Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm


13.3.2  Three-Dimensional Sound

The most common acoustical sense on Earth is 3-D sound perception. Such sound waves normally don’t carry much energy. For example, the pressure variations in a room due to people talking are only about 10-6 atm. Yet our ears can detect waves with so little energy (10-16 watts/cm2) that our eardrums only vibrate 10-9 cm in response.79 It is a fact that the primary human acoustical organ is extremely well-developed, able to distinguish at least 1600 discrete frequencies and to hear the quantum hiss of molecular motion of the air -- the theoretical lower limit of sonic sensitivity.

A good sense of hearing is highly advantageous for a number of reasons. Much like scent signals, sound waves diffract around obstacles in total darkness -- at night, in dark caves, deep underwater -- or in blinding light to reach the recipient. And while pheromones are a less expensive way to transmit data over large distances, auditory signals are considerably more efficient than optical ones in biocommunication.

Unlike visual displays which require whole body motion or complex lighting patterns, or scent-talk which requires a separate glandular "voicebox" for each chemical letter in the odor alphabet, sound may be adequately generated by a single organ. A relatively simple system can produce wide variations in pitch, tone, intensity, wave shape, and timing. This means higher data flow between brain and terrain, significantly increasing the chances for survival.

Perhaps the only real disadvantage to audition is that the sonic alien must provide its own source, since natural sounds in the environment are rarely sufficient to permit a thorough surveillance of the surroundings. But sonic senses may be highly directional with fine resolution: Even humans, who have no echolocation system at all, can accurately separate distinct sound sources located only 10-20° apart.

Animals with built-in sonar fare much better.* Many can virtually "see" with sound. For instance, bats are small mammalian avians able to navigate at high speeds using ultrasonic echolocation. Although visually blind, these creatures easily avoid millimeter-wide wires strung across their path by investigators. Even when 0.3 mm wires were substituted the animals managed to avoid them more often than not. Only when tiny threads the thickness of human hair -- about 0.07 mm -- were used in the obstacle course were the bats unable to dodge them.219,2514

The upper limit for spatial resolution of targets is a function of the wavelength of sound. The tiniest object that can just be discerned has a size roughly equal to the wavelength. Smaller objects are too small to give an appreciable reflection and so remain invisible.

Typically, the sonic beings of planet Earth use frequencies from 20-100 KHz or higher for echolocation (Table 13.1). This corresponds to a wavelength on the order of millimeters in normal dry air at a range of several meters. Much higher frequencies, say, 500-1000 KHz, would permit the resolution of targets 0.1 mm in size. This is quite enough to thread needles and soldier circuit boards at distances of about 10 centimeters from the creature’s face.


Table 13.1 Range of Hearing for Terrestrial Animals48,1698
Range (Hz)
Frequency Range (Hz)
Axolotl (salamander)
up to 240
 HUMAN (adult male)
15 -- 20,000
up to 340
up to 33,000
up to 1,200
up to 34,000
up to 2,700
up to 40,000
up to 7,000
up to 40,000
up to 10,000
Guinea Pig
up to 40,000
Characinidae fish
up to 10,000
300 -- 45,000
50 -- 10,000
430 -- 45,000
Cricket (Gryllus)
250 -- 10,000
30 -- 50,000
100 -- 12,000
Bat (Plecotus)
20 -- 80,000
up to 13,000
Deer mouse
50 -- 95,000
Harvest mouse
up to 17,500
Bat (Myotis)
20 -- 120,000
100 -- 19,000
100 -- 200,000
Cricket (Acridium)
up to 20,000
up to 240,000



Of course, aliens using sound waves with that kind of resolution would take a great deal of energy to produce. They would be "decidedly dangerous to human explorers," according to science fiction writer Hal Clement:

A story could be built on the unfortunate consequences of the men who were mowed down by what they thought must be a death ray, when the welcoming committee was merely trying to take a good look.

