What is Loudness?
Loudness is the subjective perception of sound intensity by the human ear. While related to the physical amplitude and intensity of a sound wave (often measured in decibels), loudness is a psychological attribute that also depends on the frequency of the sound and the individual's hearing sensitivity. A sound's objective intensity might be constant, but its perceived loudness can vary if its frequency changes, due to the non-linear response of the human auditory system.
Loudness vs. Intensity
Sound intensity is an objective physical measure of the sound power per unit area, typically quantified in decibels (dB) and directly related to the amplitude of the sound wave. Loudness, conversely, is a perceptual quantity. Two sounds with the same physical intensity but different frequencies (e.g., a low bass note and a high whistle) might be perceived as having different loudnesses because the human ear is most sensitive to sounds in the mid-frequency range (around 2,000 to 5,000 Hz).
A Practical Example
Imagine a car stereo playing music. If you adjust the 'bass boost' control, you are increasing the intensity of low-frequency sounds. While the decibel level for those frequencies increases, your perception of overall loudness might change dramatically, particularly if your ears are more sensitive to those boosted frequencies. Similarly, a smoke detector's high-frequency alarm, even at a moderate decibel level, is perceived as very loud and jarring because those frequencies fall within the peak sensitivity range of human hearing.
Importance and Applications
Understanding loudness is crucial in fields like audio engineering, environmental noise control, and audiology. Audio engineers use knowledge of human loudness perception (often represented by Fletcher-Munson or equal-loudness contours) to mix music so it sounds balanced to listeners. In noise control, regulations often focus on perceived loudness rather than just decibel levels to assess discomfort. Audiologists use loudness measurements to diagnose hearing loss, as hearing impairment can affect how different frequencies are perceived as loud.