We dealt with beats, Doppler Effect, and shock waves. Shock waves develop whenever an object moves faster through a medium than waves move through the medium. A commonly observed example of a shockwave is the bow wave of a boat moving quickly through water. The same thing happens when objects travel through air faster than sound travels. Such things are said to be "supersonic." Problems we worked with: Ch 12 P: 25, 26, 27, 33; Ch 12 P: 39, 41, 42; Ch 12 P: 49, 50, 51
We explored the gist of yesterday's activity. The point was to understand the resonances that we heard from the columns of air. One way to help understand the resonances is to sketch their standing wave diagrams.
Assignment:
We collected and dealt with the lengths of air columns that resonated with our various tuning forks in an effort to determine the wavelengths of the sounds these tuning forks emitted. We were able to get different lengths to resonate, and these lengths were either λ/4, 3λ/4, 5λ/4, etc.
Assignment: same as on Friday...
We considered what is required to support standing waves in columns of air confined to pipes. We saw that it matters whether both ends are open or only one end of the column is open. In one case, only odd harmonics could resonate; the even harmonics kept cancelling themselves out so that there was no resonance for them. In the other case all the harmonics would resonate. You should know which kind of pipe supports all harmonics and which supports only the odd harmonics.
And beyond that, we are planning to measure the lengths of the air columns that support standing waves in PVC pipe in order to determine the wavelength of these waves. That's for Monday. Assignment:
We worked on consolidating our understanding of relationships among sound level, sound intensity, and distance from a sound source. Intensity tells us how many joules of acoustic energy flow through each square meter of area per second. It is really a measure of how concentrated acoustic energy is. As this energy spreads out from the sound source, the intensity decreases. The more area the energy is spread across, the lower its intensity. A change in intensity I of a factor of 10 changes the sound level β by 10 dB. A change in intensity I by a factor of 2 results in a change in β of very nearly 3 dB (actually 3.01 dB, but 3 dB is close enough).
Here are the problems we worked on.
Assignment:
A trumpet plays a note that has a certain intensity and creates for you a sound level of 70 dB as you stand 5 m away.
We dispatched our assignment expeditiously today.
Assignment: Study Ch 12 Section 12-4 about standing waves in columns of air (open and closed pipes). A trumpet plays a note that has a certain intensity and creates for you a sound level of 70 dB.
So began our class today. Calculating sound levels in decibels and sound intensities in W/m^2 were today's blue plate special. Students should have left class today understanding that intensities are proportional to the energy inputt. For example, ten trumpeters will increase the intensity of the sound you receive by a factor of ten compared to a lone trumpeter.
On the other hand, sound levels vary with the logarithm of the intensity. So increasing the intensity of a sound by 10 times will result in a sound level that increases by 10 dB. Every 10 dB increase in sound level is a result of an increase of ten times the intensity. Therefore, a 20 dB increase in sound level is the result of 10 dB + 10 dB, which is two increases of ten times in the intensity of the sound input. That's an increase in intensity of 100-fold. We also saw that doubling the intensity results in a 3 dB increase in sound level. Increasing the intensity of a sound causes a 3dB + 3dB increase in sound level, or 6dB. Assignment:
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Physics IIMr. Swackhamer Archives
May 2019
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