How to get better measurements with sound

Sonar Ruler times a sound and turns that time into a distance. So better measurements come down to three things: give it one clear sound to time, make sure it's timing the right one, and keep still while it does.

The short version

1. If you have a second phone, use Two Phone Mode.
2. With one phone, stay out of small rooms and aim at a big, hard, flat wall.
3. Step toward the wall. Its echo is the one that moves.
4. Cut the background noise, stay still and quiet, and wait for STEADY.
5. Keep to sensible distances: about 50 cm to 3 m with one phone, or 10 cm to 3 m with two.

Use two phones if you can

Single Phone Mode (Live Wall in the app) hears everything the room sends back: the wall you're aiming at, but also the floor, the ceiling, the other walls and the sofa. The phone can't tell which echo is which, so that part is up to you.

Two Phone Mode (Two Phones in the app) doesn't use echoes at all. The phones take turns chirping, and each one listens for the other. The straight line between them is the shortest way there, so the chirp that goes straight arrives first, and that's the one they go by. The room's echoes come in later and make little difference.

One phone

Two phones

That makes it better in a few ways:

• There's no echo to find. No stepping and no tapping. Lay the phones down and wait.
• The sound is stronger. An echo makes the trip twice and loses some at the bounce. A chirp between two phones makes the trip once.
• It doesn't need a wall. It measures between two spots, so a soft sofa or an open doorway is no problem: put a phone there.
• It works closer. Down to about 10 cm, where one phone can't hear past its own sound.

The catch: the other phone needs the app too (Share App puts a QR code on screen for it). Lay them down with their bottom edges facing and nothing in between, and that includes you. A person standing between the phones can block the straight path.

To measure to a wall this way, lay the second phone with its top edge against the wall, then add that phone's length to the reading.

Avoid small rooms

With one phone, the main problem is too many echoes. Every surface sends the chirp back, and each corner sends back an echo of its own, a little behind the wall's.

In a big room the other walls are far away, so their echoes land well clear of the one you want. In a small room they all land together, and the wall's echo gets lost in the crowd. A bathroom, a closet or a narrow hall is about the hardest place to measure.

Small room

Big room

• Use the biggest room you can.
• Stay out of corners, and aim at the middle of the wall.
• Move closer to the wall you care about. A nearer wall makes a stronger echo.
• Clear the way. Furniture and doorways make echoes too.

Find the wall's echo by stepping toward it

Echoes all look alike on the screen, so don't guess. Step slowly toward the wall, just a few centimetres at a time, and watch. The wall's echo moves with you. The floor, the ceiling and the side walls stay where they are, because you're no closer to them than you were. A wall behind you moves the other way.

When you've found it, tap it to track it. If you rush, the app can lose it and ask you to tap it again.

Rule out the other echoes

It works the other way round too. Move toward or away from a surface you don't care about, and its echo is the one that moves:

• Raise or lower the phone. The floor's echo and the ceiling's move. The wall's stays put.
• Step sideways. A side wall's echo moves. The wall ahead stays put.

Anything that moves now isn't the wall in front of you, so you can rule it out.

Step toward the wall Step sideways Raise the phone

Step toward the wall: its echo moves with you. The others stay put.

Keep the room quiet and stay still

Echoes are faint, and noise buries them. When it does, Single Phone Mode's labels thin out and go, and Two Phone Mode waits for a quiet moment before it starts.

• Turn off music, the TV and anything else making noise.
• Stop talking while it measures.
• Hold the phone still. Handling it makes noise too, and a moving phone moves every echo.
• Keep your fingers off the bottom edge. The speaker and the microphone are there.

Aim at something big, hard and flat

Point the bottom edge straight at the wall, with the phone flat. On an iPad it's the top edge, where its microphones are. An iPad also shows every echo with a twin about 11 cm behind it, so tap the nearer of the pair.

Hard surfaces bounce sound back, and soft ones soak it up. At 2 kHz, brick absorbs about 5% of the sound that hits it and a heavy curtain about 70%. Small things just send too little back.

• Good: walls and ceilings. Pointing up at a ceiling works just as well as aiming at a wall.
• Poor: curtains, sofas and people, and small things like a cup or a chair leg.

