The World’s Oldest High Pass Filters
During a recent trip to Argyle I paid a visit to McCaig’s Folly, a circular Colosseum like tower built on a hill in Oban to film a short video shoot. I had with me a cheap travel guitar that was purchased for thirty U.S, dollars and weighs of all of about half a kilogram. I was very shocked to find that inside the folly that little guitar sounded loud and like it was being played through a delay peddle and a reverb unit. This prompted me to take an interest in the acoustic properties of amphitheatres.
McCaig's folly is actually not an amphitheatre as such, but there is a beautiful resonance inside it, caused by the sound reflecting from the non absorbing stone walls and possibly augmented by the diffraction that will be caused by the archways in the picture below. Sadly, I was only there for half an hour and so had no time nor equipment with me to investigate further.
It turns out though that recent discoveries have been made into the properties of amphitheatres, the world's oldest known high pass filters.
An Amphitheatre consists of a series of rows of semi-circular seats called a koilon and a stage with a skene behind it that reflects sound towards the audience. The koilon has one or more horizontal structures called diazoma that divides it. The diazoma is probably not for acoustic effect, but would be needed to allow the audience to take their chairs. The stage has a flat semi-circle in front of it called the orchestra and the koilon slopes upwards from this as seen in the photograph below of the amphitheatre in Amman, Jordan.
The oldest surviving amphitheatre is in Pompeii, and was built in 80 BC. This was actually an oval shape rather than the semi-circle shape favoured by the Greeks. It's acoustic properties were used as recently as October 4th to October 7th 1971 by Pink Floyd, to shoot video of a live performance of Echoes, A Saucerful of Secrets and One of These Days.
Recent experiments on a theater dating to the fourth century B.C. arranged in fifty five semi-circular rows, the creation of Polykleitos the Younger, have been detailed in the Journal of the Acoustic Society of America. These experiments demonstrate that the koilon diffracts frequencies under five hundred hertz because of the tiered steps, which also reflect frequencies above five hundred hertz back into the audience. They also have a corrugated surface that acts as an acoustic trap.
It's estimated that audiences of up to fourteen thousand people have heard actors and musicians performing unamplified from the back row of this theatre. This is because the high pass filtering and reflection increases the clarity of the sound and the intelligibility of human speech. This, of course, assumes that the amphitheatre is not overfilled. With too many people in the koilon, too much of the non-absorbing surface would be covered.
High Pass Filters?
High pass filtering is a process that every modern sound engineer uses for the purpose of enhancing the clarity of signals, although much of this is now done with electronic signal processors instead of buildings. It consists simply of attenuating the audio spectrum below a certain frequency threshold that in modern equipment can be set by the user. Amazingly, there is no evidence that the ancient Greeks and Romans had any understanding of the psycho-acoustic principles that led to the development of these architectural structures, so how they designed them is unknown.
An interesting fact to consider is that the human voice and most musical instruments have fundamentals that are below five hundred hertz and yet the audience in the amphitheatre were not disturbed by the high pass filtering effect. There are analogies to this in the modern world. A male voice has key frequencies at one hundred and fifty hertz, while most telephones don't reproduce frequencies lower than three hundred hertz, yet a male voice still sounds male over a telephone.
This is because of a phenomenon in psycho-acoustics where human listeners were found to hear low frequency sounds that were not actually present in the sounds they were listening to. At first it was thought that this was caused by distortions produced by the ear, until experiments carried out by J.C.R. Licklider in 1954 used signals that would perceptually mask the phantom signals if they were physically present to falsify this hypothesis.
It is now known that a human brain interpreting a series of harmonics with the fundamental missing will create the missing fundamental. When a listener is played a series of harmonics starting 200Hz ,400Hz ,800Hz, 1600Hz, et cetera, the human brain will create the 100Hz fundamental. Although it has never been proven precisely how this is done, it is thought that processing based on auto-correlation is used by the brain. However it is done, it explains why for people sitting in an amphitheatre the benefits of the high pass filtering would outweigh any deleterious effects on the sound.
Digital Signal processing
In the digital era, this phenomenon has been mimicked in digital signal processing. In 1999, Meir Shashoua of Tel Aviv patented an algorithm called MaxxBass, distributed by Waves Audio. This product works by high pass filtering and then recreating the perception of bass fundamentals using synthesized harmonic overtones, making it useful in creating the perception of a strong low end where the product will be played back on systems like mp3 players and computer systems with small speakers incapable of producing a strong bass response. It has been used to this end in modern studio productions. An example of this is Lady Marmalade, the 2001 version sung by Christina Aguilera, Lil Kim, Mya, and Pink and produced by Missy Elliot.
Every sound engineer who lives in Scotland or who visits Argyle should go and have a shot at playing a guitar in McCaig's Folly so they can hear how effective the oldest and most time honoured way of practicing sound engineering can be. Materials that reflect, diffract and absorb sound waves are still used to create and enhance acoustic spaces in studios and theatres today. No matter what clever electronic and digital plugins are devised, they always will be.
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About Graeme Young
Graeme Young is a sound engineer based in the South of Scotland, starting his career after being presented with a Yamaha MD8 in the late 1990's by a friend who had despaired of figuring out how "the damned thing" worked. Brimming with enthusiasm as he learned his way around his new toy, he immediately set to work with gusto, creating some of the most abominable sound recordings of musical doodling ever committed to tape.
Nevertheless, the heady times of fun and friendship that were built up in the early days convinced Graeme to go back to college and expand his knowledge and skill set, meeting contacts and learning the tradesman's tricks from industry professionals. In the meantime student loans were spent on studio equipment to expand on the trusty MD8 while the O2R96 and the old StudioMaster at the college provided experience of working in both the analogue and digital domains.
Now, Graeme has gained years of industry experience working on a number of studio recordings and location recordings with professional musicians and in directing and editing video projects for professionals in various fields. Graeme has fronted two bands, the now discontinued Popping666Cherries alongside Gwen Smith, who had fifteen minutes of fame on the X-Factor before being told by Simon Cowell that she would "never do anything" and now his own Moonstruck Project, with a loose collaboration of musical friends.
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