Figure 12.5. Why interaural level difference (ILD) occurs for high frequencies but not for low frequencies. (a) When water ripples are small compared to an object, such as this boat, they are stopped by the object. (b) The spaces between high-frequency sound waves is small compared to the head. The head interferes with the sound waves, creating an acoustic shadow on the other side of the head. (c) The same ripples are large compared to the single cattail, so they are unaffected by it. (d) The spacing between low-frequency sound waves is large compared to the persons head, so the sound is unaffected by the head.
Figure 12.4 The principle behind interaural time difference (ITD). The tone directly in front of the listener, at A, reaches the left and the right ears at the same time. However, when the tone is off to the side, at B, it reaches the listeners right before it reaches the left ear. Inter-aural Time Difference (ITD)
eagle at 10 degrees azimuth x - y =.02m speed of sound = 331 m/s x - y = 60 microseconds i.e. ITD = 60 microseconds x y 20 cm
The barn owl (Tyto alba) For owls..... the horizontal position (azimuth) of a sound source is computed with interaural time differences (ITD) -the vertical position (elevation) is computed using interaural sound-level differences (ILD)
Barn Owls do use ITDs to localize sounds in azimuth
Barn Owls do use ILDs to localize sounds in elevation
How owls use ILDs as a cue for elevation of a sound source. Barn Owl (Tyto alba)
Figure 12.12 (a) ITD tuning curves for broadly-tuned neurons. The left curve represents the tuning of neurons in the right hemisphere; the right curve is the tuning of neurons in the left hemisphere; (b) Patterns of response of the broadly tuned curves for stimuli coming from the left, in front, and the right. Neurons such as this have been recorded from the gerbil (Adapted from McAlpine, 2005).
Directional transfer function For humans, the pinnae are important for localizing the elevation of sound sources
–performance after the molds inserted –adaptation
-direct vs. indirect sound Precedence Effect -for sounds arriving within 5 msec, the location is based on the first sound -it's not the loudness of the first sound that is important -later sounds do have an effect on sound quality
Reverberation time -time it takes for sound to decrease to 1/1000 the original pressure -too short - music sounds dead -too long - muddled -optimal is 1.5 to 2 seconds -direct vs. indirect sound
Factors that Affect Perception in Concert Halls –Intimacy time - time between when sound leaves its source and when the first reflection arrives Best time is around 20 ms. –Bass ratio - ratio of low to middle frequencies reflected from surfaces High bass ratios are best. –Spaciousness factor - fraction of all the sound received by listener that is indirect High spaciousness factors are best.
Acoustics in Classrooms Ideal reverberation time in classrooms is –.4 to.6 second for small classrooms. –1.0 to 1.5 seconds for auditoriums. –These maximize ability to hear voices. –Most classrooms have times of one second or more. Background noise is also problematic. –Signal to noise ratio should be +10 to +15 dB or more.
Vision Audition pattern of light perception of objects vibration of basilar membrane perception of auditory streams Gestalt Psychology Similarity time Principles of Auditory Grouping
Vision Audition pattern of light perception of objects vibration of basilar membrane perception of auditory streams Gestalt Psychology Similarity time auditory stream segregation
Stream Segregation in a Series of Six tones (1) Stream Segregation in a Sonata be Telemann (6)
Figure 12.18 (a) These stimuli were presented to a listeners left ear (blue) and right ear (red) in Deutschs (1975) scale illusion experiment. Notice how the notes presented to each ear jump up and down. (b) What the listener hears. Although the notes in each ear jump up and down, the listener perceived a smooth sequence of notes. This effect is called the scale illusion, or melodic channeling. (Adapted from Deutch, 1975).
easy or difficult easy auditory stream segregation Release of a two –tone target by the capturing of interfering tones (16)
Stream segregation can effect perception of timing (13) Stream segregation in African xylophone music (7, 8, 9)
Figure 12.19 A demonstration of auditory continuity, using tones. Demonstrations 28,29, 31
Good Continuation phonemic restoration effect The (cough)eel was on the orange. Percept peel The (cough)eel was on the orange. The (cough)eel was on the car. wheel The (silence)eel was on the car. eel Phonemic restoration effect