0.5 Microphone fft lab  (Page 2/2)

If the doctor wishes to calculate the patient’s heart rate (assuming it is perfectly steady), he can tryto measure the time between successive peaks on the screen (1 second in this case) and calculate the heart rate from thatinformation. However, there is an easier way!

Any signal (electrical or otherwise) can be viewed as a number of sine waves at different frequencies withvarious amplitudes and phase shifts. Simply put, a graph can be made that shows amplitude vs frequency instead of amplitude vstime. In the doctor’s case above, it would be very convenient for him to have a graph of amplitude vs heart rate frequency. If thepatient’s heart rate is approximately 1 Hz as noted above, then the amplitude vs frequency plot should show a peak somewhere near 1 Hzas well. Now, the doctor can simply glance at the graph to see the heart beat frequency.

In order to convert a time domain signal such as heart rate amplitude vs time into the frequency domain toproduce a plot such as amplitude vs frequency, the Fourier Transform can be used. Several variations of this transform exist,including the Fast Fourier Transform (FFT) algorithm that is typically used by computers. For the purposes of this exercise, thelow level mathematical details of the transform will not be needed. The experimenter does, however, need to remember the basicconcept:

Remember, any signal can be thought of as being composed of sine waves, where each frequency of sine wavewill have a given amplitude and phase shift.

Hardware and software required

• 10 Ohm resistor
• 4.7 uF capacitor
• Electret microphone element
• National Instruments Low Cost USB DAQ
• Signal Express software

Laboratory exercise

During this exercise, the experimenter will acquire a sound signal from an electret microphone element. Thissound signal will then be converted into the frequency domain using the Fast Fourier Transform to produce a chart similar to thefollowing:

1) Connect the following circuit to the Low Cost USB DAQ as shown. The microphone element can be purchasedcheaply at Radio Shack, etc. Note that the +5V power supply can be obtained directly from the National Instruments USB 6008 or 6009devices.

2) Program steps in National Instruments Signal Express software to match the sequence below. These stepswill acquire a sound signal from the circuit constructed above and compute the frequency domain representation using the FFT.

3) Drag the acquired time domain sound signal as well as the frequency domain (FFT) signal into the data viewwindow. Choose “run continuously” within Signal Express to loop the sequence.

4) Try generating various sounds by talking, whistling, etc. Make sure that you are close to the microphoneelement. Observe the FFT signal when you whistle different notes.

Post-lab questions

1) Do research in a textbook or online to determine the frequency range that the human voice can produce. Didthe FFT of your voice / whistling fall within that range?

2) Imagine you tried to use the electret microphone element outside on a windy day. What might happen ifyour tried to record your voice? How does the frequency response of the microphone play a factor here?

3) How can you tell if high frequency noise is present in your sound signal without playing it back? Hint:think about the concepts discussed in the theory section above.

4) What could be added to the Signal Express sequence above in order to attenuate any noise in your soundsignal? What frequency ranges must remain intact (assuming you are attempting to record a human voice)? What frequency ranges do younot have to be concerned about at all?