CDS Physics · Sound
Pitch, Echo and Uses of Sound
What decides pitch and loudness, how the ear responds, the octave, how to find a distance from an echo, and how ultrasound is made and used.
Why this matters
Eight CDS questions from 2016 (II) to 2023 (I). Four are about pitch and hearing, two are the same echo sum (340 m/s and 5 s, asked in 2019 (II) and 2023 (I)), and two ask how ultrasound is made or what a microphone and a speaker do.
Concept 1 of 3: Pitch, loudness and the human ear
Two sounds can differ in three ways. One can be shriller than the other: that is pitch, and it comes from the frequency. One can be louder: that is loudness, and it comes from the amplitude, how far the particles swing. And a flute and a violin playing the same note still sound different: that is quality, which comes from the shape of the wave. The ear does not hear every frequency equally well, and it judges loudness on a stretched, logarithmic scale.
Definition
Three characteristics of a sound:
- Pitch depends on frequency: higher frequency, shriller sound. A woman's voice usually has a higher pitch than a man's.
- Loudness depends on amplitude (and so on intensity). Loudness level is measured in decibels (dB).
- Quality (timbre) depends on the waveform, the mix of overtones.
- Octave: a note one octave higher has twice the frequency.
- Audible range: about 20 Hz to 20 kHz for humans. Below 20 Hz is infrasound, above 20 kHz is ultrasound.
- The ear is most sensitive at about 2 to 5 kHz and much less sensitive at very low and very high frequencies. Its response to rising intensity is roughly logarithmic, which is why the decibel scale is used.
| Characteristic of sound | Depends on |
|---|---|
| Pitch | FrequencyQ Asked twice: CDS 2020 (I) and 2022 (II). The answer is frequency, never amplitude or speed. |
| Loudness | Amplitude |
| Quality (timbre) | Waveform |
| One octave higher | Double the frequencyQ CDS 2019 (II): the note an octave above 500 Hz is 1000 Hz. |
| Sensitivity of the ear | Varies with frequency; best near 2 to 5 kHzQ CDS 2016 (II): the statement that the ear's sensitivity does not vary with frequency is the false one. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q7Easy
Example 1 · Sound · Pitch, Echo and Uses of Sound
The property of the sound waves that determines the pitch of the sound is its
Pitch is frequency; loudness is amplitude
A high-pitched sound is not a loud sound. Pitch goes with frequency, loudness with amplitude. Speed, wavelength and intensity are distractors in a pitch question.
An octave doubles the frequency; it does not add to it
One octave up means , two octaves up means . Adding half (500 to 750 Hz) or quadrupling for one octave are the usual wrong options.
Concept 2 of 3: Echo: finding a distance from the time
An echo is a sound that bounces off a far surface, like a cliff or a large building, and comes back to you. The time you measure is for the trip there and back. So the distance to the cliff is only half of speed × time. This one halving is what every echo question tests.
Definition
Echo facts:
- The sound goes to the reflector and comes back, a path of .
- So , which gives .
- The ear keeps a sound for about 0.1 s, so an echo is heard separately only if it returns after at least 0.1 s. At 344 m/s that needs the reflector to be at least about 17.2 m away.
- Reverberation: many reflections in a large hall that keep the sound going. Curtains, carpets and soft boards absorb sound and reduce it.
- The same halving gives the depth of the sea from a SONAR pulse.
Distance from an echo
- ddistance to the reflecting surface
- vspeed of sound
- ttime from the sound to the echo (there and back)
Worked example
A boy shouts in front of a tall building and hears the echo 3 s later. Sound travels at 330 m/s. How far away is the building?
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q6Easy
Example 2 · Sound · Pitch, Echo and Uses of Sound
A person burned a firecracker in front of a cliff and heard its echo 5 s after it burst. The distance of the cliff from the person, if the speed of the sound is 340 m/s, is close to
Halve speed × time
The echo time covers the trip there and back. Speed × time gives twice the distance. For 330 m/s and 4 s the full product, 1320 m, is the wrong option set beside the right one, 660 m.
