The definition of noise, in audio and therefore in PA systems, is "any unwanted sound." Since the ultimate goal is the intelligibility of the message transmitted through the loudspeakers, it is essential to understand and quantify the typical sound quality of the room where the message is transmitted. There are significant differences between an office, a shopping center, a sports hall, a laboratory, and a metalworking plant, for example.
Psychoacoustics studies the masking effect that occurs when receiving two or more sounds of different frequencies and levels. The brain processes the louder sound, which masks a lower-intensity sound, in the same way that a low-frequency sound more easily masks a high-frequency sound.
The instrument used to measure noise is the sound level meter. They have three weighted circuits to measure noises of different levels and compensate for the difference in sensory perception: scales A (less than 60 phons), B (between 60 and 80 phons), and C (more than 80 phons). The measurement must specify the scale used (e.g., 65 dBA).
The design of a PA system must take into account the noise level of the different rooms. To avoid masking, the acoustic pressure generated by the installed speaker array must be at least 15 dB SPL higher than the ambient noise. In addition, electroacoustic processing (equalization, filtering, etc.) may be necessary to improve intelligibility, typically emphasizing the mid-low frequencies and attenuating the high frequencies.
The signal-to-noise ratio (SNR) is the difference in dB SPL between the audio signal transmitted by the PA system and the noise level in the room. In PA systems, the convention is:
SNR = message level - noise level > 15 (all in dB SPL)
The following table shows some reference levels that can guide you when making a quick assessment of the required PA system:
| Hearing threshold: 20 μPa | 0 dB SPL |
| Quiet places | 35 dB SPL |
| Conference rooms | 45 dB SPL |
| Private office | 50 dB SPL |
| Conversation at 1 m: 20 mPa | 60 dB SPL |
| Departure lounges (Airport) | 55 dB SPL |
| Station platform (without train) | 60 dB SPL |
| Station platform (with train) | 75 dB SPL |
| Cinemas, theaters, exhibitions | 65 dB SPL |
| Average traffic at 20 m | 70 dB SPL |
| Restaurant, bar | 70 dB SPL |
| Heavy traffic at 20 m | 80 dB SPL |
| Bus interior | 90 dB SPL |
| Mechanical workshop, assembly | 95 dB SPL |
| Alternator turbine at 1 m | 110 dB SPL |
| Pneumatic hammer at 1 m: 30 Pa | 123 dB SPL |
| Pain threshold: 65 Pa | 130 dB SPL |
| Harmful level | 140 dB SPL |
