, I made a rough simulation using audio
software (actually done by placing delay lines in a Soundscape mixer).
This simplified model has only eight stones (below).
We must assume that sound reflecting from the stones has a fairly wide angle of dispersal. This seems reasonable - although the stones are arranged with their flattest sides facing inwards, the surfaces are actually far from flat. Using the same pair of Claves as in the other experiments, here are some mp3's demonstrating what may be heard by a listener at various positions inside the circle. Note that the listener is facing toward C - therefore the reflections from the front and back hemispheres will be the same, producing a stereo effect rather than a surround sound one. So, to a listener moving along the line AE, delays will be the same length from stones B and H; C and G; D and F. Click for mp3's.
1) At the
very centre, all echoes are equal: CENTRE
2) If the sound carries far enough for a repeating echo, it sounds like
this: CENTRE
REPEAT
3) Moving
left from the centre, 5 m towards A, the delays are now unequal:
5 m LEFT
4) Now 10 m left from centre: 10 m LEFT
5) 15 m left of centre: 15 m LEFT
6) 20 m left of centre: 20 m LEFT
7) 25 m left of centre: 25 m LEFT
Because the angles of some stones are getting rather wider as the listener moves away from the centre, the level of individual echoes must vary and I have made no attempt to simulate this. Note though, how to a listener away from centre, one click produces a series of moving echoes.
At least,
that's what I think would happen! If anyone has any better idea
then please let me know - there's an email link on my main website (links
page).