Why Does My Decibel Reading Keep Jumping?

Q: Why does my decibel meter keep changing every second?

Because real sound arrives in bursts and the live number follows them. Swings of 10-20 dB per second are normal on the instantaneous readout. Use Slow response to calm the display and quote the Leq average, which stabilises over 30-60 seconds.

Short answer: your meter is telling the truth — real sound flickers several decibels every second, and the big live number follows those flickers. Stable, quotable numbers come from the slower statistics underneath: Slow response, and above all the Leq average. Below: what each reading means, a 60-second trace showing all three at once, and a routine that gives you one trustworthy number every time.

The 30-second answer: sound flickers, averages do not

Speech syllables, passing cars, clattering dishes — everyday sound arrives in bursts, and each burst moves the live number. Swings of 10–20 dB within seconds are completely normal on the instantaneous readout; they mean the meter is fast, not broken. Professionals never quote that flickering digit. They quote time-weighted statistics: Slow (a one-second smoothing that calms the display), Leq (the energy average over the whole session — the number that goes in reports), and Maximum/Peak (the loudest event, for bangs and barks).

Rule of thumb for our online decibel meter: watch the live number, quote the Leq. If the Leq itself keeps drifting after a minute, the sound genuinely comes and goes — extend the session or split it into "with event" and "background" lines instead of chasing one number.

One minute of traffic, three ways to read it

The same 60 seconds of street sound through three lenses. The instant trace jumps 23 dB; the Slow trace breathes gently; the Leq settles on one quotable number.

Figure 1 — Illustrative 60-second traffic trace. The live number swings 23 dB; Slow calms it to a 12 dB band; Leq converges on 64 dBA — the only number worth quoting.

Fast vs Slow vs Leq vs Max: the comparison table

Five numbers, five jobs. Fast and Slow are time weightings (how quickly the display follows sound — 125 ms vs 1 s exponential averaging, per IEC 61672). Leq is an energy average over your whole session. Maximum and Peak remember the loudest moment.

Comparison of Fast, Slow, Leq, Maximum and Peak readings
Reading	How it behaves	Quote it when…
Instant / Fast	Follows every burst, jumps 10–20 dB	Never quote — watch only, for finding sources
Slow	1-second smoothing, breathes gently	Describing typical level live ("sitting near 65")
Leq	Energy average, stabilises over 30–60 s	Always — exposure, comparisons, logs, complaints
Maximum	Loudest Slow-weighted moment held	Bangs, barks, pass-bys, sleep disturbance events
Peak	Loudest instantaneous pressure, dBC/dBZ	Impacts and blasts — fireworks, gunshots, slams

One subtlety worth knowing: Leq is an energy average, not an arithmetic one — loud seconds count more than quiet ones, which is exactly why it matches hearing risk and annoyance better than "the number I saw most". Feed it into the exposure calculator with your duration to get a dose percentage, and read the Leq meter guide for the full method.

Worked example: turning a flicker into a log line

A real-feeling 60-second session outside a café on a busy street, phone on the table, mic uncovered:

Worked example turning flickering readings into a log line
What you saw	Value	What to do with it
Live number swinging	55–78 dBA	Ignore as a quote — it only shows traffic comes in bursts
Slow hovering	≈ 62–68 dBA	Fine for conversation ("typically mid-60s")
Leq, 60 s	64 dBA	The quotable number — log it with place and duration
Maximum (bus pass-by)	78 dBA	Note alongside Leq — explains the annoyance

Final log line: "Café terrace, 3 m from kerb: 64 dBA Leq / 78 dBA max, 60 s, Slow." One line like that is worth more than ten screenshots of flickering digits — and repeats from the same spot should agree within a few dB. Compare against everyday sounds and the full chart to sanity-check.

The 60-second stable-reading routine

1. Freeze the setup. Phone at ear height where people are, mic pinhole uncovered, case clear, yourself silent — your own voice adds 10–20 dB and your grip adds handling thumps. 2. Set Slow response and dBA. Slow calms the display; dBA matches perceived loudness and every exposure limit (add a dBC pass when bass dominates — see dBA vs dBC). 3. Record 60 seconds without moving. Watch Leq converge; note Leq, minimum (the background), and maximum (the events). 4. Repeat once. A second 60-second run from a nearby spot should land within ~3 dB — if it does, you have a trustworthy number; if not, something pulses nearby, so log both lines and say so.

If readings still misbehave, suspect the usual culprits before the meter: a covered mic, automatic gain control re-enabled by the phone (check the Device panel), wind outdoors (+10 dB artefacts unshielded), or calibration drift between devices — covered in the phone-and-PC guide, are apps accurate, and what affects accuracy.

Frequently asked questions

Why does my decibel meter keep changing every second?

Because real sound arrives in bursts — speech syllables, engines, clatter — and the live number follows them. Swings of 10–20 dB per second are normal on the instantaneous readout. Use Slow response to calm the display and quote the Leq average, which stabilises over 30–60 seconds.

Should I use Fast or Slow response?

Use Slow for almost everything — rooms, streets, neighbourhood logs — because its one-second smoothing matches how we perceive steady noise. Use Fast only when timing short events precisely, such as capturing the peak of a pass-by. Either way, quote Leq for the session, not the live digit.

What is Leq and why should I quote it?

Leq is the energy average of sound over your whole session — loud seconds count more than quiet ones, matching hearing risk and annoyance. It is the number professionals report, the input to exposure calculators, and the value that repeats within a few dB between runs. Quote it as "64 dBA Leq, 60 s".

My two phones show different readings — which is right?

Probably both are honestly reporting different microphones: capsules vary in sensitivity and each phone applies its own gain and noise processing, so 5–10 dB gaps on the same sound are normal uncalibrated. Calibrate each device against a known reference, then compare Leq values — not live digits — measured at the same spot.

How long should I measure for a stable reading?

At least 30 seconds for steady rooms, 60 seconds as the default, and 10–15 minutes for intermittent neighbour or traffic noise so the Leq captures the pattern. Repeat once from a nearby spot: agreement within about 3 dB means the number is trustworthy.

Sources and further reading

IEC 61672-1 — Electroacoustics: sound level meters; Fast (125 ms) and Slow (1 s) time weightings, Leq definition.
NIOSH — Criteria for a Recommended Standard: Occupational Noise Exposure (Leq-based dose, 3 dB exchange rate).
EU Physical Agents Directive 2003/10/EC — Leq-based action values and exposure limits.

Why Does My Decibel Reading Keep Jumping? Fast vs Slow vs Leq