Course:PHYS341/2018/Calendar/Lecture 11

Phys341 Lecture 11: Summary and web references

2018.01.26

Textbook: 10.1, 10.3

Slide List

1. Review: Loudness (text fig 10-1)
2. Frequency discrimination
   • Use Szynalski tone generator http://www.szynalski.com/tone-generator/ (open twice)
3. Binaural Beats
   • What if the two tones go separately into each ear?
   • Use http://onlinetonegenerator.com/binauralbeats.html
4. Two tones
   • Imagine two tones of equal intensity sounding together, starting at the same frequency.
   • The frequency of one of the tones then slowly increases.
   • Call the magnitude of the frequency difference Δf.
   • If Δf is less than about 15 Hz you beats, you hear a single tone whose frequency is equal to the mean of the two frequencies, pulsing with the beat frequency Δf.
   • e.g. for tones of 250 Hz and 256 Hz, you hear a single 253 Hz tone, beating 6 times a second.
   • If Δf is more than what is called the limit of frequency discrimination, ΔfFD , you start to hear two distinct tones, and the sound is rough.
   • If Δf is more than what is called the critical band, ΔfCB , you hear two distinct tones clearly.
   • If Δf is some simple integer fraction of the contributing frequencies, the result is harmony.
5. What is happening in the ear?
   • Frequency detection occurs on the linear basilar membrane.
   • There are ~ 30,000 receptor cells on the basilar membrane spread over a little less than 3 cm.
   • Different frequencies excite different positions along the membrane
   • These excited spots need to be about 1.2 mm apart to be recognized as different frequencies.
   • Below 1.2 mm, the brain has trouble figuring out the central frequency – hence the perceived roughness.
   • (Beating is a physical phenomenon, unrelated to the physiology of the ear).
6. Masking: will revisit next lecture.