HowLouddBA vs dBC vs dBZ: Which Weighting Should You Use?
dB(A) filters out low and very high frequencies the way the ear does at moderate levels and is the standard for noise limits and hearing risk. dB(C) keeps most of the bass and is used for peaks and loud music. dB(Z) is flat, unweighted sound pressure. The same bass-heavy sound can read 10–15 dB higher in dBC than in dBA.
Why weighting exists
A microphone hears all frequencies equally; people do not. At everyday levels the ear is far less sensitive to a 50 Hz rumble than to a 2 kHz tone of the same pressure, and slightly less sensitive above about 8 kHz. Weighting curves are filters applied inside the meter so the reading tracks perceived loudness and, more importantly, hearing risk.
The three curves
| Weighting | What it does | Use it for |
|---|---|---|
| A | Cuts bass strongly (−19 dB at 100 Hz, −39 dB at 31 Hz) and treble above 10 kHz slightly; 0 dB at 1 kHz | Almost everything: noise regulations, workplace exposure, product noise ratings, neighbour complaints, hearing-risk calculations |
| C | Nearly flat from 31 Hz to 8 kHz (−3 dB at those points) | Peak levels (the EU 140 dB(C) limit), loud music and clubs, checking hearing-protection adequacy, low-frequency complaints |
| Z | No filter (zero weighting): flat from 10 Hz to 20 kHz | Engineering measurements, subwoofer setup, comparing raw sound pressure |
The curves are defined in IEC 61672-1. B and D weighting also exist but are obsolete.
Why the numbers differ so much
Consider a club with a lot of sub-bass. A dBZ reading might be 105 dB, dBC 103 dB and dBA 95 dB. The A filter removes most of the bass energy, so the reading drops. For hearing damage the A-weighted figure is the right one to compare with the 85 dB limit, because the ear is genuinely less vulnerable to low frequencies. But if the complaint is "the bass shakes my walls", an A-weighted meter will under-report the problem, which is why many councils measure low-frequency noise in dBC or in octave bands.
The C-minus-A trick
Subtracting the dBA reading from the dBC reading tells you how bass-heavy a noise is. A difference under 5 dB means broadband noise (traffic, voices); over 10–15 dB means the energy is mostly below 200 Hz (music through walls, HVAC, compressors). Occupational hygienists use the same difference to pick hearing protection, because earplugs attenuate bass far less than treble.
What HowLoud measures
The online meter and the app read dB(A) by default using an IEC 61672-1 A-weighting filter implemented digitally, and can switch to dB(Z) flat. Note that phone microphones roll off below about 50–100 Hz, so dBZ readings of deep bass are approximate.
Frequently asked questions
Which is louder, dBA or dBC?
For the same sound, the dBC value is always equal to or higher than the dBA value, because C-weighting removes less energy. The gap is small for voice-like sounds and large for bass-heavy ones.
Should I use dBA or dBC for a noise complaint?
dBA, because that is what the rules are written in. Add a dBC reading if the problem is bass or vibration; the difference between the two supports your case.
What is dB SPL?
Sound pressure level relative to 20 µPa, with no weighting stated. In practice it usually means dBZ.