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Old 01-23-2012, 04:01 PM   #1
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Default I have a complex engineering problem...

Perhaps someone here has the knowledge needed to solve this. I tried to once, but I was told repeatedly that I was wrong.

Say you have a 1 cubic foot box made of MDF, sealed all around, with no leakage, built on a typical day when its exactly 70 degrees outside. Assume atmospheric pressure stays constant at all times.

Say you then put the box in your trunk as is, and you drive around and park on a hot summer day and the internal temperature of that box reaches 120 degrees F.

What is the resulting pressure?


Now, say you do the same thing again, but this time mount a subwoofer to that box. Assume the subwoofer has zero leakage and a free-moving surround for simplicity. Assume for simply that the sub is a stiff membrane that can be pressed inward and outward.

Again, start at 70 degrees F, and increase the internal temperature to 120 degrees F. The subwoofer has a cone area of 500 square cm. When that temperature is increased, how far out will the subwoofer be pushed, in order to normalize the internal pressure with atmospheric pressure?
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Old 01-23-2012, 06:20 PM   #2
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V1/T1 = V2/T2 where T is in Kelvin
V2 = V1*T2/T1

70F=294.26K
120F=322.04K

V2= V1*1.09

In the case of the space being constant, pressure will increase by 9%. When the speaker is in place, it will move out to accomodate the increase of 0.09 cubic feet of air. This will not be exact since the resistance of the speaker to expansion will create some increase in pressure as opposed to movement of the speaker aborbing all the incremental volume. Ignore this for now.

.5382 FT2 = 500 CM2

Dist * .5382 = .09
Dist = .1672 Ft
Dist = 5 cm

Therefore the membrane will move outward 5 cm at most, and probably less due to the increase in pressure associated with the elastic resistance.
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Old 01-24-2012, 01:19 AM   #3
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Thanks for taking the time. Much appreciated.

If anyone cares to check this (no offense), feel free to.
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Old 01-24-2012, 01:30 AM   #4
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As an engineering student, that looks good to me.
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Old 01-24-2012, 08:48 AM   #5
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As an engineering student, that looks good to me.
I have one of those rings you are working towards getting.
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Old 01-24-2012, 01:02 PM   #6
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I had a feeling you did. Mechanical?
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Old 01-24-2012, 01:39 PM   #7
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I had a feeling you did. Mechanical?

Yup. Old enough to have a son with a degree in mechanical engineering as well. He is an engine calibration engineer for Chrysler. I was invited to present his ring to him at the ceremony.
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Old 01-24-2012, 02:05 PM   #8
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Wow, that'* awesome. I've always wanted to work at somewhere like GM or Chrysler.
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Old 01-24-2012, 02:32 PM   #9
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When do you graduate?
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Old 01-24-2012, 02:36 PM   #10
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In 3 years. I'm doing engineering technology right now, but I'm probably going to get my full engineering degree after.
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