Stand Closer Than Seventeen Metres and the Echo Vanishes
Learn the laws of reflection of sound, how to calculate echo distances and the minimum distance for a distinct echo, how halls are treated for reverberation, and how ultrasound is used.
Why can you not hear an echo from a nearby wall?
Because the reflected sound arrives too soon for your ear to separate it from the original.
A sensation of sound persists in the human ear for about ** s**. If a reflected sound arrives within that window, your ear merges it with the direct sound and you hear one noise, not two.
So the sound must take at least s to make the round trip to the wall and back. Taking the speed of sound as m/s:
and since that path is there and back, the wall must be at least
away. Stand closer than that and there is no distinct echo, however hard a surface you shout at.
That factor of two for the round trip runs through every calculation on this page, from echoes to SONAR to a medical scan. It covers the third part of the CBSE Class 9 Science chapter on sound waves.
A sensation of sound persists in the human ear for about ** s**. If a reflected sound arrives within that window, your ear merges it with the direct sound and you hear one noise, not two.
So the sound must take at least s to make the round trip to the wall and back. Taking the speed of sound as m/s:
and since that path is there and back, the wall must be at least
away. Stand closer than that and there is no distinct echo, however hard a surface you shout at.
That factor of two for the round trip runs through every calculation on this page, from echoes to SONAR to a medical scan. It covers the third part of the CBSE Class 9 Science chapter on sound waves.
What laws does the reflection of sound obey?
The same two laws that light obeys on reflection.
- The angle of incidence equals the angle of reflection
- The incident sound, the reflected sound and the normal at the point of incidence all lie in the same plane
Sound reflects best from a large, hard surface. Unlike light, the surface need not be polished — a rough brick wall reflects sound perfectly well, because a sound wavelength is far larger than the roughness of the bricks.
Everyday uses of reflected sound.
- A megaphone or a speaking tube has a conical shape that reflects the sound repeatedly forward, so it spreads less and carries further in one direction
- A stethoscope carries the sounds of the chest to the doctor's ears by multiple reflections along its tubes, instead of letting them escape into the room
- A curved soundboard placed behind a speaker on a stage reflects sound out towards the audience, and the curved ceilings of concert halls do the same
- An ear trumpet or a cupped hand behind the ear collects sound from a wider area and reflects it into the ear canal
Soft surfaces do not reflect — they absorb. Cloth, foam, curtains and carpets soak up the energy of the wave rather than bouncing it back. That is not a failure of the laws; it is simply that the energy has gone into the material instead of returning.
And that distinction is exactly what the treatment of large halls depends on, two sections further on. A hall with hard surfaces has a sound problem, and the solution is to put soft ones in — so understanding which surfaces reflect is what makes that solution obvious rather than arbitrary.
- The angle of incidence equals the angle of reflection
- The incident sound, the reflected sound and the normal at the point of incidence all lie in the same plane
Sound reflects best from a large, hard surface. Unlike light, the surface need not be polished — a rough brick wall reflects sound perfectly well, because a sound wavelength is far larger than the roughness of the bricks.
Everyday uses of reflected sound.
- A megaphone or a speaking tube has a conical shape that reflects the sound repeatedly forward, so it spreads less and carries further in one direction
- A stethoscope carries the sounds of the chest to the doctor's ears by multiple reflections along its tubes, instead of letting them escape into the room
- A curved soundboard placed behind a speaker on a stage reflects sound out towards the audience, and the curved ceilings of concert halls do the same
- An ear trumpet or a cupped hand behind the ear collects sound from a wider area and reflects it into the ear canal
Soft surfaces do not reflect — they absorb. Cloth, foam, curtains and carpets soak up the energy of the wave rather than bouncing it back. That is not a failure of the laws; it is simply that the energy has gone into the material instead of returning.
And that distinction is exactly what the treatment of large halls depends on, two sections further on. A hall with hard surfaces has a sound problem, and the solution is to put soft ones in — so understanding which surfaces reflect is what makes that solution obvious rather than arbitrary.
