# Voltage of sounds of MP3 files

What is the maximum voltage for common music files?

When playing back on a smartphone, if the volume of the smartphone is set to maximum, how much voltage is sent to the earphones of the electric signal?

thanks so much for comment I’m japanese. I’m not good at English. I use google translator. I want to use a low frequency therapy device on my smartphone. I thought it was dangerous depending on the voltage. And I asked this question. any precautions or dangers?

• if you really think about what you asked, then you will realise that you asked the kind of question that is un-answerable .... you did not define the parameters of the music file contents ........ a similar un-answerable question is how loud is music? – jsotola Mar 26 at 15:07
• Perhaps this is a question about the electrical interface between an audio device and a headphone or earphone set rather than a question about sampled audio data. – Solomon Slow Mar 26 at 15:39
• The MP3 records sound intensity in a range between zero and maximum. The hardware playing the MP3 decides what "Maximum" means. – J... Mar 26 at 17:10
• It depends on the phone and the earphone impedance, but iPhone 6 produces a maximum of about 1V RMS according to this link: anandtech.com/show/8554/the-iphone-6-review/11 – Ken Shirriff Mar 26 at 17:15

Amplification

It does not depend on the music file, but at the amplifier sending the music to a speaker, or to a headphone/earplug connector.

Music file

A music file only contains 'values', and mostly in a compressed form. After uncompressing, you get values which has a certain amount on bits per value (typically 16 or more).

Conversion

A microcontroller can send these values to a speaker via an pre- and/or amplifier, which converts it into actual voltages. So it depends on the amount of amplification.

Protection

Also note that sending continuous 'max values' do not result in a loud sound, actually it would breaks the speaker (to prevent this, in the amplifier a so-called DC speaker protection is present). A wave is needed to let the speaker move outwards and inwards very fast, and this is done by sending changing values to the speaker.

Measure

If you want to measure, you can use an oscilloscope to measure the audio output while sending a sine wave with a maximum amplitude.

• Re, "...actually it breaks the speaker." If I can create a sound file that blows your speakers when you play it, the fault is not in my file. Your music player should at least be able to survive any sound file that you try to play. – Solomon Slow Mar 26 at 15:25
• @SolomonSlow Good point, I updated my answer (and learnt something myself from it). – Michel Keijzers Mar 26 at 15:34
• @SolomonSlow Some speakers won't be very happy if you play a mono-tone at a resonant frequency; And it happens quite often with sub woofer speakers; I'm sure you've heard some where the cone has been damaged. I agree the ideal is that the speaker should survive any input; but realistically if you find the right resonant frequency the damping of the speaker might not be sufficient to protect it. – UKMonkey Mar 27 at 0:43
• @UKMonkey I'd reword it to "music system should survive any sound that can be reasonably expected to happen during normal use". And I agree that many sub woofers are badly (or rather - cheaply) designed and do not. – Mołot Mar 27 at 9:42
• @Mołot no - I would say that a speaker shouldn't be damaged by the amplifier for any output of the amp within the acceptable ranges supported by the speaker. This means that if the speaker needs to be rated for a lower voltage (be it by shipping with a different amp or something else) to ensure it doesn't self destruct at 20Hz; then that's what it needs to do. – UKMonkey Mar 27 at 10:11

These days a lot of DAC chips for consumer equipment use voltage output of 2 Vrms.

The supply voltage is largely irrelevant, as a lot of DAC chips also use internal charge pumps to boost up supply voltage and to generate negative supply voltage for the audio output stage, so they can drive DC coupled loads directly.

• "2 Vrms" here means 2 volts on average, where RMS (root mean square) is the method of calculating the average. – Sjoerd Mar 27 at 12:38
• Indeed, for a digital full-scale sine wave, into a typical 10kohm load presented by a line input. So definitely it can't output the full 2Vrms into headphones that are usually few tens of ohms. – Justme Mar 27 at 15:37

Normally it's limited by the cellphone battery voltage, which is about 3.7V. However there are other limits in play like the EU volume limit of 100dB - which is specified as SPL rather than a voltage.

A line input level electrical signal typically has a voltage ranging from 0,3 to 2 Volts. "Line level" is the term for the standard used between a whole bunch of devices, namely your cell phone.

Be sure to read the other answers as they will make you aware of the specifics of your question.

The music files like mp3 have only digital logic levels (0's and 1's), First the analog signals get converted into digital logic using ADC's. The ADC works like a charm here, they slice the analog signal into some simple form, the slicing frequency (sampling frequency) depends on ADC. Now, these sliced pieces get converted into bit's (each bit represents some voltage) so the processors/controllers can understand slices and store them, usually these slices has huge data (0's and 1's) so we compress this data using mp3 compression or some other audio formats. When you play a music file the 0's and 1's go to DAC which convert them back to an analog form, the voltage here depends upon your DAC's reference (this limits the swing of analog signal) Here the signal is weak so we amplify it by an amplifier and feed it to speaker. The voltage range from earphone speakers varies with the device its connected to normally 0v-3v.

• Microvolts? Do you have a reference for this? – evildemonic Mar 27 at 16:34
• I once measured a voltage of 2.7 Vpp at my notebooks output. I doubt that microvolts are realistic – Sim Son Mar 27 at 17:27
• Any time someone says "0's and 1's" it's a gross oversimplification that just confuses the issues. Music files are more than just "0s and 1"s the the same way written english is more than just "Letters A-Z" – whatsisname Mar 28 at 1:35