Physics 434 Module 3 – Created by (and thanks to) T. Burnett 1 Physics 434 Module 3 Acoustic excitation of a physical system

The theme of the next three weeks This week: operate in the frequency domain For a given frequency, what is the response? (resonances, etc.) Next week: time domain Excite with a pulse, measure response Following week: Fourier transform Relate to frequency domain measurement Physical system (the tube) Input signalOutput signal

3 Goals for this module Control and monitor an applied frequency Detect and measure a sound wave Generate set of RMS values vs. frequency Fit resonances to determine resonant frequencies and “Q” values Check speed of sound from resonance difference

4 Step 1– Driver & receiver electronics Signal generator (for checking) speaker ‘scope: microphone Breakout box ach0/ach8 dac0 Microphone, speaker Amplifiers tube

5 Step 1 electronics, contued Adjust amplifier gains for given input (say 1 V) so that output does not “saturate” between 500 and 2000 Hz Note the observed resonant frequencies for later check (at this point, can you make an inference about the nature of the resonances?)

6 Step 2 – test the computer- controlled inputs & outputs Get SoundGenerator.llb from web Note that it contains several VIs: SoundGenerator(select) Generate a continuous signal with selectable frequency SoundGenerator(sweep) Generate a sequence of discrete frequencies acquire_test Monitor and measure the signal acquired by the DAC Look at the output on the scope, verify the frequency, feed it to your microphone, verify that it does not saturate.

Physics 434 Module 3 - T. Burnett 7 The AT-E board (ADC & DAC for this lab)

8 Step 3 – check the data acquisition Another self-contained VI, acquire_test.vi, acquires a waveform, with adjustable sampling rate and sample size. The output is graphed and analyzed by a simple-minded RMS vi. Check that the RMS of the signal generator output, or the output from the DAQ card is stable and does not vary much with input frequency ( Hz). (We do not actually measure this, but we assume that it does not change.

9 Step 4 – assemble your VI and run it Ab initio or build up from the two test VI’s Must create a table of (actual) frequencies and RMS response from the microphone, with constant input to the microphone. Display on an XY graph Write to an file with the “Write To Spreadsheet File” vi from the All Functions | File I/O menu.

Physics 434 Module 3 - T. Burnett 10 Step 5 – Analyze the resonance peaks This is a new VI that you must write, capable of reading data from the file and fitting it: see test_resonance_fit.vi, with sub-vi resonance_fitter.vi for the fitting piece that you may use. Note: it must select a range over which the resonance is valid. The formula: Watch for this guy! This part sets up the demo

11 Submit your vi’s in an llb (library) Save plots with current value Use documentation for descriptions. Analysis VI should have a table of the resonance parameters, and your estimate of the speed of sound