Agreeing on loudness
Classroom Activity for 11-14
What the Activity is for
The aim of this activity is to make the link between the loudness of a sound and the magnitude of the vibration that produces it. The louder the sound, the bigger the vibration (the bigger the amplitude of vibration). The decibel scale is then introduced as a standard scale for measuring loudness.
What to Prepare
- a wobble board
- a selection of pre-recorded sounds, or the means for making them. You will need to be able to alter the loudness of the sound.
- a laser beam, reflecting from a strip of plastic mirror stuck to a vibrating loudspeaker. Make sure the reflected beam has a safe path to a distant wall.
- an oscilloscope, or computer-based oscilloscope, connected to a microphone, with the time-base switched off.
- a decibel meter, preferably linked to a large display.
Safety note: Lasers approved for school use should be used. Ensure that the beam cannot enter anyone's eye either directly or by reflection. A laser pointer can be used under the careful control of a teacher but be aware that some are marked with an incorrect power rating and so are more hazardous than they might appear.
What Happens During this Activity
Start with the wobble board, keeping the frequency the same, but driving the board into larger vibrations. Make the connection between this extra movement and a louder sound. You may need to practice this.
Now use the loudspeaker and laser beam to make the link between the amount of movement of the loudspeaker cone (shown by the movement of the laser dot on the wall) and the loudness of the sound. As you turn up the loudness control, the laser beam will move back and forth over a greater distance on the wall.
You can then use the microphone and oscilloscope to show the increase in amplitude on the screen as the sound gets louder. Play louder and softer sounds into the scope and watch the size of the vibrations increase and decrease.
Finally, introduce the decibel scale by demonstrating how a large decibel meter can be used to measure the loudness of the sounds. It is a good idea to demonstrate the link to decrease in loudness as you move further from the source. Also make connections between your readings taken in the lab and common values between the 0 dB threshold of hearing and the 140 dB threshold of pain.