ACOUSTICS LAB

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ACOUSTICS LAB
ME4053
Objectives Study the propagation of sound waves
from a loudspeaker and compare with a simple
theoretical model
Specifically

• Measure the speed of sound
• Show that acoustic pressure falls off like 1/R in
  the far-field
• Study the speaker directivity as a function of
  frequency

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• Experimental Procedure for
• Speed of sound measurement
• 1/R acoustic pressure behavior in far-field

• Perform at specified frequency
• Keep the speaker fixed
• Vary the mic distance, R
• Record time delay
• Record rms voltage

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Experimental Procedure for Capturing Sample a
Waveform

• Save waveforms to Floppy Drive
• Move mic 26 cm from baffle
• Save input waveform
• Save output waveform
• See slide 14 for processing

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Experimental Procedure for Directivity

• Obtain Directivity Data at 2 Specified
  Frequencies
• Fix mic at 26 cm
• Rotate speaker 5 degrees
• Record SPL from o-scope
• Repeat until at 90 degrees
• Repeat with next frequency

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Experimental Procedure (contd)

• Get your notebook signed
• Turn all equipment off (especially the mic!)
• Leave the lab (you dont have to go home but you
  cant stay here)

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Time Delay RMS Voltage Measurements
_at_ xx s ? xx s Ch 2 xx V RMS
Ch 1 Input
Gating ON gives RMS value only between two cursors
Ch 2 Output
_at_
?
? should remain constant as it is simply the
period of the burst
_at_ gives distance from trigger to solid
cursor. This is an easy way to measure time
delay.
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Post-Lab Data Analysis for Sound Pressure Level

• Calculate sound pressure level (SPL) in terms of
  decibels (dB)
• Plot the SPL as a function of log(R/Rmin)
• Verify that the SPL approaches a -20 dB/decade
  slope sufficiently far from the speaker

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Sound Pressure Level (SPL) Decay
SPL decay approaches 1/R model in far field
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Post-Lab Data Analysis forSpeed of Sound

• Plot the data
• Perform a regression on the data
• Calculate theoretical value of speed of sound
• Compare theoretical and experimental values

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Post-Lab Data Analysis for Captured Waveform

• Compare input and output signals
• Plot .input and output signals using Matlab. The
  subplot command is useful for this
• Discuss the differences between waveforms
• Note t0 on the abscissa is the trigger point
  for input signal

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Post-Lab Data Analysis of Directivity

• Plot the normalized sound pressure versus q at
  2.5 kHz and 5 kHz and compare with the
  theoretical directivity pattern
• Theoretical directivity pattern
• At low frequencies (where l gt a), the function
  D(q) does not exhibit many variations as a
  function of q gt speaker is weakly directional
• At higher frequencies (when l lt a), D is more
  dependent on the values of q gt the speaker is
  highly directional

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Note Your result may not match those given
(Matlab polar and hold on commands)
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Sample code to read comma delimited text file in
Matlab
This file will read a comma delimited
file generated from the Tektronix
Oscilloscope It will then plot the
data Created by Wayne M. Johnson
17MAR03 clear all clears all variables close
all closes all figure windows adlmread('TEK00
000.CSV') reading comma delimited
file tina(,1)1e3 assigning the time
data Note that time data has been shifted to
such that t0 corresponds to the trigger point
for input signal. See Tektronix User Manual, p.
3-47 Vina(,2) assigning the voltage
data adlmread('TEK00001.CSV') reading comma
delimited file touta(,1)1e3 assigning the
time data Vouta(,2) assigning the voltage
data plotting data subplot(2,1,1) plot(tin,Vin)
grid plotting the data title('Input signal
5kHz')ylabel('Voltage (V)') subplot(2,1,2) plot
(tout,Vout)grid plotting the data title('Microph
one output at xx cm from speaker') xlabel('Time
(msec)')ylabel('Voltage (V)')
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Last minute pointers

• Your presentations should be much better than
  this one
• For help contact the TAs
• Both submissions (report/pres) will be done with
  your group