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A Practical Guide To Key Audio Calculations
Thanks to modern technology, you can do dB calculations without knowing a thing about the mathematics of logs, anti-logs, ratios, exponents, or even much about math
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VOLTS TO dBu or dBv
You have a device that puts out 8.8 volts. What is that in dBu?
Formula: dBu = 20 x Log (volts1/volts2) or 20 x Log (8.8/0.775)
Enter 8.8 (volts1)
Hit the divide key
Enter 0.775 (volts2)
Hit the = key
Your answer should be 11.35
Hit the Log key
Your answer should be 1.05
Hit the multiply key
Enter 20
Hit the = key
Your answer should be 21.10 dBu

To find dBv simply substitute “1” for “0.775” in the calculation. Your answer should be 18.88 dBv.

SPL CALCULATIONS
You do SPL calculations using exactly the same formulas as for voltages. Both SPL and voltages are “pressures” and the factor “20” is used for both. Your loudspeaker puts out a maximum of 120 dB SPL at 3.3 feet = 1 meter. What is the SPL at 60 feet?

First, you must understand that SPL drops 6 dB for each doubling of distance. Why is this? Calculate the multiplier for a pressure change of 6 dB.
Formula: Multiplier = 10^(SPL/20) or 10^(6/20)
Enter 6 (dB SPL)
Hit the divide key
Enter 20
Hit the = key
Your answer should be 0.3
Hit the 10x key
Your answer should be 1.99 = about 2

So the multiplier for a 6 dB SPL change is 2. This means if you move twice as close to the loudspeaker, it will be 6 dB louder.

Now recalculate this using another function on your calculator called the change sign key. This changes a number in the display from a plus to a minus number.
Formula: Multiplier = 10^(-SPL/20) or 10^(-6/20)
Enter 6 (dB SPL)
Hit the “+/-“
Your display should change to -6 (dB SPL)
Hit the divide key
Enter 20
Hit the = key
Your answer should be 0.3
Hit the 10x key
Your answer should be 0.50 = 1/2

So the multiplier for a -6 dB SPL change is 1/2. This means if you move so you are only 1/2 as close (meaning twice as far) to the loudspeaker, the SPL is 6 dB less.


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