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Acoustic sensors and actuators

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The ear is a sensor and actuator in more than one way. Essentially a mechanochemical sensor, it includes a moving mechanism on the hearing side of the structure. But the ear also features a gyroscope, the inner ear, responsible for stability and sense of position. The ear itself is made of the outer and inner ear. The external ear is no more than a means of concentrating and guiding the sound toward the tympanic membrane (eardrum). In humans, the external ear is a relatively small, static feature, but in some animals it is both large and adjustable. The fenec fox, for example, has external ears that are larger than its head. At the bottom of the ear canal, the tympanic membrane moves in response to sound and, in the process, moves an assembly of three bones, the malleus (connected to the eardrum), the incus (an intermediate flexural bone), and the stapes. The latter, the smallest bone in the body, transmits the motion to the cochlea in the inner ear. The three bones not only transmit the sound but also amplify it through lever advantage afforded by their structure and dimensions. The cochlea is a spiral tube filled with a fluid. The stapes move like a piston, moving the fluid that in turn moves a series of hair-like structures lining the cochlea. These are the actual sensors that release a chemical onto the auditory nerve to affect hearing. The inner ear also contains three semicircular canals arranged at 900 to each other, with two roughly vertical and one horizontal. They have a similar structure to the cochlea, including a series of hair-like structures affected by the fluid in the canals based on the position of the body. These serve to maintain balance and provide information on the position and attitude of the body. The effect of motion on these structures can be immediately seen if the body rotates as, for example, on a merry-go-round. We temporarily loose the ability to keep our balance. The ear is a uniquely sensitive structure. It can sense pressures as low as 2 × 10-5 Pa (or 10-12 W/m2; i.e., on the order of one-billionth of the atmospheric pressure) and can function at levels 1013 times higher. That means the dynamic range is about 130 dB. The nominal frequency response is between 20 Hz and 20,000 Hz, although most humans have a much narrower range. But the ear is also very sensitive to pitch and can distinguish very small changes in pitch and frequency. A 1-Hz difference between two sounds is easily detectable. The hearing in humans is binaural and the brain uses that to detect the direction of sources of sound. Many animals use the mechanical motion of the outer ear to accomplish the same function but much better than we do. It should also be noted that many animals have much more sensitive hearing than humans, with ears that respond to higher frequencies and to a wider range of frequencies.

Chapter Contents:

• 7.1 Introduction
• 7.2 Units and definitions
• 7.3 Elastic waves and their properties
• 7.3.1 Longitudinal waves
• 7.3.2 Shear waves
• 7.3.3 Surface waves
• 7.3.4 Lamb waves
• 7.4 Microphones
• 7.4.1 The carbon microphone
• 7.4.2 The magnetic microphone
• 7.4.3 The ribbon microphone
• 7.4.4 Capacitive microphones
• 7.5 The piezoelectric effect
• 7.5.1 Electrostriction
• 7.5.2 Piezoelectric sensors
• 7.6 Acoustic actuators
• 7.6.1 Loudspeakers
• 7.6.2.1 The magnetic buzzer
• 7.6.2.2 The piezoelectric headphone and piezoelectric buzzer
• 7.7 Ultrasonic sensors and actuators: transducers
• 7.7.1 Pulse-echo operation
• 7.7.2 Magnetostrictive transducers
• 7.8 Piezoelectric actuators
• 7.9 Piezoelectric resonators and saw devices
• 7.10 Problems

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