The Sound Book: The Science of the Sonic Wonders of the World (15 page)

But caves and reverberation chambers are not a natural habit for ducks like Daisy. We were curious to know what happens outdoors. To hear a clear single echo from Daisy, I would need a stretch of water with a large reflecting surface, such as a cliff, nearby. In such a place, sound would travel directly from the duck to my ear, followed shortly by the delayed reflection from the cliff. In the taxonomy of echoes, this is a
monosyllabic echo
, where there is just time to say one syllable before an echo arrives. But Daisy and I could not be too near the cliff, or my brain would combine the reflection with the quack traveling directly from her beak to my ear, and I would hear only one sound.

I must admit that my field experiments were crude. Though I could not bring Daisy, I did wander around various ponds, canals, and rivers listening to wildfowl. In none of these places could I hear a clear, audible quack separate from the original call. In the end, I came to the conclusion that the phase should say, “A duck's quack might echo, but it's impossible to hear unless the bird quacks while flying under a bridge.”

Maybe I should have taken Daisy to Lake Königssee in Bavaria, Germany's highest lake, where rock faces rise steeply out of the water. Boat captains there play short phrases on trumpets so that tourists can hear the last three notes repeat, delayed by one or two seconds, after bouncing off the surrounding alps. Or perhaps I should have taken Daisy to the place where seventeenth-century French theologian, natural philosopher, and mathematician Marin Mersenne had carried out his echo experiments. He used a
polysyllabic echo
to make the first accurate measurements of the speed of sound in air. Nowadays, Mersenne is probably best known in mathematics for his work on prime numbers, but he was also passionate about a wide variety of subjects and devoted to the need for experimentation and observation.
⁵

Unsurprisingly, Mersenne did not use wildfowl in his speed-of-sound experiments. Instead, he stood facing a large reflecting surface, saying the words “benedicam dominum,” and using a pendulum to time the sound. Mersenne must have been a quick talker, because he said this seven-syllable phrase in one second. When he stood 485 royal feet (159 meters) from a large reflecting surface,
⁶
the echo immediately followed the end of the original phrase—“benedicam dominum, benedicam dominum.” This is a polysyllabic echo because many syllables can be said before the echo returns. The echoed words had covered a round-trip of two times 485 royal feet (a total of 319 meters), allowing Mersenne to deduce that 319 meters per second is the speed of sound. This is remarkably close to the correct value of about 340 meters per second (761 miles per hour).
⁷

Now, if Mersenne had used a duck, he could have stood nearer the wall and still heard a distinct “quack, quack” because a duck's call is but one syllable. In fact, the distance to hear a monosyllabic echo like a quack is about 33 meters (36 yards) from a reflecting surface (660 duck feet?
⁸
) because at that distance an echo takes just long enough to bounce back that it can be heard separately from the original sound. To hear a quack echo, I would need to find a stretch of water with a large building or cliff about 30–40 meters (or 30–40 yards) away. But even this would not work, because a duck's quack is too quiet. Sound becomes quieter the farther you are from a source, by 6 decibels for every doubling of distance, so if a quack measures 60 decibels 1 meter (3¼ feet) from the beak, 2 meters (6½ feet) away it will have dropped to 54 decibels, 4 meters (13 feet) away it will be at 48 decibels, and so on. By the time the reflected quack has undertaken its round-trip of 66 meters (72 yards), the echo will be about 24 decibels. In a completely silent place a human could hear this, but more often other noises, such as the distant rumble of traffic or wind moving through trees, are louder, making the quack inaudible.
⁹
Sadly, even in a silent place Daisy would not be able to hear the echo, because her hearing is less sensitive than a human's. So the reason the echo from a duck's call is not heard is pure physics: a quack is not loud enough to be heard after it makes the return trip from the required distance.

