Day & Night sounds and the logarithmic scale

Daytime and night-time sounds are not considered equal!

When calculating and measuring port noise levels, experts add 10dB to all sounds that occur at night. This is because sounds at night are typically more disturbing than sounds during the day, partly because we are most likely trying to sleep.  In addition, the lower background noise levels at night means any sounds stand out that much more.  In the Dunedin City District Plan, nighttime is defined as the period between 10pm – 7am.

The decibel scale is exponential, not linear. As a result, it doesn’t behave as you might expect. A change of less than 3 decibels is likely to be imperceptible, while an increase of 10 decibels will appear to be twice as loud.

Here are some examples of sound levels at each decibel level to help you imagine what the various levels sound like.

Noise effects

• Noise complaints are primarily received at night.

• Port noise ‘hum’ is typically broadband

• Certain vessels produce significant low frequency ‘rumble’.

• Building façade sound insulation is much more effective at mid and high frequencies compared with at low frequencies.

• Inside the bedroom of a typical bungalow overlooking the port, the Rio de Janeiro would be perceived as significantly louder than other container ships.