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Why the Greeks could hear plays
from the back row
An
ancient theatre filters out
low-frequency background noise.
The
wonderful acoustics for which the ancient Greek theatre of Epidaurus
(Epidavros) is renowned, may come from exploiting complex acoustic
physics, new research shows.
The
theatre, discovered under a layer of earth on the Peloponnese
peninsula in 1881 and excavated, has the classic semicircular
shape of a Greek amphitheatre, with 34 rows of stone seats (to
which the Romans added a further 21).
Its
acoustics are extraordinary: a performer standing on the open-air
stage can be heard in the back rows almost 60 meters away. Architects
and archaeologists have long speculated about what makes the sound
transmit so well.
Now
Belgian born Assistant Professor Nico F. Declercq Phd and Cindy
Dekeyser of the Georgia Institute of Technology in Atlanta say
that the key is the arrangement of the stepped rows of seats.
They calculate that this structure is perfectly shaped to act
as an acoustic filter, suppressing low-frequency sound —
the major component of background noise — while passing
on the high frequencies of performers' voices.
It's
not clear whether this property comes from chance or design, Nico
Declercq says. But either way, he thinks that the Greeks and Romans
appreciated that the acoustics at Epidaurus were something special,
and so they copied them elsewhere.
Sound steps
In
the first century BC the Roman authority on architecture, Vitruvius,
implied that his predecessors knew very well how to design a theatre
to emphasize the human voice. "By the rules of mathematics
and the method of music," he wrote, "they sought to
make the voices from the stage rise more clearly and sweetly to
the spectators' ears... by the arrangement of theatres in accordance
with the science of harmony, the ancients increased the power
of the voice."
Later
writers have speculated that the excellent acoustics of Epidaurus
(Epidravros), built in the fourth century BC, might be due to
the prevailing direction of the wind (which blows mainly from
the stage to the audience), or might be a general effect of Greek
theatre owing to the speech rhythms or the use of masks acting
as loudspeakers. But none of this explains why a modern performer
at Epidaurus (Epidravros), which is still sometimes used for performances,
can be heard so well even on a windless day.
"The
acoustic cut-off frequency is right where you
would want it"
Nico
Declercq, Georgia Institute of Technology, Atlanta
Declercq
and Dekeyser suspected that the answer might be connected
to the way sound reflects off corrugated surfaces. It
has been known for several years now that these can filter
sound waves to emphasize certain frequencies, just as
microscopic corrugations on a butterfly wing reflect particular
wavelengths of light. The sound-suppressing pads of ridged
foam that can be plastered on the walls of noisy rooms
also take advantage of this effect.
Declercq
and Dekeyser have calculated how the rows of stone benches
at Epidaurus (Epidravros) affect sound bouncing off them,
and find that frequencies lower than 500 hertz are more
damped than higher ones.
Murmur murmur
"Most
of the noise produced in and around the theatre was probably low-frequency
noise," the researchers say: rustling trees and murmuring
theatre-goers, for instance. So filtering out the low frequencies
improves the audibility of the performers' voices, which are rich
in higher frequencies, at the expense of the noise. "The
cut-off frequency is right where you would want it if you wanted
to remove noise coming from sources that were there in ancient
times," says Declercq.
Nico
Declercq cautions that the presence of a seated audience would
alter the effect, however, in ways that are hard to gauge. "For
human beings the calculations would be very difficult because
the human body is not homogeneous and has a very complicated shape,"
he says.
Filtering
out the low frequencies means that these are less audible in the
spoken voice as well as in background noise. But that needn't
be a problem, because the human auditory system can 'put back'
some of the missing low frequencies in high-frequency sound.
"There is a neurological phenomenon called virtual pitch
that enables the human brain to reconstruct a sound source even
in the absence of the lower tones," Declercq says. "This
effect causes small loudspeakers to produce apparently better
sound quality than you'd expect."
Although
many modern theatres improve audibility with loudspeakers, Declercq
says that the filtering idea might still be relevant: "In
certain situations such as sports stadiums or open-air theatres,
I believe the right choice of the seat row periodicity or of the
steps underneath the chairs may be important."