The Sound Book: The Science of the Sonic Wonders of the World (3 page)

G
uinness
recognizes a few world-record sounds: the loudest purr of a domestic cat (67.7 decibels, in case you wondered), the loudest burp by a male (109.9 decibels), the loudest clap measured (113 decibels)—all very impressive. But as a scholar in architectural acoustics, I am more intrigued by the claim that the chapel of the Hamilton Mausoleum in Scotland has the longest echo of any building. According to the 1970
Guinness Book of Records
, when the solid-bronze doors were slammed shut, it took 15 seconds for the sound to die away to silence.

Guinness
describes this phenomenon as the “longest echo,” but this is not the right term. Experts in architectural acoustics, like me, use the term
echo
to describe cases in which there is a clearly distinguishable repetition of a sound, as might happen if you yodeled in the mountains. Acousticians use
reverberation
when there is a smooth dying away of sound.

Reverberation
is the sound you might hear bouncing around a room after a word or musical note has stopped. Musicians and studio engineers talk about rooms as being
live
or
dead
. A live room is like your bathroom: it reflects your voice back to you and makes you want to sing. A dead room is like a plush hotel room: your voice gets absorbed by the soft furnishings, curtains, and carpet, which dampen the sound. Whether a room sounds echoey or hushed is largely a perception of reverberation. A little bit of reverberation causes a sound to linger—a bloom that subtly reinforces words and notes. In very lively places, such as a cathedral, the reverberation seems to take on a life of its own, lasting long enough to be appreciated in detail. Reverberation enhances music and plays a crucial role in enriching the sound of an orchestra in a grand concert hall. In moderation, it can amplify the voice and make it easier for people to talk to each other across a room. Evidence suggests that the size of a room, sensed through reverberation and other audio cues, affects our emotional response to neutral and nice sounds. We tend to perceive small rooms as being calmer, safer, and more pleasant than large spaces.
¹

I got my chance to explore the record-holding mausoleum at an acoustics conference in Glasgow that included in its program a trip to the chapel. Early on Sunday morning, I joined twenty other acoustic experts outside the gates of the mausoleum. Constructed from interlocking blocks of sandstone, it is a grand, Roman-style building rising 37 meters (40 yards) into the air and flanked by two huge stone lions. An uncharitable observer might try to infer something about the tenth Duke of Hamilton's manhood from the building's shape, which is a stumpy, dome-topped cylinder. It was built in the mid-nineteenth century, but all the remains have long since been removed. The building sank 6 meters (20 feet) because of subsidence caused by mines, which left the crypt vulnerable to flooding from the River Clyde.

The eight-sided chapel is on the first floor and is dimly lit by sunlight shining through the glass cupola. The chapel has four alcoves and a black, brown, and white mosaic marble floor. The original bronze doors that start the world-record echo (modeled on the Ghiberti doors at the Baptistry of St. John in Florence, Italy) are propped up in two of the alcoves. Opposite the new wooden doors is a plinth, built of solid black marble, that once supported an old alabaster sarcophagus of an Egyptian queen within which the embalmed duke was laid to rest. The sarcophagus was actually a bit small for the duke, and our guide delighted in relaying gruesome stories about how the body was shortened to get it to fit. On the day I was there, the plinth was covered in laptops, audio amplifiers, and other paraphernalia for acoustic measurements.

The chapel was meant to be used for religious services, but the acoustic made worship impossible. It was like a large Gothic cathedral, and I found it difficult to talk to my acoustics colleagues unless they were close to me, since the sound bouncing around the chapel made speech muddy and indistinct. But was this the most reverberant place in the world? The record is important to me as an acoustic engineer because the study of reverberation marked the beginning of modern scientific methods being applied to architectural sound.

T
he scientific discipline of architectural acoustics began in the late nineteenth century with the work of Wallace Clement Sabine, a brilliant physicist who, according to the
Encyclopaedia Britannica
, “never bothered to get his doctorate; his papers were modest in number but exceptional in content.”
²
Sabine was a young professor at Harvard University when, in 1895, he was asked to sort out the terrible acoustics of a lecture hall of the Fogg Museum, which (in his own words) “had been found impractical and abandoned as unusable.”
³
The hall was a vast, semicircular room with a domed ceiling. Speech in the room was largely unintelligible—a muddy soup of sound more characteristic of the Hamilton Mausoleum than of a well-designed lecture hall. The most forthright critic of the space was Charles Eliot Norton, a senior lecturer in fine arts.

