Text 3. Sound as a longitudinal wave

Key terms: sound, vibrating object, source of sound, audible region, infrasonic, infrasound, ultrasonic, ultrasound, intensity of the sound, bel, decibel

How does sound travel through air? To answer this question we must first examine the source of sound. The source of every sound is a vibrating object. The sound of the human voice begins as vibrations in the vocal cord, in the larynx. The sound from a guitar is produced only while a string vibrates. A trombone sounds only while the player’s lips vibrate. Sound is transmitted through a material medium as a longitudinal wave.

There is the definition of sound as that form of energy which can be detected by the human ear. This definition is restrictive since many animals sense frequencies that we cannot hear. We will instead consider sound as energy that travels as longitudinal waves containing regions of high and low pressure. The audible region of the sound spectrum for human beings extends from about 16 Hz at the low-frequency end to about 20 kHz at the high-frequency end. Because 20,000 Hz is the upper limit of the audible range for humans, a frequency of 25,000 Hz would sound silent to the human ear. To humans, this sound has a loudness of zero even though its intensity might be 100 dB.

Frequencies below 16 Hz are called infrasonic. If we could hear infrasound, we would hear the waves on a lake before they break against the shore. We would hear the vibrations of a pendulum, and the vibrations of the wings of a bee. Recent studies suggest that many birds and animals can detect these frequencies. Birds, many kilometres inland, may hear a lake and thus know where it is. There are so many facts about the animals being disturbed before earthquakes that scientists are considering the possibility that earthquakes produce low-fre- quency infrasound before the main shock waves begin. Possibly these waves alert the animals.

Frequencies higher than 20,000 Hz are known as ultrasonic. There are many applications of ultrasound. It is used by jewellers to clean jewellery and can be used to find internal flaws in castings. Doctors use ultrasound to break up gallstones. Because ultrasound is reflected at the boundary between different types of tissue, it can be used to form internal images of the body that are in many ways more useful than those from X-rays. It is also used in therapy to accelerate the healing of damaged tissues such as muscles.

Ultrasound is used extensively by several animals. Bats locate objects in the dark by emitting ultrasonic squeaks and then listening for the returning echoes. Dolphins use ultrasound for navigation and communication.

The intensity of the sound is the amount of sound energy passing each second through a unit area. If one joule of sound energy per second passes through an area of one square metre, then the intensity is one joule per second per square metre, or one watt per square metre. Our ears are so sensitive that an intensity of one watt per square metre is too large a sound level for us to cope with. To accommodate smaller values, a unit called the bell (B), named in honour of Alexander Graham Bell, has been defined. One bell is equal to ten picowatts per square metre (1 B = 10 pW / m2). Remember that pico is the SI prefix meaning 10'12. A smaller unit, the decibel (dB) is also used (1 B = 0.1 B).

Sound intensity depends on several factors. First, it depends on the strength of the source of sound. Second, it depends on how far away the listener is from that source. And third, it depends on the ability of the medium between the listener and the source to transmit sound energy.

LEXICAL EXERCISES

I. Pronounce these international words after the teacher and translate them:

guitar, produce, trombone, vibrate, transmit, detect, human, limit, ultrasound, reflect, therapy, accelerate, accommodated, muscle, extensive, locate, object, echo ['i:kou], dolphin, navigation, communicate, vocal, factor

II. Translate these word combination into Ukrainian:

vibrating object, vocal cord, human voice, a material medium, high (low) pressure, audible region, sound spectrum, high (low) frequency, upper limit, audible range, internal image, X-ray, ultrasonic squeak, returning echo, intensity of the sound, amount of sound energy, to pass through the unit area, one joule per second, a sound level, to cope with, in honour of, several factors, shock waves.

III. Define, to what part of speech belong the derivatives of these words and translate them:

restrict - restriction — restrictive - restricted

sense - sensitive - sensible - sensitively - sensitivity

loud — loudly - loudness — loudless

jewel — jeweller - jewelleryextend - extensive — extensively - extension - extended

human - humane - humanity - humanitarian - humanism

sound - soundless - sounded - sounding

intense - intensely - intensive - intensify ~ intensity

local - localize - locate — location - localized

limit - limitless - limited - unlimited - limiting

IV. Answer these questions on the text:

1. What is the source of every sound? 2. How is sound produced by a man (guitar, trombone)? 3. What is sound? 4. What is the range of the audible region? 5. How is ultrasound used a) in medicine; b) by animals? 5. What is the intensity of the sound? 6. Give the definition of one bell. 7. What factors does sound intensity depend on? 8. What is the audible range of the average human ear? 9. What frequencies are associated with a) infrasound; b) ultrasound? 10. What is the difference between sound intensity and loudness? 11. What are the base units for sound intensity? 12. What is the difference between infrasound and ultrasound?

MULTIPLE CHOICE

I. Each of the following statements is followed by five responses. Choose the correct response in each case. For questions 1 and 2 use the same choices.

1. Which wave property causes a trumpet to sound different from a clarinet even if they are the same note with the same loudness?

a) reflection b) complexity of wave pattern

c) frequency d) amplitude

e)speed

2. Which wave property affects the intensity of sound?

a) loudness b) frequency

c)amplitude d) high pressure

e) low pressure

3. Which of the following frequencies is infrasonic?

a)12 Hz b) 60 Hz c) 1800 Hz

d) 2000 Hz e) 25 000 Hz

II. There are eight paragraphs in the text: give the title to each of the paragraphs; say what is the general idea of each of them.

III. Read text 4 without a dictionary and give its main idea.