dB is the unit of sound level. dBA is a sound level measured through an A-weighting filter, which turns down deep bass and the very highest frequencies to roughly match how human ears respond at moderate volumes. For hearing risk, noise at work and most everyday questions, dBA is the number you want. NIOSH and OSHA limits are written in dBA. Unweighted (flat) dB is for when low-frequency sound itself is the issue, like a subwoofer through the wall or a humming transformer.
The same noise can read very differently in the two modes, so it’s worth knowing which one your meter is showing.
What does the A in dBA stand for? #
The A refers to A-weighting, a standard frequency curve defined for sound level meters in IEC 61672. Your ears are much less sensitive to low frequencies than to the middle of the range where speech lives, especially at everyday volumes. A-weighting builds that into the measurement. It was derived from the 40-phon equal-loudness contour first mapped by Fletcher and Munson, a curve showing how loud tones of different pitches have to be to sound equally loud to a listener.
In practice, the meter runs the microphone signal through a filter before calculating the level. Bass gets reduced a lot, the midrange passes through almost untouched, and the result tracks perceived loudness and hearing damage risk better than a raw number.
How much does A-weighting change the reading? #
These are the standard A-weighting corrections, the amount added to the sound at each frequency before the levels are combined:
| Frequency | A-weighting | C-weighting |
|---|---|---|
| 31.5 Hz (deep rumble) | -39.4 dB | -3.0 dB |
| 63 Hz (bass) | -26.2 dB | -0.8 dB |
| 125 Hz | -16.1 dB | -0.2 dB |
| 250 Hz | -8.6 dB | 0 dB |
| 500 Hz | -3.2 dB | 0 dB |
| 1,000 Hz | 0 dB | 0 dB |
| 2,000 Hz | +1.2 dB | -0.2 dB |
| 4,000 Hz | +1.0 dB | -0.8 dB |
| 8,000 Hz | -1.1 dB | -3.0 dB |
A pure 63 Hz bass tone at 80 dB would read about 54 dBA. A 1,000 Hz tone at 80 dB reads 80 dBA. Real sounds contain many frequencies at once, so the difference between dB and dBA depends on the mix. For speech, the two are usually close. For traffic rumble, HVAC or thumping bass, the flat reading can be 10 dB or more above the A-weighted one.
When should you use dBA vs dB? #
| Situation | Use | Why |
|---|---|---|
| Hearing risk from concerts, tools, headphones | dBA | Exposure limits from NIOSH, OSHA and WHO use A-weighting |
| General “how loud is this room?” | dBA | Tracks what people perceive |
| Workplace noise surveys | dBA | Required by the standards |
| Neighbor’s bass or a humming appliance | Flat (Z) or C, plus dBA | Low frequencies are the problem, and A-weighting hides them |
| Tuning speakers or subwoofers | Flat (Z) or C | You want every frequency counted equally |
| Local noise ordinance complaints | Whatever the ordinance says | Most use dBA; some add C-weighted or low-frequency limits |
If you’re unsure, measure in both. A big gap between the flat and A-weighted readings tells you the noise is mostly bass, which is useful in itself. It explains why the neighbor’s music feels louder than the dBA number suggests.
What are dBC and dBZ? #
dBC (C-weighting) is much flatter than A. It trims only the extreme low and high ends, about -3 dB at 31.5 Hz and 8 kHz. It keeps most of the bass, so it’s used for loud sounds, where the ear’s response flattens out, and commonly for peak measurements. Precision (Class 1) sound level meters are required to offer it.
dBZ (Z-weighting, “zero”) is no weighting at all: every frequency in the meter’s range counts equally. Many phone apps call this “flat.”
Some older charts use plain “dB” when they mean dBA. When a number matters, check the weighting.
What other kinds of “dB” will you see? #
The decibel is a ratio, so “dB” always needs a reference point:
- dB SPL: sound pressure level relative to 20 micropascals, the conventional threshold of hearing. Everything on a sound meter is dB SPL, weighted or not. See what is SPL.
- dBFS: decibels relative to full scale in a digital system. 0 dBFS is the loudest level a recorder or phone can represent. A meter app has to convert this to dB SPL.
- dB HL: hearing level, used on audiograms. 0 dB HL is average normal hearing at each frequency, so the scale is different from SPL.
How do sound meters apply A-weighting? #
A proper meter applies the weighting to the signal itself, with a filter, then computes the level. Adding a correction to a single overall number afterward doesn’t work, because the correction depends on which frequencies are present.
Sound Meter dB does it the proper way. It builds the IEC 61672 A-weighting curve as a digital filter matched to the microphone’s sample rate, runs the raw audio through it, and only then calculates the level. Its code is unit-tested against the standard’s A-weighting table. In Settings you can switch between A (dBA), the default, and Flat (dB). Checking a sound in both modes takes a few seconds. The app doesn’t offer C-weighting, so if you need dBC specifically, you’ll need a meter or app that does, such as the NIOSH SLM app on iPhone.
Readings from any phone are estimates, since the microphone itself isn’t a calibrated instrument. See how accurate phone sound meters are and, for technique, how to measure noise level with your phone.
Frequently asked questions #
Is dBA louder than dB? #
Neither is louder. They’re two ways of measuring the same sound. For most real noise, the A-weighted reading is equal to or lower than the flat reading, because A-weighting reduces bass. For sound concentrated around 2 to 4 kHz, dBA can read slightly higher.
Is 60 dBA the same as 60 dB? #
Only if the sound is mostly in the midrange. For speech they’ll be close. For a bass-heavy sound, 60 dBA might be 70 dB or more unweighted. The weighting tells you which number you’re looking at.
Why do hearing limits use dBA? #
A-weighted levels correlate well with the risk of noise-induced hearing loss and are simple to measure consistently. NIOSH’s 85 dBA recommended limit and OSHA’s 90 dBA permissible limit are both A-weighted. See NIOSH vs OSHA noise limits.
Why does my meter read higher in flat mode? #
Because flat mode counts the low frequencies at full strength. Traffic, air conditioning, fridges, engines and music with bass have a lot of low-frequency energy, which A-weighting discounts. A gap of 10 dB or more between flat and A readings means the sound is mostly bass.