27th December 2007:
Avebury today is so noisy that it's hard to imagine just how far sound
might have carried when the monument was in use, thousands of years ago.
If the turf covering the site was originally kept scoured down to bare
chalk, as has been suggested, than sound would have carried even further.
This first recording test has shown that a sound can be clearly heard
when reflected from a standing stone 60 m away. That large, flat slabs
of stone will reflect sound is not at all surprising but this initial
test has proved that the Avebury stones do, in fact, reflect sound very
It has been
suggested that a stone circle may function as a 'sound mirror' focussing
sound to a single, centre point, but in the case of Avebury's Great Circle
this is not going to happen. The circle is enormous (338 m in diameter)
and soundwaves diminish in intensity with distance. Also, the Great Circle
is not actually circular - it contains several straight sections and the
straightest of these is the SW quadrant.
The recordings made in the SW quadrant are interesting though, because
the stones are quite small - at least by Avebury standards. They come
in a wide variety of shapes, yet all seem to reflect sound. But the beam
of reflected sound from each stone is quite narrow and the stones are
spaced well apart (with gaps over 12 m) hence a listener moving around
the circle hears only reflections from one stone at a time. The
NW quadrant is much more rounded in shape and that section will be investigated
Inner Circle is a different proposition - it must have had some very interesting
acoustic properties when it was intact. The circle is about 100 m in diameter
and the five stones that remain are enormous - over 2 m wide and 3 m high.
The stones appear to work together in reflecting sound, rather as the
individual prisms of a fresnel lens work together to produce a much bigger
'virtual' lens. The interesting question is whether or not the size of
the circle is too great for a 'dome' effect of repeating echoes to occur.
As only five stones remain, this will only be determined either by computer
modelling or by building a replica.
Domes are known for their 'whispering' effect, but much more spectacular
effects are produced by clapping and shouting. I have recorded in such
a space - the huge dome of the Golgumbaz in Bijapur, Southern India.
At 38 m across it is the second biggest dome in the world. A single handclap
produces repeat echoes that fade away successively - but about ten repeats
can be heard clearly.
Here is a link to a video that demonstrates the dome effect - although
the repeats are much shorter in this example:
I'm not suggesting that this is at all what Avebury would sound like but
I think to expect maybe a couple of repeat echoes is reasonable. The tempo,
incidentally, would be a great deal slower - about 101 beats per minute.
a 'dome' effect, a person moving around the circle and making sounds should
experience some strange and unearthly acoustic effects. At the very centre
of the circle, by the Obelisk, a sharp click would produce an echo that
appeared to come equally from all around the circle, as each of the 29
stones reflected the sound with exactly the same length of delay. Moving
away from the centre would produce a change in the timbre of the echoes,
as the length of the path travelled by the sound increased at one side
of the circle and decreased at the other. At first, a small difference
in the length of the delays would result in a phase shift - sounding like
the 'flanging' effect associated with 1960's records; as the difference
increased it would sound more like a complex series of echoes, but in
three dimensions. I hope to conduct some tests demonstrating this effect
in the near future.
3rd January 2008:
posting these pages and inviting feedback from the Yahoo Archaeoacoustics
Forum, the question of my unusually high Speed of Sound figures was raised.
Victor Reijs pointed out that whilst temperature drastically affects the
SOS, humidity has much less effect:
SPEED OF SOUND CALCULATOR
By using this calculator, you can see that changing humidity from 0% to
100% makes less than 1 m/s difference to the SOS. I had got this completely
wrong, thinking that humidity had as much effect as temperature. I also
consulted my neighbour, Roger Partridge, who has his own garden weather
station only three miles from Avebury. The morning of the recording was
colder than I realised: Roger recorded an air temperature of -4.4 deg
C; a grass temperature of -7.2 deg C and the fog showed as 99% humidity.
All of this should have resulted in a SOS of 329 m/s - not 346 and 338
m/s as I calculated. The on-line calculator above is admittedly not designed
for temperatures below freezing, but even so, my calculation was way out.
So in light of this, I can see no point in taking an on-site SOS reading
by measurement - I now think it's more important to carry a thermometer
and measure air temperature!
What initially caused this problem was my using a borrowed odometer to
measure distances on the ground - it turned out to be quite inaccurate,
indicating only about 97.4 % of the true distances.
This is a great shame, as an odometer is so easy and convenient to use
. Proper surveying tapes are expensive and difficult for just one person
to use alone. I don't know whether laser measuring devices have recently
improved enough for this work? Certainly a few years ago they were not
regarded as reliable enough for even interior measurements