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Inleiding

Nou dat ons die basiese beginsels van longitudinale golwe bespreek het, is ons gereed om klankgolwe meer volledig te bestudeer.

Het jy al ooit gewonder oor hoe ongelooflik jou gehoor is? Dit is eintlik heel merkwaardig. In jou omgewing is daar baie verskillende tipes klank: toeters, ’n laggende baba, ’n blaffende hond en tog kan jou brein hulle almal op een of ander manier verwerk. Alhoewel al hierdie dinge vir jou ingewikkeld mag klink, word dit eintlik nogal eenvoudig sodra jy ’n baie maklike feit geleer het. Klank is bloot ’n golf, daarom kan jy alles wat jy van golwe af weet gebruik om klank te verstaan.

Eienskappe van ’n Klankgolf

Aangesien klank ’n golf is, kan ons die eienskappe van klank in verband bring met dié van golwe. Die basiese eienskappe van klank is: toonhoogte, hardheid en toon.

Figuur 1: Toonhoogte en hardheid van klank. Klank B het 'n laer toonhoogte (laer frekwensie) as klank A en is sagter (kleiner amplitude) as klank C.
(a)
Figuur 1(a) (PG11C5_001.png)
(b)
Figuur 1(b) (PG11C5_002.png)
(c)
Figuur 1(c) (PG11C5_003.png)

Toonhoogte

Die frekwensie van 'n klankgolf is wat jou oor as toonhoogte verstaan. 'n Hoër frekwensie klank het 'n hoër toonhoogte en 'n laer frekwensie klank het 'n laer toonhoogte. In figuur 1 het klank A 'n hoër toonhoogte as klank B. Byvoorbeeld: die skril gekwetter van 'n voëltjie sal 'n hoë toonhoogte hê, maar die diep gebrul van 'n leeu sal 'n lae toonhoogte hê.

Jou oor kan 'n wye reeks frekwensies bespeur (optel). Frekwensies van 20 tot 20 000 Hz is hoorbaar deur die menslike oor. Enige klank met 'n frekwensie onder 20 Hz staan bekend as 'n infraklank en enige klank met 'n frekwensie bo 20 000 Hz word 'n ultraklank genoem.

tabel 1 gee 'n aanduiding van die gehoorbestek van party diere in vergelyking met die mens.

Tabel 1: Omvang van frekwensies
  onderste frekwensie (Hz) boonste frekwensie (Hz)
Die Mens 20 20 000
Honde 50 45 000
Katte 45 85 000
Vlermuise 20 120 000
Dolfyne 0,25 200 000
Olifante 5 10 000

Ondersoek: Bestek van Golflengtes

Deur van die inligting in tabel 1 gebruik te maak, bepaal die onder- en bogrense van golflengtes wat elke spesie kan hoor. Neem aan dat die spoed van klank in lug 344 m s-1-1 is.

Hardheid

Die amplitude van ’n klankgolf word bepaal deur sy hardheid of volume. ’n Groter amplitude beteken ’n harder klank en ’n kleiner amplitude beteken weer ’n sagter klank. In figuur 1 is klank C harder as klank B. Die vibrasie van ’n bron bepaal die amplitude van die golf. Bronne stuur energie deur die medium met behulp van vibrasies. Meer energieke vibrasies lei tot groter amplitudes. Hoe groter die amplitude, hoe groter is die agressie waarmee die molekules heen-en-weer beweeg.

Daar is ook ander fasette wat die hardheid van ’n klank beïnvloed. Een van hulle is die sensitiwiteit van die oor. Die menslike oor is meer sensitief vir sommige frekwensies as ander. Die volume wat ons waarneem hang dus van beide die amplitude van ’n klankgolf én of die frekwensie in die omvang lê waar die oor min-of-meer sensitief is.

Toon

Toonhoogte is ’n maatstaf van die kwaliteit van ’n klankgolf. Byvoorbeeld: die kwaliteit van die klank wat in ’n spesifieke musikale instrument gelewer word hang af van watter harmoniese tone daarop gesuperponeer (geplaas) is en in watter verhoudings. Harmoniese tone verwys na klankgolwe wat in ooreenstemming met mekaar is en word bepaal deur die staande golwe wat in die instrument geproduseer word. Hoofstuk (Verwysing) sal die fisika agter musiek in groter besonderhede bespreek.