Formula
How do you calculate distances from an echo?
Use the round trip. The sound travels to the reflector and back, so
where is the distance to the reflecting surface, the speed of sound in the medium and the time between making the sound and hearing the echo.
Worked example 1 — distance to a cliff. A person claps and hears the echo after s. Take the speed of sound in air as m/s.
Worked example 2 — finding the time instead. How long after a shout is the echo heard from a wall m away?
Worked example 3 — will an echo be heard at all? A boy stands m from a large wall.
That is more than s, so he hears a distinct echo. Now bring him to m:
Less than s, so no distinct echo — the reflection merges with his own shout.
**Worked example 4 — the minimum distance, with m/s.**
Using m/s gives m, as the opening section showed. Use the speed the question gives you and the answer will match the expected one.
Worked example 5 — in water. A ship sends a sound pulse downwards and receives the echo from the sea bed after s. Take the speed of sound in water as m/s.
Worked example 6 — a longer sounding. The echo returns after s in the same water.
The factor of two is the whole difficulty in this section. Forgetting it doubles every answer; applying it twice halves them. Write down before substituting, and the division looks after itself.
Check that the medium's speed matches the situation. A question about a cliff needs the speed in air; a question about the sea bed needs the speed in water, which is more than four times greater. Using the air value for a SONAR problem gives an answer wrong by that factor, and it is a mistake that reads as plausible on the page.
where is the distance to the reflecting surface, the speed of sound in the medium and the time between making the sound and hearing the echo.
Worked example 1 — distance to a cliff. A person claps and hears the echo after s. Take the speed of sound in air as m/s.
Worked example 2 — finding the time instead. How long after a shout is the echo heard from a wall m away?
Worked example 3 — will an echo be heard at all? A boy stands m from a large wall.
That is more than s, so he hears a distinct echo. Now bring him to m:
Less than s, so no distinct echo — the reflection merges with his own shout.
**Worked example 4 — the minimum distance, with m/s.**
Using m/s gives m, as the opening section showed. Use the speed the question gives you and the answer will match the expected one.
Worked example 5 — in water. A ship sends a sound pulse downwards and receives the echo from the sea bed after s. Take the speed of sound in water as m/s.
Worked example 6 — a longer sounding. The echo returns after s in the same water.
The factor of two is the whole difficulty in this section. Forgetting it doubles every answer; applying it twice halves them. Write down before substituting, and the division looks after itself.
Check that the medium's speed matches the situation. A question about a cliff needs the speed in air; a question about the sea bed needs the speed in water, which is more than four times greater. Using the air value for a SONAR problem gives an answer wrong by that factor, and it is a mistake that reads as plausible on the page.
What is reverberation and how is it reduced in a large hall?
Reverberation is the persistence of sound caused by repeated reflections, and it is reduced by replacing reflecting surfaces with absorbing ones.
In a large hall with hard walls, ceiling and floor, a sound bounces many times before its energy dies away. Each reflection arrives at a slightly different moment, and the overlapping reflections merge into a prolonged blur that continues after the speaker has stopped. Speech becomes hard to follow, because each word is still sounding when the next begins.
How reverberation differs from an echo. An echo is a single, distinct repetition, heard separately because it arrived more than s later. Reverberation is many overlapping reflections merging into one continuous sound. The physics is the same reflection; the difference is how many surfaces are involved and how their arrivals overlap.
Methods of reducing it:
- Cover the walls and ceiling with sound-absorbing material such as compressed fibreboard or rough plaster
- Hang curtains and draperies
- Lay carpets on the floor
- Use padded, upholstered seats, which absorb whether or not anyone is sitting in them
- Shape the ceiling and fit a curved soundboard so that sound reaches all of the audience directly rather than after several bounces
Everyday evidence. An empty unfurnished room sounds hollow, and your voice seems to hang in the air. Move furniture, curtains and people into the same room and it sounds normal at once — the soft surfaces are absorbing what the bare walls were reflecting. A cinema hall is deliberately full of heavy curtains, carpet and padded seats for the same reason.
