Analysis of sound usually treats sound as waves; in many ways, sound waves are analogous to ocean waves or ripples in water caused by dropping a stone in a pond or puddle. Sound has the following characteristics and properties which affect its perception:
Wavelength - This is the period between waves of sound and is also refered to as pitch or frequency. Expressed in Hertz (Hz), cycles per second, the human ear perceives frequencies ranging from 20 Hz to 20,000 Hz, although as humans age they tend to lose their ability to hear high frequency sounds. Hearing ranges for some animals include:
domestic cats 100-32,000 Hz
domestic dogs 40-46,000 Hz
African elephants 16-12,000 Hz
bats 1000-150,000 Hz
rodents 70-150,000 Hz
Amplitude - The volume, or loudness of a sound.
In this graphical representation of a sound, the x axis is time and y axis is amplitude, or the strength of the signal. A louder sound produces higher waves, but does not change the distance between the waves. The wave makes repeated full cycles; the number of cycles completed per second is the frequency in Hertz. When a microphone or recording is played into an oscilloscope, sound is represented in this way.
Intensity - The energy of the sound, measured in watts per square meter. Intensity is a function of the square of the amplitude, corrected for the density of the medium and at which the sound travels in the medium.
Dissipation - The loss of intensity as sound travels. One important factor in loss is the effect of spreading; as the sound spreads out over a larger area, less energy is present at any one point. Mathematically, this dissipation is described by the inverse-square law. Another cause of loss is absorption and scattering by the medium in which the sound is travelling.
Reflection & refraction - When sound hits a denser medium, some of the energy bounces, or echoes, off the surface of the denser medium. Echoes can be useful, as in the echolocation system of bats, or can result in confusing noise. When sound moves from a medium of one density to a medium of a different density, it refracts, or changes wavelength, as well. This results in distortions of the sound.
2. Six Qualities of Sound Perception
The study of sound perception is called pschoacoustics. Any time you hear or feel a sound, there are some amazing things that happen. For every sound your ear-brain system processes, you get information about:
Pitch - (also associated with frequency), the perception of a high or low sound.
Loudness - (also called amplitude), the intensity of a sound.
Phase - the increase and decrease in pressure cycle any single vibration.
Direction - (hearing with two ears creates left/right, high/low, front/back qualities), first come first heard by one ear or the other.
Distance - (also associated with reverberation time), perception of how near or far away a sound’s source is.
Timbre - (also called tone color), the perceived quality of any students’ multiple frequencies changing through time.
2.1. What is Pitch?
Pitch is the term we use to describe the psychological sensation or perception of a sound by people.
Pitch is sometimes confused with frequency, the term we use to describe the physical phenomena of sound energy created by a series of vibrations.
These are two different but complementary aspects of sound perception.
Typically, the first concept is the idea that sounds can be low or high, and that these sounds exist in a sound spectrum. Some animals hear lower than humans and some hear higher. Other natural things like ocean waves, earthquakes create ultra-low sounds (we feel it!) and a bat’s sonar creates ultra-high sounds. So this spectrum covers a wide range from 0.1 vibrations per second to over 10 million vibrations per second!
A science term for vibrations per second is Hertz, abbreviated Hz. This term is named after the German physicist Heinrich Rudolf Hertz (1857 - 1894).
2.2. What is Loudness?
Loudness is a quality defined as the intensity of sound energy as it comes in contact with an eardrum or other surface. Another term describing loudness is amplitude.
How do we measure sound intensity? We use a system called decibels based on ratios. One bel is the ratio between the differences in intensity of two sounds of 10 to 1, and a decibel is one-tenth of a bel. The bel was named after Alexander Graham Bell (1847 - 1922).
2.3. What is a Sound's Phase?
Every sound consists of an increase/decrease cycle. This cycle effects air pressure, increasing it, then decreasing it.
Scientists use positive numbers to describe the pressure increase and negative numbers to describe the decrease.
Your brain is sensitive to a sound wave's phase. Based on the subtle differences in a sound wave's phase between your two ears, your brain can tell what direction a sound came from.
Phase can be a regular pattern, like in a musical tone, or it can be irregular, like in a waterfall.
2.4. How Do We Know What Direction a Sound Comes From?
How does the loudness of a sound effect the way we locate the direction of that sound's origination?
Our auditory pathway rapidly calculates the difference in loudness between the sound as it enters both ears. In other words, a sound coming from your left side has less energy and is slightly softer by the time it reaches your right side.
What other phenomena effect the way we perceive sound?
The ambient noise level of any space or room - the mix of all of the random sounds (called masking).
Preceding - following sounds (expectation vs. surprise).
Wanted vs. unwanted sounds - our minds filter out a large portion of sound deemed unimportant. For example, a parent seemingly ignores their screaming baby while we may notice it.
2.5. What Does Distance Have to Do with Sound Perception?
Sounds have different characteristics based on how near or far they are heard from their source.
What is reverberation? All sound has a unique echo pattern created and shaped by its physical environment, whether it’s an ocean, subway, or phone booth.
Each sound has a unique echo signature based on the size and shape of the space it occurs in. For example, a gymnasium or the Grand Canyon will have much longer reverberation times than a living room because the sound bounces back and forth over a much larger area.
2.6. What is timbre (pronounces tam-ber)?
One caveat before we start! Timbre is the most complex quality of the six to understand, much less teach to someone. It combines the concepts of frequency and loudness through time. There is also an entire set of mathematics that supports these concepts related to the harmonic series.
The perceived quality of any sound, such as bright or dull, wooden or metallic, etc. is called timbre. Musicians use the term “tone color” to describe the way instruments sound together or separately. An 18th century scientist, named Joseph Fourier (1768 - 1830), proved mathematically that any sound is actually made up of a set of multiple frequencies called the harmonic series.
The lowest sounding tone is called the fundamental partial, in this example a Cello’s low C. It is the loudest tone of the series and the one we usually identify in music.
The other higher sounding partials are almost always softer, and our ear/brain system blends them together into our perception of a single tone, just as our eye/brain system blends different colors of light into our perception of white light.
心得感想:
當第二週輪到我個人做自我介紹部分時，老師補充說明sound perception概念，並且舉例佐證-「日本」的發音可以是「ni-hon」或「ni-bon」，而後者的權威意味多於前者，因此，以發音重聲來表示權威。此課堂過後，我便開始搜尋sound perception相關資料。聲音是藉由物質震動所發出的聲音，人發出聲音是藉由喉嚨聲帶的振動而發出聲音，而人之所以聽到聲音，是因為空氣震動所傳來的聲音被耳朵接收，然後耳內的耳膜產生共振，所產生的音訊再傳到腦裡，即是我們所聽到的聲音。人發出聲音和樂器很相似，舉例來說，管樂器是藉由管內空氣震動，弦樂器是靠弦震動發出聲音，而敲擊樂器是靠敲擊面震動發出聲音，例如: 把米粒放在大鼓上，然後敲鼓，就會發現米粒都在跳動。除此之外，人的發聲頻率在中頻 300-3K (HZ)，這是一般正常人發聲的界定範圍，而人耳的極限是在20Hz-20KHz(20.000Hz)。
References
1. http://www.animalbehavioronline.com/soundperception.html
2. http://www.cafemuse.com/kitchen/perception/six_qualities.html
