Acoustics 2007

Acoustics have been on my mind recently, and not just because of (as some might suggest) the echoing empty space between my ears.

First came the CFL Western semi-final game at Mosaic Stadium, where noise, reflected and focused by the stands, played at least some role in the Riders’ victory—and utterly failed to carry from the halftime stage in the end zone to our seats near mid-field. (It doesn’t seem to matter where they place the stage, either: the sound was just as bad at halftime at the last Grey Cup Regina hosted.)

The acoustics of B.C. Place were in the news the following weekend, and crowd noise at Rogers Place in Toronto got mentioned more than once in connection with Sunday’s Grey Cup. (Which, in case you haven’t heard, the Riders won. No, really!)

Meanwhile, I’m currently in rehearsal with Persephone Theatre in Saskatoon for an upcoming production of Beauty and the Beast, to be performed in a brand-new theatre that we haven’t even seen the inside of yet. We won’t be wearing microphones, so the theatre’s acoustics are going to be of great concern to all of us.

Acoustics comes from the Greek "akouein," meaning "to hear.” It can just mean the science of sound, but usually refers to the application of that science to the construction of spaces that enhance the hearing of speech or music.

Acoustics are notoriously difficult to get right, because sound bounces off everything in the space: floor, ceiling, walls, chairs, audience members, light fixtures, fire extinguishers...everything.

Although American physicist Joseph Henry produced the first thorough scientific treatment of acoustics in 1856, lots of halls with poor acoustics have been constructed since: the acoustics of any large space are so complex that even with modern technology and knowledge, there are no guarantees of getting it right.

The three main elements that determine a space’s acoustics are size, shape and building materials.

In a large room, sound takes more time to travel to the walls and ceiling and back again, so the sound reverberates longer: in a large cathedral, for four to five seconds. That’s great for organ music, not so great for singers or speakers, whose words get lost in the echoes.

For opera, a reverberation time of around one second is preferred. Symphony orchestras prefer a reverberation time of 1.5 to two seconds.

Most concert halls are shoebox-shaped, with the stage at one end. The close-in side walls reflect sound quickly back into the audience, amplifying it while maintaining clarity. In horseshoe-shaped halls, short balconies on the sides serve the same function.

Finally, building materials matter: carpet and other porous materials absorb sound; hard materials reflect it.

The importance of building materials to acoustics was emphasized earlier this year when researchers at the Georgia Institute of Technology finally figured out why the ancient Greek theatre at Epidaurus, carved out of the side of a hill in the fourth century B.C., has such amazing acoustics: it seats 14,000, yet every audience member can hear a whisper from anywhere on stage. (I’ve been there; it’s almost spooky.)

The researchers discovered that it’s not the angle of the slope or the curve of the seats that does the trick: it’s the limestone the seats are made of. The slightly porous limestone acts like a filter, suppressing the low frequencies of voices (thus minimizing crowd noise) while reflecting high frequencies.

Even though the limestone also filters out the low frequencies from the actors’ voices, listeners don’t realize it because their brains reconstruct the missing frequencies, a phenomenon called virtual pitch. (We experience the same thing when talking to someone on a telephone.)

Did the Greeks know something we didn’t figure out until 2007? Apparently not: they tried to recreate the design elsewhere with limited success, because they used other materials (such as wood) for the benches.

In other words, they got lucky. Modern acoustical engineers try not to rely on luck...but they’ll take it if they can get it.

Oh, and for the record, I don’t believe anyone has tried filling the 14,000 seats at Epidaurus with Saskatchewan Roughrider fans.

Somehow I suspect there’d still be plenty of crowd noise, limestone or no limestone.

These weekly columns on science appear in the Regina (Saskatchewan) Leader Post and Red Deer (Alberta) Advocate.  They are available for one-time publication or regular syndication to any interested newspapers, magazines or on-line publications.  E-mail me for details.