The aim is absorption, not blocking. A thicker wall keeps sound out of a room; it does nothing about the sound already inside bouncing around. Reducing reverberation means giving the sound energy somewhere to go, and porous soft materials do that by converting it to a very small amount of heat inside themselves.
Some reverberation is wanted. A hall treated until it absorbs almost everything sounds dead and unpleasant, and music in particular needs a little persistence to sound full. So the design target is not zero reverberation but the right amount for the hall's purpose — less for a lecture theatre where speech must be clear, more for a concert hall.
In a large hall with hard walls, ceiling and floor, a sound bounces many times before its energy dies away. Each reflection arrives at a slightly different moment, and the overlapping reflections merge into a prolonged blur that continues after the speaker has stopped. Speech becomes hard to follow, because each word is still sounding when the next begins.
How reverberation differs from an echo. An echo is a single, distinct repetition, heard separately because it arrived more than s later. Reverberation is many overlapping reflections merging into one continuous sound. The physics is the same reflection; the difference is how many surfaces are involved and how their arrivals overlap.
Methods of reducing it:
- Cover the walls and ceiling with sound-absorbing material such as compressed fibreboard or rough plaster
- Hang curtains and draperies
- Lay carpets on the floor
- Use padded, upholstered seats, which absorb whether or not anyone is sitting in them
- Shape the ceiling and fit a curved soundboard so that sound reaches all of the audience directly rather than after several bounces
Everyday evidence. An empty unfurnished room sounds hollow, and your voice seems to hang in the air. Move furniture, curtains and people into the same room and it sounds normal at once — the soft surfaces are absorbing what the bare walls were reflecting. A cinema hall is deliberately full of heavy curtains, carpet and padded seats for the same reason.
The aim is absorption, not blocking. A thicker wall keeps sound out of a room; it does nothing about the sound already inside bouncing around. Reducing reverberation means giving the sound energy somewhere to go, and porous soft materials do that by converting it to a very small amount of heat inside themselves.
Some reverberation is wanted. A hall treated until it absorbs almost everything sounds dead and unpleasant, and music in particular needs a little persistence to sound full. So the design target is not zero reverberation but the right amount for the hall's purpose — less for a lecture theatre where speech must be clear, more for a concert hall.
How are ultrasonic and infrasonic waves used?
Ultrasound is above the audible range and infrasound below it, and the uses of ultrasound all rest on the same echo calculation from earlier on this page.
Infrasonic waves have frequencies **below Hz. They come from very large, slow vibrations.
- A simple pendulum, a swinging bridge and the initial tremors before an earthquake produce infrasound
- Some large animals, including whales and rhinoceroses, communicate using it — low frequencies travel great distances
- Dogs and elephants can detect infrasound and may become uneasy before an earthquake is felt by people
Ultrasonic waves have frequencies above Hz. They come from very rapid vibrations, they travel in well-defined narrow beams, and they can penetrate solids and liquids without much spreading. Bats, dolphins and porpoises both produce and detect them.
Echolocation. A bat** emits ultrasonic squeaks and listens for the reflections. From the time each echo takes to return, it works out how far away an obstacle or an insect is — the calculation, performed continuously. Bats fly and hunt accurately in complete darkness on this alone, and dolphins do the same underwater.
SONAR — sound navigation and ranging. A ship carries a transmitter that sends ultrasonic pulses downwards and a detector that receives the echo. Timing the return gives the depth of the sea bed, or the distance to a submarine, a shoal of fish, a sunken ship or an iceberg.
Worked example. A SONAR pulse is sent and its echo received after s, with the speed of sound in sea water m/s.
The sea bed is m below. This process is called echo ranging, and it is the echo calculation with a machine doing the timing.
Medical uses.
- An ultrasound scan sends pulses into the body and builds an image from the reflections at the boundaries between tissues, which is how internal organs and a developing foetus are examined
- Ultrasound is used to break up small stones in the kidney into fragments small enough to be passed out naturally
Industrial uses.
