an introduction
“a percussion of the air”
-Boethius (6th Century)
If sound is your means of expression, then you should know something about how it behaves and how we perceive it. A basic knowledge of acoustics – the study of sound – will help you understand some of the things you will encounter while working with music technologies. Acoustics is a rich field of study. We are just covering the basics.
Five Properties of Sound
| Physical Property | Equivalent Musical Concept |
| FREQUENCY | pitch |
| INTENSITY | amplitude/dynamics (pp, ff, etc.) |
| DURATION | rhythm |
| ENVELOPE | shape/articulation/dynamics (< >) |
| TIMBRE | tone |
Concepts
sound waves – propagation – compression – rarefaction
speed of sound – time domain waveform plot
amplitude – sound pressure level (SPL) – decibels (dB)
1. SOUND WAVES
What is sound? Imagine striking a tuning fork. It produces one of the most simple and pure acoustic sounds. You probably can’t see it, but striking the tuning fork causes its tines to vibrate back and forth in a fairly simple manner. When a tine moves away from its resting point it collides with the nearby air particles and pushes them into an area of greater density (higher air pressure) called compression. When the tine moves back past its resting point, the air particles spread out into an area of less density (lower air pressure) called rarefaction. The vibrations of the tuning fork cause a chain reaction resulting in areas of higher and lower pressure propagating away from the tuning fork as a sound pressure wave.
Imagine being able to slow down the vibrations and zoom in to see air particles. The animation below shows a cylinder with a piston at one end. The piston – like the tine – moves back and forth compressing and rarefying the air. The dark areas among the particles are more dense than the surrounding air. Notice how the dots do not move very far as the wave passes though, but sway back and forth. It is the chain reaction of pressure changes – the acoustic energy of the wave – that moves from one end to the other. The curve underneath the swaying dots plots the changes in air pressure. When the curve is above the horizontal axis, the particles are being squeezed together. When the curve is below, this indicates a region of low-pressure.
Figure 1. A SOUND WAVE very much slowed down and very much zoomed in.
Figure 1. from Jack Schaedler, Seeing Circles, Signs, and Signals: A Compact Primer On Digital Signal Processing. 2. PROPAGATION But sound doesn’t just move in one direction – it propagates in all directions from the sound source. Consider the image above. It shows sound pressure waves propagating in concentric rings away from a vibrating object. A more complete picture would show sound waves propagating spherically from the sound source. Also, this image does not show a difference between sound pressure waves near the sound source and those further away. Friction in the air causes sound waves lose energy the further they propagate from the source. Regardless of how hard you strike the tuning fork, the sound pressure wave travels at the same speed. The speed of sound in air is a constant 340 m/s or 1130 ft/s. This varies depending on the temperature and humidity. Under what conditions would the speed of sound be fastest? Humid air has more moisture (more “stuff” in it) and warm air means the air particles are closer together. Both result in increased friction as a sound wave propagates. Cooler and drier it is, the faster the speed of sound. Have you ever noticed how in the cold, dry air of winter you can hear someone’s footsteps crunching loudly in the snow from a long way off? Assuming the wind isn’t howling! Sound waves in the air are slower than many other kinds of waves. Light waves are nearly 900,000 times faster. Sound waves also travel faster in liquids and faster still in solids. The slow speed of sound gives rise to echos. If you shout it will take about a half second for the sound to travel 500 feet, bounce off a concrete wall, and another half second to travel back to you. You will hear your echo one second later than you shouted. 3. SOUND WAVE PLOTSOur perception of sound – as well as our capacity to capture and modify it with technology – is directly linked to the compression and rarefaction of air particles. A graph showing changes in air pressure over time is the basis for the most common representation of sound waves: the time-domain waveform plot. Often simply referred to as “waveform.” Look at Figure 1 again. It shows air pressure changing over time. You can see how the compression and rarefaction of air particles is represented by the familiar squiggly line graph. The higher the line goes on the Y axis, the greater the change of air pressure, and thus the louder the sound. The degree to which air molecules are displaced corresponds to amplitude: a little movement = quiet, a lot = loud. 4. AMPLITUDE AND DECIBELS (dB)The amount of change in air pressure of a sound wave from the normal equilibrium pressure is called amplitude. If there is a large difference in pressure between compressed and rarefied regions of air supporting a given wave, the wave has a high amplitude. Sound level meters in audio programs often are capable of reporting two types of amplitude measurement: peak and average. Both types are determined by looking at very brief segments of sound (often called ‘windows’). When a waveform plot is swinging up and down dramatically, then the wave has high average amplitude. Greater average amplitude is associated loosely with sounds that seem louder to us than ones whose waves have lesser average amplitude, though perceived loudness also depends on other characteristics of sound. Amplitude is usually measured in decibels (dB), a logarithmic scale that compares the amplitudes of two sound waves to each other, or the amplitude of one sound wave to a reference level. A doubling of amplitude represents a difference of about 6 dB. If you took a waveform and scaled it by 0.5, so that it then had half the amplitude, you would be reducing its amplitude by 6 dB. Scale this second waveform by 0.5, and you create a difference of 12 dB between it and the original waveform. When you watch a meter in a software mixer, you’re seeing the comparison of a sound wave with a reference level that is the maximum level the system can handle. The numbers printed in dB show 0 as this maximum level, and levels below this are shown as negative decibels: -6 dB, -12 dB, -18 dB, and so on. The minimum level is sometimes labeled as negative infinity (-∞), which is silence. We use the logarithmic decibel scale for two reasons:
5. SOUND PRESSURE LEVELSometimes we see sounds described as having a sound pressure level, or SPL, labeled in decibels. This means that the reference level used for computing decibel values is something called the threshold of hearing, an empirically determined standard that is the softest tone, of a certain pitch, that an average human with excellent hearing can perceive. If that is 0 dB, then louder sounds have higher dB values. A fine point: SPL actually is measured in terms of the intensity of a sound, which refers to the power of the changes in air pressure as they contact a surface, like your ear. Intensity is measured in watts per square meter. The following table gives you a sense of the very wide variety of acoustic experiences that span the range from 0 to 130 dB SPL. Relative Intensity of Familiar Sounds
6. SOUND SAFETYWe can also use the concept of SPL to decide how long it’s safe to listen to loud sounds. Consider the table below, which uses US Government standards to guide us in determining when hearing damage might occur. US Occupational Safety and Health Administration Standards
Where do you think earbuds with the volume cranked fit on this chart? |