New research on Stonehenge just keeps on coming. This month, instead of exploring the origins of its various components (see CA 366 and 367), we are examining its acoustic properties – a first for ‘Science Notes’, and an area that is seldom considered in archaeology.
Stonehenge is one of the few archaeological sites in Britain that has previously been assessed for its sound quality – to varying success. One past project examined the acoustics of the monument as it stands today, but with many of the stones missing or fallen the current conditions are very different to those in prehistory. Another investigation examined the properties of the replica at Maryhill in the United States, but this modern model is made of straight-edged concrete blocks – an imperfect stand-in for the roughly hewn real stones, and a difference that would significantly alter the way sound waves reflect from surfaces. One researcher has used computer simulations to try to determine how sound waves would travel around Stonehenge, but the model used can only achieve approximate measurements.
To try to circumvent these flaws and recreate the characteristics of prehistoric Stonehenge as accurately as possible, Trevor Cox and Bruno Fazenda from the University of Salford and Susan Greaney from English Heritage decided to use a scale model of the monument. Assessing acoustic quality using scale modelling is not a new practice, and it has been used to examine other historic buildings and amphitheatres. By using laser-scans of Stonehenge previously produced by Historic England, the team was able to create a 1:12 replica based on Stage 4 of the monument’s proposed development – which dates to around 2200 BC.
Some decisions had to be made in this recreation, however. Since the model was 3D printed (as it would be incredibly time-consuming to chisel out an exact replica), it could not be made out of stone. As this is an acoustic scale model, however, what matters most is that the sound properties are matched. So, to mimic the impervious quality of stone, each replica was sprayed with a cellulose car spray-paint to seal up any pores that might have absorbed sound. In addition, the exact ground conditions during prehistory are not known, but the team assumed that the construction of such a massive monument would have heavily compacted the grass that overlays the chalkland of Salisbury Plain, and attempted to recreate these conditions using MDF that gave a similar absorption to other examples of compacted dense ground.
The model was then tested in a semi-anechoic chamber at the University of Salford, which absorbed all the sound that escaped from the ‘stone’ circle. In this way, this room was able to simulate the effects of the open countryside. To test the acoustics from different angles, the microphone and receivers were moved to different positions around the monument.
In terms of reverberation – which is the prolongation of sound due to reflections off the ‘walls’ (in this case, stones) of the ‘room’ (in this case, the monument) – they found that their measurements significantly differed from those taken at Stonehenge today, as well as those recorded at Maryhill. Drawing on established knowledge from sound perception, they deduced that this distinction would have been audible. In particular, they found that in their model, despite there being no roof and significant gaps between the stones, much of the sound stayed within the monument itself. So, while music and/or speech would appear to be amplified for people standing within the sarsen circle, for those outside it the sound would be quite muted.
As the monument has been altered throughout prehistory, particularly in terms of the arrangement of the bluestones, the team took measurements using different configurations of those stones as well. Interestingly, it appears that the bluestone arrangement had little impact on the acoustics, with no audible difference perceived. Overall, while the team does not believe that acoustics were a main factor that was considered in the construction of Stonehenge, it does appear that it may have affected how the monument was able to be used – with only those present within the circle itself able easily to hear any music or speech produced inside it.