Die gehalte (timbre of klanktint) van ’n gehoorde klank is afhanklik van die patroon van die inkomende vibrasies, dus, die vorm van die klankgolf. Wanneer die vibrasies meer onreëlmatig is, sal die vorm van die klankgolf ru wees en sal die klank wat gehoor word ook growwer klink.

Spoed van klank

Die spoed van klank is afhanklik van die medium waarin dit beweeg. Klank beweeg vinniger in vastestowwe as in vloeistowwe en vinniger in vloeistowwe as in gasse. Hierdie verskynsel is danksy die feit dat die digtheid van vastestowwe hoër is as dié van vloeistowwe wat beteken dat die deeltjies nader aan mekaar is en die geluid makliker oorgedra kan word.

’n Ander faktor wat die spoed beïnvloed is die temperatuur van die medium. Hoe warmer die medium is, hoe vinniger beweeg die deeltjies en dus sal die klank ook vinniger deur die medium beweeg. Wanneer ons ’n stof verhit, het die deeltjies in daardie stof meer kinetiese energie en sal dit vibreer of vinniger beweeg. Daarom kan klank vinniger en makliker deur warm stowwe gelei word.

Klankgolwe is drukgolwe. Die spoed van klank sal dus deur die druk van die medium waardeur dit gaan beïnvloed word. By seevlak is die lugdruk hoër as hoog bo-op ’n berg. Klank sal vinniger by seevlak beweeg, waar die druk hoog is, as by plekke hoog bo seespieël.

Definition 1: Spoed van klank

Die spoed van klank in lug, by seevlak, ’n temperatuur van 21C en onder normale atmosferiese omstandighede is 344m s-1-1.

Klank frekwensie en amplitude

Bestudeer die volgende diagram wat ’n musieknoot voorstel. Teken nou in jou werkboek die kurwes wat elk van die volgende klanke voorstel:

  1. ’n hoër toonhoogte
  2. harder
  3. sagter
Figuur 2
Figuur 2 (PG11C5_004.png)

Fisika van die oor en gehoor

Figuur 3: Diagram van die menslike oor.
Figuur 3 (HumanEar-GrayScale.png)

Die menslike oor word in drie hoof afdelinge verdeel: die buite-, middel en binne-oor. Laat ons nou die reis volg wat ’n klankgolf neem vanaf die pinna (buitenste deel) na die gehoorsenuwee (diepste deel) wat seine na die brein deurstuur. Die pinna is die deel van die oor waaraan ons tipies dink wanneer ons na die oor verwys. Sy hoof funksie is om ’n invallende klankgolf te versamel en op die gehoorkanaal te fokus. Die klankgolf beweeg dan deur die gehoorkanaal tot dit by die oordrom uitkom. Skommelings in die druk van die klankgolf veroorsaak dat die oordrom vibreer. Daar is drie verskriklike klein beentjies in die middeloor, die hamer (malleus), aambeeld (inkus) en stiebeuel (stapes). Hierdie beentjies stuur die klanksein deur ’n ovaal venster wat die begin is van die binneoor. Vanaf hierdie ovaal venster word die klankgolwe oorsein deur die vloeistof in die binneoor en vertolk as klanke deur die brein. Die binne-oor, gemaak van die halfsirkelvormige kanale, die koglea en die gehoorsenuwee, is gevul met vloeistof. Hierdie vloeistof laat die liggaam toe om vinnige bewegings op te spoor en balans te behou. Die slak-vormige koglea is oortrek met meer as 25 000 haaragtige senuweeselle. Verskillende senuselle vibreer teen verskillende frekwensies. Wanneer ’n senusel vibreer, stuur dit elektriese impulse na die gehoorsenuwee. Hierdie impulse word na die brein gestuur deur die gehoorsenuwee en geïnterpreteer as klank.

Ultraklank

Ultraklank is klank met ’n frekwensie van hoër as 20 kHz. Sommige diere, soos honde, dolfyne en vlermuise, het ’n bo-grens hoër as dié van die menslike oor en hul kan ultraklank hoor.

