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Chapter 11

P262 - Chapter 11

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Elizabeth Olds

PS262 – Chapter 11 – Sound, the Auditory System and Pitch Perception The Sound Stimulus  Physical definition: sound is pressure changes in the air or other medium o EX// the sound levels was 10 decibels  Perceptual definition: sound is the experience we have when we hear o EX// the sound of the trumpet pierced the air Sound as Pressure Changes  A sound stimulus occurs when the movements or vibrations of an object cause pressure changes in the air, water or any elastic medium that surrounds the object  When the diaphragm of the speaker moves out, it pushes the surrounding air molecules together a process called condensation which causes a slight increase in the density of molecules near the diaphragm o This increased density results in a local increase in the air pressure that is super imposed on the atmospheric pressure  When the speaker diaphragm moves back, air molecules spread out to fill in the increased space, a process called rarefaction o The decreased density of air molecules caused by rarefaction causes a slight decrease in air pressure  When air pressure changes, which travels through the air at 340 meters per second it is called a sound wave.  Although air pressure changes move outward from the speaker, the air molecules at each location move back and forth but stay in about the same place Pressure Changes: Pure Tones  A pure tone occurs when pressure changes in the air occur in a pattern described by a mathematical function called a sine wave o A person whistling or the high-pitched notes produced by a flute are close to pure tones  Amplitude – the size of the pressure change and the frequency – the number of times per second that the pressure changes repeat  An unit of sound, decibel which converts the large range of sound pressure into a more manageable scale  SPL = sound pressure level  Frequency, the number of cycles per second the change in pressure repeats, is the physical measure associated with our perception of pitch, with higher frequencies associated with higher pitches o Frequency is indicated in units called Hertz (Hz)  Pure tones are important because they are simple and because they have been used extensively in auditory research Pressure Changes: Complex Tones  The repetition rate of a complex tone is called a fundamental frequency of the tone  An important property of periodic complex tones is that they consist of a number of pure tones  We can build a complex tone by using a technique called addictive synthesis in which a number of sine-wave components are added together to create the complex tone  A way to represent the harmonic components of a complex tone is by frequency spectra o The position of each line on the horizontal axis indicates the harmonics frequency and the height of the line indicates the harmonics amplitude o Frequency spectra provides a way of indicating a complex tones fundamental frequency and harmonics without drawing the tone’s waveform  Note: that removing a harmonic changes the tone’s waveform but that the repetition rates remain the same  The distance between harmonics equals the fundamental frequency o When the fundamental is removed, this spacing remains, so there is still information in the waveform indicating the frequency of the fundamental Perceiving Sound  Loudness is the quality most closely relate to the amplitude or sound pressure which is also called the level of auditory stimulus o Decibels are often associated with loudness o Decibels are a physical measure whereas loudness is psychological  Pitch is defined as the attribute of auditory sensation in terms of which sounds may ordered on a musical scale  Tone height is the perceptual experience of increasing pitch that accompanies increases in a tone’s fundamental frequency o In addition to the increase in tone height that occurs, we move from low to the high end of the piano keyboard, something else happens: the letters of the notes A, B, C, D, E, F and G repeat and we notice that notes with the same letter sound similar o Notes with the same letter have the same tone chroma o Every time we pass the same letter on a keyboard we have gone up a interval called octave  The pitch is determined not by the presence of the fundamental frequency but by information such as spacing of the harmonics and the repetition rate of the waveform that indicates the fundamental frequency  The constancy of pitch is called the effect of the missing fundamental and the pitch that we perceive in tones and that has had harmonics removed is called periodicity pitch  We hear sound only within a specific range of frequencies called range of hearing  The audibility curve indicates the threshold for hearing determined by free-field presentation versus frequency o The audibility curve and auditory response area indicate the loudness of pure tones depends on only on the sound pressure but also on frequency  To determine the loudness of any tone we need to know both its dB level and its frequency  Equal loudness curve indicates the number of decibels that create the same perception of loudness at different frequencies o An equal loudness curve is determined by presenting a standard tone of one frequency and dB level having a listener adjust the levels of tones with frequencies across the range of hearing to the loudness of the standard  All frequencies between 20 Hz and 5,000 Hz sound equally loud at this level  Timbre is the quality that distinguishes between two tones tat have the same loudness, pitch and duration but still sound different o For example, when a flute and bassoon play the same note with the same loudness we can still tell the difference between these two instruments o When two tones have the same loudness, pitch and duration but sound different, this difference is a difference in timbre.  The difference in harmonics of different instruments is one factor that causes musical instruments to have different timbres o Timbre also depends on the time course of the tone’s attack and on the time course of the tone’s decay o Timbre depends both on the tone’s steady state harmonic structure and on the time course of the attack and decay of the tone’s harmonics The Ear  The auditory system must accomp
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