The recent article on lightning by Larry McKenna ("Crooked fire," Issue No. 91), contains at least one outstanding error. McKenna states that "Different frequencies of sound travel at different speeds, with different efficiencies, through air. Low tones travel more slowly, but much more efficiently, than high tones," and goes on to further describe the acoustic properties in another paragraph. In the second paragraph he states that "At greater distances, the air has absorbed the higher notes"
The first statement is incorrect. All acoustic energy, regardless of frequency, travels at the same velocity in air, the velocity being dependent on density.
The second statement is correct. In air, different frequencies are absorbed at different rates. Absorption is the dissipation of the energy in the wave by the work done compressing the air, and the higher-frequency energy dissipates faster than the lower, because it does more work for a given distance traveled. Incidentally, the absorption per meter traveled at 68° F and 80% relative humidity is about 0.00001 db at 100 Hz, 0.0014 db at 1000 Hz, and 0.17 db at 10,000 Hz; obviously high-frequency energy disappears very quickly.
Absorption is one major dissipative mechanism; another is spreading loss, which results in a reduction in signal amplitude of between 2:1 and 4:1 for each doubling of distance, depending on whether the spreading is cylindrical, spherical, or somewhere in between. Spreading loss is the same for all frequencies. Other loss mechanisms include absorption and scattering by the ground and by clouds, which have a frequency-determined component.
The drawing out of thunder into a low rumble is due to the presence of multiple propagation paths in the air and reverberation of the low-frequency sound between the ground and the sky, which also results in multiple paths. Since the paths are of different lengths, the sounds arrive at different times although they travel through the air at the same velocity.
Sy Kotler has worked for NOAA, developed sonar system for submarines, performed oceanographic and hydrographic surveys, and is now chief engineer ARD Environmental in Laurel, Md.Larry McKenna responds: Mr. Kotler’s disagreement with my statement that "Different frequencies of sound travel at different speeds" is perhaps more a difference in degree than kind. It appears we both used The CRC Handbook, that venerable compendium of invaluable, if infrequently used, physical parameters.
Tabulated values for sound velocityat 68° F and 80% relative humidityvary by frequency on the order of 0.1%. The delay produced by this difference is the same produced by multi-path propagation if the two paths differ by 2.5°.
Mr. Kotler offers an interesting elaboration on the importance of multiple pathways in modulating the lower frequency "rumble." In effect, this allows the listener to hear the same parcel of thunder more than once. Popular lore states that lightning never strikes the same place twice, but thunder apparently does.