A beat frequency
occurs when two closely matched frequencies occur with similar amplitudes.
Figure 1 shows an uneventful spectrum with a resolution of about 25 cpm
per line.
Figure 1
|
But as we
discussed in the 'Frequency' section, that spectrum resolution (25 cpm/line)
will be insufficient to separate peaks less than about 50 - 60 cpm apart
- the FFT will combine those frequencies into a single peak. Figure 2 shows
the signal from which Figure 1 was generated.
Figure 2
|
| We can see from Figure 2 that the
beat is occurring over about 4000 msecs (4 seconds) which means it is occurring
15 times per minute. With a beat frequency such as this, chances are you
would be able to hear and/or feel the beat occurring. By
timing it, you can find the 'beat' rate (15 beats per minute). From the
'Frequency' section, we know that we need a spectrum resolution of about
1/3 of that beat - 5 - 6 cpm/line of resolution - to be able to separate
the peaks on the FFT. |