But there is no fundamental reason why sentient extraterrestrials couldn’t use sound as their primary sense, and to build and create as well as any sighted being.

Audition has been developed as the primary sense modality among many aerial and land based animals on Earth. Creatures evolving on planets with unusually thick atmospheres with heavy refractive effects might tend to rely far more on hearing than on seeing. Also, there is much evidence that the sea is also a most auspicious environment for the development of sonic senses. It’s quite possible that hearing may even be the sense of choice for intelligent organisms residing in the murky oceanic media of other worlds. Why is this so?

Sound travels about four times faster in water than in normal air. This allows faster response times and greater ranges of communication and data collection. For example, at sonic frequencies of 500-1000 KHz, spatial resolution in ordinary seawater is equivalent to that in air at 100-200 KHz -- but the range is about a hundred times greater.

Sound fares well against competing senses in the watery environment. Pheromones are relatively ineffectual in water, since diffusion in liquid media generally proceeds 103-104 times slower than in air. Smell signals are emitted, but creep slowly away from the source. Ocean currents are tamer than atmospheric ones, so pheromones would have to be about a million times more concentrated in water than in air to achieve comparable results.

Vision doesn’t compare much better. Light and other electromagnetic radiation cannot penetrate most liquids to any appreciable degree and are subject to countless distorting effects. In a tropical sea at high noon under very clear water, useful visual information can be gained only out to about 30 meters. Using acoustic channels instead, this range is extended to many kilometers -- comparable to vision in air on a hazeless day.

These facts are reflected in the physiological differences between humans and dolphins. The eyeballs of a man receive an estimated 50 million bits of information each second, while his ears manage only 2 million bits/second. In porpoises the emphasis is exactly reversed: Dolphin sonar handles perhaps 40 million bits/second, while the eyes manage only 5 million bits/second of information.**201 We can easily imagine that sound may be the preferred sensory modality among many, if not most, intelligent aquatic races scattered throughout the universe.

There are many properties of sound that make it a totally unique window on reality. For instance, the ultrasonic world is very quiet in comparison to the normal range of human hearing. This is because of the relatively short range of ultrasound. There is little noise because sources remain localized.

In a dense fog, both sighted and sonic organisms are ill at ease: The former, because light is scattered away causing everything to appear visually white; the latter, because the droplets of moisture or particles are excellent ultrasound absorbers and cause the entire field of view to appear acoustically black.2514

Human vision is limited to the surfaces of objects, but sound penetrates and exposes the insides of targets to view. Objects may be scanned for composition and internal structure using almost distortion- and reflection-free sound waves. A pelagic sonic alien (perhaps modeled after the dolphin) thus views its fellows, not as a sharp contour of lines and edges and distinct boundaries, but rather more like an x-ray snapshot. Skin, muscles, and fatty tissues are virtually transparent to ultrasound. Bones and teeth, internally-trapped gas bubbles, and cartilaginous structures give good reflections. Hard parts, as well as the digestive and respiratory tracts, stand out in clear relief.

Might aliens with such sonic sight be more honest and less deceitful than humans generally? Physiological reactions to other individuals might be instantly perceived by those others, a kind of body-telepathy. At least one writer has remarked: "If your visceral reactions are obvious to everybody, you don’t waste much time trying to lie."2855

Dolphin echolocation may properly be compared to human vision. Just as people are able to see by the reflected white light of the sun, porpoises emit ultrasonic clicks and trills that illuminate the surroundings with white noise. Differences in texture and composition are as obvious to their sonic senses as to our visual ones. As Dr. Winthrop N. Kellogg points out; "Wood simply 'sounds different' from metal to a porpoise in the same way that it looks different to the human eye. It is the sound spectrum of the returning vibrations which gives the clue to the nature of the reflecting surfaces."1698 Even more sophisticated, the dolphin has sound generators on either side of its head, which allows sonic depth perception176 using binaural hearing -- what Lilly has called "stereophonation."201

Acoustically-oriented aliens will also have a rather clever use for color. The color of an object is just the frequency of radiation it emits, and we may expect that objects may take on various sonic colors. But there is more. Objects moving rapidly towards a sonic sensor will reflect sound at a higher frequency because of the Doppler effect. Likewise, objects in the field of view which are moving away will reflect lower frequencies.

