Auditory Theory: Acoustics

Lecture 008 Hearing III

Reading Assignment for Lecture 009

Before next lecture please read Sections

  • 3.1.n Musical Notes

pages 109 to 119 of Acoustics and Psychoacoustics. We may have a brief quiz on these sections at the beginning of the next class.

Brain Bullets

  • Hearing Loss
    • A loss of hearing sensitivity: the effect of noise exposure causes the efficiency of the transduction of sound into nerve impulses to reduce.
    • A loss of hearing acuity: this is a more subtle effect but in many ways is more severe than the first effect.
  • Integrated noise dose
    • The integrated noise dose is defined as the equivalent level of the sound over a fixed period of time, which is currently 8 hours.
  • Protecting your hearing
  • There are a few situations where potential damage is more likely.
    • The first is when listening to recorded music over headphones, as even small ones are capable of producing damaging sound levels. .
    • The second is when one is playing music, with either acoustic or electric instruments, as these are also capable of producing damaging sound levels, especially in small rooms with a 'live' acoustic.
  • The acoustic reflex reduces the sensitivity of your hearing when loud sounds occur. This effect, combined with the effects of temporary threshold shifts, can result in a sound level increase spiral, where there is a tendency to increase the sound level 'to hear it better' which results in further dulling, etc.
  • Perception of sound source direction
    • Interaural time difference
    • The time difference between the two ears will depend on the difference in the lengths that the two sounds have to travel.
    • Interaural intensity difference
    • The other cue that is used to detect the direction of the sound is the differing levels of intensity that result at each ear due to the shading effect of the head
  • Pinnae and head movement effects
    • Pinnae: The first is to use the effect of our ears on the sounds we receive to resolve the angle and direction of the sound. This is due to the fact that sounds striking the pinnae are reflected into the ear canal by the complex set of ridges that exist on the ear. These pinnae reflections will be delayed, by a very small but significant amount, and so will form comb filter interference effects on the sound the ear receives.
    • Head Movement: When we hear a sound that we wish to attend to, or resolve its direction, we move our head towards the sound and may even attempt to place it in front of us in the normal direction, where all the delays and intensities will be the same
  • The Haas effect
    • The ear will attend to the direction of the sound that arrives first and will not attend to the reflections providing they arrive within 30 ms of the first sound.
    • The reflections arriving before 30 ms are fused into the perception of the first arrival. However, if they arrive after 30 IDS they will be perceived as echoes.
  • Stereophonic listening
    • Delay stereo: this system is shown in Figure 2.29 and consists of two omni-directional microphones spaced a reasonable distance apart and away from the performers. Because of the distance of the microphones a change in performer position does not alter the sound intensity much, but does alter the delay. So the two channels when presented over loudspeakers contain predominantly directional cues based on delay to the listener.
    • Intensity stereo: this system is shown in Figure 2.30 and consists of two directional microphones placed together and pointing at the left and right extent of the performers' positions.
    • Binaural stereo: the third major way of providing a directional illusion is to use binaural stereo techniques. This system consists of two omni-directional microphones placed on a head, real or more usually artificial and presenting the result over headphones. Because the distance of the microphones is identical to the ear spacing and they are placed on an object which shades the sound in same way a change in performer position provides both intensity and delay cues to the listener, the results can be very effective but they must be presented over headphones because any cross-ear-coupling of the two channels, as would happen with loudspeaker reproduction, will cause spurious cues and so destroy the illusion.