Energy of Sound

In Investigation One, students explored how the energy of a marble can be used to make a flowerpot move across the table. This provided students the opportunity to discover how the potential energy of the marble can be converted to the kinetic energy of the marble, and thus transferred to the flowerpot on impact, causing the flowerpot to move. In performing these experiments, students demonstrated the Law of Conservation of Energy and learned that potential energy and kinetic energy have an inverse relationship. Students also discovered that the potential energy of an object is not a constant by changing the height of the inclined plane, thus showing that the potential energy of the marble increased. Due to the increase in potential energy, the kinetic energy increased, causing the flowerpot to move a greater distance. In Investigation Two, students will discover that sound energy is also a form of kinetic energy and can also be converted to mechanical energy.

Sound energy is an audible form of energy. It is generated by vibrations and travels in the form of waves. A sound wave is a compression wave. This means that as an object vibrates, it pushes the molecules next to it, compressing them into a smaller space. Because a sound wave must be transmitted through matter to be audible, any object that vibrates in a vacuum will not produce sound. There are no molecules in a vacuum to transmit the sound wave. Molecule spacing determines the efficiency of sound travel. Sound travels most efficiently in a solid medium because the molecules are more densely packed than in a liquid or gas medium.

Volume and Amplitude

All forms of energy have properties, much as matter has properties. Sound has properties that are unique to sound energy. Volume is one of those properties. The strength of a vibration determines the amplitude of a sound wave. The amplitude of a sound wave is its height and determines the volume of the sound. The greater the amplitude of the sound wave, the greater the volume of the sound it causes. The amplitude or volume of a number of common sounds are shown in the colorful diagram to the right. As you can see, the amplitude of sound is measured in decibels (dB), We will discuss decibels again briefly.

Figure 2.1 (below) illustrates the concept of amplitude. Scientists use this type of wave diagram to illustrate sound waves. In Figure 2.1, the amplitude of figure 2.1a is greater than 2.1b. Therefore, the volume of the sound represented in figure 2.1a is louder than the volume of the sound represented in figure 2.1b.

The metric unit for measuring sound volume is the decibel (dB), named for Alexander Graham Bell, the inventor of the telephone. Ten decibels equal 1 bel; however, decibels are used for the majority of sounds because the magnitude of a bel is so great. The decibel scale is used to rate sounds on their level of intensity. A whisper has a volume equal to 20 to 30 dB. Normal conversation is rated at 60dB. A sound volume of 120 dB is capable of producing pain in the listener.

Pitch and Frequency

The other property unique to sound is pitch. The pitch of a sound is its highness or lowness and is related to the frequency of the sound wave. The frequency of a sound wave is defined as the cycles per second of the wave. A cycle is the part of the wave generated as the result of one complete vibration. One cycle per second is one hertz (Hz). In Figure 2.2, a cycle is the section of the wave bracketed by the bars at the top of the peaks.

In Figure 2.1, the two sounds have the same pitch because they both have the same number of cycles per second. In Figure 2.3, the two sounds have different pitches. Figure 2.3a has a greater frequency and a higher pitch than the sound shown in Figure 2.3b. However, they both have the same amplitude, so their loudness or volume would be similar.

The ability to hear different frequencies varies between animal species. Humans can hear sounds ranging in frequency between 50 and 15,000 Hz. The human ear is most sensitive to sounds in the range of 3000 Hz. Hearing loss occurs when an animal is no longer sensitive to frequencies within a certain range of hearing. In humans, normal hearing is classified as being able to hear sounds across a range of frequencies in the 0 to 20 dB volume range. People who are considered profoundly deaf cannot hear any sound softer than 95 dB. There are varying degrees of hearing loss.

You can listen to several sound frequencies by clicking on the audio files below. Be sure that the volume is turned up on your device to a comfortable listening level.