- Detecting flaws in metal blocks: a crack inside reflects the pulse earlier than the far face would, so a defect shows up as an unexpected echo. Metal parts for bridges and machinery are tested this way
- Cleaning parts of machinery that are hard to reach, by suspending them in a liquid through which ultrasound is passed — the vibration dislodges dirt from spirals and corners
Ultrasound is used for these jobs because of its short wavelength, not because it is inaudible. As the previous part of this chapter showed, means a very high frequency gives a very short wavelength — under cm at the top of the audible range, and shorter still above it. Short wavelengths travel in straight narrow beams and reflect from small objects, which is exactly what is needed to locate an insect, image an organ or find a crack. A low-frequency sound with a wavelength of metres would spread out and pass small objects by entirely.
Infrasonic waves have frequencies **below Hz. They come from very large, slow vibrations.
- A simple pendulum, a swinging bridge and the initial tremors before an earthquake produce infrasound
- Some large animals, including whales and rhinoceroses, communicate using it — low frequencies travel great distances
- Dogs and elephants can detect infrasound and may become uneasy before an earthquake is felt by people
Ultrasonic waves have frequencies above Hz. They come from very rapid vibrations, they travel in well-defined narrow beams, and they can penetrate solids and liquids without much spreading. Bats, dolphins and porpoises both produce and detect them.
Echolocation. A bat** emits ultrasonic squeaks and listens for the reflections. From the time each echo takes to return, it works out how far away an obstacle or an insect is — the calculation, performed continuously. Bats fly and hunt accurately in complete darkness on this alone, and dolphins do the same underwater.
SONAR — sound navigation and ranging. A ship carries a transmitter that sends ultrasonic pulses downwards and a detector that receives the echo. Timing the return gives the depth of the sea bed, or the distance to a submarine, a shoal of fish, a sunken ship or an iceberg.
Worked example. A SONAR pulse is sent and its echo received after s, with the speed of sound in sea water m/s.
The sea bed is m below. This process is called echo ranging, and it is the echo calculation with a machine doing the timing.
Medical uses.
- An ultrasound scan sends pulses into the body and builds an image from the reflections at the boundaries between tissues, which is how internal organs and a developing foetus are examined
- Ultrasound is used to break up small stones in the kidney into fragments small enough to be passed out naturally
Industrial uses.
- Detecting flaws in metal blocks: a crack inside reflects the pulse earlier than the far face would, so a defect shows up as an unexpected echo. Metal parts for bridges and machinery are tested this way
- Cleaning parts of machinery that are hard to reach, by suspending them in a liquid through which ultrasound is passed — the vibration dislodges dirt from spirals and corners
Ultrasound is used for these jobs because of its short wavelength, not because it is inaudible. As the previous part of this chapter showed, means a very high frequency gives a very short wavelength — under cm at the top of the audible range, and shorter still above it. Short wavelengths travel in straight narrow beams and reflect from small objects, which is exactly what is needed to locate an insect, image an organ or find a crack. A low-frequency sound with a wavelength of metres would spread out and pass small objects by entirely.
Exam tip
Exam tip: write 2d = vt before you substitute
**Write down first, then rearrange. The sound makes a round trip, and forgetting the factor of two is the commonest error in the whole chapter.
Use the speed for the right medium — air for a cliff or a wall, water for a sea bed or SONAR. The two differ by more than a factor of four.
Minimum distance for a distinct echo: the persistence of sound in the ear is about s**, giving m, or m if the speed is taken as m/s.
To decide whether an echo will be heard, find the time and compare it with s: m gives s (heard), m gives s (not heard).
State both laws of reflection of sound, and note that the surface need only be large and hard, not polished.
Echo is one distinct repetition; reverberation is many overlapping reflections. Say that difference explicitly when asked.
List the reverberation remedies as absorbing surfaces: fibreboard, curtains, carpets, padded seats, and a shaped ceiling.
