Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
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Make sure the Spectrum toolbar button is active, then toggle the Generator on. Hit the F9 key to open the Volume slider dialog. The Wave and Master sliders are set all the way down by default; adjust both as needed for a comfortable listening level.
The sound you hear will be a deep roaring noise plus a high hiss. The low part is a noise band of all frequencies 880 Hz and below, while the upper part is 3520 Hz and above. Between those limits is a spectral gap where there is no sound energy, as shown on the Spectrum display.
Tip: The raw Spectrum display shows the instantaneous spectrum level at each frequency, but it is rather noisy... these are noise bands, after all! You can use Exponential Spectral Averaging to get a continuously smoothed display which is much more meaningful. Click on the thin bar beneath the toolbar Averager button to open the Spectrum Averager dialog. Click on the Exponential button, and make sure that the Frames Request is set to 32. The Averager button toggles averaging on or off.
Listen for about 10 seconds (exact time not critical) and then toggle the Generator off. You should hear a pure or perhaps somewhat buzzy tone that slowly decays to nothing. Its apparent pitch is within the gap frequency region. This is the Zwicker tone... an audio "afterimage" of a tone that wasn't there in the first place!
The Zwicker tone decays more quickly if the noise is held on for a shorter time. (Try 1 second instead of 10.)
You can automate the on/off cycle using the Burst option. Close the volume slider dialog and open the Generator control dialog if it isn't already open. Click on the Left Wave Controls button, then on the Burst dialog button. At the top of the dialog that opens, toggle the Burst On button.
When the Generator is active, the noise will be on for 5 seconds, then off for 5 seconds. To change the "on" duration, enter a different value for Burst High. The total Burst Cycle is set for 10 seconds, so if you reduce High to 1 second the "off" time will be 10 - 1 = 9 seconds.
You can also experiment with the Noise Bands. Close the Burst dialog, then click on the Wave button (near the top of the Stream dialog that remains when Burst closes). In the Wave dialog, click on Band.
The Rise Fc and Fall Fc controls determine the band edge (cutoff) frequencies. Note that since Rise is greater than Fall here, the "band" is really a "gap" in a broadband noise that would otherwise cover the entire audible spectrum. You can move the gap higher or lower in frequency, and make it wider or narrower as desired.
Since the Zwicker tone will appear within the gap, note that its apparent pitch will rise as the the gap range rises. The Zwicker tone may be "thin" and hard to hear at higher frequencies.
The Taps control sets the sharpness of the band edges, with the default setting of 512 giving the sharpest edges. As Taps is reduced, not only do the edges become more rounded, but also the depth of the gap may be reduced due to overlap. You can experiment with this to see just how shallow the gap can be and still result in a detectable Zwicker tone.
Some people are also able to hear a Zwicker tone after a lowpass noise band. Set Rise Fc to 0 to eliminate the high portion, and use Fall Fc to control the upper frequency of the band.
In general, for the gap-type Zwicker tone, the center of the gap should be in the 1000 to 5000 Hz range. The gap width should be no less than 1/3 octave, which means the ratio of upper and lower edges should be at least 1.26. (2 raised to the 1/3 power.)
Some sources specify that the noise level should be in the range of 25 to 50 dB SPL, which is an absolute value that would require calibration with a special reference microphone. Instead, you can use a crude approximation by setting the level relative to your individual hearing threshold for this particular sound.
To do that, first set the Stream Level control to (say) -30 dB. (There is a little button next to the control to toggle between dB and percent of full scale.) Next, open the Volume slider dialog (F9 key) and adjust the sliders until you can just barely hear the sound. That's your personal threshold for this sound with the sound system you are using. (If you are using speakers, that includes your current listening position as well.)
Now close the volume slider dialog, and set the Stream Level from -30 back to 0 dB. Even though you don't know the absolute level in dB SPL, the volume is now 30 dB above the level you just established as your threshold.
See also Auditory Phenomena and Experiments
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