Die mees algemene gebruik van ultraklank is om beelde mee te skep wat industriële en mediese toepassings het. Die gebruik van ultraklank om beelde mee te skep is gebaseer op die refleksie en deurstuur van ’n golf by ’n grens. Soos ’n ultraklank golf binne ’n voorwerp beweeg wat uit verskillende materiale bestaan, soos die liggaam, sal grense bereik word, byvoorbeeld tussen been en spiere, spiere en vet, elke keer word ’n deel daarvan gereflekteer en ’n ander word deurgestuur. Gereflekteerde strale word bespeur en word gebruik om ’n beeld van die objek te skep.

Ultrasound in medicine can visualise muscle and soft tissue, making them useful for scanning the organs, and is commonly used during pregnancy. Ultrasound is a safe, non-invasive method of looking inside the human body.

Ultrasound sources may be used to generate local heating in biological tissue, with applications in physical therapy and cancer treatment. Focussed ultrasound sources may be used to break up kidney stones.

Ultrasonic cleaners, sometimes called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. These cleaners consist of containers with a fluid in which the object to be cleaned is placed. Ultrasonic waves are then sent into the fluid. The main mechanism for cleaning action in an ultrasonic cleaner is actually the energy released from the collapse of millions of microscopic bubbles occurring in the liquid of the cleaner.

Opmerking: Interesting Fact:

Ultrasound generator/speaker systems are sold with claims that they frighten away rodents and insects, but there is no scientific evidence that the devices work; controlled tests have shown that rodents quickly learn that the speakers are harmless.

In echo-sounding the reflections from ultrasound pulses that are bounced off objects (for example the bottom of the sea, fish etc.) are picked up. The reflections are timed and since their speed is known, the distance to the object can be found. This information can be built into a picture of the object that reflects the ultrasound pulses.

SONAR

Figuur 4
Figuur 4 (PG11C5_005.png)

Ships on the ocean make use of the reflecting properties of sound waves to determine the depth of the ocean. A sound wave is transmitted and bounces off the seabed. Because the speed of sound is known and the time lapse between sending and receiving the sound can be measured, the distance from the ship to the bottom of the ocean can be determined, This is called sonar, which stands fromSoundNavigationAndRanging.

Echolocation

Animals like dolphins and bats make use of sounds waves to find their way. Just like ships on the ocean, bats use sonar to navigate. Ultrasound waves that are sent out are reflected off the objects around the animal. Bats, or dolphins, then use the reflected sounds to form a “picture” of their surroundings. This is called echolocation.

Exercise 1: SONAR

A ship sends a signal to the bottom of the ocean to determine the depth of the ocean. The speed of sound in sea water is 1450m.s-1m.s-1If the signal is received 1,5 seconds later, how deep is the ocean at that point?

Solution
  1. Stap 1. Identify what is given and what is being asked::
    s=1450m.s-1t=1,5sthereandbackt=0,75sonewayd=?s=1450m.s-1t=1,5sthereandbackt=0,75sonewayd=?
    (1)
  2. Stap 2. Calculate the distance::
    Distance=speed×timed=s×t=1450m.s-1×0,75s=1087,5mDistance=speed×timed=s×t=1450m.s-1×0,75s=1087,5m
    (2)

Intensity of Sound (Not Included in CAPS - Advanced)

belangrik: Advanced Section:

This section is more advanced than required and is best revisited for interest only when you are comfortable with concepts like power and logarithms.

Intensity is one indicator of amplitude. Intensity is the energy transmitted over a unit of area each second.

Intensity

Intensity is defined as:

Intensity = energy time × area = power area Intensity = energy time × area = power area
(3)

By the definition of intensity, we can see that the units of intensity are

Joules s m 2 = Watts m 2 Joules s m 2 = Watts m 2
(4)

The unit of intensity is thedecibel(symbol: dB). This reduces to an SI equivalent ofW m-2W m-2.

The average threshold of hearing is10-12W m-210-12W m-2. Below this intensity, the sound is too soft for the ear to hear. The threshold of pain is1.0W m-21.0W m-2. Above this intensity a sound is so loud it becomes uncomfortable for the ear.

Notice that there is a factor of10121012between the thresholds of hearing and pain. This is one reason we define the decibel (dB) scale.

dB Scale

The intensity in dB of a sound of intensityII, is given by:

β = 10 log I I o I o = 10 - 12 W m - 2 β = 10 log I I o I o = 10 - 12 W m - 2
(5)

In this way we can compress the whole hearing intensity scale into a range from 0 dB to 120 dB.

