## SI Units

Production of Sound

Sound is a result of vibrating objects that cause a vibration in air molecules that lead to a series of compressions and rarefactions.

Recall the sound is a longitudinal wave. Compressions are when air molecules are closest together and rarefactions are when they are furthest apart.

We hear sound when the sound waves reach our ear and cause our ear drums to vibrate. We can hear frequencies from about 20 Hz to 20 kHz – Any more or any less is very difficult for our ears to pick up.

Remember that all waves (including sound) have a frequency and an amplitude:

• The frequency of a wave is the number of waves that pass a fixed spot per second (unit=Hz). The higher the frequency, the higher the pitch of the sound.

The amplitude of the wave is the maximum displacement of the vibrating particles. The larger the amplitude, the louder the sound will be.

Speed of Sound

Speed of sound through various mediums – Sound waves cannot travel through vacuum. They must be transmitted through vibrations of particles within a medium. The closer the particles are within the medium, the faster sound will travel. For example, the speed of sound is:

• 330 m/s in gases i.e. air
• 1500 m/s in liquids i.e. water
• 5000 m/s in metals i.e. metals

Air is a gas so particles are very spread out, which is why sound does not travel very fast. Metals on the other hand are generally solids, and particles are very closer together allowing fast transmission of sound waves.

Determining the speed of sound in air – Above we state that the speed of sound is approximately 330 m/s in air. We can experimentally prove this by using this set-up:

Two microphones are separated by exactly 1m. They are connected to a digital timer that starts when it gets signal from microphone 1 and stops when it gets signal from microphone 2.

A hammer is used to hit a metal block which generates sound. The sound waves will hit microphone 1 first which starts the timer, and then hit microphone 2 which stops the timer.

Since the microphones are separated by 1m, the sound has traveled 1m in 0.003 seconds.

Since speed = distance / time, 1/0.003 = 330 m/s

Echoes

Just like light waves can be reflected off a boundary, so can sound waves. When sound gets reflected off a surface, it generates an echo.

ULTRASOUND:

IN MEDICINE:

Used to construct image of foetus or organs not surrounded by bone and kidney stones.

A Transducer (a machine) produces a beam of Ultrasound waves into body. The waves are reflected back to the transducer by boundaries between tissues in beam path. When echoes hit the transducer, it creates signals that are sent to the detector. The Detector calculates the distance from the transducer to the tissue using the speed equation (speed = distance/time). By taking many time measurements and calculating many distances for a constant wave speed, an image of the foetus can be built.

IN INDUSTRY:

Ultrasounds can help Check for cracks in metal objects

Cracks causes ultrasound wave to reflect, and for a constant speed, we take several time measurements to calculate the distance to the crack from the transducer.

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