40dB

40dB is a measurement of sound intensity level expressed in decibels. The decibel (dB) is a logarithmic unit that relates a particular sound intensity to a reference intensity, usually 10‑12 µPa for air. A 40dB sound is ten times less intense than a 50dB sound, because each 10‑decibel change represents a ten‑fold change in intensity.

Typical everyday sounds of about 40dB include a quiet office, a library, or a laptop running. At this level the sound is audible but generally not disturbing. In environmental noise regulation, 40dB often represents a threshold for background noise that can affect residential areas; many noise ordinances set limits of 40–45 dB for evening and nighttime to preserve quiet. In audio equipment, a 40dB gain provides a moderate increase in signal strength without introducing significant distortion.

In occupational settings, 40dB is well below the threshold of hearing for most people, who can detect sounds as quiet as 0 dB. However, prolonged exposure to environmental noise above 40 dB can lead to auditory fatigue and may contribute to increased heart rate for some individuals. The volume level of a 40dB sound is such that it can be picked up by most human hearing apparatus and is often used as a baseline in acoustic studies.

Because the decibel scale is logarithmic, converting 40 dB to a linear intensity requires the formula I = I0 10^(L/10), where I0 is the reference intensity of 10^(-12) W/m². Substituting L = 40 gives an intensity of 10^(-8) W/m², illustrating how sound intensity grows exponentially with each decibel increment.