Sound waves

When an object or substance vibrates, it produces sound. These sound waves can only travel through a solid, liquid or gas. They cannot travel through empty space. ( mechanical wave)

Sound waves are longitudinal waves - the vibrations are in the same direction as the direction of travel.

A  sound wave traveling through air will cause a pressure variation in the air. 
 A pressure-time plot shows the fluctuations in pressure over time

Oscilloscope traces

The graphs shown by an oscilloscope are called oscilloscope traces

The diagrams show  some typical oscilloscope traces for sound:

A snapshot from oscilloscope traces of three sounds - 1 is a shallow wave with 2 peaks and 1 trough, 2 is a normal wave with 2 peaks and 1 trough, 3 is a deep wave with 3 peaks and troughs.

Oscilloscope traces for sound waves

For a wave:

    • the amplitude is the maximum height of the wave from its resting position – the greater the amplitude, the louder the sound
    • the wavelength is the distance between the crests (tops) of two waves next to each other (or any other two identical point on waves next to each other)
    • the frequency is the number of waves per second – the higher the frequency, the closer together the waves are and the higher the pitch
  • Diagram 1 and 2 show two sounds with the same wavelength and frequency, so they will have the same pitch.
  • The sound in diagram 2 has a greater amplitude than the one in diagram 1, so it will be louder.
  • Diagrams 2 and 3 show two sounds with a different frequency. The sound in diagram 3 has a higher frequency than the one in diagram 2, so its pitch will be higher. 
  • So we can define the pitch of the sound as The frequency of a sound. Sounds with a high pitch have a high frequency.

Frequency, Pitch and Human Perception

 the human ear is capable of detecting sound waves with a wide range of frequencies, ranging between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an ultrasound

YouTube Video

Usage of Ultrasound 

Ultrasound waves have frequencies above about 20kHz (20,000Hz). This is above the normal hearing range for humans,                    so we cannot hear ultrasound

1- Ultrasonic cleaning

Is the rapid and complete removal of contaminants from objects by  immersing them in a tank of liquid flooded with high frequency sounds waves. These non-audible sound waves create a scrubbing brush action within the fluid.

YouTube Video

 Ultrasonic distance measurement by echo 
(e.g. to find the depth of the ocean floor – this is probably the simplest example) 

Reflections of sound:

When ultrasound waves reach a boundary between two substances with different densities, they are partly reflected back. The remainder of the ultrasound waves continue to pass through. A detector placed near the source of the ultrasound waves is able to detect the reflected waves. It can measure the time between an ultrasound wave after leaving the source to reach the detector. The further away the boundary, the longer the time between leaving the source and reaching the detector:

distance (metre, m) = speed (metre/second, m/s) × time (second, s)

For example, sound travels through water at about 1,400 m/s. If it takes 0.5 s for a sound to reach a boundary and reflect back to the detector, the total distance travelled is:

distance = speed × time

            = 1,400 × 0.50

            = 700m

The distance to the boundary is half this, which is 350m.

The distances and times are much smaller of course when scanning babies and in engineering jobs

  medical imaging (e.g. scans in pregnancy)  medical treatments (e.g. high intensity focused ultrasound)
Sonar is used on ships and submarines to detect fish or the sea bed.
 A pulse of ultrasound is sent out from the ship. 
It bounces off the seabed or shoal of fish and the echo is detected. 
The time taken for the wave to travel indicates the depth of the seabed or shoal of fish.


      silent dog whistle:

YouTube Video

 ultrasonic cleaning  
use of sonar to find fish (this provides a cross-curricular link to the problem of overfishing using technology)