Diagramming Sine Wave, Finding Amplitude?

For my physics class I am running my own experiment similar to the "Subwoofer Aiming" tests done on installer.com .   I tested out different positions and volumes, and recorded information both with a radioshack SPL meter and DSSF3 software. With that done, I loaded up Autocad to diagram the waves as they bounced off the trunk/interior...only to find out I had no idea what the amplitude was. After doing some research, I found that the amplitude of a sound wave is represented via the SPL measurement. The SPL measurement is a log function of the amplitude squared.

Now, lets say I had a 65hz sine wave playing at 100db(C-weighting, [should I re-measure in A-weighting?]). That would make the wavelength ~16.7231 ft, the period .0154 seconds, but what is the amplitude of the wave? Is there any easy way to find this out?

Thanks,
Austin

https://physics.mtsu.edu/~wmr/log_3.htm

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I actually saw that page but didn't make it to the equations part, thanks for re-showing me the link.

However, the equations they use give an answer in N/m2. I am at a loss as to how to convert from newton/meters squared (a unit of force) to a usable measurement of distance in order to draw the wave...should I ignore the N/m2 unit and just use feet?

Can't edit my post, but:

Does the diagram near the top of the page convey that it is approximately a 1:1 ration of the pressure amplitude (n/m2) to the sine wave amplitude? Meaning that a pressure amplitude of 20 n/m2 would convert to a 20ft amplitude on the sine wave?

No, you cannot use a linear unit of measure like "feet" for sound wave amplitude.  Sound wave is pressure (force) above ambient pressure; it is a compression wave and the amplitude is equivalent to its loudness.  The common amplitude unit is db, which can be converted to pascals, or newton/meters2, or any other unit of force.  Indeed, it would be correct in your example, if you measured 100db with your SPL meter, to use 100dbSPL as the amplitude of the wave.

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So if you were to make a rough diagram of a 65hz wave at 90db coming out of a speaker, how would you go about doing that?

Vertical = dB
Horizontal = wavelength

Here's a graph on Wikipedia. You can use ANY of them, as all of those waveforms can be measured in dB, as wall as at any given frequency.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

haemphyst wrote:

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Vertical = dB
Horizontal = wavelength

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Exactly.

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Yes I know that, but how would I draw this in a program such as Autocad so that the wave is to scale?

I must have not been clear, and therefore sounded like an idiot for asking the same question nine times. I understand everything you've said so far, I just want to know how I would draw the wave in a program such as Autocad (or if you know of a better program for this purpose, please fill me in) because you can't "draw" decibels or "n/m2."

For example, the person who ran the subwoofer aiming tests at installer.com drew this picture representation: https://www.installer.com/tech/techpics/fbox.jpg

How would I draw a pictoral representation of my own test?

You can draw it the same way if you choose.  You cannot make the amplitude "to scale" with the linear measurements associated with wavelength since it is not measured in linear units.  Draw a sine wave with the appropriate wavelength and an arbitrary height and label it in AutioCAD (or whatever else you choose to use) with the units you need to use.  Loudness is not a spatial quantity.

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