Spectral ViewRecord into Waveform View, and then use the menu selection View/Spectral View to display a time-frequency spectrogram. The menu Options/Settings and the tab <Spectral> will let you change the spectrogram window, and display log or linear intensity, etc.
Drag the cursor over a range of interest, then click on the magnifying glass + rectangle combination button, the lower left of the panel of six similar buttons to the left of the displayed digitize time.
The figure below shows several pulses which are very brief -- one cycle or less -- centred at 2 kHz. The frequency content spans a number of modes -- too many, really. The modes that are excited include the lowest few modes we're trying to excite but also several modes across the short 2-inch dimension of the waveguide. Results are better if the frequency content is arranged (by choice of central frequency and duration of pulse) to excite only modes in the 6-inch direction of the waveguide. Several modes in the 6-inch direction lie at frequencies below the lowest frequency of the 2-inch modes.
The curved lines actually show that energy at frequencies near the cutoff of each mode travels at lower and lower group velocities, as the cutoff frequency is approached. From this, the modes have long tails, actually asymptotic to the cutoff-frequency lines. The group velocities here can be only 10-20% of the speed of sound in air (without waveguide effects).
Once in Spectral View, click on the magnifying glass at bottom right of the frame to zoom in on the range 0-8000 Hz, where most of our sound signals lie. You can click and drag on the scale at right, to put the zero of the axis back in the display.
In the figure above, you can see faint echos, reflections from the end of the waveguide. The flare attached to one end provides some impedance-matching and reduces reflections. The roundtrip time is about 20 ms: you can see that near the cutoff, sound travels once down the tube while other frequencies make 3 or 5 trips the length of the tube.
Last revised: 12 March 2003 - rsm