Sound Waves. Intermediate 1 Physics. Intermediate 1 Physics. Sound and Music. Frequency. Frequency. Sound Waves. Int 1 Physics - Telecommunication 1

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1 Sound Waves Speed of Sound Using Sound Sound Waves 1 2 Sound Waves Vibrations Musical instruments produce sound when part of the instrument vibrates. A guitar string vibrates when it is plucked. Vibrations A trumpet blasts out sound when the air inside it vibrates Energy is transferred from the vibrating object to the listener by sound waves which travel through the air. These sound waves make the air vibrate. 3 4 Frequency Frequency Each note or sound has a frequency which is measured in hertz (Hz). A tuning fork has the frequency engraved on it. Frequency = number of waves produced in one second. This vibrating fork will produce 256 vibrations every second. This means that 256 sound produced in one second. waves are 5 6 Int 1 Physics - Telecommunication 1

2 Frequency Detecting Sound A whistle produces a higher frequency sound than a vibrating drum. Sound can be taken in by a microphone and a trace displayed on an oscilloscope. The whistle has a much higher pitch. The higher the pitch means the higher the frequency. tuning fork sound waves travel from fork to microphone microphone Oscilloscope 7 8 Loud and quiet Loud and quiet A loud sound transfers more energy so the oscilloscope trace will have a large amplitude. The amplitude of a wave is the distance from the middle of the wave to the top or bottom of the wave. Loud note has large amplitude 9 10 Loud and quiet A soft/quiet sound transfers less energy so the oscilloscope trace will have a small amplitude High and Low Frequency The effect of changing the frequency of a note can be seen on the oscilloscope screen. Sound A has low frequency Sound B has high frequency Quiet note has small amplitude Sound B has a higher frequency than sound A Int 1 Physics - Telecommunication 2

3 High and Low Frequency If the frequency is doubled, we say that sound B is an octave higher than sound A. Changing the Note A musician tunes a guitar by making the string tighter or looser. frequency: (Hz) one octave t _ two _ octaves While playing the guitar, the note is changed by altering the length of string which vibrates Changing the Note Short strings produce a higher frequency than long strings. Tight strings produce a higher frequency than slack strings. Changing the Note An organ produces a variety of musical notes which depend on the length of the air column which vibrates. The shorter pipes have the shorter air columns which produce the higher frequency notes Speed of Sound Speed of Sound During a thunderstorm, the lightning flash is seen a short time between the roll of thunder is heard. This is due to the fact that light travels much faster than sound through the air. In air: Speed of light = m/s Speed of sound = 340 m/s Int 1 Physics - Telecommunication 3

4 Calculating the Speed of Sound In a laboratory the speed of sound can be calculated using the formula below. Speed of Sound : Apparatus speed = distance time Hammer different microphone A electronic timer microphone B (meaning; speed equals distance divided by time) Speed of Sound A loud sound is made. As the sound reaches microphone A, the timer starts. When the sound waves reach microphone B, the timer stops. The distance between the microphones is measured with a meter stick. The experiment can be repeated to calculate a more accurate, average result. 21 Speed of Sound Example: Recorded times: s, s, s, s, s Average time = /5 = s Distance travelled = 1 metre Speed = distance/time = 1.0/ = m/s 22 Using Sound Using Sound Sound can travel through and gases. solids, liquids The only thing that sound cannot pass through is a vacuum. A vacuum is an empty space, so there are no particles to pass on the vibrations Int 1 Physics - Telecommunication 4

5 Using Sound When listening to music, you hear sounds with a wide range of frequencies. We can detect sounds that range from about 20 hertz to hertz. As we get older, the upper limit gradually falls to about hertz. Some animals can detect higher frequency sounds than humans. 25 Using Sound - Ultrasound Sound beyond the upper limit of human hearing ( Hz) is called ultrasound. Ultrasound can be used in hospitals to scan the baby in the mother's womb. This can be used by a computer to produce an image of the baby on a screen. 26 Using Sound - Ultrasound A system called sonar is used by fisherman at sea. The ultrasound signal is transmitted towards the sea bed and an echo is detected. Sound Levels The sound level of a noise means how loud it is. The sound level varies depending on the source of the sound. Shoals of method. fish are located by this Sound level is measured in decibels (db) Sound Levels Sound level (db) 130 Jet engine at 50 metres 100 Pneumatic drill 70 Rush hour traffic 0 Silence Sound Levels When sound levels rise to unacceptable levels, the problem is described as noise pollution. There are many sources of noise pollution, such as aircraft noise or pneumatic drills. Exposure to high sound levels over a long time can damage our hearing. Listening through stereo headphones will produce a harmful effect if the volume is turned to maximum Int 1 Physics - Telecommunication 5

6 Amplified Sound A stereo system can be used to listen to a compact disc (CD) or a radio programme. No matter what function is selected, the signal passes through an amplifier which turns a small amplitude electrical signal into a higher amplitude one small amplitude input signal Amplifier large amplitude output signal When announcements are made at airports and stations, the message has to be changed from sound into electrical signals and back to sound. Only the amplitude of the signal is changed. The frequency does not change. When announcements are made at airports and stations, the message has to be changed from sound into An electronic system of 3 parts is used to amplify the signal given out by the announcer sound Input microphone Process amplifier Output loudspeaker sound with increased amplitude electrical signals and back to sound Part of the system m microphone _ amplifier loudspeaker Function converts sound into an electrical signal makes the electrical signal stronger (bigger amplitude) converts the electrical signal back to sound Voltage Gain The performances of different amplifiers can be compared by finding the gain of the amplifier. 0utput voltage Voltage gain = Input voltage Int 1 Physics - Telecommunication 6

7 Voltage Gain Example: Find the voltage gain of the following amplifier. Voltage Gain Voltage gain = 0utput voltage Input voltage input 0.01V output 4V = = 400 N.B. Voltage gain is just a number. It has no unit. unit Playback A recording of your own voice will sound a bit strange when it is played back from a cassette. This is because we normally hear our own voice by: vibrations of bones in our skull and vibrations in the air being detected by our ears When we listen to a recording of our own voice we hear it by: vibrations in the air being detected by our ears only. Playback A CD (compact disc) is a source of high quality sound. As the CD spins inside the CD player, light from a laser reflects off the CD. There is no wear and tear of the disc surface and there is very little background noise End of Unit 41 Int 1 Physics - Telecommunication 7