Sounds and Their Parameters

Are you a musical person? Do you have perfect pitch? If I play this sound here (please see 00:08 in the video above), and then I play this sound (please see 00:11 in the video above), can you tell what the difference is? You probably noticed that the first sound was lower and the second sound was higher. If you have musical training, you may have recognized that the interval between the two notes was a major sixth. And, if you have perfect pitch, you may have recognized the notes as a C and an A.

But, what does 'pitch' mean when it comes to sound? We know that sound travels in waves, and that those waves are characterized by their wavelengths, amplitudes, and other parameters. If the two tones I just played come from different sound waves, then what exactly about the waves is different between the two?

In this lesson, we're going to explore the more familiar characteristics of sound, like pitch and volume. We'll talk about what these mean in terms of the sound waves we've learned about so far in this chapter. By the end, you'll have a deeper understanding of how sound works and maybe appreciate music a little bit more.

Amplitude in Sound Waves

Plucking a guitar string creates longitudinal waves in the air, which produce sound.

Let's start by recalling a few things about sound waves. While sound can travel through all types of substances, we're going to use air as the medium in our examples here. A great way to visualize longitudinal waves in air is to think of the sound coming from a guitar string. When you pluck the string, it vibrates from side to side, pushing the surrounding air molecules in a periodic fashion. The compressions and rarefactions in the air comprise a longitudinal wave, which we detect as sound. If we could look at just one air particle, we would see it oscillating back and forth. Sometimes the particles move back and forth over a very large distance. Other times, the particles oscillate just a little ways. The amount of oscillation in the particles of the medium is related to the amount of energy carried by the wave.

Can you remember which of the five wave parameters describes a wave's energy? And by wave parameters, I mean the frequency, wavelength, period, speed, and amplitude. One that describes a wave's energy is the amplitude. In a transverse wave, the amplitude can be seen as the height of the crests. But in a longitudinal wave, like sound, amplitude is a measure of how far the particles oscillate back and forth. In a high-energy wave, the amplitude is large, because the particles oscillate over a large distance. A low-energy wave has low amplitude because the oscillations are small in size. So, now we get the relationship between energy and amplitude. But, how does that translate into what we hear as a sound wave?

Intensity and Volume

The decibel scale measures the intensity of sound waves.

The energy in a sound wave has to travel over a certain area in a certain amount of time. We detect sounds as being louder when we're standing closer to the source and quieter when we're standing further away. The amount of energy we detect is known as the intensity. Intensity is measured in units of energy over the area and time. In other words, it's the amount of energy that is carried over a certain area in a certain amount of time. We describe different levels of intensity using the decibel scale, a logarithmic scale for measuring the intensity of sound waves. Normal conversation generally falls around 60 decibels. A whisper is more like 20 decibels, while a vacuum cleaner runs as loud as 80 decibels. You may be more familiar with decibels as a measure of volume. When talking about sound waves, the volume is the perception of loudness from the intensity of a sound wave. The higher the intensity of a sound, the louder it is perceived in our ears, and the higher volume it has. Since intensity is a function of energy, and energy is related to amplitude, then we can make the conclusion that the volume of a sound is proportional to the amplitude of the sound wave.

Frequency in Sound Waves

It was a little tough trying to visualize the amplitude of sound waves. Because sound is a longitudinal wave, we don't get the nice 'up and down' wave shape that makes it so easy to see the parameters. We figured out a way to see the amplitude of sound. But, what about the period and frequency? Is there a way to see the parameters that are related to time?

Let's say a sound wave is traveling toward a microphone. The microphone picks up compressions and rarefactions as the wave passes by. When a compression hits the microphone, it receives a high amount of air pressure from the particles in that space. During a rarefaction, the air pressure is very low. Over time, the microphone experiences ups and downs in pressure, which we can illustrate as an 'up-and-down' wave like this: