Hypothesis: Alternating loud and soft sounds can be heard at a distance in front of two loudspeakers

playing the same sound. The distance between the loud and soft sounds decreases if the

frequency of the audio signal generator is increased.

Aim: To investigate sound wave interference and how it varies with frequency.

Apparatus: Audio signal generator, 2 loudspeakers, connecting wires, meter rule , chalk

Diagrams:

Procedure:

1. Two speakers were connected to an audio signal generator.

2. The signal generator was started and adjusted to emit identical loud sounds from both

speakers.

3. A student, blocking one ear was allowed to stand in the center of the speakers at a certain

distance from them.

4. The student walked parallel to the speakers until the loudest sound was heard. His distance

away from the center was recorded.

5. The student returned to the center and walked in the opposite direction until the loudest

sound was heard. The distance away from the center was recorded.

6. Steps 3-5 were repeated 4 times but each time the student walked further than the distance

before until the maximum sound was heard.

7. Steps 3-6 were repeated but noting areas of the softest sound instead.

8. The frequency of the generator was increased and steps 3-8 were repeated without

recording the distances.

Results:

Constructive Interference – Regions of Maxima

Distance Away From Center (cm) Average (cm)

21 19 20

42 38 40 58 62 60

80 80 80 99 97 98

Destructive Interference – Regions of Minima

Distance Away From Center (cm) Average (cm)

10 10 10

29 31 30 48 52 50

68 72 70 88 94 91

When the frequency was increased the areas of maximum and minimum sound corresponding to those

recorded before were closer to the center and closer to each other. There were also more areas of

maximum and minimum sound along the same distance walked as before.

Analysis:

Sound waves are longitudinal and travel by a series of compressions and rarefactions.

The speakers converted electrical signal into vibrations causing compressions and rare factions of air

molecules thus allowing sound to travel to the student’s ear. By blocking an ear the student was able to

concentrate and allow the sound to be heard from only one ear.

The student noted various areas of loud and soft sounds. This indicate s that interference

was taking place. The areas of maximum sound (maxima) were where constructive interference took

place and the areas of minimum sound (minima) were where destructive interference took place.

Constructive interference occurs when two waves that are in phase combine to produce a wave twice

the amplitude of the original waves. This was where compressions overlapped each other and

rarefactions overlapped each other. Destructive interference occurs when two waves that are out phase

cancel out each other. This was where compressions overlapped rarefaction and rarefactions

overlapped compressions.

The distance between the areas of maxima and minima decreased when the frequency

was increased. Frequency is the number of waves produced per second. Since more waves are produced

per second the waves interfere more frequently, thus producing closer areas of maxima and minima.

The wavelength of sound could be calculated from this experiment. Using one area of

constructive of destructive interference, its distance away from the center (m), the angle the path of

maxima/ minima makes with the center (pheta) and the distance between the two speakers (d), it is

possible to calculate the wavelength (lamda) using the following formula: