Plastic-henge

After reading your article about Prof Trevor Cox’s work at Stonehenge (October, p22) I found myself asking, “what’s missing from this experiment?”

Many years ago, I qualified as a stonemason. We learned how to tell if a stone had any flaws: you take your chisel and bang it against the stone: the sound resonance will either ring true and clear, like flicking a crystal glass, or give a dull thud if the stone is flawed. So, how can any sound reflected from a piece of printed plastic give a true indication of the actual sound, especially from the harder bluestones from the Preseli Hills? I’m sure Cox thought of this, but it wasn’t mentioned in the article. I’d be very interested to know what he thinks.

Gary Sansom

Our experiments were not concerned with how the stones might respond to being struck, instead we determined how voices and musical sounds passing through the air were reflected from the structure. Acoustic scale models frequently use materials different from the original because we choose stuff that is easier to work with. But it is vital that the materials respond to sound in the same way as would happen in the real Stonehenge, allowing for the fact we test at 12 times the frequency in a 1:12 scale model.

To replicate the acoustic properties of stone, we used materials that were heavy and non-porous, like 3D-printed hollows filled with concrete. Matching ground conditions was more problematic because the conditions are uncertain. Ground reflects differently depending on how compacted it is and also on how saturated by water it is. We decided to model the ground as compacted, because moving many tonnes of rock around would have compressed the ground.

Prof Trevor Cox, acoustic engineer