Watch the units in the options
660 m is 0.66 km, not 6.6 km or 0.066 km. Convert the answer before matching it.
Concept 3 of 3: Ultrasound, SONAR and transducers
Ultrasound is sound pitched too high for us to hear. It can be aimed in a narrow beam and it bounces back from objects, so it is used to find the depth of the sea, to look inside the body and to clean small parts. It is made by a crystal that vibrates when a voltage is put across it. A device that turns one kind of signal into another, like a microphone, is called a transducer.
Definition
Making and using ultrasound:
- Ultrasound: frequency above 20 kHz. Bats, dolphins and porpoises use it.
- Production: a piezoelectric crystal (such as quartz) vibrates at ultrasonic frequency when an alternating voltage is applied.
- Uses: SONAR (depth of the sea, finding submarines), ultrasound scanning of the body, cleaning small parts, detecting cracks in metal.
- SONAR stands for Sound Navigation And Ranging.
- Transducers: a microphone turns sound into an electrical signal; a loudspeaker turns an electrical signal back into sound.
| Device or term | What it does |
|---|---|
| Piezoelectric crystal | Produces ultrasound when an alternating voltage is appliedQ CDS 2017 (II): ultrasonic waves are produced using piezoelectric material. |
| SONAR | Measures distance under water from the echo of ultrasound |
| Ultrasound scanner | Forms images of organs inside the body |
| Microphone | Sound to electrical signal |
| Loudspeaker | Electrical signal to sound |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (I) 2023 — General Knowledge · Q7Easy
Example 3 · Sound · Pitch, Echo and Uses of Sound
Consider the following statements about the microphone and the speaker of a mobile phone :
1. The microphone converts sound to a mechanical signal.
2. The microphone converts sound to an electrical signal.
3. The speaker converts a mechanical signal to sound.
4. The speaker converts an electrical signal to sound.
Which of the statements given above are correct?
Piezoelectric, not pyroelectric or ferromagnetic
Ultrasound generators use piezoelectric crystals, which vibrate under a voltage. Pyroelectric materials respond to heat, and ferromagnetic rods (magnetostriction) work only at much lower frequencies.
Microphone and speaker both deal in electrical signals
A microphone gives an electrical output, not a mechanical one, and a speaker takes an electrical input. The vibrating diaphragm inside is only a step on the way.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Echo: finding a distance from the time
Distance from an echo
Reference tables (2)
Pitch, loudness and the human ear5 rows
| Characteristic of sound | Depends on |
|---|---|
| Pitch | FrequencyQ Asked twice: CDS 2020 (I) and 2022 (II). The answer is frequency, never amplitude or speed. |
| Loudness | Amplitude |
| Quality (timbre) | Waveform |
| One octave higher | Double the frequencyQ CDS 2019 (II): the note an octave above 500 Hz is 1000 Hz. |
| Sensitivity of the ear | Varies with frequency; best near 2 to 5 kHzQ CDS 2016 (II): the statement that the ear's sensitivity does not vary with frequency is the false one. |
Ultrasound, SONAR and transducers5 rows
| Device or term | What it does |
|---|---|
| Piezoelectric crystal | Produces ultrasound when an alternating voltage is appliedQ CDS 2017 (II): ultrasonic waves are produced using piezoelectric material. |
| SONAR | Measures distance under water from the echo of ultrasound |
| Ultrasound scanner | Forms images of organs inside the body |
| Microphone | Sound to electrical signal |
| Loudspeaker | Electrical signal to sound |
Watch out for (6)
- Pitch is frequency; loudness is amplitude→ Pitch, loudness and the human ear
- An octave doubles the frequency; it does not add to it→ Pitch, loudness and the human ear
- Halve speed × time→ Echo: finding a distance from the time
- Watch the units in the options→ Echo: finding a distance from the time
- Piezoelectric, not pyroelectric or ferromagnetic→ Ultrasound, SONAR and transducers
- Microphone and speaker both deal in electrical signals→ Ultrasound, SONAR and transducers
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.