If you're checking a reading against a tape measure, keep the tape out of the chirp's way. A metal tape makes an echo of its own.

How far can Sonar Ruler measure?

These are rough guides from our own tests, with two iPhones and an iPad in ordinary rooms. They aren't limits or promises: your phone, your room and your wall will all change them.

Live Wall's screen opens on 0 to 4 m. Pinch to zoom out and it shows up to 6 m, where its readings stop, and a double tap takes it back. Past 3 m the wall's echo gets weak and the room's other echoes crowd it, so it's worth trying, but don't expect much.

Quiet Mode's limits

Quiet moves the chirp up to 17.5 to 20 kHz, above what most grown-ups can hear. That's handy when you don't want to bother anyone, but it costs a lot:

• It needs the volume near the top. Below 80% the app says so. At 45%, an iPhone 18 Pro heard no wall at all.
• It doesn't reach as far. See the table above.
• Background noise can break it. Its echoes are so faint that a click or a cough can swamp them.
• Its echoes are blurry. The narrower a chirp's range of pitch, the blurrier its echo. Quiet has 2.5 kHz to work with and the Sweep has 14 kHz, so Quiet's echoes show about 17 cm wide on the screen against the Sweep's 3 cm. Echoes close together merge into one.
• In Two Phone Mode, both phones need it on. A phone on Quiet hears only Quiet. Its readings also shift more with where the phones lie.
• Children and pets may still hear it.

So save Quiet for when the sound would bother someone, and use the audible chirps for your best measurement. Turn the volume down before you switch back, too: at the top of the range, the Sweep is loud.

Check the volume and the temperature

Volume. The app tells you when it's too low: "Turn the volume up." shows under about 40% in Single Phone Mode and 25% in Two Phone Mode. You don't need full volume for the audible chirps.

Headphones. Sonar Ruler needs the device's own speaker and microphone. Unplug headphones and turn off Bluetooth audio while you measure.

Temperature. Sound is faster in warm air, so a wrong temperature puts every reading off by the same share: about 3.4 cm on a 2 m reading for every 10 degrees. Tap the temperature pill and roll the wheel to the room's temperature. In Two Phone Mode, set it on both phones, because each one does its own sum.

The chirp. In Single Phone Mode, the small button in the bottom row picks it. The Sweep is the default, with the sharpest echoes and the farthest reach. Chirps are as sharp with a little less reach, and steadier while you move.

Wait for STEADY, then check the reading

Save when it says STEADY, not before. It takes a second or two of holding still.

STEADY only means the last few readings agree. It can't tell you that you're on the right echo. So check:

• Move and watch. Step back 10 cm (4 in) and see if the reading grows by about that much. If it doesn't, you're on the wrong echo: tap the red × and pick again.
• Take a few. Save readings from slightly different spots and compare them in History. If one is far from the rest, suspect an echo.
• Compare the two phones. In Two Phone Mode both show the same distance. If they don't, check that they're set to the same temperature.
• Keep the setup the same. A case changes the sound near the speaker a little, so leave it on or off for all your readings.
• Know where it measures from. Single Phone Mode measures from the speaker at the bottom edge. Two Phone Mode measures between the two bottom edges. If you want the distance from the top of the phone, add its length.

When it has to be exact

Sonar Ruler is for fun and learning. Even after all this, a reading still depends on the room, the air and the echoes. That's what makes it fun to play with. Use a tape measure when it has to be exact.

Sources

• C. Peng, G. Shen, Y. Zhang, Y. Li and K. Tan, BeepBeep: A High Accuracy Acoustic Ranging System using COTS Mobile Devices, ACM SenSys 2007 (two phones timing each other's chirps, and going by the earliest arrival)
• Wikipedia, Absorption (acoustics) (the 2 kHz figures, from B. Parker, Good Vibrations: The Physics of Music, 2009)
• Handbook for Acoustic Ecology, Absorption Coefficient
• Wikipedia, Pulse compression (why a wider sweep gives a sharper echo)
• The ranges, volume levels and echo widths are from Sonar Ruler's own test notes, on an iPhone 18 Pro, an iPhone 13 Pro and an 11-inch iPad Pro, October and November 2026