Frequency and Amplitude
The frequency of a wave is a measure of the number of vibrations per second the particles of the medium undergo as the wave passes through the medium. A full vibration is referred to as a cycle because the particles of the medium don’t really go anywhere; they simply vibrate back and forth in a repeating motion as the wave passes through the medium. Frequencies are measured in hertz (Hz), where 1.0 Hz = 1.0 cycle per second. The frequency of a sound wave determines the ‘pitch’ of the sound, i.e. how high or low the sound is. The frequency determines the wavelength of the sound waves as they travel through the air. Higher frequencies have a shorter wavelength than lower frequencies. i.e. high pitch sounds have shorter wavelengths while lower pitch sounds have longer wavelengths.
The amplitude is the maximum displacement of the particle from its equilibrium position and for sound waves corresponds to the maximum difference in the air pressure at the centre of a compression or rarefaction when compared with the original ambient air pressure. The amplitude of a wave is a direct measure of the amount of energy being transferred by the wave. For a sound, the amplitude of the sound wave determines the volume of the sound.
Questions & Answers
1. Identify what determines the energy carried by a particular wave.
Higher frequency the more energy the wave has.
2. Identify the property of a sound wave that determines the:
(a) volume of the sound > amplitude
(b) pitch of the sound > frequency
3. If the waves are travelling in the same medium, compare waves of different frequency.
The wave of the higher frequency wave has more force behind it. It will travel further at a higher speed
influence the sounds produced.
Workshop Day Term 1
Experiment 2: Frequency and amplitude
Introduction
From the syllabus:
- perform a first-hand investigation to gather information about the frequency and amplitude of waves using an oscilloscope or electronic data-logging equipment
The frequency of a wave is a measure of the number of vibrations per second the particles of the medium undergo as the wave passes through the medium. A full vibration is referred to as a cycle because the particles of the medium don’t really go anywhere; they simply vibrate back and forth in a repeating motion as the wave passes through the medium. Frequencies are measured in hertz (Hz), where 1.0 Hz = 1.0 cycle per second. The frequency of a sound wave determines the ‘pitch’ of the sound, i.e. how high or low the sound is. The frequency determines the wavelength of the sound waves as they travel through the air. Higher frequencies have a shorter wavelength than lower frequencies. i.e. high pitch sounds have shorter wavelengths while lower pitch sounds have longer wavelengths.
The amplitude is the maximum displacement of the particle from its equilibrium position and for sound waves corresponds to the maximum difference in the air pressure at the centre of a compression or rarefaction when compared with the original ambient air pressure. The amplitude of a wave is a direct measure of the amount of energy being transferred by the wave. For a sound, the amplitude of the sound wave determines the volume of the sound.
In this experiment you will use a microphone attached to a cathode ray oscilloscope (CRO), or a data-logger with computer, to examine the display when sounds of different frequency and volume are detected by the microphone.
Purpose
To conduct an analysis of sounds of different frequency and different volume using a microphone attached to a CRO or data-logger.
Required equipment
o cathode ray oscilloscope or suitable data-logger and computer
o suitable microphone or sound sensor
o audio oscillator or signal generator and speaker
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Discussion points
The audio oscillator contains an electric circuit which can produce vibrations over a wide range. When the speaker is switched on, it converts the electrical waves from the circuit into sound waves. A signal generator also uses an electric circuit to produce different frequencies. By attaching a speaker to the signal generator, the electrical wave is converted into a sound wave. When using a signal generator, ensure that it is set to produce a sine wave. |
Procedure
o Set up the CRO or data-logger according to instructions and attach a suitable microphone.
o Check the display to determine that the microphone is sending a signal to the CRO or data-logger.
o Adjust the CRO or data-logger as required so that a clear display of the sound detected by the microphone can be observed.
o Set the audio oscillator or signal generator to a low frequency (100-200 Hz) and adjust the signal strength to a maximum value. Switch on the speaker so you can hear the sound being created. Adjust the signal strength to produce a volume convenient for your situation.
o Place the microphone near the speaker and adjust the display of the CRO or data-logger until the wave is easily seen in the display.
o In the space provided in the Results section, note the frequency on the audio oscillator or signal generator and draw a sketch to show the appearance of the display.
o For the second trial do not change the frequency of the audio oscillator or signal generator.
o With the speaker on and the microphone in the same position as in the first trial, and while observing the display on the CRO or data-logger, slowly reduce the strength of the signal from the audio oscillator or signal generator. Note any variation in the sound you hear from the speaker.
o In the space provided in the Results section, draw a sketch to show what happened to the appearance of the CRO or data-logger display, and note any effect in the sound heard as the signal strength was reduced.
o For the third trial, adjust the signal strength of the audio oscillator or signal generator to the original value so the sound produced is clearly audible.
o Position the microphone and then, while observing the display on the CRO or data-logger, slowly increase the frequency being produced by the audio oscillator or signal generator until it is about twice the original frequency, i.e. it will now be about 200-400 Hz depending on your original choice.
o In the space in the Results section, draw a sketch of the CRO or data-logger display and record any observations you made on the effect of increasing the frequency of the sound.
Results
For each trial, draw a simple neat sketch to show the appearance of the screen caused by the sound and make a short note on other observations.
Figure 1.5 Appearance of screen for low frequency sound that is easily heard.
Frequency of sound: ___________200__________________________________________
Description of sound: _____________loud________________________________________
Figure 1.6 Appearance of screen for same frequency sound as signal strength reduced.
Observation of sound: ________quieter______________________________________
__________________________________________________________________
Figure 1.7 Appearance of screen for higher frequency sound that is easily heard.
Frequency of sound: ________________500_____________________________________
Observations: _____________________wave length smaller______________________________
Analysis
1. On the basis of your observations in Figures 1.5 and 1.6, describe what effect changing the amplitude of a wave has on:
(a) the volume of the sound >becomes louder
(b) the pitch of the sound>nothing
2. Describe what occurred in the pitch of the sound as the frequency of the signal was increased.
When the frequency was increased it went into a higher pitch
3. Explain how the appearance of the display on the CRO or data-logger could be used to identify sounds with:
(a) the same frequency but different volume
The height of the wave increased but the wave length stayed the same
(b) the same volume but higher pitch
the wave length became shorter but the amplitude stayed the same
Questions
1. Identify what determines the energy carried by a particular wave.
Higher frequency the more energy the wave has.
2. Identify the property of a sound wave that determines the:
(a) volume of the sound > amplitude
(b) pitch of the sound > frequency
3. If the waves are travelling in the same medium, compare waves of different frequency.
The wave of the higher frequency wave has more force behind it. It will travel further at a higher speed
Conclusions
Write a brief conclusion to your experiment, being sure to identify how changes in volume and frequency influence the sounds produced.
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