The note called middle C is considered by physicists as having 256 vibrations a second. This would give the following _actual vibration_ numbers to the remaining notes of the major scale that begins with "Middle C" D.-288, E.-320, F.-341.3, G.-384, A.-426.6, B.-480, C'.-512. Musicians, however, usually make use of a scale of slightly higher pitch. The _international_ standard of pitch in this country and in Europe is that in which "A" has 435 vibrations per second. This corresponds to 261 vibrations for middle C. =333. The Relation between Speed, Wave Length, and Number of Vibrations per Second.=--Since the notes from the various musical instruments of an orchestra are noticed to harmonize as well at a distance as at the place produced, it is evident that notes of all pitches travel at the same rate, or have the _same speed_. Notes of high pitch, having a high vibration rate produce more waves in a second than notes of low pitch, consequently the former are shorter than the latter. The following formula gives the relation between the speed (_v_), wave length (_l_), and number of vibrations per sec. (_n_): _v_ = _l_ × _n_, or _l_ = _v/n_ that is, _the speed of a sound wave is equal to the number of vibrations per second times the wave length, or the wave length is equal to the speed divided by the number of vibrations per second_. This formula may also be employed to find the _number_ of vibrations when the wave length and speed are given. Important Topics 1. Difference between noise and music. 2. Factors affecting intensity: area, amplitude, density, distance. 3. Pitch, major scale, relative vibration numbers. 4. Relation between speed, wave length and vibration rate. Exercises 1. Give an illustration from your own experience of each of the factors affecting intensity. 2. Write the relative vibration numbers of a major scale in which _do_ has 120 vibrations. 3. What is the wave length of the "A" of international concert pitch at 25°C.? Compute in feet and centimeters. 4. At what temperature will sound waves in air in unison with "Middle C" be exactly 4 ft. long? 5. Explain the use of a megaphone. 6. What tone has waves 3 ft. long at 25°C.? 7. What is the purpose of the "sounding board" of a piano? 8. Two men are distant 1000 and 3000 ft. respectively from a fog horn. What is the relative intensity of the sounds heard by the two men? 9. The speaking tone of the average man's voice has 160 vibrations per second. How long are the waves produced by him at 20°C.? (4) MUSICAL SCALES AND RESONANCE