**Infrasonic is below Hz; ultrasonic is above Hz.
Name the ultrasound uses with a reason: echolocation, SONAR (echo ranging), medical scans and stone breaking, flaw detection and cleaning.
And say why ultrasound suits those jobs — its short wavelength** gives narrow beams that reflect from small objects.
Use the speed for the right medium — air for a cliff or a wall, water for a sea bed or SONAR. The two differ by more than a factor of four.
Minimum distance for a distinct echo: the persistence of sound in the ear is about s**, giving m, or m if the speed is taken as m/s.
To decide whether an echo will be heard, find the time and compare it with s: m gives s (heard), m gives s (not heard).
State both laws of reflection of sound, and note that the surface need only be large and hard, not polished.
Echo is one distinct repetition; reverberation is many overlapping reflections. Say that difference explicitly when asked.
List the reverberation remedies as absorbing surfaces: fibreboard, curtains, carpets, padded seats, and a shaped ceiling.
**Infrasonic is below Hz; ultrasonic is above Hz.
Name the ultrasound uses with a reason: echolocation, SONAR (echo ranging), medical scans and stone breaking, flaw detection and cleaning.
And say why ultrasound suits those jobs — its short wavelength** gives narrow beams that reflect from small objects.
Did you know
Why a bat can catch an insect in total darkness
A bat hunting at night is solving the same problem a ship's SONAR solves, with the same arithmetic, thousands of times a second.
It emits a short ultrasonic pulse and waits. When the pulse strikes a moth, part of it returns, and the bat measures how long the round trip took. Halve the product of the speed of sound and that time, and the distance to the moth is known. Repeat as the moth moves and the bat also knows which way it is going and how fast.
The choice of ultrasound is essential and not incidental. A high frequency means a short wavelength, and a short wavelength does two things a long one cannot: it travels in a narrow beam rather than spreading out, and it reflects from small objects. A sound with a wavelength of several metres would sweep past a moth as though it were not there.
The same reasoning sets a limit on medical ultrasound. Finer detail needs a shorter wavelength, so scanners use very high frequencies — and there the trade-off appears, because higher frequencies are absorbed more strongly and do not reach as deep into the body. Every scan is a compromise between how deep and how detailed.
And it explains the shape of a flaw-detection test. A pulse sent into a steel block returns from the far face at a predictable time. An echo arriving earlier than that means it bounced off something inside — a crack or a cavity — and its timing says how deep the defect lies. Nothing has to be cut open.
So the bat, the ship, the scanner and the engineer are all using . Only the medium and the instrument change.
It emits a short ultrasonic pulse and waits. When the pulse strikes a moth, part of it returns, and the bat measures how long the round trip took. Halve the product of the speed of sound and that time, and the distance to the moth is known. Repeat as the moth moves and the bat also knows which way it is going and how fast.
The choice of ultrasound is essential and not incidental. A high frequency means a short wavelength, and a short wavelength does two things a long one cannot: it travels in a narrow beam rather than spreading out, and it reflects from small objects. A sound with a wavelength of several metres would sweep past a moth as though it were not there.
The same reasoning sets a limit on medical ultrasound. Finer detail needs a shorter wavelength, so scanners use very high frequencies — and there the trade-off appears, because higher frequencies are absorbed more strongly and do not reach as deep into the body. Every scan is a compromise between how deep and how detailed.
And it explains the shape of a flaw-detection test. A pulse sent into a steel block returns from the far face at a predictable time. An echo arriving earlier than that means it bounced off something inside — a crack or a cavity — and its timing says how deep the defect lies. Nothing has to be cut open.
So the bat, the ship, the scanner and the engineer are all using . Only the medium and the instrument change.
Exam relevance
How do echo and ultrasound questions feed into JEE Main and NEET?
This page is the foundation for the Class 11 Physics chapter Waves, examined in JEE Main and in NEET Physics, and its numerical technique survives unchanged.