Imagine Norton standing at the front of this vast hall trying to expound on the arts—formally dressed and sporting a large mustache, sideburns, and receding hair. His students would first get the sound traveling directly from the professor to their ears—the sound that goes in a straight line by the shortest route. This direct sound would then be closely followed by reflections—the sound bouncing off the walls, domed ceiling, desks, and other hard surfaces in the room.

These reflections dictate the
architectural acoustics
—that is, how people perceive the sound in a room. Engineers manipulate acoustics by changing the size, shape, and layout of a room. This is why acousticians like me have an uncontrollable desire to clap our hands and hear the pattern of reflections. (My wife was appalled when I clapped my hands in a crypt of a French cathedral. This must go down as one of the more esoteric ways of embarrassing your spouse.) After clapping my hands, I listen for how long it takes the reflections to become inaudible. If sound takes a long time to die away—if it reverberates for too long—then speech will be unintelligible as consecutive words intermingle and become indecipherable. As Henry Matthews wrote in a nineteenth-century text on sound, reverberation “does not politely wait until the speaker is done; but the moment he begins and before he has finished a word, she mocks him as with ten thousand tongues.”
⁴
This is what happened whenever Norton tried to lecture. Students might quip that most lectures are incomprehensible even before the speech is mangled by the room, but Norton was a good communicator and a popular teacher. In this instance it was indeed the fault of the room and not the performer.

Large spaces with hard surfaces, such as cathedrals, the Hamilton Mausoleum, or the cavernous lecture hall at the Fogg Museum, have reflections that persist and are audible for a long time. Soft furnishings absorb sound, reducing reflections and speeding the decay of sound to silence. Wallace Sabine's experiments involved playing around with the amount of soft, absorbing material in the lecture hall—a method that makes him seem like an overenthusiastic fan of scatter cushions. Sabine took 550 one-meter-long (about 1 yard) seat cushions from a nearby theater and gradually brought them into the lecture hall in the Fogg Museum to observe what would happen. He needed quiet, so he worked overnight after the students had gone home and the streetcars had stopped running, timing how long it took sound to die away to nothing. He did not use clapping, maybe because it is difficult to clap consistently unless you are a professional flamenco musician, but instead used a note created by an organ pipe.

Sabine called the time it took for the sound to wither to silence the
reverberation time
, and his work established one of the most important formulations in acoustics. The equation shows how the reverberation time is determined by the size of the room, which is measured by its physical volume, and the amount of acoustic absorbent like the seat cushions from Sabine's experiments or the wall covering of one-inch-thick felt that he ultimately used to treat the acoustics of the lecture hall. One of the crucial decisions engineers make when designing a good-sounding room—whether a grand auditorium, a courtroom, or an open-plan office—is how long the reverberation time should be. Then they can use Sabine's equation to work out how much soft, absorbing stuff is needed.
⁵

Alongside reverberation time, a designer has to consider frequency, which directly relates to perceived pitch. When a violinist bows her instrument, the string behaves like a tiny jump rope, whipping around in circles. If she plays the note that musicians call
middle C
, the jump rope turns a full circle 262 times every second. The vibration of the violin radiates 262 sound waves into the air every second, which is a frequency of 262 hertz (often abbreviated to Hz). The unit was named after Heinrich Hertz, the nineteenth-century German physicist who was the first to broadcast and receive radio waves. The lowest frequency a human can hear is typically around 20 hertz, and for a young adult the highest is about 20,000 hertz. However, the most important frequencies are not at the extremes of hearing. A grand piano has notes from only about 30 to 4,000 hertz. Outside that range we cannot easily discern pitch, and all notes start to sound the same. Beyond 4,000 hertz, melodies are turned into the mindless whistling of someone who is tone-deaf. The middle frequencies where musical notes reside are also where our ears are most efficient at amplifying and hearing sounds. Most speech falls into this range as well, which is why in rooms where music will be played, acoustic engineers concentrate on designing for a frequency range of 100 to 5,000 hertz.

