CHAPTER 11 Sound stimulus Sound physical stimulus or perceptual response 1 2 physical stimulus sound is pressure changes in the air or other medium perceptual definition sound is the experience we have when we hear occurs when the movements or vibrations of an object cause pressure changes in air water or any other elastic medium that surrounds the object changes in air pressure vibration travels in waves condensation pushes surrounding air molecules together causes slight increase in the density of molecules near the diaphragm results in a local increase in the air pressure that is superimposed on the atmospheric pressure rarefaction diaphragm moves back in air molecules spread out to fill in the increased space decreased density of air molecules caused by rarefaction causes a slight decrease in air pressure repeating this process creates a pattern of alternating high and low pressure regions in the air as neighboring air molecules affect each other sound wave pattern of air pressure changes pure tones occurs when pressure changes in the air occur in a pattern described by a mathematical function called a sine wave tone with this pattern of pressure changes are occasionally found in the environment amplitude the size of the pressure chance different in pressure b w the high and low peaks of the sound wave associated with our experience of loudness higher amplitudes associated with louder sounds amplitude physical loudness perceptual decibel a unit of sound which converts the large range of sound pressure into a more manageable scale SPL sound pressure level indicates we have used the standard pressure of 20 p0 level sound level when referring to decibels or sound pressure of a sound stimulus frequency the number of times per second that the pressure changes repeat the number of cycles per second the change in pressure repeats physical measure associated with our perception of pitch higher frequencies associated with higher pitches hertz Hz units 1 Hz is 1 cycle per second humans can perceive frequencies ranging from about 20 Hz to 20 000 Hz and most sensitive at frequencies b w 2 000 and 4 000 Hz range of frequency important for understand speech frequency physical and pitch perceptual pressure changes complex tones periodic tone property of repetition for complex and pure tones fundamental frequency repetition rate of a complex tone additive synthesis use to build a complex tone a number of sine wave components are added together to create the complex tone starting point a single pure tone which has a frequency equal to the complex tone s fundamental frequency then add the fundamental frequency that is a multiple of the fundamental frequency harmonics additional tones to a tone first harmonic fundamental first adding the first harmonic and higher harmonics the waveform of the complex tone frequency spectra represent the harmonic component of a complex tone distance between harmonics fundamental frequency pitch perceiving a tone as high or low perceiving sound loudness quality most closely related to the amplitude or sound pressure also called the level of an auditory stimulus often associated with decibels decibels physical measure and defined in terms of pressure not perception loudness psychological pitch perceptual quality described as high or low the attribute of auditory sensation in terms of what sounds may be ordered on a musical scale closely related the physical property of frequency low fundamental frequencies are associated with low pitches and vice versa tone height perceptual experience of increasing pitch that accompanies increases in tone s fundamental frequency tone chroma notes with the same letter octave a higher interval every time we pass the same letter tones separated by octaves have the same tone chroma notes with the same chroma have fundamental frequencies that are multiples of one another effect of the missing fundamental constancy of pitch even when the fundamental or other harmonics are removed periodicity pitch pitch that we perceive in tones and that has had harmonics removed range of hearing the specific range of frequencies we hear sound in audibility curve the human range of hearing is depicted indicate the threshold for hearing determined by free field presentation listening to a loud speaker versus frequency auditory response area area about audibility curve can hear tones that fall in this area below audibility curve can t hear tone upper boundary of the auditory response area is the curve marked threshold of feeling tones with these high amplitudes are ones we can feel become painful and can cause damage to the auditory system loudness audibility curve and auditory response area indicate the loudness of pure tones from sound pressure and frequency to determine loudness of tone need to know both its dB level and frequency equal loudness curve explains the relationship b w loudness and frequency indicate the number of decibels that create the same perception of loudness at difference frequencies timbre quality that distinguishes between two tones that have the same loudness pitch and duration but still sound different closely related to harmonic structure of a tone difference in harmonics of different instrument is one factor that causes musical instruments to have different timbres attack the buildup of sound at the beginning of the tone decay the decrease in sound at the end of the tone pure tones and tones produced by musical instruments are all periodic sounds pattern of pressure changes repeats aperiodic sounds sound waves do not repeat the ear 1 must deliver the sound stimulus to the receptors 2 3 it must transduce this stimulus from pressure changes into electrical signals it must process these electrical signals so they can indicate qualities of sound source such as pitch loudness timbre and location divided into 3 divisions outer middle and inner outer ear pinnae structures that stick out from the sides of the head important in helping us determine the location of sounds and important for those who wear eyeglasses part of the ear we could most easily do without capture sound localize sound sound waves first pass through the outer ear consists of pinna and auditory canal auditory canal tubelike structure protects the delicate structures of the middle ear from the hazards of the outside world along with its wax protects the tympanic membrane or eardrum at the end of the canal helps keep this membrane and the structures in the middle