Homelab 1

This homelab is an introduction to Raven Lite, the software we will use to record and analyze sounds. Click on the Software tab on our web site and read about Raven Lite. Following the instructions, download and install the program. Then launch Raven Lite and work through the first video tutorial on this page.

Open the Chestnut Sided Warbler example file again and examine the upper part of the sound window. You will see a graph of the sound pressure versus time, with the x-axis labeled in seconds. Zoom in the horizontal time axis until you can see the individual oscillations in the sound recording. Then zoom in even more until you can see dots representing the individual samples in the sound file. Raven Lite normally samples at 44,100 times per second, or in other words it uses a 44.1 kHz (kilohertz) sampling rate. This is the same sampling rate used in audio CDs. The y-axis of the graph represents sound pressure. The program cannot display the sound pressure directly in Pascals because it does not know the sensitivity of your microphone or the gain of the amplifiers in your computer, so it represents the sound pressure in arbitrary units that are proportional to pressure. It calls these units U or kU, where kU = 1000*U.

The lower part of the window is a spectrogram representing the frequencies or pitches present in the sound and how they vary with time. The x-axis of the spectrogram is time measured in seconds, the same as for the upper graph of pressure versus time. The y-axis represents frequency in Hz or kHz. If you use the grey-scale color map, dark regions in the spectrogram show what frequencies are present in the sound at each time. There is a cursor readout at the lower left of the main window that gives the frequency and sound power (in decibel units) at each location in the spectrogram.

Use the Play button in the upper tool bar to play the sound file. Try setting the Rate to 0.2 to play the sound more slowly. This will make it easier to see how features in the spectrogram correspond with the sounds you hear. Note how lines sloping downward in the spectrogram correspond to falling pitch and lines sloping upward to rising pitch.

When you explore your own sounds it will be critical to understand the function of the Focus slider in the spectrogram tool bar. The Focus slider is the third slider just to the right of the Brightness and Contrast sliders. It determines the number of adjacent samples the program uses to calculate the frequencies displayed in the spectrogram. The default value is 256. Try sliding the control to the left to make the value less than 256. You will see that the spectrogram becomes broader in the y or frequency direction. This is because a short section of the sound only has a few oscillations and its frequency cannot be determined precisely. Now slide the control to the right and notice that the spectrogram becomes sharper in the frequency direction but broader in the x or time direction. When we try to determine the frequencies present in a longer section of the sound, we can do so more accurately, but only if the frequencies do not change with time. Sounds in music, like sounds made by birds, often vary rapidly in time, so we cannot increase the Focus slider's value too much without smearing the spectrogram. For this example, you should find that a value of about 500 is best. Be sure to experiment with the Focus slider (as well as the Brightness and Contrast sliders) every time you explore a sound using Raven Lite.

Next you will study three notes with Raven Lite and record your activities on this form.

Choose New > Recorder... from the File menu, and click OK in the Configure New File window that pops up. This will open a window for recording sound from the microphone in your computer. Now click the little green triangle at the lower left of the window to start recording.

Sing any steady note for about three seconds and then click the green arrow again to stop the recording. Observe the sound in the time window and in the spectrogram window. In the time window, zoom in until you can see the individual oscillations and measure the period of your note, using the cursor. You will probably get a result in the range 1 to 10 ms (milliseconds). The cursor resolution is only 1 ms, so instead of measuring the period of one cycle it is better to measure the period of 10 cycles and then divide by 10. This way you can get an accuracy of 0.1 ms. Record your measured period T for the first note in the first row and column of the table on the form. Then calculate the frequency using f=1/T and record the result in the second column. Convert your period from milliseconds to seconds before you use the formula, so that your result will be in Hz and not kHz. Now go back to Raven Lite and measure the frequency of your note in the spectrogram window using the cursor. To do this, zoom the frequency scale so it covers the range 0 to 1000 Hz, set the Focus to about 8,000, and adjust the Brightness and Contrast sliders until you see dark parallel horizontal lines in the spectrogram. The frequency of the lowest of these lines is the frequency of your note. (The other lines are called harmonics, and we will discuss them later in the course.) If you put your cursor over the lower line you can read its frequency in the cursor readout at the lower left of the window. Record this 'cursor' frequency in the last column of the table. It should be close to the frequency you calculated from the period. If it isn't, figure out what you did wrong and try again.

Repeat the above for two more notes with different pitches (different frequencies), and enter your results in the second and third row of the table. If you have a very low voice, it is possible that the lowest notes you can sing cannot be recorded very well by your computer's microphone. In that case, use notes in the middle and upper part of your vocal range.

Plot your results as three dark dots on the graph provided on the form, using the calculated frequency f=1/T as the x-coordinate and the cursor frequency as the y-coordinate. Note that the axes are labeled clearly and that the units are given in parentheses. This is a standard style for scientific graphs. The scale range given of 0 to 1000 Hz might not be the best for your data. In later homelabs, you will choose the scale ranges and label the axes yourself.

Using a ruler or other straight edge, draw a straight line that goes though the origin (the point x=0, y=0) and that comes as close as possible to your three points. If you measured your periods correctly, calculated your frequencies correctly, used the cursor correctly, and plotted all the points in the right location, you should see that your points fall close to a straight line with a slope of one that goes through the origin.

Scan your homelab to a pdf file using a scanner or a smartphone app, and upload it to the dropbox on our Canvas site before it is due.