[Keywords] Sound

Important Facts That You Must Know First!

1. You need to first identify whether the question is asking you about production or transmission of sound. When you discuss about production of sound, it is essential to identify the vibrating source and substantiate why the source is vibrating. For transmission of sound, it is important to describe the longitudinal nature of sound waves in terms of the processes of compression and rarefaction.
2. Speed of sound depends on medium it is in, temperature and humidity. Sound travels faster in higher temperature. For instance, at 0 degree Celsius, speed of sound in air is about 331 m/s. At room temperature, the speed increases to about 340 m/s. To understand the change, you need to think about the mechanism of transmission of sound.
3. Sound can be reflected by large, flat and hard surfaces like walls and cliffs. Note that large obstacle is essential so that sound waves do not pass around the edges which will cause refraction of sound. Being large, it means the sound will be reflected, absorbed and/or transmitted.
4. When solving questions involving echo, we ignore the echo coming off the ground beneath our feet because the duration for that to happen is too fast for our ears to detect. This is because any two identical sounds reaching our ears within 1/15 second or less cannot be distinguished as two separate sounds.
5. When quoting a frequency of sound that can be considered as ultrasound, the value must be greater than 20,000 Hz. Stating 20,000 Hz will be considered as wrong since that value is our upper limit of hearing frequency.
6. The Moon has no atmosphere and thus sound cannot travel/propagate.

Commonly Seen Questions

• Explain how sound is produced by a vibrating tuning fork and can be heard by a person nearby.

Production: When a tuning fork is struck (that’s why it is vibrating), the two prongs vibrate (identify the source of vibration) with repeated inward-outward (or forward-backward) motions. These motions cause the layers of air nearby to be displaced by getting pushed and pulled repeatedly.

Transmission: The displaced layers of air around the vibrating tuning fork displaces the neighbouring air molecules and creates a series of compressions and rarefactions that travels parallel to the direction of the vibration of the air particles, forming a longitudinal wave with air molecules transferring energy from molecule to molecule until reaching the ears of the person.

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• Figure above shows how ultrasound is being used to study an unborn baby. Describe how the vibrations of the source produce waves of ultrasound and how these waves are transmitted through the body tissues to the receiver.

Production: When the ultrasound source vibrates with repeated inward-outward motions, these motions cause the layers of body tissue nearby to be displaced by getting pushed and pulled repeatedly at the rate of more than 20, 000 complete oscillations per second.

Transmission: The displaced body tissues displace neighbouring tissues and create a series of compressions and rarefactions that travel parallel to the direction of the vibration of the source. Ultrasound travels as a longitudinal wave through the body tissues. When ultrasound hits the denser body tissues, it is reflected back through the body tissues and detected by the receiver.