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Name: Erik
Status: student
Grade: 6-8
Location: MN 
Country: N/A
Date: 10/16/2005


Question:
How does a sub-woofer work?


Replies:
Erik-

I thought a sub-woofer was pretty much the same as any other speaker, except it is physically bigger so it can hit lower frequencies.

Some properties of low sonic frequencies:

- because the wavelength is long compared to the room, there is much less directionality to the sound within the room. There is only one instantaneous pressure value, about the same at most parts of the room. More like squeezing a closed empty plastic soda bottle, less like shouting across a valley. The speed of sound is about 1000ft/sec, and the room is about 20ft wide, so it takes 1/50 sec to cross the room, so for frequencies less than 25Hz, the entire room is feeling the same half-cycle of the sine wave. So only one speaker is needed for this frequency band. No point in doing stereo.

- it is easy for any cheap 10KHz microphone to carefully track the instantaneous pressure of a 20Hz or 200Hz sine-wave. So some sub-woofers have such a microphone, telling the amplifier to drive harder of softer to get exactly accurate reproduction. This is called feedback, or closed-loop control. Some sub-woofers have it now. Probably no tweeters do.

- because one side of a sine-wave lasts a long time, you need to move a large volume of air to keep up the pressure during that time. so the speaker needs to be large and have long throw, and the empty box behind the speaker's diaphragm or cone needs to be large. If the box has a hole in the back, that hole is likely to be pulling in air when the speaker pushes it out, resulting in some cancellation. So sub-woofers have a closed box or a very long tubular pathway inside. Usually a closed box.

- it is hard to push the mass of the speaker-cone back and forth fast, so high-frequency speaker must have light cones and coils. Sub-woofers have less of this problem, so the cone and coil can be relatively hefty.

- many of the body's natural resonances, whether of air in and out of the lungs or of fluid and sinew in the gut, are at very low frequencies that a sub-woofer would reach best. This definitely accounts for some of the pounding felt in your chest.

- vibrations ~10-20Hz can be partly followed through cycles by our tactile senses, not just detected by secondary effects. This may account for some of the pounding felt in your chest.

Hope that covers it for you.

Jim Swenson



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