Reverberation

by Kay Leather

Stonehenge and Stonehenge Aotearoa

Stonehenge Aotearoa is not a replica of Stonehenge England. Their sizes are slightly different, they have a different number of pillars, the pillars are not evenly spaced and the height of the henge is different from its English counterpart. Ours has an obelisk just off-centre and there are no bluestones. The pillars are hollow and their exterior has concrete spray texture. However, the sounds observed by many visitors and performers at Stonehenge Aotearoa suggests that their acoustics have much in common. Many of the statements made by R. Till in connection with Stonehenge England also have relevance to Stonehenge Aotearoa, though the sound at Stonehenge Aotearoa is yet to be scientifically measured. 

“In attempting to understand the acoustics of a monument, whether it be an enclosed or open space, we may consider two factors: the physical laws of acoustics, and the human perception, or ‘aural experience’ of the monument. Acoustic principles are constant, unchanging with time; human perception is quite the opposite! 

So there are several types of measures to be investigated, which can be divided in two main groups related to hearing and listening. The first group holds the quantitative (hearing) measures. Some measures are based on physical laws, while others are related to the combination of the ear and brain (psychoacoustics). 

Measures based on physical laws are, for instance, sound pressure level, clarity, reverberation time and early decay time (Bradley 2011, 715; Cross and Watson 2006, 109-11). 

As experiences in monuments cannot always be explained by physical laws, a more subjective explanation can be provided by including psychoacoustics. Examples of these psychoacoustic measures are perceived loudness rather than sound pressure level, and timbre – the character of a sound rather than its frequency and intensity (Cross and Watson 2006, 111- 13).

 The second group holds more qualitative (listening) aspects which depend mainly on the cultural environment of the listener by considering, for instance, semantics and aesthetics (Schafer 1977, 146-50; Storr 1933, Chapter I). This is concerning the mind.” 

– Reijs & Mussik

Stonehenge England

“Let us consider the most obvious paths of sound in a circular stone building. If standing in the middle of an open field, when one makes a sound it leaves you and keeps traveling away, thus there is no reverberation.  If  one  stands  in  a rectangular  building  and  makes  a  sound,  it  leaves  you,  hits  various  walls  at  different  times,  and  reflects  back  to  you  at  different  times,  creating  reverberation  as  the  echoes  created  merge  together.  If  standing  at  the  centre  of  a  circular  building,  when  one  makes  a  sound  it  leaves  you,  hits  and  reflects  straight  back  from  the  wall  at  all  points  of  the  circumference  simultaneously,  and  these  reflections  return  to  you  at  the  same  time.  In a large enough space this would produce a prominent echo and reverberation from the combination of a number of ‘echoes’, or reflections.  If  standing  at  the  edge  of  a  circular  space,  when  one  makes  a  sound  it  will  leave  you,  reflect  back  off  the  wall  directly  opposite  and  return  as  a  slightly  later  echo; travel to one side reflect off the wall twice making a triangular shape and return a little later still; hit a wall three times making a square shape and return a little later; hit five walls forming a pentangle and return a little later; and so on. The overall effect would  be  echoes  (in  a  large  space)  or  resonance/reverberation  (in  a  smaller  space  where   the   differences   in   distance   are   small   and   so   the   sounds   are   not   distinguishable).  In circular stone spaces the size of Stonehenge… there would actually be both reverberation and echoes…

Stonehenge could  be  made  to  resonate  further  by  producing  sound in time with the echoes in the space. One could make Stonehenge resonate, much like blowing over the top of a bottle to make it hoot, or like running one’s finger around  the  top  of  a  glass;  or  hitting  the  skin  of  a  drum.  This  would  work  by  making  the air in the space vibrate at its fundamental resonant frequency. Measurements of the  diameter  of  Stonehenge  told  us  that  this  frequency  would  be  about  10Hz.  Stonehenge  is  of  course  not  a  simple  circle,  it  is  a  complex  monument  involving  a  number of geometric shapes. However despite this, theoretically it should still have a strong fundamental resonance at this frequency. The circle of stone lintels placed on the outer ring of upright sarsen stones protrudes largely above any other stones, and would  provide  an  almost  unheeded  clear  ring  of  stone,  and  any resonant  effects  could be supported by the upright stones that support the ring of lintels.

