Picturing Sound: The Physics behind Waveforms and Spectragraphs
Good Vibrations: How Sound is Created
Good Vibrations: How Sound is Created
Every sound we hear is produced by something vibrating. The vibrations have enough energy to push against the surrounding air or water, making that medium also vibrate. These vibrations travel in every direction. When they reach our ears, they make our eardrums vibrate, and that is how we hear them. Sound travels slower than light, so we see a flash of lightning before we hear the thunder produced at the exact same moment. Most sounds we hear travel through gas or liquid, but sound can also travel through solids. That's why you could put your ear to the ground and hear galloping hooves, or to the rails of a train track to hear an approaching train.
Measuring Sound by Frequency
Even a single drumbeat produces sound waves. The sound of a single drumbeat lasts about 0.15 seconds. We can picture what the waves look like on a special graph called a waveform. Notice that the waves start big and get smaller as the sound ebbs. There are approximately 103 waves per second with this drumbeat. So this sound has a frequency of about 103 Hz. We call one vibration per second 1 Hertz, or Hz. If something vibrates a thousand waves per second, we can say it as 1000 Hz or 1 Kilohertz or 1 KHz, which means that the frequency of a single drumbeat is also 0.103 KHz. Compare the drumbeat to a pure digital tone of exactly 103 Hz.
Lowdown on High Notes and Low Notes
Slow vibrations produce sounds that sound low-pitched to our ears, like the tone or drumbeat. These are called low frequency sounds because they produce a low number of waves every second. Sounds can vibrate MUCH faster than 103 waves per second. Sounds that vibrate thousands of waves per second are called high frequency sounds and sound high-pitched to our ears.
It is impossible for human ears to detect a sound that is only 1 Hz. Children's ears can perceive sounds that range from about 20 to 20,000 Hz. As adults get older, their eardrums lose some of their resiliency and flexibility, and so they usually lose some hearing at the highest and lowest frequencies. Small songbirds and hummingbirds can hear at higher frequencies than humans can, and large whales can almost certainly hear at lower frequencies than we can. To help their hearing, some whales have a bony plug in the ear canal or even use part of their jaw!
That drumbeat we listened to is a natural sound. When a drummer hits a drum, the drumhead vibrates, and the snares at the bottom of the drum also vibrate, along with the metal around the rim and the drum stick. Sometimes other things in the room may even start vibrating as the waves hit them. So the sound of a drummer hitting a drum isn't just one pure frequency. To see all the different frequencies in a sound, we look at a spectragraph. The X-axis of the spectragraph shows time, so following a spectragraph from left to right we can see what the frequencies are at every moment.
Let's compare the spectragraphs of the same drumbeat and digital tone. Notice how there are so many frequencies in the drumbeat that we can't even see where one ends and the next begins! The way the frequencies all mix together is what gives the drumbeat its special sound, and explain why it doesn't have a pitch like a musical note. The pure tone is a straight line, because the sound is all on one frequency at every moment.
Now look at and listen to these other pure tones. Depending on the speakers of your computer, you might not be able to hear the 50 Hz sound (headphones work better for this sound). That's because a lot of speakers are less sensitive than your ears, and can't feel such a slow vibration. But you should be able to easily hear the difference (and see it) between a 500 Hz and a 5000 Hz pure tone.
Measuring Sound by Volume
Sound isn't only measured in frequency. We can also perceive how loud sounds are, and measure that as the sound's amplitude. We measure this in decibels. We can see how loud a sound is by looking at how big the waves are on the waveform. Waveforms show the actual sound waves as they oscillate above and below a middle line.
Normal sounds made by people, animals, musical instruments, and just about everything else are nearly always made up of different sound waves all happening at once. Look at the waveform and spectrograph of a metronome making a ticking sound every half second. Notice how there is empty space between the sounds on the X-axis, to show the silence between sounds, but the sounds themselves are produced at many frequencies. Compare the even metronome to a drumroll. Which has the most noticeable changes in volume?
The Sound of Music
Musical notes from most instruments are made up of a basic tone and harmonics, patterns of frequencies that work together to make the sounds our ears hear as musical. These patterns of frequencies are produced by various vibrating surfaces in a well-tuned instrument. The waveform of a single piano note shows a loud amplitude when the key is struck, which fades away. The spectragraph of that exact same note shows several different frequencies that work together to make the sound we hear.
When we play three or more musical notes at the same time, we call it a chord.. Some chords sound very pleasing to our ears. The waveform of a single piano chord looks a lot like the waveform for the single piano note, because when pianist hits one key or three, the sound starts loud and quickly dies out. Each of the three notes has its own pattern of frequencies, but they overlap so the spectragraph doesn't look as clear as the spectragraph of a single note. But if you look carefully, you'll see that there is a pattern, which is part of the reason a chord has a musical effect.
For what reasons might scientists find waveforms and spectragraphs useful?
Testing, One, Two, Three! Can you read a song with me?
One of these Mystery Songs is "Twinkle Twinkle Little Star," one is "Chopsticks," and one is an octave scale ("Do Re Mi Fa Sol La Ti Do"). Click on each waveform and spectragraph if you want to see it in a bigger format. Can you tell which goes with which song?
To see spectragraphs and waveforms for a variety of animal sounds, from frogs and birds to whales and wolves, see Picturing Sound Part 2.
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