Course:PHYS341/2018/Calendar/Lecture 12

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Phys341 Lecture 12: Summary and web references

2018.01.28

Textbook: 10.5-10.6

Slide List

  1. Project ideas
    • Simple acoustic measurements on an instrument, the more unusual the better
    • In this course at present, I have no examples of First Nations instruments, African instruments, and South Asian instruments. One of these instruments would be especially interesting.
    • Singing styles
    • All my examples are operatic; I have nothing from jazz, blues, or any popular style
    • There are many musical instruments in MOA, and they are generally poorly described
    • (e.g. a Chinese banhu is labelled simply as “violin”). Here’s an opportunity for an organological project.
    • A great project is one that teaches me things I did not know. Musical acoustics is so large a field, that this is not as hard as you might think.
    • See pages on the erhu, tambourica and room acoustics at http://acoustics.phas.ubc.ca/
    • Send me your ideas and we can hone them into suitable projects (in the coming week).
    • If you have a partner you wish to work with, let me know.
    • If you don’t have a partner, let me know, and I can pair you off with another class member.
  2. Masking
    • When one tone is played fairly loud, it becomes impossible to hear a range of tones of a higher frequency, unless they are played really loud.
    • This is called Masking:
    • For example, if a tone of 800 Hz reaches a listener with a SPL of 80 dB, a tone of 1000 Hz will have to be played at 50 dB above the normal threshold of hearing to be heard at all.
    • This physiological effect is exploited in sound recording, where sound files are compressed to 10% of their original size by excluding sounds that we cannot hear due to masking (e.g. the mp3 format).
    • Needs two separate function generators for effective demonstration
  3. Compression
    • In an era of FFTs and fast computers, masking allows sound to be analyzed before being recorded :
    • Parts of the sound spectrum that are fully perceived are recorded with high precision.
    • Parts of the sound spectrum that are masked by other parts of the spectrum can be recorded with low precision.
    • Frequency bands recorded separately in a recognized format (e.g. mp3).
    • On playback the frequency spectra are reconstructed and inverse transformed back into a time-sequence of binary numbers.
    • This time sequence is passed through a digital-analog-converter (DAC) and the voltage output is passed to a speaker.
  4. Hearing loss
    • Audiologists measure hearing loss in dB, the difference between the patient’s threshold of hearing compared to that of a standard issue ISO 226 human being.
    • Measurements are made at seven octave bands:
    • Central frequencies 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.
    • The low tones especially have to be pure, otherwise the more easily heard harmonics will spoil the measurement.
    • The threshold difference for considering hearing loss is ~ 25 dB
    • Hearing loss is caused by age, noise exposure, disease, poverty, congenital conditions...
  5. Cochlear implants
    • How can 22 frequency bands replace 15,000 hair cells connected to 30,000 nerve fibres?
    • “Like playing a piano with boxing gloves on”.
    • Low frequencies (deepest in the cochlea) are a problem.
    • Need to use pitch cues.
    • Note: we perceive the full range of colours with just three colour receptors in the eye.
    • https://www.youtube.com/watch?v=SpKKYBkJ9Hw
    • http://www.soundonsound.com/sound-advice/implanting-awareness