=351. How we Hear.=--Our hearing apparatus is arranged in three parts. (See Fig. 344.) _The external ear_ leads to the _tympanum_. _The middle ear_ contains three bones that convey the vibrations of the tympanum to the _internal ear_. The latter is filled with a liquid which conveys the vibrations to a part having a coiled shell-like structure called the _Cochlea_. Stretched across within the cochlea are some 3000 fibers or strings. It is believed that each is sensitive to a particular vibration rate and that each is also attached to a nerve fiber. The sound waves of the air transmitted by the tympanum, the ear bones and the liquid of the internal ear start sympathetic vibrations in the strings of the cochlea which affect the auditory nerve and we hear. The highest tones perceptible by the human ear are produced by from 24,000 to 40,000 vibrations per second. The average person cannot hear sounds produced by more than about 28,000 vibrations. The usual range of hearing is about 11 octaves. The tones produced by higher vibrations than about 4100 per second are shrill and displeasing. In music the range is 7-1/3 octaves, the lowest tone being produced by 27.5 vibrations, the highest by about 4100 per second. [Illustration: FIG. 344.--The human ear.] The tones produced by men are lower than those of women and boys. In men the vocal cords are about 18 mm. long; in women they are 12 mm. long. The compass of the human voice is about two octaves, although some noted singers have a range of two and one-half octaves. In ordinary conversation the wave length of sounds produced by a man's voice is from 8 to 12 ft. and that of a woman's voice is from 2 to 4 ft. Important Topics 1. Tone quality. Fundamental and overtones. Chladni's plate. 2. Manometric flame apparatus. 3. Phonograph recorder and reproducer. 4. Air columns and wind instruments. 5. How we hear. Exercises 1. What determines the pitch of the note of a toy whistle? 2. The lowest note of the organ has a wave length of about 64 ft. What is the length of a closed pipe giving this note? Of an open pipe? 3. What is the first overtone of C? What are the second and third overtones? Give vibration numbers and pitch names or letters. 4. Why is the music of a band just as harmonious at a distance of 400 ft. as at 100 ft.? 5. A resonant air column 60 cm. long closed at one end will respond to what rate of vibration at 10°C.? 6. Can you find out how the valves on a cornet operate to change the pitch of the tone? 7. How is the trombone operated to produce tones of different pitch? 8. The lowest note on an organ has a wave length of about 64 ft. What must be the length of a closed pipe giving this note? 9. What is the approximate length of an open organ pipe which sends out waves 4 ft. long? Review Outline: Sound Sound--definition, source, medium, speed, nature. Waves--longitudinal, transverse, illustrations.