Sound

Background:

When something makes a sound, it is vibrating -- moving back and forth. As it moves it pushes the air that is around it compressing the air. This increases the pressure of the air. As the object moves back, it leaves an empty space where it was. This decreases the pressure of the air. These compressions and decompressions are called the sound wave and the wave propogates outward from the sound source. If the sound wave reaches your ear(s), you may (or may not) perceive it as a sound.

Whether you hear the sound wave or not depends on several physical and psychological factors. In normal humans, the auditory system is sensitive only to a relatively small range of frequencies -- approximately 20 to 20,000 Hertz. A Hertz, or Hz, is one complete cycle of a wave per second. A sound source needs to vibrate (move back and forth) between 20 and 20,000 times for a young, normal human to hear the sound wave that the source produces. If the sound source is vibrating less than 20 times per second or more than 20,000 times per second, the wave that it produces will not be heard. Sound waves with lower frequencies are usually heard as lower pitches (like those produced by keys on the left side of a piano keyboard) while higher frequencies are usually heard as higher pitches (like those produced by keys on the right side of a piano keyboard.)

Whether you hear the sound wave or not also depends on how much the sound source moves. If the sound source moves very little, the compression and decompression of the surrounding air will be very slight. If the sound source moves a lot, the compression and decompression of the surrounding air will be much more. The difference between the highest air pressure (caused by the compression) and the lowest air pressure (caused by the decompression) is called the sound wave's amplitude. If the amplitude is too small, you probably will not perceive the sound wave. If the amplitude is too big, it will blow your head off your shoulders and perhaps separate you into little, tiny pieces. Let's hope that that doesn't happen! In between is a range of amplitudes that is sufficiently large that you can perceive the sound wave at a given distance from the sound source, but not so large as to cause your auditory system physical harm. At the same distance from the sound source, lower amplitudes will usually be perceived as less loud while higher amplitudes will usually be perceived as louder.

There are several psychological and physiological variables that also play a role in whether you will perceive the sound wave or not. Repetitive noises, such as the ticking of a clock, are often not perceive after a few minutes of exposure. Physiological arousal -- whether you are ready to fight or flee a situation vs being asleep -- can influence whether a given sound wave is perceived or not. Whether you are attending to the sound wave or not can influence whether it is perceived or not. Exposure to loud sounds can temporarily reduce your ability to hear soft sounds. Really loud sounds can trigger the acoustive reflex which will temporarily reduce your ability to hear soft sounds.

When psychologists study the perception of sounds, they frequently use pure tones. A pure tone is a sound wave with a single frequency. Most things in nature produce complex tones -- tones with many different frequencies at different amplitudes. Why use pure tones when nature produces complex tones? Pure tones are simpler to create and study. Very early in the auditory system it decomposes complex tones into their pure tone counterparts. Thus, most of what occurs with complex tones can be described as the sum of what happens with the pure tones that they are composed of.

One type of complex tones is a musical note. Musical notes usually (depending on the instrument that produced them) consist of numerous frequencies called the fundamental frequency and the harmonic frequencies. The fundamental frequency is the lowest frequency present in the sound wave and is associated with the note's name in a musical score. For example, the fundamental frequency of middle C is approximately 262 Hz, no matter what (properly tuned) instrument produces the note. If only the fundamental frequency was present, the note would be a pure tone and, in many people's opinion, not a very pleasing sound. When a note, such as middle C, is played on most instruments, additional frequencies called harmonics are also produced. The frequenciesy of the harmonics are integer multiples of the fundamental frequency. That is, the frequency of the second harmonic of middle C is 2 X 262 Hz = 524 Hz. The frequency of the third harmonic of middle C is 3 X 262 Hz = 786 Hz. These harmonic frequencies are the same no matter what instrument produces the middle C. The amplitudes of the harmonics are often lower than the amplitude of the fundamental. Unlike the harmonic frequencies, the harmonic amplitudes can be very different for different instruments. The harmonic amplitudes are a major component of timbre (pronounced "tamber") -- the quality of a sound that makes a particular instrument sound like that particular instrument. That is, you can easily tell whether a middle C note is produced by a piano or by a trumpet -- that is because the two instruments have different timbres and the amplitudes of the harmonics will be different between the piano and trumpet. There are other features of the sound wave that also influence the timbre of the instrument.