The reflection of sound becomes the basis of standing waves in that chapter. When a sound wave reflects and overlaps the incoming wave, the two combine to form a stationary pattern — which is how an air column in a pipe or a stretched string produces a definite note. JEE Main questions on organ pipes and resonance columns are extremely common, and they all begin with the reflection studied here.
The echo calculation reappears whenever a wave makes a round trip. It is the same structure as the time-of-flight reasoning used for radar, for laser ranging and in Class 12 for the distance to a reflecting surface using light. The factor of two never goes away.
Reverberation and absorption connect to the treatment of intensity and its fall with distance in Class 11.
Ultrasound's short wavelength is the entry point to one of the most reused ideas in physics: resolution depends on wavelength. The explanation given above for why a bat needs ultrasound is exactly the argument used in Class 12 Wave Optics for why a microscope's resolving power is limited by the wavelength of light — and it is why an electron microscope can see what a light microscope cannot, as the cell chapter of this course noted.
The Doppler effect in Class 11 builds on echolocation directly: a bat can tell whether its target is approaching from the change in frequency of the returning echo, and that is a standard JEE Main topic.
What the questions look like. Numericals dominate, and the standard shapes are find the distance from the echo time, find the echo time from the distance, and decide whether a distinct echo is heard. For NEET, assertion-reason items favour the statements that ultrasound is used because of its short wavelength, and that reverberation is reduced by absorption rather than by blocking. Match-the-column questions pair an application with the kind of wave used.
How board and competitive emphasis differ. A board paper asks you to define echo and reverberation, list the methods of reducing reverberation, and compute one echo distance. A competitive paper sets the numerical with the medium changed, or asks for the minimum distance with a stated speed, or combines it with so that the wavelength must be found first. Board papers reward the list of remedies; competitive papers reward the round-trip factor and the correct medium speed.
The single trap that costs the most marks. Omitting the factor of two. instead of doubles every answer, and the result looks perfectly reasonable on the page. Writing as the first line makes it impossible to forget.
A second trap worth naming. Using the speed of sound in air for a SONAR or underwater problem. Water carries sound at roughly m/s against air's m/s, so the error is large — and a question that tells you the medium is telling you which speed to use.
The reflection of sound becomes the basis of standing waves in that chapter. When a sound wave reflects and overlaps the incoming wave, the two combine to form a stationary pattern — which is how an air column in a pipe or a stretched string produces a definite note. JEE Main questions on organ pipes and resonance columns are extremely common, and they all begin with the reflection studied here.
The echo calculation reappears whenever a wave makes a round trip. It is the same structure as the time-of-flight reasoning used for radar, for laser ranging and in Class 12 for the distance to a reflecting surface using light. The factor of two never goes away.
Reverberation and absorption connect to the treatment of intensity and its fall with distance in Class 11.
Ultrasound's short wavelength is the entry point to one of the most reused ideas in physics: resolution depends on wavelength. The explanation given above for why a bat needs ultrasound is exactly the argument used in Class 12 Wave Optics for why a microscope's resolving power is limited by the wavelength of light — and it is why an electron microscope can see what a light microscope cannot, as the cell chapter of this course noted.
The Doppler effect in Class 11 builds on echolocation directly: a bat can tell whether its target is approaching from the change in frequency of the returning echo, and that is a standard JEE Main topic.
What the questions look like. Numericals dominate, and the standard shapes are find the distance from the echo time, find the echo time from the distance, and decide whether a distinct echo is heard. For NEET, assertion-reason items favour the statements that ultrasound is used because of its short wavelength, and that reverberation is reduced by absorption rather than by blocking. Match-the-column questions pair an application with the kind of wave used.
How board and competitive emphasis differ. A board paper asks you to define echo and reverberation, list the methods of reducing reverberation, and compute one echo distance. A competitive paper sets the numerical with the medium changed, or asks for the minimum distance with a stated speed, or combines it with so that the wavelength must be found first. Board papers reward the list of remedies; competitive papers reward the round-trip factor and the correct medium speed.