If sonic beings divide their audible frequency spectrum into colors, then the sonic Doppler effect will cause approaching targets to appear "bluer" and retreating targets to appear "redder." As objects pass in front, their color alters noticeably depending on relative speed, angle of approach, and distance. If the medium is in motion, as with gusts of wind or surging water currents, the apparent color will flicker in frequency -- a phenomenon quite alien to normal human visual experience.

The linguistic significance could be enormous. Much as the Eskimos have more than fourteen different ways to say "snow" (depending on its firmness, wetness, age, etc.), intelligent marine ETs will have words without analogue in our language. There might be terms describing approaching-red, receding-red, stationary-red, stereophonic-red, circulating-red, gulfstream-red, and dozens of other subtle distinctions we cannot begin to imagine. Or perhaps these sea-folk communicate in a manner which is possible only among creatures who use the same sense to see as to talk. Many researchers believe that the language of dolphins consists not of words as we understand them, but rather of a series of sonic images transferred into speech:

In this view a dolphin does not "say" a single word for shark, but rather transmits a set of clicks corresponding to the audio reflection spectrum it would obtain on irradiating a shark with sound waves in the dolphin’s sonar mode. The basic form of dolphin/dolphin communication in this view would be a sort of aural onomatopoeia, a drawing of audio frequency pictures -- in this case, caricatures of a shark. We could well imagine the extension of such a language from concrete to abstract ideas, and by the use of a kind of audio rebus -- both analogous to the development in Mesopotamia and Egypt of human written languages. It would also be possible, then, for dolphins to create extraordinary audio images out of their imaginations rather than their experience.2552

It is difficult to emphasize too strongly how unfamiliar the world may appear when viewed through other senses. Extraterrestrials will have experiences and capabilities that are difficult for humans to fully appreciate. A case in point in the perception of a peculiar sea phenomenon known to oceanographers as the sofar channel.

The speed of sound in water increases with pressure and decreases with temperature. Moving downward from the surface the temperature plummets, and about 100 meters down the speed of sound reaches a minimum value of 1480 m/sec. At greater depths the temperature remains fairly constant (close to freezing) but the pressure begins to build, causing the speed of sound to go back up. This region of minimum sound speed is called the sofar channel.

Sonic radiation, due to the physics of refraction, is actually attracted towards the channel. As with astronomical black holes (from which light waves cannot escape), acoustical signals sent from within the sofar channel likewise can’t get out. Transverse waves cannot cross it easily, which must cause it to appear sonically dark and foreboding when "viewed" from above or below.

If sufficient courage can be mustered, the channel has one very interesting property as regards long range marine communications. Sounds generated at the proper depth remain trapped in the zone, much like light in a strand of optical fiber. Messages may travel virtually unattenuated for literally thousands of kilometers in all directions. Sentient oceanic creatures of other worlds may regularly broadcast long-distance calls to other "universes" in the sea.


* Part of the difficulty is ear design -- most of the energy in sound is reflected away at the surface of the structure. There are various solutions to this problem. Star Trek’s Mr. Spock’s ears are adaptations to the thin air of his home planet Vulcan. They permit greater directionality of sonic reception by utilizing a back-curving pinna ("pointed" ears). Another solution commonly found on Earth is the independently targetable ears of goats, cats, dogs, and others.

** Lilly’s figures are probably overoptimistic. Homer Jacobson and others have done a more careful analysis, and have concluded that the human eye is capable of transmitting only 4.3 million bits/second and the human ear only about 50,000 bits/second maximum.955,979,980


Last updated on 6 December 2008