Measuring white noise dB at the crib using a free smartphone app

Published 2026-05-18 · 8 min read

Informational reference, not medical advice. Smartphone apps are adequate for nursery checks against the AAP 50 dB ceiling. For clinical audiometric assessment, see a pediatric audiologist.

The AAP 2023 sound machine guidance specifies a number (50 dB at the baby's head) and a distance (at least 7 ft from the crib). Both are operationally meaningless unless you actually measure. Manufacturer spec sheets quote dB at the device face, which is not where the baby is. Volume knob positions are not labelled in dB and vary by machine. The only reliable verification is to measure at the head position with whatever machine you have, at the volume you actually use. You do not need a professional meter for this; a free smartphone app is adequate for the precision the AAP guidance requires. This page walks through both major options screen by screen.

The recommended tool on iOS is the NIOSH Sound Level Meter (SLM) app, free, developed by the U.S. National Institute for Occupational Safety and Health and validated against laboratory reference instruments. On Android, Decibel X is the closest consumer-grade equivalent. Both will give you a usable answer in under 2 minutes per check.

Why measurement matters more than spec sheets

Three reasons spec sheets fail. First, manufacturer dB ratings are typically measured at the device face (often 30 cm or 1 ft), not at typical placement distance. A machine rated “65 dB at 1 ft” reads roughly 59 dB at 2 ft and 53 dB at 4 ft, following the inverse square law. The same machine is either compliant or non-compliant depending only on placement.

Second, volume knob positions are not standardised. “Medium” on one machine is 45 dB; “medium” on another is 58 dB. Rotary knobs have no labelling at all. Stepped digital volumes (1-10, 1-30) have no fixed dB mapping.

Third, the room matters. A small carpeted nursery with curtains absorbs sound; a hard-floor room with bare walls reflects it. Your nursery's acoustic signature can swing the head-level dB by 2 to 4 dB versus an identical machine in a different room. Spec sheets cannot account for this; measurement does, by definition.

iOS: NIOSH Sound Level Meter walkthrough

Download the NIOSH Sound Level Meter (SLM) from the App Store (search “NIOSH SLM”; publisher is the U.S. National Institute for Occupational Safety and Health). It is free, ad-free, and has no in-app purchases. The app was developed in collaboration with EA LAB and has been validated against IEC 61672 Class 2 standard reference instruments in published peer-reviewed work.

Open the app. You will see a large dB readout, a level history graph, and three primary metrics: instantaneous SPL, LAeq (time-weighted average), and Lmax (peak). The default weighting is A; the default time response is Slow (1-second integration). Both defaults are what you want.

Procedure:

  1. Start the sound machine at your normal nightly setting.
  2. Wait 30 seconds for the sound to stabilise.
  3. Place the iPhone at the baby's head position: mattress level, head end of the crib, slightly off centre, microphone end up and free of the mattress.
  4. In the SLM app, tap the start (play) button.
  5. Hold position for 60 seconds (without moving the phone or talking near it).
  6. Tap pause. Read the LAeq number, not the instantaneous SPL. The LAeq is what the AAP guidance refers to.
  7. If LAeq is at or below 50 dB, you are compliant. If above, reduce volume or move the machine further away.

The SLM app also supports calibration against an external sound source if you want to verify accuracy. The default uncalibrated reading is within +/- 2 dB of laboratory reference for modern iPhones (per NIOSH validation studies), which is more than adequate for nursery checks.

Android: Decibel X walkthrough

Decibel X is the closest Android equivalent. It is free with an optional paid upgrade for additional logging features; the free tier covers everything needed here. It is not laboratory-validated to the NIOSH standard, but is reasonably accurate in independent testing against Class 2 reference meters.

Open the app. You will see a similar layout: large dB readout, history graph, instantaneous SPL plus average and maximum. Default weighting is A, default time response is Fast (125 ms). For nursery checks, switch the time response to Slow in settings if your version supports it; if not, the Fast default is acceptable provided you let the average stabilise.

Procedure mirrors NIOSH SLM: start the machine, wait 30 seconds, place phone at head position, start recording, hold for 60 seconds, read the average (Lavg or LAeq depending on Decibel X version). At or below 50 dB is compliant.

Android phone microphones vary more than iPhones, so individual-device accuracy can be off by +/- 3 to 5 dB. To check your phone's offset, take a reading next to a known reference (a conversation at normal volume should read 60 to 65 dB; a quiet bedroom at night should read 30 to 40 dB). If your phone reads systematically high or low, mentally adjust the nursery reading by the same offset.

Common measurement mistakes

Measuring at the wrong height. Sound intensity falls with distance from the source. A measurement taken at the side rail of the crib (often at 50 to 80 cm height) reads higher than one at the mattress level where the baby's head actually is. Use mattress level, head end, every time.

Talking or moving during the recording. The microphone picks up everything. A whispered comment to a partner during the 60 second window will spike the instantaneous reading and pull the average up. Stay silent and still.

Reading the instantaneous SPL instead of the LAeq. The instantaneous number flickers; the LAeq is the time-weighted average and is the meaningful figure for chronic exposure. AAP guidance refers to average exposure, not instantaneous peaks.

Measuring once and considering the job done. The baby's position can change (rolling, growing into a different crib position), the machine's volume can drift on certain models, and the room can change acoustically (new curtains, moved furniture). Re-measure every 2 to 3 months and any time you change anything about the setup.

What to do if you measure above 50 dB

Three escalating responses, applied in order.

First, reduce volume on the machine. Most readings 51 to 60 dB can be brought into compliance by stepping the volume down one or two notches. Re-measure to confirm.

Second, increase distance. If the minimum volume on your machine still reads above 50 dB at the current position, move the machine further away. Each doubling of distance drops dB by about 6, so a machine reading 53 dB at 5 ft typically reads about 50 dB at 7 ft and 47 dB at 10 ft.

Third, replace the machine. If the lowest setting at the maximum reasonable distance (limited by room size) still reads above 50 dB, the device is too loud for nursery use. Many older or adult-oriented machines (the Marpac/Yogasleep Dohm Classic at higher settings, for example) fall in this category. Adult-focused devices were not designed around the AAP infant ceiling; baby-targeted models like the Hatch Rest 2nd gen ship with factory presets explicitly below 50 dB.

Frequently asked

Is the iPhone microphone accurate enough?

Yes, per NIOSH validation. The NIOSH SLM app paired with a modern iPhone reads within +/- 2 dB of Class 2 reference instruments. That is more than sufficient for the 50 dB threshold check.

What about a smart speaker or Echo Dot built-in noise meter?

Echo devices do not have a calibrated SPL meter. They have a microphone for voice input, but no public API for accurate dB readings. Use a phone app instead.

Can I leave the meter running all night?

You can. Both apps can log overnight. The result is the LAeq across the full window, which is the most clinically meaningful number. For first-time setup verification, a 60-second sample at the chosen setting is enough.

Does the AAP require me to measure?

The statement does not mandate measurement, but treats the 50 dB number as the operational threshold. Without measurement you cannot verify compliance; you are relying on manufacturer claims and intuition, neither of which is reliable.

Sources

Baby Safety overview →The 50 dB ceiling explained →The 7 ft placement rule →All-night vs sleep timer →Sound machine buyer's guide →

Updated 2026-04-27