I don't know how many of you actually pay attention to this. However, a large volume output can really damage your hearing, even further, anything over ~75-85 dB can cause your ears to tune out low and high frequencies. This in turn can actually

decrease the quality of your music (Inner Fidelity). Anything over 90 dB can damage your hearing if exposed too long.

Ideal listening levels should be around 60-80 dB. I generally listen myself around 70 dB (calculated). So how exactly do we calculate this (estimated) listening level? Well it's simple, first we need some information though:

Maximum voltage output of your device (search google for this), the iPod Touch 4G puts out a maximum of 1.110 v (60 mW max or 30 mW per channel).
Headphone specs (specifically the impedance and the sensitivity ratings) - Find it on the box of your headphones, or online through the manufacturer's website (or even the reseller's site sometimes)
Disclaimer: this guide will not get you exact numbers. It will merely give you a ballpark estimate of how loud you are actually listening. To get actual numbers, obtain a mic, dummy head and test your own headphones.
So let's start by calculating the actual voltage that comes out of your headphone jack. When you push up on your volume, you actually don't increase in a linear, straight manner. Instead, it's squared. To get the voltage your increasing use this formula below:

Actual Voltage (volts) = Max Output Voltage (volts) * (volume level fraction)^2*
The volume level fraction is actually the amount of the total volume you are outputting. So if you listen at half the max volume of the iPod, your volume level fraction is 1/2 = .5. If you listen to it at 30%, then the fraction would be 30/100 = 3/10 = .3. If you listen at 3/8, then your fraction would be 3/8 = .375. It's important that you keep this a fraction under 1.

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The next thing we need is actually the current going through your headphones. The current is generated using the formula below for current through a

parallel circuit.

Current = Voltage * (1/R1 + 1/R2 + 1/R2 + 1/R4...); where R1, R2, R3, etc are resistors. Your headphones have two of these both rated the same. So we can simplify that whole statement to:
Current (amps) = Voltage (volts) * (2/R); Where R = Impedance rating (ohms) of the headphones. Thusly, we get the equation:
Current (amps) = Voltage (volts) * 2 / Impedance (ohms)*

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With the voltage and current we can calculate power generated by your device at a given volume level. This is why we calculated the current and voltage. So use the equation below:

Now to get the actual sound pressure level (SPL) coming out of your headphones (note this is in Pa), we can use the formula below:

Now, this number is going to range anywhere from 0 to 15 (maybe higher if you have special headphones). We need to convert this number into one we can use. To convert the SPL (Pa) to SPL (dB) to get the decibel rating we want you can do one of two things:

Use this converter *

Or you can use this formula:

SPL (dB) = 20 log(SPL (Pa) / (2*10^-5)) / log(10) (dB)

*We are stating that the log has a base of ten when we divide by log(10). On any given calculator, the log button has a default base of 10 (check this for your self), that is, log(10) = 1. Ln is the natural log which has a base of e. In the equation above, log can be replaced with ln, but you'll have to divide by ln(10) still since ln(10) is not 1.

So we get the final equation as:

SPL (dB) = 20log(SPL (Pa) / (2*10^-5)) dB*

Plugging in what SPL (Pa) is, you get the long equation:

SPL (dB) = 20 log(Sensitivity * (Voltage (volts) * 2 / Impedance (ohms)) * (Max Output Voltage (volts) * (volume level fraction)^2) / (2*10^-5))

^^If you do this equation, don't type it in wrong

And double check your work).

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Please note you'll get more accurate results (since you'll make less human errors) if you just calculate each thing and right down the number it produces and plug those into the small equations (which are marked with a

star ,

* , at the end of them.. The big equations scare me (especially the last one)... If you are using the bigger equations, just skip to the final equation; plug and chug it all down and hope you didn't type anything in wrong

And there you have it, the actual listening level you are listening at (approximated). Now let's see how scary your numbers are... Happy (safe) listening everybody.

Sources used: http://www.innerfidelity.com/content/loud-music-sucks

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