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Speaker Set Up...Math/Geometry Experts Please Help

Hey guys, I need your help. I am helping a friend of mine set up his speakers and can't figure something out.

His speakers need to be tilted back to time align the drivers. According to the graph supplied from the manufacturer, we need a rake (tilt) of a 1/2 inch. This is measured using a plumb bob hanging from the back of the speaker, (which is perfectly flat). You measure the distance from the string to the back bottom of the speaker. The problem we are having is that even if we let the plumb bob settle, as soon as we try to measure the distance, the string moves just from the air disturbance around the string. We are having trouble getting an exact reading.

My question is: Can you convert the 1/2 inch distance to a degree? I have a really nice digital level that we can put on top of the speaker, start with it perfectly level and zero out the level. My level only reads in degrees, so I need to know how many degrees correspond to 1/2 inch tilt.

I've actually done this a few times in the past and always used the plumb bob method, but with current technology, was hoping to get it perfect.

Maybe this can't be converted? I've tried looking it up online, but Geometry was always my weak subject.

Thanks guys!
128x128mofimadness
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almarg
9,670 posts
 
Hi Mofi,

First, let me make sure I understand how the 1/2 inch tilt is being defined. I think what you are saying is that the speaker should end up tilted back such that if the level is rested against the top front edge of the speaker, and the level is placed perfectly horizontally, the top back edge of the speaker would be 1/2 inch below the point where that level passes above it.

If so, the required angle would be arcsin(0.5/the depth of the top of the speaker in inches). "Arcsin" = "the angle whose sine is," also known as "inverse sine."

If my interpretation of the requirement is correct, and you let me know the depth of the top of the speaker, in inches, I have a scientific calculator which can easily compute the answer. Or you can use the calculator that is built into Windows, under "Accessories." You would set the calculator to scientific mode under its "view" menu. You would then press the "inv" button to cause the inverse sine button to appear.

Best,
-- Al
mofimadness's avatar
mofimadness OP
4,991 posts
 
Al...thank you for responding. I was hoping you would know the answer.

I think what you stated above is correct. The digital level would be placed on the top of the speaker, but facing from front to back not side to side?

The depth of the top is 10 inches. I tried to use the calculator you suggested, but again, not my forte. I think I got an answer of 2.3 degrees. If you could check that, that would be great.

Thanks again sir!
almarg's avatar
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almarg
9,670 posts
 
Yes, I was envisioning that the level would be oriented front to back, not side to side.

I get 2.866 degrees, based on the inverse sine of (0.5/10).

Best,
-- Al
mofimadness's avatar
mofimadness OP
4,991 posts
 
Al...thanks! I knew I wasn't using that damn thing right. I did finally get the same answer.

This will make it way easier to set up.

Happy Halloween!!!
minkwelder
255 posts
 
"According to the graph supplied from the manufacturer, we need a rake (tilt) of a 1/2 inch. This is measured using a plumb bob hanging from the back of the speaker, (which is perfectly flat). You measure the distance from the string to the back bottom of the speaker."

Actually, the above quote indicates that the measurement is made from a plumb line extending down from the back of the top edge of the speaker, so the 1/2" is measured at the very bottom of the back.

This would change the angle considerably from what Al has calculated. For instance, if the speaker is 36" tall, the calculation would be arcsin(0.5/36) = 0.796 degrees or 0 degrees, 47 minutes, 45 seconds (a little over 3/4 of a degree).
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