What A-Weighting Does
A young person with normal hearing can hear from about 20 Hz to 20,000 Hz, but the ear is not equally sensitive across that range. OSHA describes A-weighting as the best fit to the ear's frequency response: it is most responsive from 500 Hz to 4,000 Hz.
The filter leaves 1,000 Hz unchanged and cuts lower frequencies more and more. These are the A-weighting values from the IEC 61672 standard, as published by NTi Audio:
- 1,000 Hz: 0 dB
- 100 Hz: −19.1 dB
- 63 Hz: −26.2 dB
- 31.5 Hz: −39.4 dB
So a deep hum reads far lower in dBA than in unweighted dB.
dBA vs dBC vs dBZ
Sound level meters offer three weightings, all defined in IEC 61672:
- A-weighting (dBA): approximates the loudness of tones relative to a 40 dB tone at 1,000 Hz. It is the scale for hearing safety, and OSHA notes it also correlates with how annoying people find a noise.
- C-weighting (dBC): approximates the loudness of tones relative to a 90 dB tone at 1,000 Hz. It is close to flat and keeps the low frequencies: at 31.5 Hz it cuts 3.0 dB, against 39.4 dB for A. NTi Audio lists it as the weighting for peak levels.
- Z-weighting (dBZ): no filter at all, flat from 10 Hz to 20,000 Hz. It became an international standard in 2003 and is used to analyze a noise source rather than its effect on people.
If a reading in dBC is well above the same reading in dBA, most of the sound is low-frequency, because A-weighting removes much more of the bass.
Which Weighting to Use
- Hearing safety and workplace noise: A-weighting with the slow response. OSHA's noise standard measures on the A scale at slow response, and its 85 dBA action level and 90 dBA limit are both 8-hour averages.
- Bass-heavy noise, such as music or engines: compare a C-weighted reading with the A-weighted one to see how much of the level is low-frequency.
- Short bangs and impacts: a peak reading. OSHA says impact noise should not exceed a 140 dB peak.
- Analyzing a machine or a room: Z-weighting, so no frequencies are hidden.
For how long each dBA level is safe, see how loud is too loud.
Fast and Slow Response
Response is how quickly the meter follows changes. OSHA gives the standard time constants: fast is 125 milliseconds and slow is 1 second.
Fast catches quick changes, so a fluctuating sound usually shows a higher maximum on fast. Slow steadies the reading, and OSHA, the Mine Safety and Health Administration, and ACGIH all require it for noise dosimetry.
Measure dBA on Your Phone
The decibel meter reads in dBA with the fast response by default. Open Calibrate and Settings to switch to C or Z weighting, or to the slow response. The workplace noise meter uses the slow response and shows how long the current level is safe.
A phone mic gives an estimate, so see how accurate a phone decibel meter is and how to calibrate it. For compliance, OSHA treats a Type 2 sound level meter as accurate to ±2 dBA. Our sound level meters guide compares handheld Class 2 meters.