[Virginia Tech Department of Physics]

Lecture Demonstrations

Waves & Sound
   

W52: Interference

 

Purpose: To demonstrate constructive and destructive interference using sound waves.

Description:

 

Audio oscillator, 2 speakers, Pasco sound sensor with 2 connected 8 pin DIN cables, Pasco 750 interface, and computer with data studio software loaded.

Connect the two 8 pin DIN cables and plug into the sound sensor. Connect the sensor to the Analog B slot of the Pasco Lab Interface. Place sound sensor on its stand in front of the speakers. Open the data studio file "S:/Demonstration files/Wave & Sound/w52_Interference.ds". Display the Data Studio software on the wall so the class can see.

Connect the oscillator to each speaker separately. The small inner knob on the far right adjusts the amplitude. Use the high impedence slot so the tone is loud enough (Clamp your portable microphone to the stand with the sound sensor if you still don't feel the tone is loud enough). The Freq Mult knob turns he oscillator on. Set the knob to x1K for a good range of frequencies. Adjust the large frequency dial on the far left, and pick a pleasant frequency. Students should hear the same tone and see the same type of sine wave pattern on the screen.

Now connect the oscillator to both speakers. By varying the distance d, students can hear a tone of the same frequency but higher or lower intensity. They can also see the differences on the screen.

Complete destructive interference cannot be produced because of reflections, but it is easy to show that you get constructive interference if:

       d
      --- = n = 0, 1, 2, 3, 4, ...
       λ
      
and destructive interference if:
       d          1  3  5  7  9
      --- = n =   -, -, -, -, -, ...
       λ          2  2  2  2  2


          speed of sound in air
     λ = ------------------------
          frequency of oscillator

 
Equipment List: Storage Location: Hahn N 130A

 

Wavetek Audio Oscillator

E3.4
  Two Speakers E3.2
  Pasco Sound Sensor and stand B2.1
  Two 8 pin DIN cables C.H

References:

Manual: None

  Setup Notes: W52 PIRA #:
Manufacturer(s): Pasco, WaveTek, VT Other school's Demonstration web pages

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