“Frequency,” Dr. Mindstein continues, “refers to the number of vibrations the sound generating device makes per second. The unit of measurement we use is the hertz, or Hz, which is named for Heinrich Rudolf Hertz, a 19^th-century German physicist. Your ears, if you haven’t been to too many rock concerts, are capable of hearing sounds from a low of about 20 Hz to a high of about 20,000 Hz, or 20 kilohertz (kHz), as it’s sometimes expressed. The frequency of a sound is related to its pitch; low frequency is perceived as a low-pitched sound and high frequency as a high-pitched sound.”

At this point, the professor startles the class by saying, “This is a low-pitched sound,” in as deep a voice as he can muster, “and this is a high-pitched sound,” which he speaks in a voice that sounds like he has just inhaled some helium. Several of your classmates laugh.

“As some of you may realize, some animals can hear sounds we cannot. For example, dogs, cats, and rats can hear higher-pitched sounds than we can, but they aren’t the champions by a long shot. Bats and harbor seals can apparently hear sounds as high as 100,000 Hz or higher.”