Introduction to Sound
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INTRODUCTION TO SOUND Course Reader | Autumn 2013 | Seth Kim-Cohen It's as if perfectly capable curators in the visual arts suddenly lose their equilibrium at the mention of the word sound. These same people who would all ridicule a new art form called, say, 'Steel Art' which was composed of steel sculpture combined with steel guitar music along with anything else with steel in it, somehow have no trouble at all swallowing 'Sound Art'. In art, the medium is not often the message. If there is a valid reason for classifying and naming things in culture, certainly it is for the refinement of distinctions. Aesthetic experience lies in the area of fine distinctions, not the destruction of distinctions for promotion of activities with their least common denominator, in this case sound. Much of what has been called 'Sound Art' has not much to do with either sound or art. - - Max Neuhaus, “Sound Art?” (2000) Table of Contents First Reading What is sound? A Primer (2003) Second Reading Alan Licht, “What Is Sound Art?” (2005) “When Does Sound Become Art?” Art Review Magazine (2005) Third Reading Michael Nyman, “Seeing, hearing: Fluxus” (1974) Fourth Reading Alvin Lucier, selected texts and scores (1965 - 1994) Fifth Reading Luigi Russolo, The Art of Noises (1913) Sixth Reading John Cage, excerpts from Silence (1961) Seventh Reading Michael Nyman, “Towards A Definition Of Experimental Music” (1974) Steve Reich, “Music As A Gradual Process” (1968) Eighth Reading Jonathan Sterne, “Hello?” from The Audible Past (2003) Bonus Resource Craig Dworkin, Unheard Music (2009) First Reading What is sound? A Primer (2003) WHAT IS SOUND? (WHAT IS SILENCE? WHAT IS NOISE?) Adapted primarily from Prof. Jeffrey Hass Center for Electronic and Computer Music, School of Music Indiana University, Bloomington, Indiana For the full, more technical version, go to: http://www.indiana.edu/~emusic/acoustics/acoustics.htm We live at the bottom of a sea of air. As with the ocean, the farther down one goes, the greater the density of molecules and the greater the pressure it exerts on us. Sound is produced by a rapid variation in the average density or pressure of air molecules above and below the current atmospheric pressure. We perceive sound as these pressure fluctuations cause our eardrums to vibrate. When discussing sound, these usually minute changes in atmospheric pressure are referred to as sound pressure and the fluctuations in pressure as sound waves. Sound waves are produced by a vibrating body, be it an oboe reed, guitar string, loudspeaker cone or jet engine. The vibrating sound source causes a disturbance to the surrounding air molecules, causing them to bounce off each other with a force proportional to the disturbance. The energy of their interaction creates ripples of more dense (higher pressure) to less dense (lower pressure) air molecules, with pressures above and below the normal atmospheric pressure. When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound waves. Amazing factoid #1: The threshold of human hearing, or the softest perceptible sound, corresponds to a pressure variation of less than a billionth of the current atmospheric pressure (though the threshold of hearing varies according to frequency, as you will see below). Sound waves in air are longitudinal waves, in that the pulsating motion of the air is in the direction the sound wave travels. Physicists classically demonstrate this with the “Slinky” model, in which a quick push on one end of a slinky will cause a longitudinal wave to travel down its length. A sound wave is also a form of a traveling wave, in that the air molecules disturbed by the sound source are unlikely to be the ones hitting your eardrum, but transfer their energy to other neighboring molecules. A sound wave, which is not impeded by another object, propagates (or spreads) out from the source as a sphere. How fast does sound travel? The speed at which sound propagates (or travels from its source) is directly influenced by both the medium through which it travels and the factors affecting the medium, such as altitude, humidity and temperature for gases like air. Bad news for Star Wars fans—there is no sound in the vacuum of space because there are too few molecules to propagate a wave. It is important to note that sound speed in air is determined by the air itself. It is not dependent upon the sound’s amplitude, frequency or wavelength. For comparison, the speed of light in a vacuum is 299,792,458 meters per second or 186,000 miles per second (669,600,000 mph), which is roughly 870,000 times faster than the speed of sound. The difference between the speed of light and the speed of sound is why you see lightening before you hear it (unless you are