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Human range of hearing, , scale, Beats, Doppler Effect. Human Range of Hearing • Remember: The human ear is built to sense pressure fluctuations above and below atmospheric pressure. It will sense fluctuations in the 20Hz – 20,000Hz range. Check out other organism’s hearing. A decibel, or one unit of Decibel (dB) Scale (abbreviated as dB,) is used to measure the intensity of a sound wave. Measurement of a decibel is tricky, 0dB is the equivalent of an inaudible sound, 10dB is a 10 times larger pressure than an inaudible sound and 20dB is 100 times more than an inaudible sound. This is a .

Most people perceive a 10dB increase in sound level as about twice as loud as the original level.

0 dB to 10dB 0 dB to 20dB

10 times greater 10X10=100 times pressure than the greater pressure reference than the reference amplitude. amplitude. Decibel (dB)Scale

• The decibel scale is a logarithmic scale of . • Since it is logarithmic, an increase of 10 dB results in a ten fold (10X) increase in pressure on the ear drum. • Zero dB is considered the Threshold of Human Hearing. • at this point is 1x10-12W/m2 Some common pressure & dB ratings

Ha Ha!!! Above and Below • Frequencies above 20 kHz (20,000 Hz) are called Ultrasonic frequencies, • And those frequencies below 20 Hz are called Infrasonic frequencies. Beat Frequency • When sound waves from two different similar frequency sources overlap, interference occurs, both constructive and destructive. • A beat frequency is heard based on how often the sound waves interfere. • The more beats heard, the greater the difference between the two sound sources. • Used to tune instruments. • http://www.acs.psu.edu/drussell/De mos/superposition/superposition.ht ml Beat Frequency The beat frequency is the frequency of a sound which is created from two similar sound sources interfering.

Example: If two tuning forks are set into vibration side to side, and the frequencies of each are 100Hz and 105Hz, the two waves will cause constructive and destructive interference and the beat frequency will be 5Hz.

Check the math! The frequencies of the red and blue sound waves are slightly Beat Frequency (Hz) = │F1 – F2 │ different and their constructive & destructive interference Where the beat frequency is equal determines the beat frequency, to the absolute value of the shown by the green wave. difference between the frequencies of the two tuning forks. Doppler Effect • When either a sound source is moving or a person hearing a sound source is moving, there is a perceived shift in frequency for the listener. • Higher perceived frequency for approaching, lower perceived frequency for leaving. What What do I do I hear? hear? Simulations