Low—pitched sounds have longer wavelengths, so the peaks are more spread out. Humans cannot hear very high-pitched sounds. These are called ultrasound. Ultrasound is used for medical scanning and testing materials.
Doctors use ultrasound to check the health of a baby while it is still in the womb. A computer uses the reflected sound waves to create a black-and-white picture of the growing baby. The picture here has been colored in to show the baby more clearly.
Describe what pitch is and how it varies. Describe what volume is. Materials Per Class: tuning forks rubber mallet or the rubber bottom of a shoe resonance box optional If using this as an activity, provide the materials above for each pair of students. Key Questions Why do different sized tuning forks produce different sounds? What To Do Show the class a selection of different tuning forks. Ask the class to predict which fork will have the highest and lowest sound pitch. Strike the forks one at a time to determine the answer.
Brainstorm with the class on how the tuning fork could make a louder sound. Strike the fork with more force. Strike the fork and place the handle on the table. Optional if you have a resonance box : Strike the fork and place the handle on the box. Brainstorm with the class on how the tuning fork could make a softer sound.
Extensions Get the students to touch various objects with their tuning fork to see whether the sound becomes louder or softer. How do instrument builders decide the shape, size, and material of their instruments? You can model how the length of a tuning fork affects the frequency by placing a plastic ruler over the edge of a table. Try it with a shorter length. What do you notice? If a particle of air undergoes longitudinal vibrations in 2 seconds, then the frequency of the wave would be vibrations per second.
A commonly used unit for frequency is the Hertz abbreviated Hz , where. As a sound wave moves through a medium, each particle of the medium vibrates at the same frequency. This is sensible since each particle vibrates due to the motion of its nearest neighbor. The first particle of the medium begins vibrating, at say Hz, and begins to set the second particle into vibrational motion at the same frequency of Hz. The second particle begins vibrating at Hz and thus sets the third particle of the medium into vibrational motion at Hz.
The process continues throughout the medium; each particle vibrates at the same frequency. And of course the frequency at which each particle vibrates is the same as the frequency of the original source of the sound wave.
Subsequently, a guitar string vibrating at Hz will set the air particles in the room vibrating at the same frequency of Hz, which carries a sound signal to the ear of a listener, which is detected as a Hz sound wave.
The back-and-forth vibrational motion of the particles of the medium would not be the only observable phenomenon occurring at a given frequency. Since a sound wave is a pressure wave , a detector could be used to detect oscillations in pressure from a high pressure to a low pressure and back to a high pressure. As the compressions high pressure and rarefactions low pressure move through the medium, they would reach the detector at a given frequency.
For example, a compression would reach the detector times per second if the frequency of the wave were Hz. Similarly, a rarefaction would reach the detector times per second if the frequency of the wave were Hz. The frequency of a sound wave not only refers to the number of back-and-forth vibrations of the particles per unit of time, but also refers to the number of compressions or rarefactions that pass a given point per unit of time.
A detector could be used to detect the frequency of these pressure oscillations over a given period of time. The typical output provided by such a detector is a pressure-time plot as shown below. Since a pressure-time plot shows the fluctuations in pressure over time, the period of the sound wave can be found by measuring the time between successive high pressure points corresponding to the compressions or the time between successive low pressure points corresponding to the rarefactions.
As discussed in an earlier unit , the frequency is simply the reciprocal of the period. For this reason, a sound wave with a high frequency would correspond to a pressure time plot with a small period - that is, a plot corresponding to a small amount of time between successive high pressure points.
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