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later as she approached from behind and to the left of us.
    The last thing to notice starts at about 30 seconds and continues through the end of this recording: quiet spectral bands very far down in the background noise, alternating between about 200 and 500 Hz. Although at this scale they seem continuous, there are short gaps in the frequency bands, and the two bands in fact alternate. These are the footsteps of a runner approaching from behind and to our right, passing us at about 34 seconds. While it may seem odd that there would be such a difference between the left (lower-frequency) and right (higher-frequency) footfalls, this gives another interesting insight into acoustical recognition. If the runner had been running with perfectly symmetrical strides and feet perfectly aligned front to back, there would have been very little difference between them. However, I happened to notice as he passed us that the runner had a very distinct outward turn of his right foot and landed strongly on his left heel. The heel strike and full-foot roll off the left foot created less of a slap than the turned-out right foot (which had less surface area to work with and hence put moreenergy into a smaller footprint, generating a slightly louder sound at a higher frequency). Many years ago, there was a student in my lab who was interested in the question of whether you could identify someone just from the sound of his or her footsteps. She carried out a very neat little experiment where she had people of similar weights and heights wearing similar footwear walk and run down a hall while she recorded them. She then played these recordings back to listeners who had heard the subjects run previously. She found that people were remarkably good at identifying individuals just using these simple sounds—another example of your brain being able to carry out extremely complicated identification and analyses based on subtle acoustic cues. And if you think this is just an interesting academic exercise, bear it in mind the next time you hear footsteps behind you on a dark street, and realize that you too probably would not have to turn around to determine whether you are being chased by a stranger or by your roommate looking for the keys.

Chapter 3
Listeners of the Low End: Fish and Frogs
    Just as every place has its own acoustic signature, every listener has its own plan for hearing what it needs to. There are about fifty thousand kinds of listeners in the vertebrate world, each with its own solution to the problem of what to listen to and usually very closely tied to the acoustics of its normal environment. Of all these, maybe one hundred have been explored scientifically (and most data are drawn from about a dozen, including zebrafish, goldfish, toadfish, bullfrogs, clawed toads, mice, rats, gerbils, cats, bats, dolphins, and humans).
    At one level this is okay. Hearing in all vertebrates is based on using hair cells in some configuration to detect changes in pressure or particle motion and converting this into useful perceptions to help guide behavior. Once you get past the ears, vertebrate brains derive from a similar general plan—hindbrain receiving and sending much of the raw sensorimotor information, midbrain integrating both incoming and outgoing information, thalamus acting as a relay center to forward brain regions, and forebrain governing intentional behavior. But on the other hand, every species has developed its own solution towhat it should hear. And to make it worse, every individual shows differences from that species’s version of “normal,” not only through genetic variation but also by what it has been exposed to over the course of its own life. So trying to understand hearing and all its variations from such a small sample of species can be very frustrating.
    But people who study hearing ultimately want to understand their subject from a human perspective. We humans are primarily concerned with the human experience. Even