Smartphone sound level meter: measuring sound levels

A sound level meter is an instrument that measures the sound level, a quantity related to the intensity of a sound, expressed in decibels (dB) on a logarithmic scale. A conversation is about 60 dB, a blender 90 dB, and the pain threshold lies between 120 and 140 dB depending on the individual.

Discover FizziQ

How to measure it in class

On a smartphone, the sound level meter is a software construction built on the microphone: the acoustic pressure signal is digitised, then converted into a level expressed in decibels displayed in real time. FizziQ displays the sound level from 0 to 110 dB and tracks its evolution over time; a companion instrument, the noise level, gives the average of the ambient level. Absolute accuracy however depends on the device’s microphone, which is not calibrated.

Steps:

Open FizziQ and select the Sound level meter among the microphone measurements.
Place the smartphone in a quiet spot and read the background level (often 30 to 40 dB).
Produce a sound (voice, hand clap, sustained note) and watch the level rise in dB.
Move the source away then closer to observe the level fall and rise with distance.
Start a recording to plot the sound level over time and export it.
Compare two environments (corridor, classroom, playground) by reading their average levels.

Scientific activities on this topic

Possible extensions with FizziQ: study of the decrease of the sound level with distance to the source, comparison of the sound environments of a school, or measurement of the attenuation provided by an insulating material. See the smartphone sensors guide.

Learn more: how it works

The microphone converts acoustic pressure variations into an electrical signal, which the analog-to-digital converter samples, typically at 44,100 Hz. From these samples, the software computes the root mean square (RMS) value of the pressure over a short time window, then derives a level in decibels through the logarithmic relation. This last step is where the problem lies: converting an electrical signal into an exact absolute pressure requires knowing the microphone’s sensitivity precisely. But it varies from one smartphone to another and is not certified. The software sound level meter is therefore not calibrated to the standard of professional devices (IEC 61672): it is excellent for comparing levels and measuring differences, but approximate in absolute terms.

Orders of magnitude:

Common audio sampling rate: 44,100 Hz.
Audible range: 20 Hz to 20,000 Hz.
Reference pressure of the sound level: p0 = 20 µPa (hearing threshold at 1 kHz).
Quiet library: about 30 dB; conversation: about 60 dB; heavy traffic: about 80 dB.
Hearing risk threshold for prolonged exposure: from 85 dB; pain threshold: above 120 to 140 dB.

Formula

The sound level L is computed from the effective acoustic pressure p:

L = 20 log(p / p0), with L in decibels (dB), p and p0 in pascals (Pa) and p0 = 20 µPa.

Multiplying the pressure by 10 corresponds to a rise of 20 dB; doubling the pressure corresponds to about +6 dB.

Application examples

Compare the sound level of a full and an empty classroom.
Check the effectiveness of noise-cancelling headphones or an insulating panel.
Measure the decrease of the level when moving away from a loudspeaker.
Track the ambient noise of a playground over the day.
Illustrate the logarithmic nature of the decibel scale by adding two sources.
Raise awareness of hearing risks by reading the level produced by audio headphones.

FAQ

Q: Is my smartphone’s decibel reading reliable? A: For comparing levels and measuring differences, yes. In absolute terms, no: the microphone is not calibrated to the IEC 61672 standard, so the displayed value can differ from the true value by several decibels.

Q: Why does the level never drop to 0 dB? A: 0 dB corresponds to the hearing threshold, an extremely small pressure. There is always residual background noise, including from the microphone itself, which keeps the level above a few tens of decibels.

Q: Can I directly add two levels in decibels? A: No. The scale is logarithmic: two independent 60 dB sources do not give 120 dB but about 63 dB, because it is the sound intensities that add up, not the decibels.

Decibel - Sound intensity - Sound level - Microphone - Noise pollution