One way that sound waves can be represented graphically is to plot time across the X axis and the amount of compression (values above the X axis)/ decompression (values below the X axis) across the Y axis. After you generate a sound in the following activity, such a graphical depiction of the sound wave will be displayed.

Always Practice Safe Listening:

Exposure to loud sounds can lead to permanent damage to the auditory system. Always practice safe listening -- turn the volume level all the way down on your computer and gradually increase it until the sound is at a comfortable level. If you are using earbuds or earphones, if someone standing next to you can hear the sound, it is too loud and you must reduce the volume. The damage caused by exposure to loud sounds is cumulative -- you may not notice an effect today, or tomorrow, but continued exposure to loud sounds will eventually lead to permanent hearing loss.

Activities:

This activity is known to work on Firefox. It does not work on Internet Explorer. There is a known bug with Chrome that can break the activity.

This activity allows you to generate sounds consisting of up to three notes. For each note, you can specify its fundamental frequency and the amplitude of the fundamental frequency and up to four harmonic frequencies.

Pure Tone Activities:

For these activities, play only one note and set the harmonic amplitudes to zero. This will produce a pure tone.

1. Adjust the frequency of a pure tone and play it. What psychological property of the sound changes as you adjust the frequency? How does the graphical representation of the sound wave change as you adjust the frequency?
2. Adjust the amplitude of the pure tone and play it (use the "Amplitude of fundamental" slider.) What psychological property of the sound changes as you adjust the amplitude? How does the graphical representation of the sound wave change as you adjust the amplitude?
3. Pick a low frequency. Depending on the quality of your speakers / earbuds, they may not be able to play really low frequencies. 220 Hz works on the el cheapo speakers in my office (thanks UD!) -- that is the fundamental frequency of the A below middle C. Play the note. Increase the frequency by 1 Hz to 221 Hz (typing it works better than using the slider in this case.) Play the new note. Do the two notes sound different? Try it again, separately playing 220 Hz and 222 Hz notes. Do these two noes sound different? How far apart do the frequencies need to be before you can distinguish the two notes as different pitches? That is, what is the difference threshold for frequency discrimination? Repeat with a higher pitch, say 880 Hz (two As above middle C.) Is the difference threshold for the higher frequency smaller, the same as, or larger than the difference threshold for the lower frequency? Why? What does Weber's law have to say about that?

Complex Tone Activities:

By selecting to play a single note but setting the harmonic amplitudes to non-zero values, you can produce a type of complex tone, a note.

1. Compare a pure tone and a complex tone with only the second harmonic's amplitude greater than zero. Psychologically, how do the notes differ? How does the graphical representation of the sound wave change as you increase the amplitude of the harmonic?
2. Increase the amplitude of additional harmonics, one by one. How do the notes differ?
3. With the amplitudes of the fundamental and harmonics set to non-zero values, play the note. Now set the amplitude of the fundamental frequency to 0 and play the note again. Can you still hear the missing fundamental? What does the perception of the missing fundamental tell us about the auditory system?

Two or More Notes:

1. Play two or more notes together. Adjust the frequency of one of the notes until you find a pair of notes that sound pleasing together. Such notes are called "consonant." Adjust the frquency of one of the notes until you find a pair of notes that do not sound pleasing together. Such notes are called "dissonant." Can you find a relation between the fundamental frequency of consonant notes? That is, can you find a rule that lets you predict whether a pair of notes will be consonant or dissonant?
2. Play two notes together. Adjust the frequencies of the two notes so that they are very similar, but not identical to each other -- say no more than 10% different. What is the perceptual consequence? Can you hear the oscillation in the loudness of the sound -- like a wah-wah? While it is really hard to do, especially with such short sounds, can you relate the frequency of the wah-wah with the difference in the frequencies of the two notes? How might that be of use to a piano tuner?