Looking across the stone circle of Easter Aquorthies.
 
 

Acoustics at Easter Aquhorthies

Easter Aquhorthies is a recumbent stone circle, one of a distinctive group of monuments found in the north-east of Scotland. Like many recumbent circles, the monoliths are graded in height so that the tallest stones stand to either side of a large ‘recumbent’ stone. The stones display a variety of colours and textures, including quartz banding. Two stones project from the recumbent inwards towards the centre, creating an alcove. During a site visit in 1994, I realised that this setting was echoing sounds made within the circle, and sounds were also ricocheting between the other stones.

 
The large recumbent and its flanking stones at Easter Aquhorthies.
 
 

Working with acoustician David Keating, a method was developed to measure these subtle experiences. In 1996 I returned to the site and placed a loudspeaker in the alcove created by the recumbent and its flanking blocks. This emitted pink noise (a combination of frequencies that are reminiscent of a waterfall) at a constant volume, while I used a digital tape recorder to systematically captured this sound on a grid across the circle.

Measuring the archaeoacoustic properties of Easter Aquhorthies in 1996.

The loudness of each recording was plotted onto a plan of the site. This showed that the recumbent stone does indeed throw echoes across the centre of the circle. The same equipment was then used to measure the same sound source in an open area. This control reveals that, with no stones present, sound diffuses equally in all directions.

The plan on the right shows how sound is reflected by the recumbent stone at Easter Aquhorthies, contrasted against a control plot on the left that shows the same equipment being used in an open area. 

Since the acoustic work at Easter Aquhorthies, Richard Bradley has excavated a number of recumbent stone circles across Aberdeenshire. This revealed that these monuments were constructed in a series of phases towards the end of the Neolithic, around 2000 BC. The earliest feature at each was a low circular ring cairn, within which funeral pyres were burnt. The standing stone circle was added to enclose this cairn, which continued to be a focus for the treatment and deposition of the dead.

While Easter Aquhorthies has not been excavated in recent times, discoloured grass during very dry summers hints that there was likely to have once been a ring cairn inside. The measured sound reflections would have been most obvious at the centre of the ring cairn, the focal point of the monument.

Easter Aquhorthies is likely to have been a venue for gatherings of people, perhaps as part of funerary rituals. The arrangement of stones around the recumbent might have acted rather like a stage in a theatre, being both a visual backdrop and reflecting sound in unusual ways.

Further reading

Watson, A. and Keating, D. 1999. Architecture and sound: an acoustic analysis of megalithic monuments in prehistoric Britain. Antiquity 73, 325-36.