struck by it, in which case it may be simultaneous!) What are the characteristics of sound waves? Sound waves can be characterized by three basic qualities, though many more are related: Frequency, Amplitude, and Wave Shape Some sound waves are periodic, in that the change from equilibrium (average atmospheric pressure) to maximum compression to maximum rarefaction back to equilibrium is repetitive. The 'round trip' back to the starting point just described is called a cycle. Periodic motion is classically demonstrated by observing the motion of a pendulum. The amount of time a single cycle takes is called a period. Simple harmonic motion, the motion described by the pendulum example above, is represented in sound as a sine wave, which traces the mathematical shape of it namesake. A sinusoidal wave (which also includes a cosine wave) is the only wave shape that produces a single frequency, as we will see in the waveform chapter. With any minute deviations in the sine shape, additional frequencies will be generated. Noise is characterized as being aperiodic or having a non-repetitive pattern. There are many different types of noise, depending primarily on the random distribution of frequencies. For example, some types of noise may sound brighter than others. Some periodic waveforms can be complex enough to be perceived as noise if our ears cannot detect perceptible pitches. Many real-world sounds, such as the "chiffy" attack of a flute note contain some combination of periodic and aperiodic components. What is frequency? The number of cycles per unit of time is called the frequency. For convenience, frequency is most often measured in cycles per second (cps) or the interchangeable Hertz (Hz) (60 cps = 60 Hz), named after the 19th C. physicist. 1000 Hz is often referred to as 1 kHz (kilohertz) or simply '1k' in studio parlance. The range of human hearing in the young is approximately 20 Hz to 20 kHz—the higher number tends to decrease with age (as do many other things). It may be quite normal for a 60-year-old to hear a maximum of 16,000 Hz. Amazing factoid #2: For comparison, it is believed that many whales and dolphins can create and perceive sounds in the 175 kHz range. Bats use slightly lower frequencies for their echo-location system. The perceived pitch of a sound is our ear/mind's subjective interpretation of its frequency. A frequency lowered by 400 Hz will not be perceived by us as a change equivalent to a pitch to raised by 400 Hz. Therefore, frequency and pitch should not be considered interchangeable terms. One particularly interesting frequency phenomenon is the Doppler effect or Doppler shift. You've no doubt seen movies where a police siren or train whistle seems to drop in pitch as it passes the listener. In actuality, the wavelength of sound waves from a moving source are compressed ahead of the source and expanded behind the source, creating a sensation of a higher and then lower frequency than is actually being produced by the source. What is amplitude? Amplitude is the objective measurement of the degree of change (positive or negative) in atmospheric pressure (the compression and rarefaction of air molecules) caused by sound waves. Sounds with greater amplitude will produce greater changes in atmospheric pressure from high pressure to low pressure. Amplitude is almost always a comparative measurement, since at the lowest-amplitude end (silence), some air molecules are always in motion and at the highest end, the amount of compression and rarefaction though finite, is extreme. In electronic circuits, amplitude may be increased by expanding the degree of change in an oscillating electrical current. A woodwind player may increase the amplitude of their sound by providing greater force in the air column i.e. blowing harder. Discussions of amplitude depend largely on measurements of the oscillations in barometric pressure from one extreme (or peak) to the other. The degree of change above or below and imaginary center value is referred to as the peak amplitude or peak deviation of that waveform. When using audio gear or software, it is important to know whether your meter is a peak-reading meter or averaging meter (or neither). While there are many good reasons to keep an eye on a signal’s peak, the average is far more akin to the way we hear. Once you have an understanding of dB’s described below, the markings on the meters should make more sense. Decibels: While power is measured in watts, the most-used acoustic measurement for intensity is the decibel (dB). Named in honor of Alexander Graham Bell, a decibel = 1/10 of a bel. A decibel is a logarithmic measurement that reflects the tremendous range of sound intensity our ears can perceive and closely correlates to the physiology of our ears and our perception of loudness.