What if there were no echoes?

Stand in a big, empty hall and clap. Clap... clap! You might hear it come back.

That is an echo. It is sound bouncing back. What if sounds never bounced back?

Clap in a big empty gym or shout into a canyon, and a moment later you may hear your sound again. That's an echo: sound that has bounced off a wall or a cliff and come back to your ears.

Echoes might seem like a fun trick, but many animals, ships and doctors depend on them. This lab is a thought experiment. What if sound could never bounce back?

Sound is a pressure wave. When it meets a boundary where the material changes, such as air meeting rock, part of it is reflected. OpenStax notes that sound is most strongly reflected where density changes are greatest, and that the time for an echo to return is directly proportional to the distance of the reflector.

This thought experiment removes reflection of sound and follows three threads: echoes in caves and halls, echolocation by bats and dolphins, and sonar mapping of the sea floor.

Bats call out as they fly. The sound bounces off bugs and comes back. Then the bat knows where the bug is!

Ships send sounds to the sea floor and listen for them to come back.

In air, sound travels about 343 meters every second, OpenStax explains. So the farther away a wall is, the longer an echo takes to come back.

Animals use this. The National Park Service explains that bats send out sounds too high for us to hear, called ultrasound, which bounce off objects and return to their ears. NOAA's Ocean Today describes dolphins sending clicks from the rounded part of their head, called the melon; the sounds bounce off things and return, giving the dolphin a picture of what's around it.

At 20 °C sound travels at 343 m/s in air (OpenStax). A bat uses echoes to find its way and catch prey; the echo delay is directly proportional to the distance, so a reflector 1.7 m away returns an echo in about 10 ms (our arithmetic: 2 × 1.7 / 343).

NOAA's active sonar emits a pulse into the water; if an object is in the path, the sound bounces off and returns, and the time between emission and reception gives its range. Medical ultrasound uses the same principle in the body.

Imagine sounds never bounce back.

A big cave might sound very quiet. Bats might have trouble finding bugs. Ships might not know how deep the water is.

Imagine sound could never bounce. You could still hear a friend talking to you, because that sound comes straight to your ears. But a canyon would never shout back, and a big gym might sound strangely flat.

Bats that hunt by echoes might struggle to find insects at night, and dolphins might need to rely more on their eyes. Ships might not be able to measure the sea floor with sonar, and doctors might not be able to use ultrasound to look inside the body. These are ideas.

Direct sound would still reach listeners, so speech and music would carry, but rooms would lose their reflected sound, and every technique based on timing echoes would fail: bat and toothed-whale echolocation, active sonar and diagnostic ultrasound.

Passive sonar, which only listens for sounds coming toward it, would still work, since NOAA notes it emits no signal of its own. How animals and people would adapt is speculation; these consequences are reasoned ideas.

Sound is a wiggle that travels through the air.

When it hits something hard, like a cliff, it bounces back, like a ball off a wall.

Sound travels as a wave, a push passed from particle to particle through the air. When that wave reaches something very different from air, such as a rock wall or a cliff, much of it bounces back instead of going in.

The bounced sound travels back at the same speed it went out, so if you know how long the echo took, you can work out how far away the wall is. Bats, dolphins and ships all use that trick.

OpenStax explains for ultrasound that the intensity of a reflected sound is related to changes in density, with the strongest reflections where density changes most, which helps explain why air-to-rock or water-to-seabed boundaries echo strongly. The round-trip time t gives the distance d = vt/2.

OpenStax adds that bats and dolphins may also sense the velocity of prey from the Doppler shift of their echoes. This lab uses only these qualitative ideas; no model computes the consequences.

Doctors use echoes to make pictures inside your body. It doesn't hurt at all!

Ships use echoes to map the bottom of the sea.

NOAA scientists use sonar to make charts for ships, find hazards and shipwrecks, and map the sea floor. Sound is used because it travels farther in water than radar or light.

Doctors use echoes too. OpenStax explains that medical ultrasound sends short beeps into the body and records the echoes from different depths. Because echoes return later from deeper places, the machine can build a picture, for example to check that a baby is growing well before it is born.

NOAA uses active sonar for nautical charts, hazard detection and seafloor mapping, because sound travels farther in water than radar and light. Passive sonar, by contrast, only listens.

In medicine, OpenStax describes an ultrasound transducer that doubles as speaker and microphone: brief bleeps go out, echoes are recorded from various depths, and the delay of each echo, directly proportional to the reflector's distance, maps tissue noninvasively. Doppler-shifted echoes even measure blood flow.