Ref: http://www.aip.org/isns/reports/2004/014.html
Sounds of the Subway

New York City is famous for the broad diversity of its cultural
offerings, particularly world-class performance halls like Carnegie
Hall and Lincoln Center. But the city's most popular performance
space can be found in any number of its vast underground network of
subway stations.

"Ask any New Yorker and you'll get an earful of recommendations on
the best stations, the best players, and how much to tip," says Alex
Case, director of Fermata Audio + Acoustics, Portsmouth, NH, and a
professor at Berklee College of Music in Boston. Case himself is
something of a connoisseur when it comes to subway acoustics. He's
done extensive studies of local musicians who regularly play in
subway stations, using portable digital recorders to capture these
on-the-fly performances.

Not all subway stations are created equal. Subway buskers choose
their locations carefully, avoiding stations with a steady stream of
announcements, or major hubs with more than one line running through
them. Delays might be irritating for commuters, but for the
performers they are godsends, giving them an extended performance
period in between trains. And Case has found that the buskers
instinctively seek out locations near hard walls and under low
ceilings, so their music is amplified above the din of the station.

He doesn't find this at all surprising. Subway walls are typically
made of rigid heavy materials like tile, stone, steel and concrete.
These sorts of materials are better at reflecting sound waves,
allowing sound pressure (volume) levels to build up naturally, with
no need for microphones or loudspeakers. As a result, subway
listeners are immersed in a bath of echoed sound known as
reverberation. The same sound, heard up close, has much less
reverberation.

Reverberation is the critical element in the design of all
performance spaces, whether mainstream opera houses or alternative
spaces like the subway. It's the same reason why so many people enjoy
singing in the shower. "It just so happens that the sort of space
that is durable, easy to clean, and graffiti resistant also happens
to be sound reflective," says Case. "The musicians wallow in it. The
passengers variously savor, ignore or avoid it."

So if subway stations are such terrific acoustical environments, how
come it's so hard to understand the announcements over the public
address system? Case says that's because amplifying speech requires
far less reverberation than music. The same phenomenon that sustains
musical notes by building up sound reflections causes speech to
become mushy and unintelligible. The reflections all mix together, so
that individual words can't be deciphered. Add in the electronic
amplification, and the reverberation is so strong that the announcer
might as well be speaking with a mouthful of marbles.

That's why acousticians like Case tailor their designs to the
specific needs of performance spaces. Large opera houses like Boston
Symphony Hall or Carnegie Hall have different acoustical needs than,
say, Broadway theaters. For the latter, says Case, he designs spaces
with more sound absorption, lowering the amount of sustaining
reverberation the space adds to a speaking voice. "This makes it
easier for listeners to follow the spoken word, syllable by
syllable," he says.

Concert hall design isn't just about the strategic conservation of
sound energy inside the performance space. A great deal of effort is
also spent on suppressing the noise and vibration of the surrounding
city, not to mention noise from modern amenities -- the heating
system, air conditioning equipment, elevators and plumbing - all of
which can seriously detract from the pleasure of a performance. No
one wants to hear a subway rumbling or a toilet flushing in the
middle of Wagner's "Ring" cycle.

For Case, the most appealing feature of alternative performance
spaces like the subway is their broad accessibility, compared with
conventional opera houses, which tend to be somewhat elitist. He
estimates that some 7 million passengers ride the New York subway
system every day. Even if only 1 in 10 passengers pay attention to
the music, it still adds up to about 700,000 listeners per day, from
every conceivable social demographic. Carnegie Hall and Lincoln
Center would have to sell out 54 shows every day, on all six main
stages, just to compete. The subway is the opera house of the people.