Hi,
So I downloaded the NIOSH sound measure app on my iPhone. I was playing around with it and my colleague coughed. The Lmax said it was 86 dB while the LCpeak said it was 115 dB. Is that number real? 115 dB sound very loud for a cough! I tried to read up on the difference between these values and read something about square roots etc. but I didn’t understand any of it. So can somebody please ELI5?
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You are right, the difference has something to do with square roots, specifically what is called the “Root Mean Squared” or RMS of the value.
Lpeak represents the actual maximum value of the pressure that was measured, and Lmax represents the RMS of that same value. Mathematically, the RMS is literally the value divided by sqrt(2). So if you take Lpeak and divide by sqrt(2) = 1.414… you will get Lmax.
In fact, we do the same thing with AC electrical power. If the plugs in your home are 120V, that is actually the RMS value of the voltage, and the actual voltage that is provided is around 169V.
Why do we do this? Because when you are dealing with waves (of pressure, electricity, or otherwise), it turns out that the power (energy per second) contained in that wave is more easily related to the RMS value rather than the actual size of the wave itself. This has to do with the nature of sinusoidal waves and is beyond an ELI5 explanation.
Now, if you checked the math you might realize that 86 is not the RMS of 115, and that’s because LCpeak is a tweaked version of Lpeak. LCpeak is weighted to consider certain frequencies of sound more significant than others. Probably those within human hearing (20Hz – 20kHz), although I’m not 100% sure.
Thank you for the response.
I just had a little accident while rearanging the my fridge freezer a drawer fell about 20 cm to the floor. This made a huge bang. Since I have tinnitus I am quite carefull about safe sound exposure. I put on ear protection and recreated the event while measuring with the app. The result was an Lmax of 107 dB, and LCpeak of 125.2 dB. Which one of these values would be correct to use in regards to possible ear damage and noise exposure limits?
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