M
arin Mersenne's acoustic work extended beyond the speed of sound; he also debunked fanciful tales about 400 years before myth busting became a popular pastime on television. One of the more extravagant acoustic claims in classical literature is a supposed
heterophonic echo
that, when spoken to in French, replied in Spanish. Marsenne knew this could not be true, but as Professor Fredrick Vinton Hunt wrote in his seminal book
Origins in Acoustics
, Marsenne “almost convinced himself that one could devise a special series of sounds whose echo might lead a listener to think he had heard the response in a different language.”
¹⁰
The term
heterophony
comes from musicology and denotes a melody being simultaneously played with an elaborated variant, so I can only imagine that a heterophonic echo might augment the French words to make them Spanish. Unfortunately, no one knows for sure what was meant by the term, and there are no examples of heterophonic echoes. Luckily, there are other literary games that can be more successfully played with echoes, as I found out in France.

One hot, sunny day in 2011, I was cycling with my family in the Loire valley, and we arrived at the Château de Chinon. The heart of the castle was built by Henri Plantagenêt, who would later become King Henry II of England. But I was more interested in a very unusual road sign just outside the castle walls. It points up a small lane and simply says, “Écho.” How could a collector of sonic wonders resist this invitation? A few hundred yards up the lane was a small, raised turnout and a sign indicating that this was the place to test the acoustics. I yelled and yodeled and appreciated the fine echo.
¹¹
What made the experience very satisfying was that the side of the château, which was reflecting the sound, was partly hidden by an orchard, so the clarity of the echo was surprising. I could not resist attempting a traditional piece of echo humor from my guidebook to the Loire:
¹²

Me: “Les femmes de Chinon sont-elles fidèles”

Echo: “Elles?”

Me: “Oui, Les femmes de Chinon”

Echo: “Non!”

This translates into English as:

Me: “Are the women of Chinon faithful?”

Echo: “Them?”

Me: “Yes, the women of Chinon”

Echo: “No!”

Given the right enunciation, with an unnatural stress placed on the last syllable in each phrase, such as the “non” in
Chinon
, the rhyme worked. By that I mean that the partial words echoed back from the north side of the château and were clearly audible. The sentiments expressed in the poem could not be readily verified.

There are other echo stories. Here is a nineteenth-century account from
Wonders of Acoustics
by Rodolphe Radau (with a Latin translation in square brackets):

Cardan tells a story of a man who, wishing to cross a river, could not find the ford. In his disappointment he heaved a sigh. “Oh!” replied the echo. He thought himself no longer alone, and began the following dialogue:

Onde devo passar? [Hence I have to pass?]

Passa. [pass]

Qui ? [here?]

Qui. [here]

However, seeing he had a dangerous whirlpool to pass, he asked again

Devo passar qui? [I have to pass here?]

Passa qui. [pass here]

The man was frightened, thinking himself the sport of some mocking demon, and returned home without daring to cross the water.
¹³

Wonders of Acoustics
includes much about Athanasius Kircher, a seventeenth-century Jesuit scholar based in Rome who wrote extensively about theater acoustics and other marvels. He was intrigued by
manifold echoes
—echoes that produce multiple distinct reflections. Included in this category are repeating echoes caused by elaborate structures that turn one word into a whole sentence. For his two-volume masterpiece
Musurgia Universalis
, from 1650, Kircher produced a drawing of large upright panels, spaced at various distances from a talker, to generate a series of reflections arriving one after another. One such edifice had five panels and was designed to take the word
clamore
and break it down to echo
clamore
from the first panel and
amore
from the second, followed by
more
,
ore
,
re
from the third, fourth, and fifth surfaces, respectively. So, if you shouted the question “
Tibi vero gratias agam, quo clamore?
” (“How shall I cry out my thanks to thee?”), the echoes from the last word would reply with a Latin phrase “
clamore, amore, more, ore, re
,” which roughly translates as “with thy love, thy wont, thy words, thy deeds.”
¹⁴

I thought this seemed very unlikely to work, but the idea was intriguing enough to inspire a quick test. Not having five large panels lying around, I decided to try out the idea by simulating the situation on my computer. I recorded myself saying “
clamore
” and then, using a piece of prediction software, estimated the reflections returning from each of the panels shown in Kircher's picture. I played around with how far the panels were from the speaker and the loudness of the reflections in an attempt to produce the echo pattern.
¹⁵
Much to my surprise, the echo phrase actually worked, but perhaps only because my brain was fooled into hearing patterns I wanted to hear.

