This section will cover the following topics:

Laws of Reflection

The laws of reflection are the same for all types of waves, including light and sound. The diagram below shows light reflecting in a mirror:

Laws of Reflection

  1. The angle of incidence is equal to the angle of reflection
  2. The incident ray, the reflected ray, and the 'normal' all lie in the same plane

Note: the 'normal' is a line drawn at 90 degrees to the surface of the reflector, at the point where the incident ray hits. Angles of incidence and reflection are normally (no pun intended!) measured between the ray and the 'normal', not between the ray and the surface of the reflector.

This all works OK for flat surfaces - normal mirrors around the house, or flat walls reflecting sound waves in an acoustic application. What if the 'mirror' is not flat?

Curved Reflectors

If a curved surface is used to reflect waves, they can be focused onto a point. The diagrams below show both a spherical and parabolic mirror shape:

Diagrams showing two curved mirrors. A Spherical mirror with a rounded curve focuses the waves at a central point in front of the mirror. A Parabolic mirror with a steaper curve focuses the waves at a central point much closer to the front of the mirror.

Parabolic mirrors are especially useful, and they have one focus point. If a bulb or LED is placed at the focus, a straight 'beam' of light can be formed which travels for long distances (e.g. in torches and car headlamps). Curved satellite dishes are used to transmit or receive radio waves. Even sound can be focused with a curved surface - for instance whispering galleries.

Elizabeth Palmer Centre

The above photo shows a music recital room. There is a big concave surface forming one of the walls, which without treatment, would have caused sound to be focussed to a particular place in the room leading to the sound being uneven across the room. To deal with this, a surface diffuser has been added, if you look carefully you can see that the surface is wiggly. This breaks up the focussing and removes the distortion that the curved surface would have caused by reflecting the sounds into lots of different directions. The diffuser used to treat the recital room is shown below:

The diffuser used to treat the recital room

Echoes

When you shout near a tall building or under a bridge, the sound is reflected back from the walls. You hear this reflected sound as an echo. The time it takes for the echo to reach you can be used to calculate your distance from the wall. If the sound takes one second to go to the wall and back again how far away is the wall? Speed of sound = 330ms

ANSWER: Time taken for sound to travel to the wall = 1 seconds / 2 = 0.5 seconds

Speed = 330ms

Distance = speed x time taken = 330 x 0.5 = 165 metres

Echoes are a problem in large concert halls. If a trumpet plays on the stage, the sound can reflect off the back wall and return to the front of the seating (stalls) still quite loud. Sometimes this sound can be heard by the audience as an echo. To overcome this problem, absorbs can be put on the rear wall to stop the sound reflecting, as was done in the Royal Festival Hall, London shown below. Nowadays, it is more normal to use diffusers to disperse the reflection causing the echo.

RFH

Echoes are used in Sonar and Radar. Both systems send out waves (sound waves in water and electromagnetic waves in air, respectively) and record the time it takes for the reflections to arrive back. From this they can detect nearby reflective objects. 'Stealth' devices are those which attempt to make an object non-reflective - if the waves do not bounce off your aircraft, then no-one will know it is there! In a way, glass is a 'stealth' technology - light waves pass through it, and if it is very clean (and also if there are no reflections from the surface, as is sometimes the case depending on the angle of incidence of the light) we may walk straight into it before realising that it is there!

You also get sound radar, called SODAR which is used to detect wind speed and temperature in the atmosphere.

SODAR image

Sodar image from Antarctic research

And, yes, a duck's quack does echo

Ultrasound

An adaptation of sonar is used to create images of structures inside the human body - this is called medical ultrasound. Ultrasound is very high-frequency acoustic energy, typically between 1 and 3MHz - much higher than the frequency range audible to humans (20-20kHz). This high frequency means that the patient is not disturbed by the noise, and high amplitudes can be used, but primarily ultrasound is chosen because high frequency sounds have small wavelength. This small wavelength allows a very high degree of image detail to be recorded.

Ultrasound scan

The speed of sound in air is much less than that in water (and the human body is mostly water!). This means that there is an acoustic impedance difference between the air and the body. This difference would normally mean that a large part of the ultrasound energy is reflected away from the body and wasted. To prevent this, the transducer that produces the ultrasonic waves is placed on the skin using a special gel. The speed of sound in the gel is part way between that in air and water, and it therefore creates a smooth transition for the sound waves resulting in less reflected (wasted) energy. This is an example of impedance matching.

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