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SMC|Academic Programs|Physical Sciences|Sound

Sound

AIM : To measure and calculate the frequencies of some common sounds, and to investigate the frequency spectrum of sounds through the Fourier transform.

Part 1

Plug the microphone into the ULI and turn on the ULI. Start LoggerPro and open "Experiments/Physics with Computers/ Expt21/ Expt21". This starts the program that allows us to analyze sounds. The program just records a short sample of your sounds - so you will want to make sure they are constant or uniform so that the sample you get is indicative of the general sound.

You will first investigate five different tuning forks. For the first, strike it with the rubber mallet (or on your shoe) to make it ring. Bring it close to the microphone and then push "Collect". You shou7ld see a clean "sine-wave" shape repeated across the screen. (If not, try hitting again, softer or harder.) Draw a rough sketch of what you see.

  

 

 

 

Once you have a clean wave, click "Analyze" and then "Examine" . Measure the time at one peak (t1) and then the time at the fifth peak after the first one you picked (t2) Record them in the table below and find the difference between them - this is the time for 5 complete cycles, or 5 periods. Divide by 5 to get the period. The frequency of the wave is just 1/period. Record this. Compare each calculated frequency to the true tuning fork frequency by calculating the percentage difference.

Fork Frequency t1 t2 t2 - t1 Measured Period Measured Frequency % Difference
1.            
2.            
3.             
4.            
5.            

Part 2

Now, click on "Window/New wide window/FFT/FFT graph" . This will bring up a window that allows you to see all the frequencies that are actually present in the wave, and their relative amplitudes. This is called the "Fourier Transform" of the sound wave. Change the horizontal frequency scale to go from 0 to 2000 Hz. Change the vertical axis to go from 0 to 0.5. Take the tuning fork that was closest to the "true" frequency, strike it and record a clean wave. Draw a rough sketch of what you see in the "FFT"  (Fourier Transform) window. 

  (Tuning fork frequency:                   )

 

 

 

 

 

 Explain what you see.

 

 

 

Now, sing a medium pitch note - yes, sing! Record the wave's shape:

 

 

 

 

 

Measure the frequency by recording the peak to peak times for one complete repeated shape. 

Medium Pitch
t1 t2 Period Frequency
       

Use the Fourier transform to see what frequencies were really present. Sketch it.

Medium Pitch

 

 

 

 

Repeat for the lowest note your group can sing. Draw the wave.

Low Pitch

 

 

 

 

Measure the frequency by recording the peak to peak times for one complete repeated shape. 

Low Pitch
t1 t2 Period Frequency
       

Use the Fourier transform to see what frequencies were really present. Sketch it.

 Low Pitch:

 

 

 

 

Repeat for the highest note your group can sing. Draw the wave.

High Pitch:

 

 

 

 

Measure the frequency by recording the peak to peak times for one complete repeated shape. 

High Pitch
t1 t2 Period Frequency
       

Use the Fourier transform to see what frequencies were really present. Sketch it.

High Pitch:

 

 

 

 

Repeat for some other sound (whistle, bottle, ...). Draw the wave.

 Other Sound:

 

 

 

 

Measure the frequency by recording the peak to peak times for one complete repeated shape. 

Other Sound:                                            
t1 t2 Period Frequency
       

Use the Fourier transform to see what frequencies were really present. Sketch it.