Steeper Seats
I don't know what it is like now, when tourism to Jerash has taken off and the place is crowded with chattering Americans in ghastly checked trousers calling to "Muriel" to pose right where you want to take a photograph "just one more time". Back in 1958, when I first visited the place, the ruins were totally deserted and we actually drove up the main street from the Oval Forum to the Tetrapylon in our car.
One of the "treats" of a visit to Jerash was to stand on the stage of the Great Theatre while some cooperative companion climbed to the top-most row of seats, seemingly miles away. Yet you could speak in a perfectly normal voice and be heard clearly by your distant friend - though he had to shout to be heard by you. It was an impressive piece of acoustical engineering.
In part it is helped by the fact that the theatre is a semi-circle, so unlike a modern theatre, which is basically a box with a stage at one end and corners and flat walls which distort the sound, in an ancient theatre everyone was within the same radius of the stage and there are no flat surfaces to bounce the sound around and create acoustic dead spots.
In fact, as I discovered to my surprise when I guided the Diggings tour around Rhodes, there is a "sweet spot" in the orchestra which is exactly in the centre of the semi-circle and anyone standing there has his voice dramatically amplified. Very impressive, but I can't see actors jumping back and forth in and out of the sweet spot as the dialogue goes on and I'm also uncertain whether the effect would be as impressive when all the seats were occupied by humans whose clothing and flesh would effectively damp any reflected sound.
The other feature is the fact that the seating is raked - that is, the seats rise up so that those furthest away from you are also highest. Again, it meant that there was pretty much a straight line between the speaker on the stage and the furthest auditor with no heads or hats or balconies to interfere.
This problem of dead spots and echoes and sweet spots is quite a serious one. The last thing you want is to spend several million building a wonderful opera house or concert hall only to discover at your first concert that the place is unusable because of the echoes reverberating around the space. Acoustic engineers have all sorts of "rules of thumb" and rely on experience and intuition when designing such buildings, but recently they have found a new tool.
The most photo-realistic computer generated scenes use a technique called "ray-tracing" in which the computer traces every ray of light that hits the viewer's eye back to its source, modifying it as necessary along the way. There are all sorts of complicated mathematical formulae to describe what happens when a ray of light hits a mirror or a piece of black felt or water or anything else you can imagine.
The most complicated surface of all is human skin. Various formulae were tried until someone realised that skin cannot be treated as a single surface: not only is there the reflection from the surface but there is also light from layers beneath the surface which vary depending on the skin colour, age, and all sorts of other factors.
Some time ago some smart sound engineer realised that what worked for light rays would also work for sound "rays", and various software packages are now available which allow you to specify the shape of a building and the position and materials of the walls, seats, lighting and anything else which might affect the sound. Set it running and the program traces the audio signals and tells you - or even demonstrates for you - what a listener in any particular seat will hear.
Recently Jian Kang and Kaliopi Chourmouziadou of the University of Sheffield applied this software to six Greek and Roman theatres, accurately recreating the size and shape of the auditoria. The earliest theatres were simply rectangular spaces with the stage on one of the long sides and the team didn't worry about those. However around 500 BC the Greek theatres adopted the semi-circular form, though at first the seats were only slightly tiered.
The Romans not only made the seating steeper, but they also installed a wall behind the stage to act as a sort of rudimentary scenery. Despite the pillars, doorways and porticoes, the wall was made of solid stone, so it also had the effect of reflecting sound towards the audience.
Kang and Chourmouziadou found that as the seating got steeper and the proscenium higher, so the sound quality improved - as the theatre at Jerash demonstrates.
Clever chaps, these Greeks and Romans.
© Kendall K. Down 2009
