dBA vs dBC: A, C, and Z Weighting Explained

Why weighting exists

A microphone does not hear like a person. It responds to pressure changes across frequencies, while human hearing is much less sensitive to deep bass and very high treble at moderate levels. Weighting curves adjust the measured signal before the meter reports a number. That is why the label matters: 70 dB(A), 70 dB(C), and 70 dB(Z) can describe different acoustic realities.

The online sound meter lets you switch between A, C, and Z weighting, as well as Fast and Slow response. It also shows Leq, Min, Max, Peak, a timeline, and an octave spectrum. Those features help you see not only how loud a sound is, but what kind of sound it is.

Weighting is standardized for sound level meters in IEC 61672. A phone is not a certified IEC 61672 instrument, but using the same concepts makes readings easier to compare with guidance from WHO, NIOSH, OSHA, and local environmental-noise rules.

A-weighting: the everyday default

A-weighting, reported as dB(A) or dBA, reduces low-frequency sound strongly and slightly adjusts high frequencies. It was designed to approximate the ear’s sensitivity at moderate loudness. Because it correlates reasonably with many annoyance and hearing-risk situations, it became the default for environmental noise, occupational exposure, appliance ratings, and many ordinances.

Use A-weighting for most everyday questions: bedroom noise, classroom levels, office noise, traffic outside a window, or whether a room is generally quiet. WHO 1999 community-noise guidance and WHO 2018 environmental-noise guidance commonly discuss A-weighted levels. NIOSH and OSHA occupational exposure limits are also expressed in dBA.

A-weighting has a weakness: it can understate low-frequency annoyance. A subwoofer, mechanical hum, idling truck, or HVAC rumble may feel intrusive even when dB(A) looks modest. In those cases, add C-weighted readings or look at the octave spectrum.

C-weighting: useful for bass and peaks

C-weighting, reported as dB(C) or dBC, is much flatter than A-weighting through the low and mid frequencies. It still rolls off at the frequency extremes, but it does not discount bass nearly as much. That makes it useful for loud music, machinery, and impulse-noise peak assessment.

If dBC is much higher than dBA, the sound contains substantial low-frequency energy. For example, a room might read 42 dB(A) but 58 dB(C) when a bass-heavy source is present. That difference helps explain why the sound feels stronger than the A-weighted number suggests.

C-weighted Peak is also common in impulse-noise contexts because very brief events can be hazardous even when the average level is not high. Phone microphones and operating systems may miss true impulse peaks, especially above roughly 110 dB, so treat phone peak readings as screening information, not certified safety data.

Z-weighting: the flat reference view

Z-weighting means zero frequency weighting over the specified bandwidth. It is the modern flat option, replacing older labels such as Linear on many instruments. Z-weighted readings are useful when you want to inspect the raw broadband level without A or C correction.

Z is not automatically better. Human impact, regulations, and guidance often specify A or C weighting for a reason. Use Z when you are comparing spectra, checking equipment, or trying to understand what the microphone is receiving before applying human-hearing weightings.

On a phone, Z-weighting is still limited by the microphone, browser, and operating system. It is not a laboratory flat response. It is best read alongside the octave spectrum.

IEC 61672 A and C weighting values

The table below shows standard weighting corrections at common octave-band center frequencies. Negative values mean that frequency is reduced before the meter reports the weighted level.

Frequency	A-weighting	C-weighting
31.5 Hz	-39.4 dB	-3.0 dB
63 Hz	-26.2 dB	-0.8 dB
125 Hz	-16.1 dB	-0.2 dB
250 Hz	-8.6 dB	0.0 dB
500 Hz	-3.2 dB	0.0 dB
1 kHz	0.0 dB	0.0 dB
2 kHz	+1.2 dB	-0.2 dB
4 kHz	+1.0 dB	-0.8 dB
8 kHz	-1.1 dB	-3.0 dB

The contrast at 31.5 Hz is the big lesson. A-weighting subtracts 39.4 dB at that band, while C-weighting subtracts only 3.0 dB. This is why a low-frequency thump can seem to disappear in dB(A) but remain obvious in dB(C) and in the octave spectrum.

