The source of the rhythm transfers the vibrations of the rhythm to the surrounding area, whether it is air, water, or the ground. Whenever these vibrations disturb the medium in a regular periodic way, they create wave motion. The portion of a rhythm that is repeated repeatedly represents one cycle of the periodic motion, or one wave. A wave has a high point and a low point, just as a wave in the ocean has a crest and a trough.
The first kind of wave we will consider is called a transverse wave. In a transverse wave, the particles in the medium vibrate perpendicular to the direction in which the wave is traveling. Imagine attaching a rope to a wall, standing away from the wall just enough to give the rope some slack, and then giving the rope a quick up-and-down jerk with your hand.
The movement of your hand sends a wave traveling horizontally along the rope, while the rope itself moves up and down, perpendicular to the direction of the wave's movement.
A violin string, when plucked, works just like the rope. The pluck, rather than the hand jerk, generates the wave. While the wave travels along the string horizontally, the string itself, and, therefore, the air particles around it, move in the vertical direction.
The other kind of wave to consider is called a longitudinal wave. In a longitudinal wave, the particles in the medium vibrate parallel to the direction in which the wave is traveling. An example of a longitudinal wave is a Slinky spring toy hung from the ceiling with a weight attached to the end. If you pull on the weight and then let go, the whole system bobs up and down. The wave and medium move parallel to one another.
The Slinky will eventually stop bobbing and come to rest. However, until then, the motion is periodic - a new wave starts traveling along the Slinky every time the weight reaches its lowest point. That means that as the wave travels along the Slinky, its coils will be close together at some places (called points of compression), and farther apart at others (called points of rarefaction).
Sound waves are also longitudinal. The source of a sound sends a vibration outward into the air. The vibration temporarily shoves the air particles away from their equilibrium (original) positions, each particle moving, on average, 1/100,000 inch.
At the points of compression, there are many air molecules crowded together and the pressure is high. At the points of rarefaction, the air molecules are more spread out and the pressure is low. The vibration, or sound wave, creates these compressions and rarefactions as it travels through the air. The wave that travels along a violin string when it is plucked is transverse, but the sound wave that the string transmits to the air is longitudinal. The longitudinal wave travels through the air, hits your eardrum, and allows you to hear the note.
So what makes one sound wave different from another? Although the decision about what is music and what is noise is in reality personal and subjective, music is generally distinguished from noise by the regularity of the sound waves that create musical sounds. The regularity of a wave is determined by certain characteristic quantities called amplitude, frequency, and wavelength.
Recall the example of the rope given above. The distance from the top of the crest of the wave to the equilibrium (original) position of the rope, or from the bottom of the trough or the wave to the equilibrium position, is called the amplitude (A) of the wave.
The wavelength is the distance between any point on one wave and the corresponding point on the next one; that is, the distance the wave travels in one cycle.
The frequency (f) is the number of waves, or vibrations, that pass a given point per second. The frequency of the wave is the same as the frequency of the source. Frequency is measured in Hertz (Hz), where 1 Hz = 1 vibration/second, after Heinrich Rudolf Hertz, a German physicist who lived during the 1800s.
The period (T) is the time it takes for one whole wave or cycle to pass a given point. Therefore, the period (number of seconds per wave) and the frequency (number of waves per second) are reciprocals of one another, and T = 1/f.
These quantities - amplitude, frequency, wavelength, and period - are all measurable physical characteristics of any musical tone.
The loudness of a tone is the listener's evaluation of the amplitude - the larger the amplitude, the louder the tone; the smaller the amplitude, the software the tone. Loudness is measure in decibels.
The pitch of a tone is the listener's evaluation of the frequency, and represents how high or low a note sounds. The higher the frequency (the most vibrations per second), the higher the pitch; the lower the frequency (the fewer vibrations per second), the lower the pitch.
A musical tone lasts long enough and is steady enough to have pitch, quality, and loudness. A drum usually makes a short, sharp sound that encompasses many different frequencies, never settling in on one. Consequently, the sound disappears quickly and doesn't have a discernible pitch; therefore, it is not a tone. A pure tone has constant frequency and amplitude and has the shape of a sine curve.
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