THE BECKHAMPTON COVE, AVEBURY - an archaeoacoustics experiment
www.stevemarshall.org.uk
 
Right: the four stones of the Beckhampton Cove. Only the Eastern stone, known as 'Adam' remains today. Note that the actual setting is skewed to the west: the long axis points more NW/SE.

When the experiment was first proposed in June 2009, Victor Reijs was the first to respond. He suggested computer modelling as a first step and others agreed. Victor quickly modelled the Cove using CARA - a programme really designed for indoor spaces; he still manged to produce a useful animated Sound Pressure Level diagram (swept from 5 Hz to 20 kHz), showing the 'standing waves' with multiples of 12 Hz (east/west) and 25 Hz (north/south):

STANDING WAVES VIDEO

This was encouraging, as the original assumption had been that we could expect to find resonant frequencies within the setting, as dictated by its dimensions.

Stephen Allan then visited the actual Beckhampton Cove and made an impulse recording from the one remaining stone 'Adam' using a balloon burst. The recording confirmed that the surface of the stone is indeed highly reflective of sound.

Stephen proposed still building a physical model, but instead of placing plywood sheets around 'Adam' he suggested modelling the entire setting with some available chipboard. He found a suitable site - a large sports field on the University of the West of England campus near Bristol. Using the impulse recording, Victor Reijs did further modelling in CARA, confirming that chipboard reflects sound very similarly to concrete:

CONCRETE/CHIPBOARD COMPARISON VIDEO

Stephen Allan, Richard Pearson and myself met at the site on 12th July 2009 on a very wet Sunday morning. Stephen had already marked out the positions of the four stones, but by using 8' x4' sheets vertically, each 'stone' would only be half its proper width. When the rain stopped we erected a single sheet using a bungee and guy rope on each side. An impulse recording was made from the single sheet, before the other three sheets were put in place.


Weather conditions were bad. High winds continued for the whole day, which caused problems as the sheets were bending in the wind. A series of impulse recordings using ballon bursts was made, using two Edirol recorders in a variety of positions, both inside and outside the setting.

Model as seen from the south end:

Model seen from the north end:

 

RESULTS

Using a variety of musical instruments and human voices, sounds were played inside the setting - searching for resonances or any other unusual effects. Voices from within the setting sounded slightly 'roomy' to a listener outside, but generally, any effects were minimal. The expected flutter echo along the N/S axis was not heard: this appears to be a result of the placing of the N & S stones. Although parallel, both stones are skewed a little to the west (see diagram above). All the experimenters felt this placing was a deliberate attempt to reduce flutter echoes.

Then a very strange effect was noticed: the Cove functions as a megaphone!

If sounds are made from the position marked roughly with a red spot in the diagram left, they are projected south by stones E & W.
A listener outside the setting, standing behind stone S, can hear the sound source clearly - even though it is hidden from view! The stone appears to 'speak'.

This effect also works in reverse, almost like an ear-trumpet: a listener at the red spot can clearly hear a sound source hidden behind stone S.

The plot below demonstrates the megaphone effect very clearly.
A balloon burst at the red spot position inside the Cove was recorded from a position beyond stone S, where the sound source was hidden from view.

The recorder is shielded from the source by the sheet of chipboard representing stone S. The first impulse is the direct sound, the second impulse 7ms later is the sound as reflected by the E and W stones. The reflection is surprisingly high in level being almost the same level as the original impulse. There is some smearing of the reflected impulse probably caused by a slight inequality in the length of the paths travelled by the reflected sounds. The initial impulse has either travelled through the chipboard representing stone S or has been refracted around it.

Note that a sheet of chipboard only blocks the upper frequencies: bass frequencies will pass straight through the sheet.

Another series of recordings was then made, with a djembe drum played at the red spot position. Here are two mp3s that demonstrate the difference:

RECORDING FROM BEHIND STONE S - megaphone effect

RECORDING FROM BEHIND STONE N - no megaphone effect

For further evidence, see Stephen Allan's page of recordings and diagrams:

FULL RESULTS

WHAT NEXT?

If the Beckhampton Cove is indeed, as it appears to be, a Neolithic megaphone, further investigation is needed - both by computer simulation and by building a better physical model. It is proposed that eventually this should be as realistic as possible, using concrete slabs cast to the size and shape of the original stones of the Cove. Although sheet materials such as chipboard reflect high fequencies well, they are transparent to the lower frequencies. At 12 Hz and 25 Hz the standing waves inside the Cove are below the lower limit of human hearing but may still produce an effect on the body. Perhaps by exciting the standing waves with drums, or even a bull-roarer, further effects will become apparent?

There is also the question of alignment. Why would the Late-Neolithic builders have wanted to direct sound in that particular direction? They seem to have had a preoccupation with more ancient sites, such as the Windmill Hill causewayed enclosure and the West Kennet long barrow - the 'places of the ancestors'. There are unusual sight-lines and alignments between the monuments of the Avebury complex that suggest that a culture was affirming its own origins.
The central long axis of the Beckhampton Cove is quite difficult to determine exactly from the available excavation plan, but so far appears to point towards the Knapp Hill causewayed enclosure, 6.5 miles to the south-east. It may also be a lunar or solar alignment.

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Photographs & text Copyright Steve Marshall 2009. All rights reserved.