Sound

EXPERIMENT TO FIND THE SPEED OF SOUND

Sound travels much more slowly than light in the air. We can use this to measure how quickly the sound does travel. starting pistol:

Person A makes a loud sound and produces a visual signal at the same time, this could be by firing a starting pistol or by banging large cymbals together.

Person B starts a stopwatch when they see the sound being made and stops the stopwatch when they hear the sound. They can work out the

speed of sound using this formula:

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Distance between the two people/m

speed of sound = time measured/s

Properties of sound waves

Sound waves fly through the air at about 340 m / s – much slower than light speed. That’s why you almost always see the lightning flash before you hear the thunder crack.

The echo is due to waves, a violin or cello front, or the echo.

Within a trumpet, the column of air. When using a loudspeaker, you can observe the loudspeaker’s cone going in and out and changing the air pressure. The tone is longitudinal.

The sound goes along. Tiny changes in air pressure arise from compression waves in sound waves.

Sound waves move by liquids more rapidly than by air. Sound moves through solids most easily. The explanation is that particles are most closely related to solids.

However, remember that the sound must be viable on a medium. In comparison to electromagnetic waves, the sound does not move into a vacuum.

Sounds with a high pitch have a high frequency. Examples of high pitch sounds include bird songs and all the sounds you hear when the volume is too high from someone else’s player.

Sounds of the low pitch have a low frequency. The horn of a big ship and the bass guitar are examples of low-pitch effects. Sounds with pitches between 20 Hz and 20 000 Hz can be heard by the human ear.

The sound is known as ultrasound with frequencies above this range. Bats for sailing and doctors for looking at newborn infants are used for the ultrasound.

The ear is far more easily damaged than most people realize, and care needs to be taken both with the volume of sound and the length of time that the ear is exposed to it.

The damage is cumulative, and so you don’t notice it at first. Many older rock stars have serious hearing problems, and younger ones often wear earplugs. Loud sounds have high amplitude whereas quiet sounds have a low amplitude.

The unit in which we measure the loudness of sounds is the decibel (dB). Decibels are used to measure various electrical quantities as well, but when they are used for sound, 0 dB is defined to be the quietest sound that can be heard.

This then makes a quiet room at night about

40 dB; a noisy classroom is 60 dB; the sound 1 m from a vacuum cleaner is 80 dB; a loud disco could be 100 dB, which would be illegal in many countries; sounds of 120 dB are extremely painful, and windows break at 160 DB.

The ears start to be damaged at 85 dB, a level that some personal players

can reach without difficulty

Sound waves can be displayed on an oscilloscope by using a microphone. This produces a voltage-time graph on the oscilloscope.

• amplitude

• vibrating

• loudspeaker

• microphone

• oscilloscope

PROPERTIES OF WAVES, INCLUDING LIGHT AND SOUND

Typical results are shown in the graphs below.

(a) A loud sound of low frequency

(b) A loud sound of high frequency

(c) A quiet sound of low frequency

(d) A quiet sound of high frequency.

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ECHOES

Hard surfaces reflect sound waves. An echo is a sound that has been reflected before you hear it. For an echo to be heard, the obstacle needs to be large compared with the wavelength of the sound.

So, you will hear an echo if you make a loud noise when you are several hundred meters from a brick wall or a cliff, for example. You will not hear an echo if you are several hundred meters from a pole stuck in the ground.

There will still be an echo, even if you are much closer to the wall, but because sound travels very quickly, the echo will return in such a short time that you will probably not be able to distinguish it from the sound that caused it.

Measuring the speed of sound by an echo method

The following worked example illustrates how echoes may be used to measure the speed of sound.