THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

556 18. Music and Musical Instrumentscovered at each end by a stretched membrane or parchment or skin, is struck witha softheaded stick; its vibration is quite complex, owing to nodal contributionsby the air column and the stretched membrane. Thumbscrews supply the meansof adjusting the stretch of the drumskin. The bass drums are constructed in sizesranging from 61 cm to as much as 3 m, with approximately 78 cm being the mostcommon diameter. The military drum is a smaller rendition of the bass drum, beingabout 40 cm in diameter and about 30 cm in depth. At nearly 36 cm in diameter,the snare drum is even smaller than the military drum. Across its lower head, cordsof catgut are stretched so these cords vibrate when struck by the membrane. Thisresults in a sound output that is buzzing and rattling.The tambourine consists of a hoop of wood or metal with a single membranestretched over one end. Smaller circular metal disks are inserted in pair in thehoop and loosely strung on wires. During performance, the tambourine is heldin one hand and the membrane struck with the fingers or palms of the otherhand.A cymbal is a brass circular disk that is concave at its center. Its vibration is thatof a circular plate supported at its center. Cymbals equipped with handles can behold, one in each hand, and struck together. A single cymbal can be supported ina fixed horizontal position on a stand or on some other support and struck with adrumstick. A sock cymbal consists of a pair of horizontally positioned cymbals,one being fixed and the other being moved by a pedal. Cymbals are made in varioussizes, from as little as 5 cm in diameter to 51 cm.Castanets, used to accentuate Spanish-style music, are hollow shells of hardwood. These are hold in the hand and clapped together. They can also be mountedon a handle in such a manner that shaking the handle clacks the castanets.The gong is a round plate of hammered bronze, with the edges curved up so thatthe shape takes on the appearance of a shallow pan. The gong is usually suspendedby strings attached to a frame. It is excited by striking with felt-covered hammer,with the sound output resembling a heavy roar. The diameter of the gong rangesfrom about 45 cm to 90 cm.18.12 Electrical and Electronic InstrumentsA tone can be generated by a number of electrical means. The means of producing atone can be achieved by interruption of an air stream (as with a siren that consists ofa rotating perforated wheel), an electrically driven diaphragm (as in the automotivehorn), and electronic alternator and loudspeaker combination (electric organs),an electronic oscillator–amplifier–loudspeaker combination (the electric piano isan example). The electric guitar is an adaptation, where an electromechanicaltransducer is attached to the bridge of the instrument. The transducer convertsthe mechanical vibrations into a correspondingly varying electrical signal that istransmitted to an amplifier. The amplifier increases the strength of the signal andsends it on to a loudspeaker that converts the electrical signal into the correspondingacoustical output. A volume control in the amplifier or integrated into the guitar