Overview

• Reflection of waves
  
• Continue to work in the same groups
  

Project

• Overview of how to do the project:
• You will need one file for this project
• For the project
  
• You will measure resonant modes in two acoustic transmission lines
• Find two tubes of approx. 1.5 inch diameter and preferable one longer than 24 inches (lengths should differ by at least 25%)
• For best results, try to avoid tubes shorter than 10 inches
  

• Paste the following code into a maatlab script file ( .m file) and run it
• See below for further instructions
  

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% first, connect your bluetooth speaker and a microphone (an ear phone mic will work)
% then, paste this into a maatlab script file ( .m file) and run it
%
% audioSweep test
%
%copyright 2020 by Thomas P. Weldon
% all rights reserved
%
%  outputs a sinusoidal sweep to a speaker
%  reads microphone input
%  displays signal on oscilloscope along with a voltage=frequncy/1000
%  works best using the microphone on earphones at test location
%

clear all
close all hidden
clc

minFreq=150       % begin freq sweep ~20% below mode n=1
maxFreq=8*minFreq % this is likely high enough for you to see modes n=1,2,3
fstep=minFreq/20;
tfreq=0.05          % time spent at each freq
amplitude=0.2     % sine out amplitude
scopetimewidth=30 % scope width seconds
sampleRate=44100; % audio sample rate

%microphone input
deviceReader = audioDeviceReader;
micdevices = getAudioDevices(deviceReader)
deviceReader=audioDeviceReader(sampleRate,'Device','Built-in Microphone')
deviceReader.Device='Built-in Microphone'; %
deviceReader.SampleRate=sampleRate;
deviceReader.SamplesPerFrame=1024*2;

deviceWriter = audioDeviceWriter;
spkrdevices = getAudioDevices(deviceWriter)
deviceWriter.Device='S111 BT';   % bluetooth speaker purchased for this course
%deviceWriter.Device='Default';
%deviceWriter.Device='Built-in Output';
deviceWriter.SampleRate=deviceReader.SampleRate;
deviceWriter.ChannelMappingSource='Property';
deviceWriter.ChannelMapping=1;
samplesPerFrame = deviceReader.SamplesPerFrame;

sineSource  = audioOscillator( ...
      'Frequency', minFreq, ...
      'Amplitude', amplitude, ...
      'SignalType', 'sine', ...
      'SampleRate', deviceWriter.SampleRate, ...
      'SamplesPerFrame', deviceReader.SamplesPerFrame);

scope = dsp.TimeScope( ...
    'NumInputPorts',1, ...
    'LayoutDimensions',[2,1], ...
    'SampleRate',deviceReader.SampleRate, ...
    'TimeSpan',scopetimewidth, ...
    'BufferLength',deviceReader.SampleRate*scopetimewidth, ...
    'YLimits',[-0.1,0.1], ...
    'TimeSpanOverrunAction',"Scroll");
scope.NumInputPorts=2;
scope.YLimits=[-0.1,0.1];
 
 sineFreq = sineSource.Frequency
 sineSource.Frequency = sineFreq;
  
    tic
    while toc<3  
    x = sineSource(); deviceWriter(x);
    signal = deviceReader();scope(signal.*0,sineFreq/1000)
    end
 
scope.reset

  tic
    while toc<1
    x = sineSource(); deviceWriter(x);
    signal = deviceReader();scope(signal.*0,sineFreq/1000)
    end
   
while true
        %sineFreq = 1.025*sineFreq  ; % could use exponential sweep
        sineFreq = sineFreq +fstep  ;
        disp(['frequency=' num2str(sineFreq) ] )
        if sineFreq > maxFreq
            break
        end
        sineSource.Frequency = sineFreq;
    tic
    while toc<tfreq
    x = sineSource(); deviceWriter(x);
    signal = deviceReader();
    scope(signal,sineFreq/1000)
    %scopout=sineFreq/1000
    end
    %pause(1)  
end

release(deviceReader)
release(deviceWriter)

•  To begin this experiment:
• Connect your bluetooth speaker (see course syllabus for model number and vendor)
• Plug a set of earphones with microphone into your computer
• make sure that the bluetooth speaker and earphone mic are the defautl sound output and input of your computer (use your computer settings)
  
• load the maatlab script above
• Run the script, and you should hear an audio sweep tone, and and oscilloscope trace should pop up
• If you receive an S11BT error, your bluetoot speaker was not found:
  
• instead try the "deviceWriter.Device='Default';" line that is commented out in the code
• Place a tube with both ends open betwen the speaker and microphone as shown below

Fig. 01

• For best results,
• use a tube with diameter of approx 1.5 inches andlength greater than 24 inches
  
• cover the microphone and speaker with towels or pillows to redice noise as shown below

Fig. 02

• Run the maatlab script,  and you should see peaks at the modes of the transmission line as shown below
• press the rescale button (magenta arrow in left picture below)
• After the trace is rescaled, it will appear as the picture on the right below
• the modes of the acoustic transmission line with both ends open will appear as peaks
  
• the second trace equals the frequency/1000, and is next used to find the frequencies of the peaks
  

     
Fig. 03

• Use the cursor (cyan arrow above) to measure the mode frequencies, by finding the peaks as shown below (cyan arrows below)
  
• Make sure to select the lower trace channel 2 (magenta circle below) to read out frequency/1000 (magenta arrow below)
  
•  Save a picture of each measurement of each peak for your report, and record the first 3 frequencies (n=1, 2, 3)
• In the example below, the first resonance is the n=1 fundamental mode, and 0.5762 volts would correspond to 576.2 Hz

Fig. 04

• Repeat the measurement for the n=2 mode at the second peak as illustrated below

Fig. 05

PROJECT REPORT

• For your report:
• First, USE THIS TEMPLATE.
• Make any needed changes including:
• Formula for fundamental mode
• 3 figures
• fill in all values in the table
•  Saveyour word file
• Then export-as-pdf
  
• DO A FINAL CHECK of your pdf before turning it in.
• Submit your report
  

• DONT FORGET TO SAVE YOUR FILES!

Copyright  2020 T. Weldon