Questions? Contact us. If you have significantly changed your sound system or have added sound abatement, or if you'd like an Entertainment Commission Inspector to sync your sound monitoring device with our sound meter, contact us at entertainment.enforcement@sfgov.org
What are decibel readings and what do they measure?
Decibel readings for amplified sound are typically done with two different measurements: dBA and dBC. These are also called A-weighted (dBA) and C-weighted (dBC) decibels. They both measure sound pressure level, but in order for decibel measurements to correspond to perceived loudness, different frequency weights are used (ex. dBA and dBC). Using different weights accounts for the high and low frequencies that the human ear is less sensitive to.
What is the difference between A-weighted and C-weighted decibels?
Oftentimes, sound pressure level is measured using A-weighted decibel limits. In theory, this is supposed to be a realistic representation of human hearing since our ears are more sensitive to midrange frequencies. This is why it is often used as a standard.
However, A-weighted decibel readings only work when sound is at lower volumes and do not consider the loud low-end frequencies (bass) that occur during performances with amplified sound. C-weighted measurements include a wider range of frequencies. When experiencing louder volumes, this is actually a closer representation of human hearing.
Why does the Entertainment Commission use both A-weighted and C-weighted decibel limits?
To ensure sound limits are appropriate for each unique space, both A-weighted frequencies and C-weighted frequencies may be listed in the required sound limits in the entertainment permit.
What are ambient sound measurements?
Ambient sound can be defined as background noise of any given outdoor environment. Ambient sound measurements are used as a point of reference when studying a separate intrusive sound source. The Entertainment Commission uses a 10-minute decibel measurement to determine an average value of the consistent background noises in the immediate area. These ambient noises often include, but are not limited to, traffic sounds, people, and ventilation systems.
Suggested devices for sound monitoring
Sample Handheld Decibel Meters
- Tadeto SL720
- Protmex PT6708 Sound Level Meter
- Grainger EXTECH, General Purpose Sound Level Meter
Sample Smartphone App Decibel Meters
- For iPhone (iOS):
NIOSH Sound Level Meter. Free app. It is capable of measuring both A and C weighted frequencies. Includes live readouts and logging of dBA and dBC. iOS only.
- For Android and iPhone (iOS):
Decibel X. Paid subscription required. Measures all weights needed. Has a camera-based logging function which is useful for proof of monitoring. This app has the most features after in-app purchases and can be calibrated with EC meters.