by 105 Student
25/365 Days - Violin by athena.
When an object is struck, the energy from the strike transfers to the molecules of the object, causing them to vibrate. These vibrations produce sinusoidal waves that carry to our ears and are then interpreted as sound in our brain (Taylor, 1992). But when the object vibrates, it is actually doing so at multiple frequencies at once, producing many waves our brain perceives as one sound. So contrary to the single note we hear, the sound is actually composed of waves vibrating at many frequencies simultaneously, producing multiple pitches. Of these pitches, the lowest one (the slowest frequency produced) we refer to as the “fundamental frequency”: the pitches above this frequency are called “overtones.” Mathematically, these overtones are integer multiples of the fundamental frequency (at least when the sound is harmonic, meaning it contains a distinctive pitch/tone). For example, if a string vibrates 110 times a second (110Hz), the overtones would be 2×110 (220Hz), 3×110(330Hz), and so forth until the overtones became too high for us to hear. Again, though, our brain perceives all of these frequencies as one pitch, the fundamental frequency(Levitin, 2007).
In fact, our brains are so adapted to this phenomenon that, even without the presence of the fundamental frequency (such as when a note is artificially simulated with all pitches except the fundamental), we would still perceive the sound as the original note, the fundamental pitch. For example, if a scientist wanted to create a note with the fundamental frequency of 210Hz, the scientist could actually create this sound by simulating 210’s overtones (420Hz, 630Hz, 840Hz, …) to create a hybrid note that, while lacking the fundamental, still sounds just like the fundamental pitch, 210Hz. This phenomenon where our brain fills in the missing sound is called restoration of the missing fundamental (Levitin, 2007).
In an experiment by Petr Janata (1997), electrodes were placed in the inferior colliculus (part of the auditory system) of a barn owl. A version of Strauss’s “The Blue Danube Waltz” simulated with all but the notes’ fundamental frequency was then played for the owl. The electrodes output (what the owl’s brain was essentially perceiving), was played back via a small amplifier. What was heard was “The Blue Danube Waltz,” fundamental frequency and all, indicating that the brain, with or without the actual pitch, will hear the fundamental frequency. The neurons in our inferior colliculus actually fire at the same rate as the missing frequency.
This concept has a practical application in our telephone system. In most current telecommunications systems, the bandwidth (width or capacity of a communications channel, measured in Hz) is not large enough to cover the vocal range of male and female voices. Males and female voices range from about 90-100Hz to 1100Hz. Most phones, however, transmit from about 300Hz to 3400Hz. But when we talk on the phone, we still perceive the other’s voice quite clearly and (for the most part) distinctively. This is thought to be due to the restoration of the fundamental frequency, in which the overtones of lower voices are transmitted over phone lines and reproduced in “original form” in our brains. While we perceive the other’s voice one the other end of the line, we are actually hearing just their overtones(Kontio, 2004).
References
Taylor, C. (1992). Sound. Retrieved March 6, 2009, from Grove’s Music Dictionary Online Web site: http://www.oxfordmusiconline.com/subscriber/article/grove/music/26289?q=sound&search=quick&pos=1&_start=1#firsthit
Levitin, D. (2007). This is your Brain on Music. New York, NY: Plume.
Janata, P. (1997). Electrophysiological studies of auditory contexts. Dissertation Abstracts International: Section B: The Sciences and Engineering, University Of Oregon
Kontio, J. (2004). Neuroevolution Based Artificial Bandwidth Expansion of Telephone Band Speech Master’s. Retrieved March 1, 2009, from http://www.acoustics.hut.fi/publications/files/theses/kontio_mst.pdf
