Noise is considered to be any audible sound not produced by a performer. A performer is someone who is supposed to be making sound whether through speech, singing, or instrumentally. Some have compared having noise in a room to looking through a dirty window. You may not notice the dirt (noise) directly, but the view is clouded (sound is not clear). When you clean the window (eliminate or reduce the noise), the image (sound quality) can be considerably different and more enjoyable.
Nearly every acoustical parameter in a space is affected by noise. Taking care of noise issues is one of the first steps in solving many acoustical problems.
For proper intelligibility in a church sanctuary, we need a signal-to-noise ratio of at least 25dB. Although some may claim less is okay, we must keep in mind there may be visitors who do not know the “church lingo”, and therefore they cannot fill in the words they do not hear or understand. This means that noises in the room must be at least 25dB lower in level than the sound(s) we want to hear. Comfortable speech levels are achieved around 65dB. Therefore, we need a noise floor no higher than 40dB-flat. This is to say, no noise should be louder than 40dB at any frequency.
Noise curves have been used for years to allow manufacturers, architects and builders to assign a number to the noise level in a space or emitted by a product. These curves originate from the Fletcher Munson equal loudness curves which compare objective sound levels in decibels with actual perceived loudness for sounds at varying frequencies. Our ears are less sensitive to low frequencies, therefore we require low frequencies at higher amplitude levels (than high frequencies) to sound as loud as a higher frequency.
We cannot forget that microphones are dumb. If there’s noise in a room, a microphone will “hear” it and allow the audio system to amplify it even more. When they pick up sound, they will pass the signal on to the mixing console, processors, amplifiers and speakers – such that the noise level has now been amplified. Microphones cannot distinguish between the intended “signal” and the less-desired “noise”; both receive equal treatment. Thus, if a person is speaking in a normal voice at 65dB and the noise level is at 60dB, it will send the signal of each equally through the system to be amplified. This is one way the audio system can actually increase the noise level in a room when it’s being used.
Controlling both inside and outside noise sources is an important aspect of achieving good acoustics. Doing so also improves the performance of the audio system, and, in effect, provides better intelligibility for those listening.