 Space… the sonic frontier

Perception of Direction  Spatial/Binaural Localization  Capability of the two ears to localize a sound source within an acoustic space  Localization – ability to place a sound in a specific location

Sound Location cues  The ear uses 3 cues to determine the location of a sound  Interaural Intensity Difference (IID)  Aka: spectral shadow  Interaural Arrival Time Difference (ITD)  Aka: time delay  Effects of the Pinnae

Interaural Intensity Difference

 Mid to high frequency sounds coming from the right will reach the right ear at a higher intensity level than the left ear  Head casts an acoustic block,or shadow  Sound travels farther to other ear, thereby losing energy (i.e. intensity is reduced with distance)

IID, continued  Works best above 1500 Hz  Doesn ’ t work as well with lower frequencies -- WHY NOT?

Interaural Arrival Time Difference

 Sounds coming from the right will reach the right ear sooner than the left  Time difference occurs because the acoustic path length to one ear is longer than the path to the other ear  Helps give lateral localization cues

ITD (contd.)  Works better below 1500 Hz -- WHY?

Effects of the Pinnae

 Pinnae = outer ears! (Pinna = 1)  Its ridges introduce minute time delays between direct sound and reflected sounds  Help determine:  whether something is behind us  or on a vertical plane

Perception of Space  The ear helps us determine  Distance  Physical sense of the space in which a sound occurs  Sound propagates AWAY from a source in directions determined by the nature of the source and its surroundings  Nature of source – frequency, amplitude, spectral content  Surroundings – reflective surfaces, room absorption, etc.

Perception of space  Waves coming into contact with objects behave in 2 possible ways:  Refraction  Reflection

Directional observations  If there is no difference between what the left and right ear hears, the brain…  Assumes that the source is the same distance from each ear  3 distance cues allow us to position sound left/right and monophonically

Sound propagates for everyone!

Sound Propagation  Described in three stages: 1. Direct Sound 2. Early Reflections 3. Reverberation

Sound Propagation

Direct Sound  The percentage of sound that reaches the listener directly  Determines source ’ s location  aka precedence effect or Haas effect  Also helps determine source ’ s size and true timbre  Timbre of a sound will change as it propagates through a space  Why? Loss and/or attenuation of spectral content over time

Early Reflections  Sounds that bounce off surfaces and reach the listener second  Reflections off the largest boundaries in a room  Provide clues about:  Reflectivity, size and general nature of acoustic space  Direct sound tells us about the sound source, early reflections tell us about the sonic space

Early Reflections (contd.)  Arrive < 50 ms after brain perceives direct sound  Time between direct sound and early reflections gives info about the size of a space  (farther the boundaries, longer the delay before sound is reflected)

Temporal Fusion  When early reflections arrive within 35 ms of the direct sound, the early reflections fuse with the direct sound  Can ’ t distinguish early reflections from original sound  Makes sound seem louder & fuller

Reverberation Diagram

Reverberation  Sound that persists after the source stops  Sound bounces off so many surfaces they reach listener as a continuous stream from all directions  Reverberation = densely spaced reflections

Reverberation (contd.)  Reach listener > 50 ms after direct sound  Gradual decrease in amplitude  Sense of added warmth or reverb  Timbre usually different  High frequency rolloff (gradual attenuation)  Slight bass emphasis

Reverberation TIME  The time it takes for the sound to decrease to 60dB below its original level  This is dependent upon the room ’ s absorptive characteristics

Reverberation Graph

Sound Absorption  Reverberation is frequency dependent!  Rooms can be described based on the frequencies they absorb, and those they allow to reverberate  Depends on materials and objects in the space

Sound Absorption (contd.)  Small obstacles  Reflect high frequencies  Large obstacles  Reflect low frequencies

Absorption Coefficient  Calculation that determines what material types absorb what frequency types

Absorption/Reflection Types  Highly Reflective surfaces = low absorption coefficient  Heavy, stiff materials  Concrete  Shower tiles  marble

Absorption/Reflection Types  High Frequency Absorbers ( Hz)  Soft porous materials  Curtains  Clothing  Carpeting  Thicker, more porous materials will also absorb lower frequencies

Absorption/Reflection Types  Low Frequency Absorbers ( Hz)  Materials that can vibrate  Act as resonators for frequencies near their resonant frequencies  Windows  Plaster walls on widely spaced beams  Wooden floors

Absorption/Reflection Types  Mid Frequency Absorbers ( Hz)  Combination of high and low frequency absorptive materials  Wall of porous material with regularly spaced wood panels on it

Absorption chart