# Subwoofer in a car = mathematically useless?

Discussion in 'Archived Threads 2001-2004' started by Phil Kim, Mar 10, 2003.

1. ### Phil Kim Stunt Coordinator

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First, the speed of sound is commonly measured at 968 feet per second.

The fundamental calls for f = v / 2L, where f is frequency, v is velocity, and L is length.

For a 20 Hz note, this equation becomes 20 = 968 / 2L. After simple algebra, you get 24.2 feet for L. In other words, a 20 Hz note will require 24.2 feet to complete an oscillation.

A typical mid-size sedan has an interior length of 8 feet (front-to-back windshields). That's enough for 60 Hz note at best.

Now, I understand that most cheap car speakers can't reproduce 60 Hz note properly, and so-called subwoofers for cars certainly can't reproduce 25-30 Hz note that most mid-end home theater subwoofers can, but isn't it still an overkill to get a subwoofer?

Or am I missing something (e.g., bad math)? It's been awhile since I took physics, so please be kind.

2. ### Walt N Second Unit

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"...And 20Hz, the lowest frequency we’re supposed to be able to hear, would clock in around 55 to 59 feet long. Do you need a room with at least one 60-foot dimension in it to hear a real 20Hz in the room? No. Your ears actually pick up sound in a different way, reacting to the compressions and rarefactions that happen in the air as the sound propagates though the room. Twenty Hertz creates 20 compressions and rarefactions per second and your ear will pick that up even if you are listening to headphones that respond to frequencies that low. Otherwise your ear canal would have to be 60 feet long -- we would look rather odd if our heads were 60 feet wide."

http://www.soundstage.com/maxdb/maxdb021999.htm

More good stuff here as well, hope this helps.

http://www.avguide.com/how_to/bass.html

3. ### John Watson Screenwriter

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You guys are good!

My only thought has been - what does the vibration do to the brains of the guys driving those cars?

4. ### David_Moechnig Stunt Coordinator

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This example is so common, and so obviously flawed. You do not need to perceive the entire wave at one time to "hear" a sound-- otherwise we could hear very few freq.

You don't hear based upon having enough room for the full wave- rather you hear based up on a wave passes a given point. Whether in a car or in an ope field- the wave compression is "heard" - not by hearing the entire wave at once- but by it passing your given point.

If the contained space size would effect the sounds we hear- headphones would never work. With a pair of heaphones, you would only have a "space" of less than an inch- so you would get no low end or mid range.

-Vince

6. ### Kris Coffin Stunt Coordinator

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QUOTE]My only thought has been - what does the vibration do to the brains of the guys driving those cars?[/quote]

I wish I could think of a response to this, but mine turned to jello due to my Van stereo a long time ago.

KC

7. ### Tim Morton Stunt Coordinator

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Ya know the soundwave can fire back towards the car...and then do the whole "pong" thing bouncing back and forth between front and back window several times to form that 20hz 60' wave.....of course with that comes standing waves and cancelation...but factor in the 50-60db cabin gain at certain freq's and you will see what I might have built at least 15 different subwoofer boxes for maybe 5 or six different sets of subwoofers trying to get the bass right in my car...and why i built 1 box with 1 driver for my home theater and it sound GREAT!!!

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9. ### Eric Kahn Guest

when the speaker cone moves, it is actually compressing the air in the car/room, your ear responds to pressure levels, not waves, since a car is so small compared to a room, you can use an inefficient setup and still get good bass
if you were to get a room the "length" of the sound wave, you would get resonance at that frequency from what I understand (might be wrong here, not into math with letters and wierd symboles)

A wave form drawn on a chart is just a way to show relationships between frequencies in a way everyone can understand

besides, someone forgot to tell my car sub that it can not reproduce those sounds so it is doing a really good job from the hatch in the back of my New Beetle

10. ### Mary M S Screenwriter

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Obviously, they do have trouble compensating for the tiny acoustic enclosure a car represents. To overcome this sad state of affairs, they make sure its rattling my teeth 6 cars back at a light.

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