The single trap that costs the most marks. Omitting the factor of two. instead of doubles every answer, and the result looks perfectly reasonable on the page. Writing as the first line makes it impossible to forget.
A second trap worth naming. Using the speed of sound in air for a SONAR or underwater problem. Water carries sound at roughly m/s against air's m/s, so the error is large — and a question that tells you the medium is telling you which speed to use.
Key takeaways
Reflection of sound, echo, reverberation and ultrasound: quick revision
- Laws of reflection of sound: the angle of incidence equals the angle of reflection, and the incident sound, reflected sound and normal lie in one plane.
- The surface must be large and hard, but need not be polished. Soft surfaces absorb rather than reflect.
- Uses of reflection: megaphone, stethoscope, curved soundboard, ear trumpet.
- Echo formula: , so — the sound makes a round trip.
- A clap echoing after s at m/s means a cliff m away. A wall m away returns the echo in s.
- **Persistence of sound in the ear is about s**, so the minimum distance is m, or m at m/s.
- At m the echo takes s and is heard; at m it takes s and is not.
- In water at m/s, an echo after s means a depth of m, and after s, m.
- Use the speed for the right medium — air and water differ by more than a factor of four.
- Reverberation is the persistence of sound from many overlapping reflections; an echo is one distinct repetition.
- Reduce reverberation by absorption: fibreboard, rough plaster, curtains, carpets, padded seats, and a shaped ceiling or soundboard.
- Absorption, not blocking — and a little reverberation is desirable, especially for music.
- Infrasonic is below ** Hz — pendulums, earthquake tremors, whales and rhinoceroses. Ultrasonic is above Hz — bats, dolphins, porpoises.
- Echolocation**: a bat times its returning ultrasonic pulses and applies .
- SONAR (echo ranging): a transmitter and detector on a ship find the depth, or locate submarines, shoals, wrecks and icebergs.
- Medical: ultrasound scans image organs and a foetus, and break kidney stones.
- Industrial: detecting internal cracks — a flaw returns the pulse early — and cleaning hard-to-reach parts.
- Ultrasound suits these jobs because of its short wavelength, which gives narrow beams that reflect from small objects.
Calculate the depth of the sea from a SONAR time and then the distance to a cliff from an echo time, using the right speed for each — getting both the factor of two and the medium right is the whole of this topic.
- The surface must be large and hard, but need not be polished. Soft surfaces absorb rather than reflect.
- Uses of reflection: megaphone, stethoscope, curved soundboard, ear trumpet.
- Echo formula: , so — the sound makes a round trip.
- A clap echoing after s at m/s means a cliff m away. A wall m away returns the echo in s.
- **Persistence of sound in the ear is about s**, so the minimum distance is m, or m at m/s.
- At m the echo takes s and is heard; at m it takes s and is not.
- In water at m/s, an echo after s means a depth of m, and after s, m.
- Use the speed for the right medium — air and water differ by more than a factor of four.
- Reverberation is the persistence of sound from many overlapping reflections; an echo is one distinct repetition.
- Reduce reverberation by absorption: fibreboard, rough plaster, curtains, carpets, padded seats, and a shaped ceiling or soundboard.
- Absorption, not blocking — and a little reverberation is desirable, especially for music.
- Infrasonic is below ** Hz — pendulums, earthquake tremors, whales and rhinoceroses. Ultrasonic is above Hz — bats, dolphins, porpoises.
- Echolocation**: a bat times its returning ultrasonic pulses and applies .
- SONAR (echo ranging): a transmitter and detector on a ship find the depth, or locate submarines, shoals, wrecks and icebergs.
- Medical: ultrasound scans image organs and a foetus, and break kidney stones.
- Industrial: detecting internal cracks — a flaw returns the pulse early — and cleaning hard-to-reach parts.
- Ultrasound suits these jobs because of its short wavelength, which gives narrow beams that reflect from small objects.
Calculate the depth of the sea from a SONAR time and then the distance to a cliff from an echo time, using the right speed for each — getting both the factor of two and the medium right is the whole of this topic.