The Universal Sense by Seth Horowitz Page A

if you take your earbuds out and move your head around to listen, you can actually get an idea what the outer ear does.
    The outer ear is basically a flattened cone of relatively stiff flesh ending at the entrance to the ear canal or external auditory meatus , the place past which the Q-tip shall not go. If you look at most sources that talk about its function, the pinna is described as a sound-gathering device for low-frequency sounds, increasing the volume of vibration-bearing space that can funnel sound into the ear canal and increasing the relative gain of sound by up to 20 dB. This alone would make it an impressive passive listening aid; remember that dB are logarithmic in nature and every 6 dB is a doubling of the sound pressure, so a 20 dB gain gives you a lot more sound to work with. (If you look at antique hearing aids from the nineteenth century—or old cartoons—you usually see what was called an ear trumpet, a device that looks like a long metal horn with the small end fitted into the ear. It was basically an enlarged prosthetic outer ear.) But this change in gain only applies to sounds below about 4–5 kHz, the range at which all other vertebrates hear quite well.
    High-frequency sounds, on the other hand, get horribly attenuated in the air due in part to their short wavelength. They tend to get degraded into thermal noise over relatively short distances unless they are extremely loud. So to use them, you need something that gathers more sound than either a small hole in the side of the head or even an eardrum stuck out on the edgeof your skull, as with frogs. You need the equivalent of an audio telescope lens, something with a larger sound-gathering area. And not only do you need something to gather more sound, but you have to have some way of discriminating subtle changes in the sound based on the direction the sound is coming from, hopefully without having to swivel your head back and forth constantly to try to figure our where that high-pitched sound is coming from. 14
    Think about what the outer ear actually looks like—look across the room at someone else’s ear or gently feel your own. It’s not just some simple cartoonish cone. It is an extremely individualized shape, replete with ridges and usually a small tab that points slightly forward just over the opening to the ear canal (this structure, the tragus , is usually much larger in smaller mammals). For higher-frequency sounds, particularly those above about 6–8 kHz for humans, the ridges and valleys act as little blockades that slightly reduce the amplitude of certain frequencies of sounds, creating one or more spectral notches. The pattern of notches in the spectrum of the sound is specific to the shape and position of your ear. This “pinna notch” helps you localize high-frequency sounds, especially in the vertical plane. A sound coming from above or below your head hits these ridges and the tragus at different angles, with the result that different frequencies are slightly blocked. You can check this out yourself, especially if you go outside on a summer day when there are cicadas or other loud insects around. If you hold your outer ears flat against the side of your head (including the little flap in front of the ear canal (without, of course, blocking the canal itself), you’ll find it’smuch harder to figure out if a sound is coming from above, below, or level with your head.
    The pinna notch has another interesting function, one that to my knowledge has not been ever studied, though it’s rather obvious once you spend too many years thinking about things auditory. It just requires watching a mammal listen to a new sound that may be of interest to it—dogs are great ones to try this out on. Find your subject, be it a dog, a small child, a kitten, or a roomful of students, and say some nonsense words at it, but intone them as if you are saying something meaningful, like “Do you want a treat?” or “This will be on the final.”