Tabel 2: Examples of sound intensities.
Source Intensity(dB) Times greater than hearing threshold
     
Rocket Launch 180 10 18 10 18
Jet Plane 140 10 14 10 14
Threshold of Pain 120 10 12 10 12
Rock Band 110 10 11 10 11
Subway Train 90 10 9 10 9
Factory 80 10 8 10 8
City Traffic 70 10 7 10 7
Normal Conversation 60 10 6 10 6
Library 40 10 4 10 4
Whisper 20 10 2 10 2
Threshold of hearing 0 0

Notice that there are sounds which exceed the threshold of pain. Exposure to these sounds can cause immediate damage to hearing. In fact, exposure to sounds from 80 dB and above can damage hearing over time. Measures can be taken to avoid damage, such as wearing earplugs or ear muffs. Limiting exposure time and increasing distance between you and the source are also important steps for protecting your hearing.

Discussion : Importance of Safety Equipment

Working in groups of 5, discuss the importance of safety equipment such as ear protectors for workers in loud environments, e.g. those who use jack hammers or direct aeroplanes to their parking bays. Write up your conclusions in a one page report. Some prior research into the importance of safety equipment might be necessary to complete this group discussion.

Summary

  1. Sound waves are longitudinal waves
  2. Thefrequencyof a sound is an indication of how high or low thepitchof the sound is.
  3. The human ear can hear frequencies from 20 to 20 000 Hz.Infrasoundwaves have frequencies lower than 20 Hz.Ultrasoundwaves have frequencies higher than 20 000 Hz.
  4. Theamplitudeof a sound determines itsloudnessor volume.
  5. Thetoneis a measure of thequalityof a sound wave.
  6. The speed of sound in air is around 340 m.s-1-1. It is dependent on the temperature, height above sea level and the phase of the medium through which it is travelling.
  7. Sound travels faster when the medium is hot.
  8. Sound travels faster in a solid than a liquid and faster in a liquid than in a gas.
  9. Sound travels faster at sea level where the air pressure is higher.
  10. The intensity of a sound is the energy transmitted over a certain area. Intensity is a measure of frequency.
  11. Ultrasound can be used to form pictures of things we cannot see, like unborn babies or tumors.
  12. Echolocation is used by animals such as dolphins and bats to “see” their surroundings by using ultrasound.
  13. Ships use sonar to determine how deep the ocean is or to locate shoals of fish.