I once saw a great demonstration of a similar effect by author Simon Singh, based on the accusation that Led Zeppelin hid satanic messages in “Stairway to Heaven.” If you play the track backward, you supposedly hear, “Oh here's to my sweet Satan. The one whose little path would make me sad, whose power is Satan. He'll give those with him 666, there was a little toolshed where he made us suffer, sad Satan.” So concerned were some religious groups that various US states introduced legislation requiring records to carry warning labels.
¹⁶
These claims further implied that even when records were listened to normally, with the sound playing forward, the listener would subconsciously decipher the meaning of the backward satanic messages.
¹⁷

Several groups of psychologists have tested the claims using proper scientific methods. Experiments showed that if you listen to “Stairway to Heaven” backward with your eyes closed, what you actually hear is gibberish. These satanic lyrics are heard only if you have a printed version in front of you. (You can try this out yourself; plenty of websites are dedicated to
backward masking
with sound samples.) The brain has to make sense of incomplete information all the time, so it is very adept at finding patterns and fitting together different sources of information. But sometimes the brain gets it wrong, in this case matching the written lyrics to the otherwise incomprehensible backward murmurings.

The same thing happens with the “
clamore, amore, more, ore, re
” echo. When I listened very carefully for this pattern of words, I could pick out the phrase. The effect was especially strong if the echoes were faint and I was forced to strain to hear them. But if I closed my eyes and listened more holistically and analytically, the dominant effect I could hear was many repetitions of “
re
.” The clever wordplay disappeared.

A
multiple echo, or
tautological echo
, is almost the same as a manifold echo, except the same words or syllables are repeated many times. An episode of the TV show
The Simpsons
features this phenomenon as a piece of aural slapstick. Marge is in church and yet again is being embarrassed by Homer. She cries out, “Homer, your behavior is heinous” and a tautological echo replies “anus, anus, anus, . . .”
¹⁸

Athanasius Kircher was also interested in echo pranks. He describes being amused at a friend's expense in the Campagna at Rome, a lowland plain surrounding the city. His friend cried out, “
Quod tibi nomen?
” (“What is your name?”), and the echo impossibly replied, “Constantinus.” The conceit was achieved using an accomplice hiding near a cliff where normally there was no echo. The accomplice would shout out the reply after hearing the question, impersonating the improbable sound reflection.
¹⁹

A more impressive practical joker is Bob Perry, who has taught himself to impersonate an echo. He does an impressive performance of John F. Kennedy's inaugural address, complete with multiple renditions of each word as though caused by a public address system. With a little practice, you could teach yourself to do this. Select a spoken passage in which the space between syllables is a little longer than normal, as in JFK's speech, because of the slow delivery, and then say every syllable twice: “Ask ask not not what what you you can can do do . . .”. To make it convincing, the echo word should be a bit quieter.

It is not usually the architecture that renders big speeches and announcements in train stations unintelligible; often electronics are to blame. Bad public address systems send out sound too loudly from too many sources. You hear words from two or more loudspeakers arriving separately because these sources of sound are different distances from you. One engineering solution is to change the position and orientation of each loudspeaker to make sure you hear only one at a time. Engineers can also use loudspeakers that illuminate defined areas rather than speakers that radiate in all directions—the aural equivalent of using a spotlight instead of a general-purpose lightbulb. But targeted illumination is not always possible, in which case engineers add electronic delays to each speaker to ensure that you hear all versions of the speech arriving at roughly the same time. Your brain will then lump the speech from the different speakers into one louder sound, minimizing the confusing cacophony of repetition.