Page 11 - Spring 2006

power that we heard...louder than they had been heard through the rainless atmosphere. ...This observation is entirely opposed to prevalent notions...”
While Tyndall found many examples of good sound transmission when optical visibility was poor, he also observed poor sound transmission when the air was clear. These latter observations were fatal to the transparency argu- ments. Regarding his inability to hear a powerful siren on a clear day, Tyndall wrote: “...what...could so destroy [the air’s] homogeneity as to enable it to quench in so short a dis- tance so vast a body of sound?...As I stood upon the deck of the ‘Irené’ pondering it, I became conscious of the exceeding power of the sun beating against my back and heating the objects near me. Beams of equal power were falling on the sea, and must have produced copious evaporation. That the vapour generated should so rise and mingle with the air as to form an absolutely homogeneous medium I considered in the highest degree improbable. It would be sure, I thought, to rise in streams, breaking through the superincumbent air now at one point now at another, thus rendering the air floc- culent with wreaths and striae...At the limiting surfaces of these spaces, though invisible, we should have the conditions necessary to the production of partial echoes and the conse- quent waste of sound.”
Fig. 2. John Tyndall’s concept of flocculence supplanted earlier scattering theories because it seemed to explain poor propagation of sound in optically clear condi- tions. Tyndall believed that convection currents and streams of vapor would cre- ate enough discontinuities in density or sound speed to scatter (and absorb) sound. He might have drawn a figure like this one to illustrate sound waves (red) passing through regions of vapors (shaded). Even though the idea of refraction developed at about the same time as Tyndall’s work, Tyndall believed that the loss processes were the primary processes by which sound amplitude was reduced. This sort of scattering and absorption does occur but it is far too weak to explain the large fluc- tuations in sound level that were often observed.
While such processes in the atmosphere do scatter sound, the effects are not strong enough to explain the dra- matic decrease in sound audibility in many conditions. But Tyndall was confident that he had found the answer: “The real enemy to the transmission of sound through the atmos- phere...has been proved to be not rain, nor hail, nor haze, nor fog, nor snow...but water in a vaporous form, mingled with the air so as to render it acoustically turbid and floccu- lent...Thus, I think, has been removed the last of a congeries
of errors that for more than a century and a half have been associated with the transmission of sound by the atmos- phere.” But Tyndall had replaced one error with another4.
He even went so far as to devise an intricate apparatus in to demonstrate the absorption of sound by flocculence. He arranged a series of tubes that fed carbon dioxide (“carbonic acid”) into a duct from above and another series of tubes that fed methane (“coal gas”) in from below. This produced alter- nating layers of heavy and light gas in the duct that success- fully attenuated sound passing through the duct. But the “flocculence” was exaggerated far beyond anything that would occur naturally.
Fig. 3. Since experiments were difficult to perform in the field, Tyndall set out to prove that inhomogeneities in the atmosphere could extinguish sound. He built this intricate apparatus in which carbon dioxide (“carbonic acid”) was introduced through many tubes above a duct and methane (“coal gas”) was introduced through many tubes beneath. The heavier carbon dioxide flowed downward through the acoustic duct and the lighter methane flowed upward. The tube arrangement cre- ated alternating layers of light and heavy gas. The sound detector was a sensitive flame shown at the right. The apparatus produced very high losses in sound level but the inhomogeneity was exaggerated far beyond what would occur in the atmos- phere. (From Tyndall’s On Sound.)
More observations raised more questions. Sound seemed to travel better at night than during the day. Perhaps the sounds just seemed louder because background noise was lower at night. This was an admirable guess—even today, the loudness of a signal is often confused with the ability to distinguish a signal from surrounding noise. However, Baron von Humboldt provided convincing, con- trary evidence. On an expedition to South America, he wrote that the sounds from a waterfall on the Orinoco River were stronger and clearer at night than during the day even though, in the jungle, insect and animal noises were much louder at night than during the day. Humboldt’s explanation was similar to Tyndall’s, though: Humboldt reasoned that, during the day, uneven heating of the ground caused strong upward and uneven currents of air that scattered the sound.
The Tyndall-Humboldt hypothesis was credible and contains some truth: turbulence in the atmosphere does scat- ter sound. However, plausibility is no guarantee of accuracy. The effects observed by Humboldt and Tyndall were too strong to be explained by scattering from turbulence; these effects had their roots in another cause entirely.
Refraction of Sound in the Atmosphere 9