How to Measure Your Sound Machine’s Volume With Your Phone

Why measure at all

Most sound machines and apps, including ours, have a volume knob or slider but no idea how loud they actually are in your room. The same setting can be gentle on one speaker and loud on another, and the level your baby hears depends on where the speaker sits. A 2014 study in Pediatrics found that at maximum volume, all 14 infant sleep machines it tested were louder than 50 dBA at 30 cm, and 13 of 14 still were at 200 cm. You can read more about that in how loud a baby sound machine should be.

The good news is that you probably already own a usable sound meter. The American Speech-Language-Hearing Association (ASHA) notes that you can use “a sound level meter application (e.g., on your smartphone) to measure sound levels.”

A quick word on dBA

Decibels (dB) measure sound level. The scale is logarithmic, so the numbers don’t add up the way you’d expect, and a few decibels can matter. Most meters offer several “weightings.” For this job you want A-weighting, shown as dBA or dB(A). In ASHA’s words, “the term ‘dBA’ describes the way human hearing perceives sound.” All the benchmarks you’ll see for babies and hearing, including the 50 dBA nursery figure and the 85 dBA workplace limit, are A-weighted. So your reading has to be too, or you aren’t comparing like with like.

Which app to use

On an iPhone

The free NIOSH Sound Level Meter app comes from the US National Institute for Occupational Safety and Health, part of the CDC. It was built to measure workplace noise, which is overkill for a nursery, but it is free and comes from the same agency whose researchers tested the accuracy of phone sound apps (more on that below). It reports A, C and Z weightings and an average level called LAeq, among other readings.

On Android

NIOSH offers its app only on iOS. The agency says testing and verifying “an Android-based app in our laboratory is not currently possible,” because so many manufacturers use different hardware. Android sound meter apps exist and can still give you a useful ballpark. Just know that NIOSH’s own testing found high variation between devices, so treat the number more loosely.

How accurate is a phone, really?

The best evidence comes from two NIOSH studies by Kardous and Shaw in the Journal of the Acoustical Society of America.

In the first (2014), they evaluated more than 130 iOS apps. Only 10 met their selection criteria for testing. Of 62 Android apps, only 4 could be tested. They played pink noise at levels from 65 to 95 dB in a lab and compared each app to a professional reference meter. The best two iOS apps were very close, with average differences of 0.07 dB (unweighted) and −0.52 dB (A-weighted). Two others were within ±2 dB. The Android apps “revealed a high variance in measurements of similar apps between different devices.”

In a 2016 follow-up, they tested four iOS apps with calibrated external microphones plugged in. The results were within ±1 dB of the reference. The authors concluded that an external microphone “greatly improves the overall accuracy and precision of smartphone sound measurements.” The NIOSH app’s own page says it is “accurate within ± 2 dBA” and meets a Type 2 sound level meter standard “with calibrated external microphone.”

The part that matters for a nursery

Look at the test range. Both studies tested from 65 to 95 dB, because they were about workplace noise. The level you care about at the crib is around 50 dBA, which is below anything they tested. The 2016 paper also notes that the tiny microphones in phones have limits in dynamic range and signal-to-noise ratio. So these studies tell us good phone apps are accurate for loud sounds, but they don’t tell us how accurate they are at quiet ones.

That doesn’t make your reading useless. It means a phone is good for answering “is this roughly 50 or roughly 65?” and not for telling 49 from 52. If you get a borderline result, turn it down a little. That is the cautious direction anyway.

Step by step

1. Set the app to A-weighting. Look for dBA or dB(A). If the app has a slow or averaging mode, or shows LAeq, use that rather than the fast-jumping number.
2. Take a baseline with everything off. Turn off the sound machine and stay quiet. Lay the phone in the crib where your baby’s head goes and note the reading. This tells you how loud the room is on its own. For reference, ASHA lists a quiet room at 40 dBA.
3. Set up exactly as at bedtime. Put the machine or speaker in its usual spot, at the volume you actually use. Close the door, run the fan or heater, whatever you normally do at night.
4. Place the phone at your baby’s head. Lay it on the mattress where your baby sleeps, with the microphone uncovered. Don’t measure next to the speaker. That only tells you about the speaker.
5. Step back and stay quiet. Your voice, breathing and footsteps all count. Leave the room or stand still at a distance.
6. Watch for about a minute. Sounds like waves or rain rise and fall. An average over a minute is more meaningful than a single moment. Note the average and the highest reading.
7. Compare and adjust. If the average is above about 50 dBA, turn the volume down or move the speaker farther away, then measure again. Repeat until you’re at or below it, or as low as still masks the noise that wakes your baby.
8. Write it down. Note the setting and distance that gave a good result, so you don’t have to guess next time.

If the baseline is already close to your reading

If the room reads, say, 45 dBA with nothing on and 47 dBA with the machine running, the machine isn’t adding much. Your phone is mostly hearing the room, and you are almost certainly in a gentle range. On the other hand, if the machine reading is far above the baseline, the machine is the main thing your baby is hearing, and the number is worth paying attention to.

When to measure again

• You switch to a different speaker, phone, tablet or laptop.
• You move the crib, the speaker or big furniture.
• You turn the volume up during a rough patch (teething, a noisy neighbour, travel).
• You change to a different sound or mix. Measure the one you actually use.
• You’re in a new room, such as a hotel or grandparents’ house.

How loud are everyday sounds?

ASHA publishes a chart of average levels for common sounds. These values are copied from ASHA’s page. They are typical levels, not exact figures for any one appliance.

Two entries stand out for parents. A real vacuum cleaner is listed at 70 dBA and a real hair dryer at 94 dBA, and both are popular soothing sounds. Running the actual appliance next to a baby is a very different thing from playing a recording or a synthesized version at a low volume. Our Hairdryer and Vacuum sounds are generated in your browser, and how loud they are depends only on your slider and your speaker. The name of a sound tells you nothing about its level. Only measuring does.

ASHA’s chart calls levels up to 70 dBA safe for any amount of time, which matches NIDCD’s statement that sounds at or below 70 dBA are unlikely to cause hearing loss even after long exposure. That is about hearing damage in general, though, not a target for a sleeping baby. For babies, the cautious benchmark is still around 50 dBA at the head, with the machine kept well away from the crib.

Common mistakes

• Measuring at the machine. The reading next to the speaker is always higher than at the crib.
• Using an unweighted reading. dB(Z) or “flat” readings aren’t comparable with the dBA benchmarks.
• Reading the peak instead of the average. A wave crash or thunder clap spikes the number. The average gives a steadier, fairer picture.
• Talking while you measure. Normal conversation is around 60 dBA on ASHA’s chart, which will swamp a quiet machine.
• Measuring once and forgetting. Any change to the speaker, room or volume changes the answer.

What we don’t know yet

Nobody has tested phone sound meter apps specifically at the quiet levels used in nurseries. The best studies looked at 65 to 95 dB, and they were run years ago on the phones of the time. Newer phones and apps may be better or worse. Until someone checks, think of your phone as a sensible smoke detector for “too loud,” not a lab instrument. If you want more confidence, the NIOSH studies suggest a calibrated external microphone is the single biggest improvement. For most families, a phone reading plus a cautious margin is plenty. You can find all the related advice in our white noise safety checklist.