These hair cells, and their attached neurons, transform the motion of the tectorial membrane into electrical signals that are sent to the brain. The motion of the tectorial membrane stimulates tiny cilia on specialized cells called hair cells. This creates pressure waves in the fluid that cause the tectorial membrane to vibrate. As the middle ear bones vibrate, they vibrate the cochlea, which contains fluid. This protective reaction can also be triggered by your own voice, so that humming during a fireworks display, for example, can reduce noise damage.įigure 14.12 shows the middle and inner ear in greater detail. They react to intense sound in a few milliseconds and reduce the force transmitted to the cochlea. Two muscles in the middle ear protect the inner ear from very intense sounds. The lever system of the middle ear takes the force exerted on the eardrum by sound pressure variations, amplifies it and transmits it to the inner ear via the oval window. The middle ear converts sound into mechanical vibrations and applies these vibrations to the cochlea. The outer ear, or ear canal, carries sound to the eardrum protected inside of the ear. The body part normally referred to as the ear is technically called the pinna.įigure 14.11 The illustration shows the anatomy of the human ear. The ear converts sound waves into electrical nerve impulses, similar to a microphone.įigure 14.11 shows the anatomy of the ear with its division into three parts: the outer ear or ear canal the middle ear, which runs from the eardrum to the cochlea and the inner ear, which is the cochlea itself. The sound wave that hits our ear is a pressure wave. The way we hear involves some interesting physics. The perception of frequency is called pitch, and the perception of intensity is called loudness. Sounds below 20 Hz are called infrasound, whereas those above 20,000 Hz are ultrasound. You may have noticed that dogs respond to the sound of a dog whistle which produces sound out of the range of human hearing. Dogs can hear sounds as high as 45,000 Hz, whereas bats and dolphins can hear up to 110,000 Hz sounds. Other animals have hearing ranges different from that of humans. Humans can normally hear frequencies ranging from approximately 20 to 20,000 Hz. It can give us plenty of information-such as pitch, loudness, and direction. A voice becomes louder when air flow from the lungs increases, making the amplitude of the sound pressure wave greater. A voice changes in pitch when the muscles of the larynx relax or tighten, changing the tension on the vocal chords. This vibration escapes the mouth along with puffs of air as sound. As air travels up and past the vocal cords, it causes them to vibrate. These folds open and close rhythmically, creating a pressure buildup.

People create sounds by pushing air up through their lungs and through elastic folds in the throat called vocal cords. Power is the rate at which energy is transferred by the wave. In general, the intensity of a wave is the power per unit area carried by the wave. Figure 14.10 shows such a cartoon depiction of a bird loudly expressing its opinion.Ī useful quantity for describing the loudness of sounds is called sound intensity. In cartoons showing a screaming person, the cartoonist often shows an open mouth with a vibrating uvula (the hanging tissue at the back of the mouth) to represent a loud sound coming from the throat. But in a traffic jam filled with honking cars, you may have to shout just so the person next to you can hear Figure 14.9.The loudness of a sound is related to how energetically its source is vibrating. In a quiet forest, you can sometimes hear a single leaf fall to the ground. Sound intensity is defined as the sound power per unit area, whereas amplitude is the distance between the resting position and the crest of a wave. While sound intensity is proportional to amplitude, they are different physical quantities. Students may be confused between amplitude and intensity.

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