Binaural Hearing. Reading: Yost Ch. 12

Transcription

1 Binaural Hearing Reading: Yost Ch. 12

2 Binaural Advantages Sounds in our environment are usually complex, and occur either simultaneously or close together in time. Studies have shown that the ability to perceive sounds with two ears increases performance in a variety of tasks. In particular, listening with two ears give an advantage in: loudness summation localizing sounds reverberant environments background noise

3 Binaural Loudness Summation Binaural loudness summation refers to the fact that sounds received in two ears are louder than sounds received in one ear. Recall that binaural minimal audible field (MAF) thresholds are ~6 db lower than monaural minimal audible pressure (MAP) thresholds obtained with headphones: binaural advantage: threshold is about 3 db lower when listening with both ears. physiological noise: when the ear is covered, the subject can hear body noises, such as heart beat, and these may have a masking effect. The binaural advantage of ~3 db (perfect binaural addition) is maintained (if not increased) at all supra-threshold levels and suggests sounds are twice as loud under binaural than monaural conditions.

4 Binaural Sound Localization Cues Sounds have no spatial dimensions; localization depends on sounds interacting with the head and ears to generate cues. If a sound is lateral to the midline, then the initial wave front of the sound arrives at the near ear before far ear. This creates interaural (between the ear) differences in time (ITD) and level (ILD) that serve as binaural cues for sound source azimuth. Azimuth

5 Monaural Sound Localization Cues In addition to generating binaural sound localization cues for azimuth, the head and pinna add spectral cues for vertical localization in the form of high-frequency (6-8 khz) notches in directional transfer functions. The notch shifts systematically to higher frequencies with increasing elevation. Elevation Spectral notches provide vertical localization cues for broadband, highfrequency complex sounds, which can convey the spatial cues in their spectra. Directional transfer functions also resolve most front/back, and up/down confusions.

6 Binaural vs Monaural Localization Listeners locate well broadband sounds in both azimuth and elevation when binaural, as well as monaural, sound localization cues are available. Binaural Performance is substantially degraded when one ear is plugged (acute). Elevation localization is preserved, but sounds are all pulled to the unplugged side. Acute monaural Over time, individuals can learn to localize sounds reasonably well with only one functional ear, suggesting that monaural spectral cues alone are sufficient; however, accuracy does not achieve that of binaural listening. Monaural

7 Effects of Acute Ear Plug Under normal binaural hearing, azimuth localization is nearly perfect because the binaural difference cues are reliable. With a plug, however, the binaural difference cues (ITD and ILD) break down. This induces a decrease of the contribution from binaural difference cues (arrows), and an increase in the gain of the spectral cues of the contralateral normal-hearing ear (HRTF c ) (eventually, monaural spectral cues can support some azimuth sensitivity).

8 Binaural Precedence Effect The precedence effect or law of the first wavefront is a binaural psychoacoustic effect that is important for listening in enclosed, reverberant spaces. When a sound is followed by another sound separated by a sufficiently short time delay (below the listener's echo threshold), listeners perceive a single fused auditory image whose perceived spatial location is dominated by the location of the first-arriving sound. Sounds are said to be: direct (a), reaching the listener s ear straight from the source; or indirect (b-d), reaching the listener s ear after being reflected off of environmental surfaces

9 Reverberation Reverberations that occur in a room can severely distort localization cues. One strategy that listeners unconsciously employ to cope with this is to make their localization judgments instantly based on the earliest arriving waves in the onset of a sound. This strategy is known as the precedence effect, because the earliest arriving sound wave (I.e., the direct sound with accurate localization information) is given precedence over the subsequent reflections and reverberation that convey inaccurate information. Reverberant talk clap

10 Studying the Precedence Effect Precedence effect demonstration with two speakers reproducing the same sound. The pulse from the left speaker leads in the left ear by a few hundred microseconds, suggesting that the source is on the left. The pulse from the right speaker leads in the right ear by a similar amount, which provides a contradictory localization cue. Because the listener is closer to the left speaker, the left pulse arrives sooner and wins the competition the listener perceives just one single pulse coming from the left, but its location depends on both sources.

11 Precedence Effect on Localization Sounds produced in areas with multiple surfaces give rise to reflections. Many copies of a sound reach a listener s ears. The direct sound arrives first. With complex sounds like speech, early reflections tend to perceptually fuse with the direct sound (the Haas effect). The direct sound dominates localization of the sound (the precedence effect); effects include summing localization (one object located between the sources), and localization dominance (one object located at the source of the leading sound). Beyond the echo threshold, both objects are localized to their respective locations.

12 Sound Reinforcement Systems The precedence effect allows sounds to be localized well in enclosed spaces. The effect can be exploited in sound reinforcement systems (e.g., concert halls, public address systems). Here, the signal for loudspeakers placed at distant locations from a stage may be delayed electronically by an amount equal to the time sound takes to travel through the air from the stage to the distant location, plus about 5 to 20 milliseconds and played at a level up to 10 db louder than sound emanating from the stage. The first arrival of sound from the source on stage determines perceived localization whereas the slightly later sound from delayed loudspeakers simply increases the perceived sound level.

13 Binaural Spatial Release from Masking In everyday life we often listen to one sound, such as someone's voice, in a background of competing sounds. In these environments, perception of target speech is assisted by a listener s ability to segregate the multitude of sounds into separate auditory streams, one cue to which is the angle of incidence of different sounds and the resulting binaural cues to sound localization. Masker Target Binaural spatial release from masking can increase to upwards of 15 db, one of the largest known psychoacoustic advantages of binaural processing.

14 The Cocktail Party Effect The cocktail party effect is the phenomenon of being able to focus one's auditory attention on one stimulus while filtering out a range of other stimuli, much the same way that a partygoer can focus on a single conversation in a noisy room. The binaural aspect of the cocktail party effect is related to the localization of sound sources. As soon as the auditory system has localized a sound source, it can extract the signals of this sound source out of a mixture of interfering sound sources. same different Auditory scene analysis (i.e., segmentation) can also be based on other aspects of sound, for example timber and pitch.

15 Spatial Release from Masking Real environments are noisy Energetic masking: occurs at periphery; target signal is blocked by masker (e.g., speech in background speech) Informational masking: occurs centrally; target signal competes with masker for auditory attention (e.g., own name mentioned) Spatial release from masking: Detection and comprehension is enhanced if target and masker are spatially distinct Mechanisms of spatial release: Better ear arises from head's acoustic shadow Classic binaural cues (ITD, ILD) allow spatially focused attention

16 MLD (db): MoSpi - MoSo Binaural Masking Studies have shown that the masked threshold of a signal is the same when the stimuli are presented in a monotic (one ear only) or diotic condition (identical stimuli to both ears). If the masker and signal are presented in a dichotic situation (different stimuli presented to the two ears from different spatial locations), then the signal has a lower threshold then either the monotic or dichotic condition. 16 Diotic: M o S o Dichotic: M o S p Monotic: M m S m Dichotic: M o S m The masking level difference (MLD, dichotic minus diotic) decreases as a function of increasing frequency, but not to zero. It is thought that the MLD reflects processing of the interaural time and level difference cues (MLD is stronger in the ITD pathway) ,000 10,000 Fequency - Hz

17 Summary Studies have shown that the ability to perceive sounds with two ears increases performance in a variety of tasks. In particular, listening with two ears give an advantage in: loudness summation (x2) localizing sounds (left vs right) reverberant environments (precedence effect) background noise (spatial release)