Binaural recordings are made using a pair of microphones spaced exactly as human ears or even inside human ears. This captures a detailed 3-D soundfield that could be described as an 'audio hologram'. When played back on headphones, the spatial imaging exactly recreates the original location, with sounds appearing to come from all around - even from above.

There's much more about binaural and surround sound at my BILOCATION website.

Most of my recent recordings have been made with mics inside my own ears, and that's how I attempted to record the first performance at 11am on Saturday 14th July. However the recording was unusable, due to excessive wind and traffic noise in a position near the busy A4 road. I returned for the 3pm session with a dummy head equipped with wind shields and chose a better location away from the road - marked as position 1 on the map. I recorded a commentary, but the hill was too far away to see the instruments properly and some were identified wrongly. All were clearly audible though, except the wooden pipes.

At 6am on Sunday 15th July, I recorded another performance from position 2 on the map. Near the top of Waden Hill, I was almost exactly level with the top of Silbury and just about half a mile away. The near absence of traffic noise meant that I could hear absolutely all the instruments, although rather quietly.

THE SILBURY ECHO
For my last recording, at 11am on Sunday 15th July, I chose position 3 - where I guessed that an audience may have stood when the hill was in use 4000 years ago. In the winter, a wide moat forms around the hill and this appears to be part of the original design. I stood just north of the area that floods, and from there the hill appears much bigger than it does from the road. All of the instruments could be heard clearly - even some conversation. As observed in the other recording positions, sounds appeared to travel somehow over the top of the hill: musicians can be heard even when they are not visible. This is not an effect of the wind, which sometimes blew in the opposite direction.
The braying, high notes of the Celtic Trumpet produced a surprising effect. All the instruments were played in four positions - facing N, S, E & W. The Celtic Trumpet is very directional (as are the animal horns) and extremely loud. When Simon played facing east, clear repeat echoes were heard - resounding from two different locations on Waden Hill. The echoes are between a third and half a second in length. Similarly, when he played the trumpet facing south, slightly shorter echoes were heard - this time from the hills to the SW of Silbury. I understand that the musicians on the hill were also aware of echoes, but I have no details.
These tests were made under poor conditions: strong winds, birdsong, planes and traffic all made listening quite difficult. On a misty and still day in the winter, sounds will carry much further and it is possible that quieter sounds will echo - maybe even the calls of Rooks and Buzzards. My feeling is there is very likely a 'sweet spot' somewhere between recording positions 1 and 3 where the echoes will be at their most dramatic; and that is where an audience would have been positioned for maximum theatrical effect.