That  the  fundamental  resonant  frequency  of  Stonehenge  was  10Hz  raised  a  number  of  interesting  questions.  Different  types  of  brain  waves  that  are  present  during  various  different  mental  states  have  specific  typical  dominant  frequency  ranges. 10Hz is a frequency that when detected in the brain is described as an alpha wave pattern. As alpha waves are often associated with relaxation, altered states of consciousness, meditation.

Clarity  and  definition  in  the  space  were  found  to  be  slightly  lower  than  ideal,  while  level  and  envelopment  were  higher.  These  figures  indicate  the  space  was  better  for  music  than  for  speech.  It  also  indicates  that  the  acoustics  would  support  rhythmic  music  better  than  sustained  musical  sounds.  The  acoustics  also  act  to engage   those   within   the   stone   circle   and   exclude   those   outside.   The   strong   envelopment  would  be ideal  for  activities  in  which  a  high  level  of  engagement  and  participation  was  required,  rather  than  where  some  people  present  were  able  to  remain  detached  from  the  activities…

Evidence  has  been  found  that  low  frequency  modal  resonance  and  standing  waves  could  indeed  have  been generated  at  the  site,  perhaps  by  large  numbers  of  participants  playing  small hand percussion instruments such as clay or wooden drums, or pieces of wood or  stones.  It  also  seems  likely  that  these  low  frequency resonances  could  be  produced by strong winds. This study revealed that there was a hierarchy of position in  the  space,  implying  that  the  different  circles  around  the  centre,  such  as  the  large  bank  and  various  stone  circles,  demarcated  different  levels  of  significance,  with  the  centre   the   most   important   position.   Being  within   the   sarsen   stone   ring’s   circumference would have produced a powerful sense of inclusion and involvement. The  stones  outside  the  central  circle  seem  to  have  had  significant  acoustic  effects  associated  with  them,  which  would  have  linked  them  in  perceptually  to  the  main  circle.”

— R. Till

Stonehenge Aotearoa

The acoustics of Stonehenge Aotearoa have not been scientifically tested but the builders and many visitors have noticed remarkable acoustic effects at Stonehenge Aoteroa.

Before we commenced the construction I had come across a summary of Dr Rupert Till’s investigation of Stonehenge, England but had forgotten exactly who was involved. However, we were curious to see if similar sound effects would be observed as we built Stonehenge Aotearoa.

Stonehenge Aotearoa is situated in the Wairarapa countryside:

  • Latitude: 41 degrees, 06 minutes, 0,48 seconds, South
  • Longitude: 175 degrees, 34 minutes, 24 seconds, East
  • 94m above mean sea level

It has a wonderful view across the valley toward the distant hills.

It was designed to be an accurate outdoor observatory and a teaching tool for teaching both ancient and modern astronomy. It is 30m in diameter with 24 pillars, topped with lintels.

The main causeway is through the Sungate toward the centre of the henge. The causeway is aligned to the equinox sunrise. 

The centre of the henge has a tiled square with a brass plaque at the centre. The hole in the centre of the plaque marks the exact centre of the henge. This point is where astronomical observations should be made. From this point the causeway, flanked by the Sungate marks the position of the equinox sunrise.

There is an obelisk just off-centre. The obelisk is at the head of a pictorial zodiacal analemma. The obelisk casts a shadow which falls on the analemma and traces the path of the sun through the stars of the zodiac. The analemma stretches from the obelisk to the outer circle of pillars. The centre of the analemma is aligned exactly north/south. Along the same alignment but beyond the circle of stones is the statue of Artemis. She marks the most southerly position of the moon.

There are six heelstones outside the circle of stones. They mark the position of the rising and setting sun at the solstices and equinoxes as viewed from the centre of the henge.

When building the henge, the first task was to exactly mark the centre of the henge. From that time on, the centre of the henge was a protected position.

The compass rose at the centre of the henge
The centre of the henge looking down the causeway

The site itself was slightly sloping east-west, requiring some shaping on the east and building up for the heelstone on the western equinox position. The causeway approaches the henge from the East.

When the first six or so pillars were erected, with their lintels, there were strong sound effects. The pillars appeared to be reflecting sound toward the central position. This was very noticeable when people were talking to each other or when hammering. It became even more effective as more pillars were erected.

