Creating a Cochlear Excitogram

1. If you have not already done so, install the program Octave on your computer. Start the program by typing octave on the command line. Also, download the archive of source code required for this lab, and uncompress the archive into the directory in which you will be running Octave.

2. We need to load a sound into the Octave environment. For this purpose, we have created an Octave function to load a sample sound for you. On the Octave command line, type the following:

     > x = loadLoudnessTestSound();
   

   This creates a test sound, and stores it in the Octave variable x.

3. Next, it is necessary to apply a filter to the sound to simulate the effects of transmission through the outer and middle ear. To do this, enter the following command:

     > xFilt = transmissionOuterMidEar(x);
   

4. We next wish to determine the short-time spectra of the signal as they vary over time, which may be found using a form of Short-Time Fourier Transform (STFT). Compute this by typing the following:

     > xSTFT = cochlearInput2fftInt(xFilt);
   

5. Finally, create and plot a cochlear excitogram using the following command:

     > [xExcitogram,bandCentersHz] = fftInt2ExcitPat(xSTFT);
   

   In this command, the excitogram is stored in the Octave variable xExcitogram, and the variable bandCentersHz gives the frequencies at which the excitogram is computed. This command may require a minute or so to finish on your computer.

   You can create a plot of the excitogram using the following:

     > excitPatPlot(xExcitogram, bandCentersHz);
   

   The plot should resemble that shown in Fig.5.

Figure 5: A sample cochlear excitogram.

How many sound objects do you see on the excitogram? Approximately what ERB numbers do these sound objects correspond to? What frequencies in Hz do these ERB numbers correspond to? Which sound object creates the largest cochlear excitation? Which creates the smallest cochlear excitation?