Many sources give different recommendations for background noise levels. Due to the differences in the recommendations, it is important to understand the basis of the recommendations, how to interpret the recommendations, and what actions to take.
The sound pressure level is measured at a known distance from the source and reported in decibels (dB). For example, conversational speech is typically considered to be 60 dB at a 1-meter distance from the talker. If the measurement were taken at a different distance from the person speaking, the level would change. For example, doubling the distance in a wide-open environment will decrease the measurement by 6 dB.
Background noise measurements are intended to capture the average level of ambient noise in a space. Ambient noise includes the noise generally heard from the following sources:
- Building HVAC systems
- Fan noise from computers, displays, projectors, and other devices
- All other sources of sound that are heard when the space is free of people
Most often the background noise measurement is taken from several distinct positions in a room and averaged.
Frequency weighting
When using a sound meter to determine the background noise level in a room, many weightings are used for different situations. This article references A-weighting, which is the sound meter weighting that focuses on the frequency range where the human ear is most sensitive.
Signal to noise ratio
The ratio of speech compared to the background noise is a key factor in ensuring signal comprehension. This is the signal-to-noise ratio. Industry guidelines describe the minimum signal-to-noise ratio for conference room spaces to be in the order of 20 dB. So, if a person spoke at a 60 dB SPL A-weighted level, then the room should ideally have a background noise level of no more than 40 dB SPL A-weighted.
Room size
Vocal effort is related to distance, noise & room properties. Noise levels being equal, people in smaller rooms are closer and require less vocal effort. In larger rooms, people are farther away, so more vocal effort (higher level) is required to maintain an adequate SNR for distant listeners.
In a smaller room people are generally closer together and can have softer conversations than when in a large room. In a smaller space, the conversation decibel level is often lower than the referenced 60 dB. Due to this, smaller spaces should have an appropriate SNR to support these quieter conversations. An example is if a person is speaking at 54 dB SPL A-weighted, then the room would ideally be no more than 34 dB SPL A-weighted.
In larger rooms, people are farther apart and tend to speak louder. This does not mean that the background noise level can be higher. The amplitude of the speech, the signal, decreases as it travels through the room while the background noise remains the same. To maintain an adequate SNR the background noise level still needs to be approximately 20 dB lower than the signal, or speech. If a person were to be speaking at a level of 65 dBA measured at 1 meter in free space, the measured level at 4 meters away would be 53 dBA. In this case, an ideal background noise level would be 33 dB to support easy listening and to prevent the person speaking from having to raise their voice even more.
Measuring background noise levels
Nureva devices measure background noise differently than a single measurement capture used by generic sound level meters.
Our audio devices utilize Microphone MistTM technology and take the quietest reading found in a 1-minute period from continuous 500-millisecond captures. This is done over 5 minutes and then the values are averaged. For every minute that moves forward, the average is based on the last 5 minutes. This captures the essence of the background noise. It attempts to omit speech events or sounds that are not consistently there.
Similar values can be obtained with a data-logging sound level meter as follows:
- Set the sound level meter to capture values over a 1-minute period
- Log the slow response, A-weighted minimum value
- Repeat and average the results for 5 minutes
Acceptable background noise levels
Nureva advises a maximum background noise level of around 40 dBA. If Nureva Console is reporting a background noise level beyond 45 dBA for a sustained period, it is recommended that steps be taken to review the room and potentially take action to improve the level.
Improving background noise levels
If excessive background noise is reported from within the room, the following causes should be investigated.
- Ensure that your microphone and speaker bar are not installed too close to equipment with a fan or fans
- Check the HVAC system and/or ventilation fans to see if the noise can be reduced through system maintenance or settings changes
- Check other electronics in the room for fan noises and move them into cabinets
- Check neighboring spaces for equipment that generates loud noise
If excessive background noise is coming from outside the room, sound dampening might be required.