Characteristics of Sound

Sound, once produced, is perceived by a listener through three distinct, subjective CHARACTERISTICS -- loudness, pitch, and quality -- even though, physically, sound itself is fully described by just its intensity, frequency, and the mixture of frequencies (waveform) present.

LOUDNESS is the human perception corresponding to the physical quantity INTENSITY (energy crossing unit area per unit time, itself proportional to the square of amplitude); besides amplitude, intensity as actually perceived also depends on the listener's distance from the source, the motion and density of the intervening medium, the surface area of the vibrating (sounding) body, and the presence of other resonant objects nearby. Crucially, the ear's RESPONSE to a given intensity is not linear but roughly logarithmic/exponential -- which is precisely why sound level is conventionally expressed not directly in terms of intensity I, but on the logarithmic DECIBEL scale, $L=10\log_{10}\left(\dfrac{I}{I_0}\right)$ (Eq. 6.40), where $I_0=10^{-12}$ W/m^2 is a fixed minimum reference intensity, chosen as roughly the faintest sound (I = $I_0$) that a normal human ear can just detect, at which $L=0$ dB by definition. On this scale, ordinary whispering registers around 10 dB and normal speech around 60 dB (both measured at roughly 1 m from the source), while the maximum sound level a human ear can tolerate without pain is around 120 dB; correspondingly, the actual pressure variation caused by a sound wave ranges from as little as $2.0\times10^{-5}$ Pa for the faintest audible whisper up to about 28 Pa for the loudest tolerable sound. (Ten decibels make one BEL, the coarser unit for loudness difference met already in Class XI.) Because loudness for the SAME intensity is also perceived differently at different frequencies, a further unit, the PHON, is used specifically for loudness: one phon is defined as the loudness, in decibels, of an equally-loud PURE TONE at the reference frequency of 1000 Hz.

PITCH is the human perception corresponding to FREQUENCY: a higher-frequency sound is perceived as higher in pitch, which is exactly why a typical female voice (generally higher in frequency) sounds higher-pitched than a typical male voice. …