For everyday examples across the scale, use the decibel chart or check specific references such as how loud is 80 dB and how loud is 100 dB.

Fast, Slow, and Impulse response

Frequency weighting shapes pitch content. Time response shapes how quickly the displayed level reacts. Fast response uses a 125 ms time constant. It follows changing sound closely and is useful for speech, traffic pass-bys, tools, and short events. Slow response uses a 1 second time constant. It smooths fluctuations and is easier to read for rooms, appliances, and background noise.

Impulse response was historically used for very short sounds, with a fast rise and slower decay. You may still see it on some sound level meters. Modern standards and regulations often specify exact metrics such as peak level, event level, or time-weighted averages instead. If you are documenting impulse noise, record Peak and Max, and note the response setting.

For a phone meter, Slow is usually the best default because it reduces display chasing. Fast is better when you care about how quickly the sound changes. Peak is useful, but phone peak values can be limited by microphone clipping and audio processing.

Leq, Lmax, and Lpeak

Leq, Lmax, and Lpeak answer different questions.

Metric	Meaning	Good use
Leq	Equivalent continuous sound level over the measurement period	Overall exposure or average annoyance
Lmax or Max	Highest time-weighted level during the period	Loudest ordinary moment in a session
Lpeak or Peak	Highest instantaneous pressure level	Impulse and impact sounds

Leq is central because sound damage and many environmental effects depend on energy over time. A steady 70 dB sound and a varying sound with the same acoustic energy over the period can have the same Leq. Lmax shows the loudest moment but not how long it lasted. Lpeak captures instantaneous pressure and is especially important for impacts, blasts, or sharp claps.

For hearing-risk assessment, NIOSH uses an 85 dBA 8-hour Recommended Exposure Limit with a 3 dB exchange rate. OSHA uses a 90 dBA 8-hour Permissible Exposure Limit with a 5 dB exchange rate. Those are time-and-level rules; a momentary Max is not enough. For practical listening guidance, see safe decibel levels for listening.

What to use in common situations

Situation	Weighting	Response	Main metric
Bedroom background noise	A	Slow	Leq and Max
Neighbor bass	A plus C	Slow	Leq, Max, octave spectrum
Classroom noise	A	Slow or Fast	Leq and Max
Concert or gym	A plus C	Fast	Leq, Max, Peak
Power tools	A	Fast	Leq and Max
Impulse or impact	C	Fast plus Peak	Peak and Max
Technical spectrum check	Z	Fast or Slow	Octave spectrum

If a rule, standard, or lease specifies a metric, follow that wording exactly. A limit written as 45 dB(A) Slow is not the same as 45 dB(C) Fast. If no rule is specified, document what you used.

For classroom display use, the classroom noise monitor turns sound levels into a simple visual cue. For phone setup and calibration, read how to measure decibels with your phone.

How calibration interacts with weighting

Calibration adjusts the level offset for a microphone. It does not make a phone microphone perfectly flat, and it cannot remove operating-system processing. The most practical method is to calibrate using the same weighting and response you plan to use, usually A-weighted Slow, against a reference sound level meter exposed to the same steady sound.

On this site, tap Calibrate, type the reading from the reference meter, then choose “Match now.” You can also use the slider for small adjustments. Calibration is stored per microphone. If you switch microphones, recalibrate.

After calibration, steady sounds in the phone’s comfortable range may be within about ±1–2 dB of the reference. Without calibration, expect larger differences, often around ±3–7 dB. Below about 30 dB and above about 110 dB, physical microphone limits become more important than the weighting choice.

The takeaway

Use A-weighting when you want the standard everyday answer. Add C-weighting when bass, loud music, or peaks matter. Use Z-weighting when you need a flatter technical view. Use Slow when you want readability, Fast when you want time detail, Leq for exposure, Max for the loudest ordinary moment, and Peak for sharp events.

A decibel number without these labels is incomplete. Once you include them, your measurements become much easier to compare, explain, and trust.