Frequency response from speakers of different sizesFeatured science projectScience project video

popular science fair projects
Complexity level:
9
Project cost ($):
180
Time required:
2 days to prepare, 1 day for experiment
Material availability:
Easily found
Safety concerns:

Basic safety requirements

Abstract

This science fair project was performed to investigate how the frequency response of a speaker changes when as the size of the enclosure changes.

Hypothesis

Reducing the size of the speaker enclosure will cause the dB of some frequencies to be higher compared to the rest of the frequencies.

Background

Speakers

The speaker consists of an enclosure, a driver and a crossover. The enclosure of speaker is normally made from hard material like plywood or MDF (Medium Density Fiber). The enclosure will help to hold the speaker "driver" in place and also absorb the vibrations from the driver.

The diaphragm of the driver moves back and forth to create sound. The forward movement of the diaphragm will the air pressure inside the enclosure to fall and the backward movement of the diaphragm will cause the pressure inside the enclosure to rise.

Scientific Terms

Diaphragm, decibel, pressure

Materials

The materials required for this science fair project:
- 3 pieces of plywood 1meter x 1meter in dimension
- Hammer and nails (quantity as required)
- Glue
- Silicon filler
- Three 8-inch speakers
- Electric drill and drill bits
- Saw
- Decibel meter
- CD player
- Test CD with various audio frequencies

Procedure

1. For this experiment, the independent variable is the side of the enclosure and audio frequency played. The dependent variable is the decibel reading of the frequency. The reading is measured using a Decibel meter. The constants (control variables) are the materials used to construct the speaker, the size of the speaker and the distance of the decibel meter from the speaker.

2. The plywood planks are cut and constructed according to the speaker enclosure sizes shown in figure 2. The speakers are marked A for 500mm depth, B for 400mm depth and C for 300mm depth. The plywood planks are constructed into the enclosure sizes using glue and nails.

3. The areas where the two plywood pieces meet are sealed using silicon filler to make the enclosure air tight. Using a drill and  saw, an 8-inch hole is cut in the center of the enclosure to mount the driver.

4. A small hole is made at the back of the enclosure using the drill. The wire from the speaker driver is taken out through this hole and connected to the CD player.

5. The decibel meter is placed 1 meter in front of the speaker A. The speaker wire from enclosure A is connected to the CD player. The CD player is made to generate audio sounds at 30Hz, 50Hz, 80Hz, 150Hz, 300Hz and 600Hz. The reading on the decibel meter is recorded in the table below.

6. The procedure above (Step 5) is repeated using speaker B and speaker C. The results are all recorded in the table below.

Observation

The results show that the frequency response of  speakers A and B  does not go beyond the 0 dB level, but the smallest speaker C will produce some frequencies that are louder than the others. Thesefrequencies  show a  response reading higher than 0dB.

Speaker

dB reading taken at decibel meter

30Hz

50Hz

80Hz

150Hz

300Hz

600Hz

Speaker A

-14

-7

-3

0

0

0

Speaker B

-22

-9

-1

0

0

0

Speaker C

-30

-12

1

6

3

1

Conclusion

The hypothesis that reducing the size of the speaker enclosure will cause the dB of some frequencies to be higher is proven to be true.  With a  depth of  300mm the sound of speaker C  at  the 150Hz  frequency  was  6 times  louder than the sound level of the other speakers.

This type of air tight speaker enclosure is called a sealed enclosure. When the diaphragm of the speaker moves inwards, air pressure will build up inside the speaker enclosure and push the diaphragm back out. Similarly if the diaphragm of the speaker moves outwards, the air pressure inside the enclosure will drop and pull the diaphragm back. This spring like effect will cause the sound  produced  to be more precise. However the audio electrical signal will require boosting by the amplifier circuit.

Also consider

This science fair project may be repeated, this time,by checking the frequency response  using different types and thickness of wood for the enclosure.

Modify the science project experiment, using using different sizes of speakers.

References

Speaker enclosure design - http://www.niksula.hut.fi/~mdobruck/siililand/mini/diy/alien/ice/speaker.html

How speakers work - http://electronics.howstuffworks.com/speaker9.htm

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