Exercises

  1. Choose a word from column B that best describes the concept in column A.
    Tabel 3
    Column AColumn B
    pitch of soundamplitude
    loudness of soundfrequency
    quality of soundspeed
     waveform
  2. A tuning fork, a violin string and a loudspeaker are producing sounds. This is because they are all in a state of:
    1. compression
    2. rarefaction
    3. rotation
    4. tension
    5. vibration
  3. What would a drummer do to make the sound of a drum give a note of lower pitch?
    1. hit the drum harder
    2. hit the drum less hard
    3. hit the drum near the edge
    4. loosen the drum skin
    5. tighten the drum skin
  4. What is the approximate range of audible frequencies for a healthy human?
    1. 0.2 Hz200 Hz
    2. 2 Hz2 000 Hz
    3. 20 Hz20 000 Hz
    4. 200 Hz200 000 Hz
    5. 2 000 Hz2 000 000 Hz
  5. X and Y are different wave motions. In air, X travels much faster than Y but has a much shorter wavelength. Which types of wave motion could X and Y be?
    Tabel 4
     XY
    Amicrowavesred light
    Bradioinfra red
    Cred lightsound
    Dsoundultraviolet
    Eultravioletradio
  6. Astronauts are in a spaceship orbiting the moon. They see an explosion on the surface of the moon. Why can they not hear the explosion?
    1. explosions do not occur in space
    2. sound cannot travel through a vacuum
    3. sound is reflected away from the spaceship
    4. sound travels too quickly in space to affect the ear drum
    5. the spaceship would be moving at a supersonic speed
  7. A man stands between two cliffs as shown in the diagram and claps his hands once.
    Figuur 5
    Figuur 5 (PG11C5_006.png)
    Assuming that the velocity of sound is 330 m.s-1-1, what will be the time interval between the two loudest echoes?
    1. 1616s
    2. 5656s
    3. 1313s
    4. 1 s
    5. 2323s
  8. A dolphin emits an ultrasonic wave with frequency of 0,15 MHz. The speed of the ultrasonic wave in water is 1 500 m.s-1-1. What is the wavelength of this wave in water?
    1. 0.1 mm
    2. 1 cm
    3. 10 cm
    4. 10 m
    5. 100 m
  9. The amplitude and frequency of a sound wave are both increased. How are the loudness and pitch of the sound affected?
    Tabel 5
     loudnesspitch
    Aincreasedraised
    Bincreasedunchanged
    Cincreasedlowered
    Ddecreasedraised
    Edecreasedlowered
  10. A jet fighter travels slower than the speed of sound. Its speed is said to be:
    1. Mach 1
    2. supersonic
    3. isosonic
    4. hypersonic
    5. infrasonic
  11. A sound wave is different from a light wave in that a sound wave is:
    1. produced by a vibrating object and a light wave is not.
    2. not capable of travelling through a vacuum.
    3. not capable of diffracting and a light wave is.
    4. capable of existing with a variety of frequencies and a light wave has a single frequency.
  12. At the same temperature, sound waves have the fastest speed in:
    1. rock
    2. milk
    3. oxygen
    4. sand
  13. Two sound waves are traveling through a container of nitrogen gas. The first wave has a wavelength of 1,5 m, while the second wave has a wavelength of 4,5 m. The velocity of the second wave must be:
    1. 1919the velocity of the first wave.
    2. 1313the velocity of the first wave.
    3. the same as the velocity of the first wave.
    4. three times larger than the velocity of the first wave.
    5. nine times larger than the velocity of the first wave.
  14. Sound travels at a speed of 340 m s-1-1. A straw is 0,25 m long. The standing wave set up in such a straw with one end closed has a wavelength of 1,0 m. The standing wave set up in such a straw with both ends open has a wavelength of 0,50 m.
    1. calculate the frequency of the sound created when you blow across the straw with the bottom end closed.
    2. calculate the frequency of the sound created when you blow across the straw with the bottom end open.
  15. A lightning storm creates both lightning and thunder. You see the lightning almost immediately since light travels at3×108m s-13×108m s-1. After seeing the lightning, you count 5 s and then you hear the thunder. Calculate the distance to the location of the storm.
  16. A person is yelling from a second story window to another person standing at the garden gate, 50 m away. If the speed of sound is 344 m s-1-1, how long does it take the sound to reach the person standing at the gate?
  17. A piece of equipment has a warning label on it that says, "Caution! This instrument produces 140 decibels." What safety precaution should you take before you turn on the instrument?
  18. What property of sound is a measure of the amount of energy carried by a sound wave?
  19. How is intensity related to loudness?
  20. Person 1 speaks to person 2. Explain how the sound is created by person 1 and how it is possible for person 2 to hear the conversation.
  21. Sound cannot travel in space. Discuss what other modes of communication astronauts can use when they are outside the space shuttle?
  22. An automatic focus camera uses an ultrasonic sound wave to focus on objects. The camera sends out sound waves which are reflected off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 344 m s-1-1) returns to the camera 0,150 s after leaving the camera, how far away is the object?
  23. Calculate the frequency (in Hz) and wavelength of the annoying sound made by a mosquito when it beats its wings at the average rate of 600 wing beats per second. Assume the speed of the sound waves is 344 m s-1-1.
  24. How does halving the frequency of a wave source affect the speed of the waves?
  25. Humans can detect frequencies as high as 20 000 Hz. Assuming the speed of sound in air is 344 m s-1-1, calculate the wavelength of the sound corresponding to the upper range of audible hearing.
  26. An elephant trumpets at 10 Hz. Assuming the speed of sound in air is 344 m s-1-1, calculate the wavelength of this infrasonic sound wave made by the elephant.
  27. A ship sends a signal out to determine the depth of the ocean. The signal returns 2,5 seconds later. If sound travels at 1450 m.s-1-1in sea water, how deep is the ocean at that point?

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