Classroom Noise Meter

What is sound? In technical terms, sound is the movement of air in the form of a pressure wave. This movement of air is usually caused by a vibrating object. For example, this is how musical instruments create their sound. A drummer hist a drum to make its membrane vibrate; an oboe player blows into a reed, causing it to vibrate; and a guitar player plucks a string to make it vibrate. Humans can also generate sounds with their voice. Although you cannot see the vibrations that cause your voice's sound, you can feel them when you put your hand on your throat while humming or talking. This is where your voice box, also called larynx, is located. Within the voice box we have our vocal folds, also called vocal cords, as shown in Figure 1.

Figure 1. Cross-sectional view of a human head and throat that shows the location of our voice folds (voice cords) in our voice box.

These vocal folds are key for creating sound with our voice. The vocal folds are two bands of elastic muscle tissue stretched horizontally, from back to front, across the voice box as seen in Figure 2 on the left. They are located side by side just above the windpipe (trachea). The vocal folds are able to close and open our windpipe by vibrating back and forth as shown in Figure 2 on the right. When we are not speaking or inhaling, our vocal cords are open, so the air we breathe in can make it into our windpipe. When we speak, they start to vibrate and thus create a sound. But why do they start vibrating?

Figure 2. Top view (left) and cross-sectional view (right) of the vocal folds (vocal cords) opening and closing.

The vibrations are caused by the air we breathe. Our breath is basically the fuel for our voice. When exhaling, air is pushed from our lungs through the narrow opening between the vocal folds. The force of this air causes the vocal folds to vibrate. As the vocal folds open and close, the air flow will be alternately interrupted and allowed to pass. This results in a fluctuation in air pressure that produces a sound wave. A sound wave consists of a repeating pattern of high-pressure and low-pressure regions in the air, as shown in Figure 3, traveling through the air in the form of a pressure wave. You can illustrate the low-pressure (rarefaction) and high-pressure (compression) zones of a sound wave in a graph showing pressure versus distance or time. The sound wave in Figure 3 shows how the pressure at a single point in time changes over a distance.

Figure 3. The vibrations of the vocal folds create sound waves. The sound wave can be represented with dots showing air particles. The dots are closer together in the high-pressure zones (compressions) and farther apart in low-pressure zones (rarefactions). The sound wave can also be represented using a graph with pressure on the y-axis and distance on the x-axis, where a positive y-axis value corresponds to higher pressure (compression) and a negative y-axis value corresponds to lower pressure (rarefaction). Both methods represent the sound wave at a single snapshot in time.

When the sound waves reach our eardrums, they cause the bones in our middle ear to vibrate, and the vibrations are transmitted to fluid in our inner ear. Then, the vibrations travel to the inner ear hair cells and to the nerves that carry the signal to our brains where we interpret the signal as sound.

When sound waves enter a human ear they travel past the outer ear until they hit the ear drum. The ear drum vibrates air trapped in the inner ear and a nerve sends a signal to the brain which can decipher the vibrations as different sounds.

Figure 4. Our ear translates sound waves into a sound that we can hear.

This explains how we can make sounds with our voice and how we can hear sounds, but it does not explain why there are so many different sounds with various pitches and volumes. To answer this question, we have examine the properties of a sound wave in more detail. In addition to speed, you can describe waves by their frequency, period, and amplitude (Figure 5). Let's start with frequency (f). The frequency of a wave describes how many cycles of the wave occur per unit of time. This is dependent on how fast the object that creates the sound wave (such as the vocal folds) are vibrating. Frequency is measured in Hertz (Hz), which is the number of cycles per second. Figure 5 (on the left) illustrates examples of sound waves of two different frequencies. Note that the graphs in this figure show time on the x-axis, not distance as in Figure 3. These graphs show how the pressure at a single point in space (a fixed distance) changes over time. The frequency of a sound wave determines the pitch of a sound. The higher the frequency, the higher the perceived pitch. On average, the frequency range for human hearing is from 20 Hz at the low end to 20,000 Hz at the high end. Figure 5 also shows the period (T) of the wave, which is the time it takes for one complete wave to pass a given point. The period is simply the reciprocal of the frequency (T = 1/f).

Diagrams showing the impact of sound wave frequency and amplitude on pitch and volume of sound. The diagram on the left shows the impact of frequency on pitch-- as low frequency waves produce lower pitch sounds, while high frequency waves produce higher pitch sounds. The diagram on the right shows the impact of amplitude on volume-- as lower amplitude waves produce quieter sounds while higher amplitude waves produce louder sounds.

Figure 5. Illustration of sound waves with different frequencies and amplitudes, representing sound waves of different pitches and volumes respectively.

Finally, the amplitude of a wave is the distance from the center line to the top of the peak or the bottom of the trough, which for a sound wave is measured in units of pressure. For a sound wave, the amplitude is connected to the loudness of the sound you hear. Figure 5 (on the right) shows examples of waves with two different amplitudes. The higher the amplitude, the louder the sound. The intensity of sound is measured in decibels (dB). Figure 6 shows the decibel ratings of some common sounds. Decibels are a logarithmic scale, not a linear scale. This means that for every increase of 10 dB, the sound intensity increases by a factor of 10. For example, sound with an intensity of 40 dB is 100 times as intense as 20 dB, not twice as intense. However, while we may use the terms interchangeably in everyday speech, loudness and intensity are not the same thing (see this article from Georgia State University for a more detailed explanation). Most of us perceive a sound to be "twice as loud" as another one when they are about 10 dB apart. Sound levels above 80 dB can cause hearing damage over long periods of time, and sound levels above 120 dB can cause immediate damage.

A bar graph showing the decibal levels of common sounds with the loudest at the top to the softest at the bottom. A gunshot is the loudest common sound with a value of 140 decibels, a normal conversation has a value in the middle of the graph of 60 decibels, and the sound of breathing is the quietest with a value of 10 decibels.

Figure 6. Decibel levels of some common sounds. Remember that the decibel scale is nonlinear. Every increase of 10 dB corresponds to roughly doubling the perceived loudness of the sound. So, for example, a chainsaw (100 dB) does not sound twice as loud as moderate rainfall (50 dB); it sounds 32 times as loud!

In this lesson plan, your students will determine the appropriate sound levels for different classroom scenarios such as working in groups, working independently, or listening to the teacher. To do this, they will investigate how the sounds they make with their voices translate into different sound intensities. To be able to measure different sound levels, students will use their mobile phones and a specific sensor app, which uses the microphones built into smartphones to measure sound. The app helps your students to record and visualize the intensity of the sounds they create with their voices, allowing them to discover the relationship between the properties of the sound waves and what they hear.