Sonar
How submarines see in the dark
Below about 200 m the sea is dark, so ships do what Sonar Ruler does: make a sound and time its echo.
ReadSonar Ruler 2 · for iPhone and iPad
Measure with sound.
Your iPhone chirps and listens for the echo. The time it takes becomes a distance.
Get Sonar Ruler App StoreHow does it work?
Sonar Ruler measures two ways: from one phone to a wall, or between two phones.
1 In the app: Live Wall
Single Phone Mode
Aim your phone's bottom edge at a wall. It chirps from its bottom speaker and listens, and every echo shows up under the tape. The wall's echo is the one that moves when you step closer.
- Point the bottom edge at a wall, or up at the ceiling.
- Echoes glow under the tape. Step closer: the wall's moves with you.
- Tap it to track it. Save when it says STEADY.
2 In the app: Two Phones
Two Phone Mode
Lay two phones down with their bottom edges facing. They take turns chirping, about three times a second. Each one hears both chirps, so their own delays cancel out.
- Open Sonar Ruler on both phones. The other one needs the app too.
- Lay them down, bottom edges facing, nothing in between.
- Wait for STEADY, then tap Save.
Works between an iPhone and an iPad too.
3 In the app: Quiet, in either mode
Quiet Mode
Quiet moves the chirp up to 17.5 to 20 kHz, above what most grown-ups can hear. Kids, and your dog, may disagree.
Silent preview of Quiet
It starts quiet, so turn it up slowly. Laptop speakers often can't play Quiet at all.
How to turn it on
In Live Wall, pick Quiet as the chirp. In Two Phones, both phones need Quiet on.
The catch
It needs the volume near the top, it doesn't reach as far, and background noise can break it.
The science
Sound travels about 343 metres a second in room-temperature air. Sonar Ruler times the round trip, from the speaker to the wall and back, halves it, and turns the time into a distance.
The echo takes 5.31 ms there and back. Half of that is 2.655 ms, and 2.655 ms times 343.2 m/s is 0.911 m.
Sound is faster in warm air.
v=331.3 × 1 + T273.15 m/sthe app's formula
=331.3 × 1 + 20273.15at 20°C
=343.2 m/sthe speed of sound
At 20 °C sound travels 343.2 m/s, so an echo that takes 5.31 ms is 0.911 m away.
Other features
- History
- Save a reading, add a note, share it.
- Temperature
- Set the room's temperature on a little wheel. Sound is faster in warm air.
- Metric or imperial
- Metres and centimetres, or feet and inches.
- iPhone and iPad
- Either one, or one of each.
- Private
- No accounts, no ads, no tracking. Nothing leaves your phone.
Read more about sound
Ultrasound
Seeing inside the body with sound
A baby scan is sonar too: about a thousand times higher in pitch than the app's chirp, with its echoes back in millionths of a second.
ReadPhysics
Why sound is faster on a warm day
Sound gains about 0.6 m/s for every degree, which is why a ruler made of sound asks for the temperature.
ReadGuide
How to get better measurements with sound
Which mode to use, how to find the right echo, how far Sonar Ruler reaches, and what gets in the way.
ReadMore soon.
For fun and learning. Use a tape measure when it has to be exact.