Construction of the first pillars and lintels

The construction of the obelisk was begun before the henge circle was completed and because it is taller than the lintels of the henge it offered a good viewpoint. It did not pay to make careless comment anywhere in the henge. A quiet comment made while working on the upper part of the obelisk or a lintel was very clearly heard elsewhere in the henge, almost as if the speaker was standing right next to the listener. We had a lot of fun with that effect.

First pillars, sheathed with Hardibacker board

When the construction of the circle was complete the effects were even more pronounced. The centre of the henge became the prime focal point. It seemed as if anyone speaking at the centre could be heard easily without raising their voice. Sound that was uninterrupted and continuous like singing rounders or clapping seemed to build on itself.

Wire & battens over the Hardibacker, being sprayed with concrete. Analemma under the plastic in the foreground
The final effect taking shape

Away from the centre a time delay could be observed, especially obvious with clapping or other sharp sounds. People placed in different parts of the henge heard the same event at different times. Later, when the henge hosted full moon drumming events this was especially noticeable. Experienced drummers spent some time moving back and forth, in and out from the centre altering the timing of their drumbeat in an effort to put other experienced drummers off their beat. Inexperienced drummers often moved away from the central position only to find that doing so made it harder for them to keep time. Noticing this one night, I put a man, with a very large drum right at the centre. That soon cured the problem as the sound waves seemed to move the air in your lungs and it was impossible not to stay in time.

Another effect is the way the henge itself reflects sound. We have had some large musical events at the henge. At several, the stage was not in the centre of the henge, but outside the circle with its back almost touching the Artemis statue. The sound control tent was in the middle of the field on the eastern side of the henge, some 50m or more away.

When setting up the sound, the sound controller would call instructions to his workers in and around the stage. Every command was echoed back perfectly by the henge. I wondered how they would get on during the performance but it they must have controlled the echoes as it did not cause a problem. 

When the large stage was set up, the large speakers were just next to the stage, facing away from the henge. During the set up, the sound was so loud that I found it uncomfortable at the house full paddock distant. However, the quietest place was at the centre of the henge. Here it was possible to have a conversation without difficulty.

I have often been amused by mothers located a 100m away from the henge, near the Visitors’ centre but with a clear line of sight to henge, who call their children who are still in the henge area. The henge amplifies their voice, and they looked so shocked at the noise they have made. Quite often they cover their mouths and look around to see if anyone noticed.

When mustering sheep we have also noticed that in some places what is said in the henge can be easily heard over 100m away. I have heard my husband talking to workers at the henge from the gate to the property. I was probably 200m from where he was. The strange thing is that if you move even a little bit, it is not possible to hear anything from the henge.

One of the first big events we had after the opening, was a Druid event. During this ceremony, the Druids march in a circle within the circle of stones, chanting their Arwen chant. At this ceremony the TV crew came to photograph the Druids. 

The first thing we noticed, after the chant began, was all the birds in the area for some distance, flying into the air. This is unusual in New Zealand and we do not have the sort of mass migrations that happen in Europe. 

The next thing that occurred, was that all animals that were, at that time, down the hill came racing up to the fence around the henge. A bull in the herd started bellowing, and was answered by another in an adjoining property. An old horse that came with the cattle started to behave as it was training in the Spanish Riding School. It pranced up and down, lifting is hooves as if to music, head held high and tail swishing. It was quite a sight. The TV camera operators did not seem to know what to photograph – Druids or animals. I was late arriving, due to processing late arrivals, and I just enjoyed the entire spectacle.

We encourage everyone who visits the henge to experiment and enjoy the sound effects in the henge and universally they are surprised at the effects they obtain there. Henges are fun places really. I am sure our ancestors used them to great dramatic effect.

References:

Acoustic and aural research at timber and stone circles Authors Victor Reijs, ma.victor.reijs@gmail.com, ORCID: http://orcid.org/0000-0003-1429-0750, Reinhard Mussik, reinhard.mussik@gmail.com

Songs of the Stones: An Investigation into the Acoustic Culture of Stonehenge Dr. Rupert Till University of Huddersfield, UK.

Watching the equinox sunrise