Auditory Theory: Acoustics

Lecture 007 Hearing II

Reading Assignment for Lecture 008

Before next class please read Sections

• 2.5.n Noise Induced hearing loss
• 2.6.n Perception of Sound Source Direction

pages 79 to 108 of Acoustics and Psychoacoustics. We will have a brief quiz on these sections at the beginning of the next class.

Brain Bullets

• Human hearing: 20Hz - 20kHz
  • Healthy young children may have a full hearing frequency range up to 20,000 Hz,
  • by the age of 20, the upper limit may have dropped to 16,000 Hz.
  • From the age of 20, it continues to reduce gradually to approximately 8,000 Hz by retirement age.
• Hearing loss
  • can also be induced by prolonged exposure to loud sounds especially via headphones
  • The ear's sensitivity to sounds of different frequencies varies over a vast sound pressure level range
  • The maximum average sound pressure level which is heard rather than perceived as being painful is 64 Pa.
• Loudness Perception
  • The ear is a pressure sensitive organ that divides the audio spectrum into a set of overlapping frequency bands whose bandwidth increases with frequency
  • These are both objective descriptions of the amplitude and the function of the ear.
  • However, they tell us nothing about the perception of loudness in relation to the objective measures of sound amplitude level
• Fletcher Munsen
  • These contours, originally measured by Fletcher and Munson (1933) and by others since, represent the relationship between the measured sound pressure level and the perceived loudness of the sound. The curves show how loud a sound must be in terms of the measured sound pressure level to be perceived as being of the same loudness as a 1 kHz tone of a given level.
  • There are some humps and bumps in the contours above 1 kHz. These are due to the resonances of the outer ear. This is a tube about 25 mm long with one open and one closed end. This will have a first resonance at about 3.4 kHz and, due to its non-uniform shape a second resonance at approximately 13 kHz
  • The second effect is an amplitude dependence of sensitivity which is due to the way the ear transduces and interprets the sound and, as a result, the frequency response is a function of amplitude.
  • the frequency response of the ear is not flat and is also dependent on sound level.
  • Tones of different frequencies therefore have to be at different sound pressure levels to sound equally loud and their relative loudness will be also a function of their absolute sound pressure levels
• Phons
  • The phon scale is a subjective scale of loudness based on the judgements of listeners to match the loudness of tones to reference tones at 1 kHz.
  • The curve for N phons intersects 1 kHz at N dB(SPL) by definition,
  • The relative loudness of different frequencies is not preserved, and therefore the perceived frequency balance of sound varies as the listening level is altered
• Measuring loudness
  • These effects make it difficult to design a meter which will give a reading which truly relates to the perceived loudness of a sound,
  • two frequency weightings which are commonly used to perform this compen sation: termed 'A' and 'C' weightings
  • The 'A' weighting is most appropriate for low amplitude sounds as it broadly compensates for the low level sensitivity versus frequency curve of the ear
  • The 'C' weighting on the other hand is more suited to sound at higher absolute sound pressure levels and because of this is more sensitive to low frequency components than the' A' weighting
• Loudness of simple sounds
  • The peak sensitivities shown in this figure are equivalent to a sound pressure amplitude in the sound wave of 10 μPa or: about -6 dB(SPL)
  • For sounds presented on the listening side of the head there is a rise in peak sensitivity of about 6 dB due to the increase in pressure caused by reflection from the head
  • However, for broad-band noise, or sounds with several harmonics, it generally accepted that a change of about 10 dB in SPL corresponds to a doubling or halving of perceived loudness.
  • we must increase the number of violinists in the string section by a factor of ten in order to double their volume.
• Effect of duration
  • when we listen to sounds which vary in amplitude the loudness level is not perceived significantly by short amplitude peaks, but more by the sound level averaged over 200 milliseconds.
• Loudness of complex sounds
  • The first effect is that the ear seems to lump all the energy within a critical band together and treat it as one item of sound
  • As the ear is sensitive to sound pressures, the sound intensity is proportional to the square of the sound pressures
• Fatigue and Adaptation in the Hearing System
  • one tone following after a loud organ point of 5 sec duration at the same pitch will be heard after 1 ½ sec with a loudness of 5 db less than the original tone
  • with increasing beating rate the fatigue effect declines, and completely disappears for a beat of 15/sec
  • a musician has no absolute measure of objective loudness, just as he has no absolute measure of time
  • a constant tone, which, for example, swells in 2 minutes to double the sound pressure, finally attains scarcely half of its initial subjective loudness
  • On the other hand, the reduction to half of the sound pressure results in the same subjective loudness as in the previous experiment