Download Physics 1240: Sound and Music - Understanding Pitch, Pressure, and Waves - Prof. Steven J. and more Study notes Physics in PDF only on Docsity! Phys 1240: Sound and Music www.colorado.edu/physics/phys1240 LAST: pitch/frequency Speed of sound TODAY: pressure and the nature of sound NEXT: waves and oscillations READ: Hall 2.4, 2.5 CAPA Thursday… Question 1 got chopped, fixed it ~1 hr after class last Thurs, Grab a corrected copy if you want, or just read the screen!) steven.pollock@colorado.edu CT 1.2.3 What determines pitch? Length/size? “Bendability”/rigidity ? Heaviness/density? (maybe) ? The above may influence it, but it’s really all about the frequency of vibration! What about amplitude? This impacts the loudness, not (generally) the pitch. CT 2.1.2 When the amplitude of an oscillating object is doubled, the period becomes: a) twice as big b) 1/2 as big c) Stays the same d) 1/4 times as big e) Not enough information to decide When the frequency is doubled, the period becomes… CT 2.1.4 A vibration has a frequency of 100 cycles/second, what is the period (or the amount of time for one cycle)? a) 100 sec b) 1 sec c) 10 msec d) 100 msec e) None of these Is this tone audible? If so, is it high or low? How fast (frequently) can you tap the desk? CT 2.1.3 If I lower the frequency of this tone what happens to the pitch? a) It decreases b) It increases c) It remains the same d) Not enough information What if I make the same sound louder (keeping the same pitch), the frequency… Production, propagation perception • Instrument (string/tube/etc) wiggling (at some frequency) => air pressure a little high, then a little low, over and over. • Air pressure “wiggles” too! • Frequency of pressure changes is the SAME as the frequency of the instrument • You detect varying pressure at your ear: frequency => pitch Brief detour: Air pressure • Pressure is not force… it’s force divided by area. • P = Force/Area Lots of pressure from the air in this room! Called “1 atm” of pressure = 14.7 pounds/in2 (sea level) = 100,000 N/m2 (sea level) (P ~ 12.5 pounds/in2 in Boulder, about .85 atm) CT 1.5.2 I stand on a scale with both feet and measure my weight. If I stand on one foot, does the reading: a) Increase b) Decrease c) Remain the same I weigh 500 N, what is the force on each foot? a) 500 N each b) 250 N each c) It depends on the area of my foot CT 1.5.2 What is the area of my feet in m2 (Hint: my feet are very roughly 25 cm x 8 cm)? a) 200 b) 20 c) 2 d) .2 e) .02 CT 1.5.2 What is the approximate pressure on each foot? a) .125 N/m2 b) 125 N/m2 c) 1250 N/m2 d) 1.25x104 N/m2 e) Not enough information Convert this to atmospheres a) 1.25x10-5 atm b) 1.25x10-3 atm c) .125 atm d) 125 atm e) 1.24x105 atm