In 2005, Brian Katz and Ewart Wetherill used computer models to explore the effectiveness of Sabine's treatment in the Fogg Museum. They programmed the size and shape of the lecture hall into a computer and employed equations that describe how sound moves around a room and reflects from surfaces and objects. They then added virtual materials to the walls and ceiling of their simulated lecture hall to mimic Sabine's felt treatments. Although the absorbers improved the acoustics, the intelligibility of speech was still poor in places. As one student reported, there were seats where hearing was easy, and conversely, “there were dead spots where hearing was often extremely difficult.”
⁶
Though the treatments were imperfect, Sabine's experiments opened the door for a wide variety of acoustic exploration. His equations remain the foundation of architectural acoustics to this day.

I
love walking into a concert hall and hearing the contrast between the small entrance corridor and the huge expansive space of the auditorium. From the claustrophobic passageway, one enters a palpably vast room, passively perceiving the quiet chatter of anticipation among the audience and the occasional loud sound stirring the mighty reverberation. Entering Symphony Hall in Boston is particularly exciting for me. Symphony Hall is Mecca for many acousticians, for it was in this very hall that Wallace Sabine applied his newfound science to create an auditorium still considered to be one of the top three places to hear classical music in the world. Completed in 1900, it has a shoe box shape—long, tall, and narrow—with sixteen replica Greek and Roman statues set into the walls above the balconies. On my visit I settled into one of the creaky black leather seats while the Boston Symphony Orchestra was tuning up on the raised stage in front of the gilded organ. As the first piece began, I could immediately understand why audiences and critics wax lyrical about the place. The hall beautifully embellishes the music, having a reverberation time of about 1.9 seconds.
⁷
When the orchestra stopped playing at the end of a moderately loud phrase, it took nearly 2 seconds for the sound to become inaudible.

At an outdoor concert, an orchestra might play from a tented stage while the audience enjoys a picnic. The night often ends with bottles of champagne and fireworks exploding overhead. These concerts are fun, but the orchestra sounds thin and remote. In contrast, within a great venue like Symphony Hall the music appears to fill the room and envelop the audience from all sides. The reverberation inside amplifies the orchestra, allowing for more impressive loud playing. It also makes sounds linger a little, enabling musicians to make smoother transitions from note to note. Reverberation helps create a more blended and rich tone. As the twentieth-century conductor Sir Adrian Boult put it, “The ideal concert hall is obviously that into which you make a not very pleasant sound and the audience receives something that is quite beautiful.”
⁸

The transformational effect of reverberation is not restricted to classical music; it is also used extensively in pop. The 1947 number one hit “Peg o' My Heart” (a slow instrumental played on giant harmonicas), by Jerry Murad's Harmonicats, was the first recording to use reverberation artistically.
⁹
Since then, “reverb” has become a ubiquitous part of the music producer's tool kit. It makes voices sound richer and more powerful, mimicking what would happen if the person were singing from the stage of a theater. On many television programs where people with lousy voices attempt to sing, as soon as the person hits the first note you can hear the audio engineers slathering on reverberation to rescue the sound.

Reverberation is not the only important feature of a good auditorium. The most infamous concert hall failure is probably the original Philharmonic Hall at Lincoln Center in New York, which opened in 1962 (and was later rebuilt as Avery Fisher Hall). Acoustician Mike Barron describes it as “the most publicized acoustic disaster of the twentieth century.”
¹⁰
Influential music critic Harold C. Schonberg was particularly vocal, describing the hall as a “great big, yellow, $16,000,000 lemon.”
¹¹
Acoustic expert Chris Jaffe described how Schonberg “had a field day writing article after article on the acoustics of the hall as a sort of
All My Children
–type soap opera.”
¹²
Ironically, the acoustic consultant for the hall was Leo Beranek, possibly the most influential architectural acoustician of the twentieth century, and also the only person famous enough to be pursued by groupies at acoustics conferences. I remember my first meeting with Leo over breakfast at a conference when I was a young academic. It was a brilliant chance to talk about my research into concert hall acoustics with this superstar. Unfortunately, he greeted me with a question about why I had been measuring echoes from duck calls (see Chapter 4).