Perception of sound
10 What drives perception
Vibration properties drive perception
The air that vibrates our eardrums starts a complex process that ends in our brain. The properties of the vibrations at the eardrum (such as amplitude, frequency, spectrum shape, actual duration) are turned into electrical signals which the brain turns into perceptions (such as loudness, pitch, timbre and perceived duration). It is the properties of the vibration of our eardrum that drive our perceptions of the sound.
The complete relationship between properties of the eardrum’s vibration and human perception of sound (called psycho-acoustics) is complicated. The diagram below summarizes important links between the physical properties of a vibration and the impressions they create in the human mind. Notice that there are some very strong links (like between frequency and pitch). Notice too that concepts that seem unified in our mind (like loudness) are actually influenced by multiple properties of the vibration. (This diagram is explained in more detail below).
Loudness perception (and amplitude)
Human perception of loudness is based mostly on a sound’s pressure amplitude at the eardrum. If all other factors are equal, higher amplitude sounds are perceived as louder- “bigger” vibrations sound louder.
Amplitude is the most important factor in loudness perception, but it’s not the only factor- frequency also plays a role. For most people, a sound with a frequency of 3000 Hz will sound louder than a 20 Hz sound sound with exactly the same amplitude. Human hearing is optimized for sounds in the 2000 Hz to 4000 Hz range. Sounds in this frequency range simply sound louder than other sounds with the same amplitude. If the frequency is too low (below about 20 Hz for most people) or too high (above about 20 kHz for most young people), sound is completely inaudible, no matter how big the amplitude is!
The upper frequency limit of human hearing decreases with age. “Mosquito” ring tones for cell phones take advantage of this- the ultrahigh frequency sound is too high for most old people to hear. High frequency sounds have also been used to drive away young people to prevent loitering. The sounds are loud and annoying for some (mostly young) people and inaudible to others.
Duration also plays a minor role in how we perceive loudness, as do other factors.
Pitch perception (and fundamental frequency)
If a sound vibration is quasi periodic, the human ear/brain system produces a sensation called pitch. Pitch perception is based primarily on the fundamental frequency of the sound. Vibrations with higher fundamental frequencies (like flutes) are perceived as higher pitched than vibrations with low frequencies (like tubas). Each musical note corresponds to a certain frequency- the higher the note, the higher the frequency. The note the orchestra tunes to has a frequency of 440 Hz. Notes on the piano have frequencies that go from from about 28 Hz to about 4200 Hz. Characteristics of t