# How does electricity represent sound?

I understand that AC electricity can travel through wires to represent sound. But, to be honest, I can't find a good explanation of it.

Here are my questions:

How exactly is this setup? Does voltage represent decibels or W/m2? Does the frequency of the current have a 1:1 ratio with the frequency of the sound waves (1Hz = 1Hz)? I assume that there are probably different standards. (line level and speaker level, for example).

• It's usually convenient to consider voltage as analogous to pressure and current as analogous to volume velocity. Power is the product of both measurements in either system, and (neglecting transducer inefficiencies) the same in either case. Commented Jan 30, 2014 at 6:13
• This is like asking "how does ink on paper represent sounds?" - it could be anything from musical notation, written words, to light/dark patches run under an optical sensor or a QR code with a URL to an MP3 download. Commented Jan 30, 2014 at 20:22
• @JohnU You obviously didn't read the whole question. In the first sentence, I specified that I was asking about AC electricity. In the second paragraph, you can infer that I'm talking about analog (in case you didn't see the tag). Commented Feb 1, 2014 at 1:37
• Pretty much any information encoded electrically is AC, and everything is analogue at some level or, as my boss says, there would be no capacitors on digital circuits. Commented Feb 3, 2014 at 8:50

In the case of audio, energy must be translated from electric current to pressure waves (sound waves) by some sort of transducer (a speaker). Speakers produce sound by physically pushing air out of the cone, and thus producing the required pressure waves. An electric current is passed through the speaker coil to produce a magnetic field which either pushes the cone outwards or pulls it inwards. Thus the instantaneous magnetic field (or speaker cone position) is directly related to the instantaneous value of the current, thus the frequency of the outputted sound will be the same as the frequency of the current in the coil.

To answer a couple of your questions directly, the voltage across the speaker terminals will affect the current, but decibels (dB) are a measure of gain on a logarithmic scale, e.g. 20 dB is an increase of 100 times. However, there is also the dBm which measures power compared to 1mW, so a 3 dBm increase is equivalent to doubling the power. Lastly, "line level" just refers to a set voltage range for the audio signal (more about line level here).

• Note that the decibel is a ratio of the power in something. To be an absolute measure it is a ratio with a reference value. They are real pain unless you use them all the time. A change in voltage by a factor of 10 is 20 dBV where the 'v' denotes a ratio of voltages (thus use in amplifier voltage gain). Anything with amplitudes will use the scale of 20, including sound pressure. A ratio of power equaling a factor of 10 is a 10 dB change. Power ratios will always use the scale of 10 (unless they don't). Now I'm getting confused. A bunch of things can be expressed in dB with various refs. Commented Jan 30, 2014 at 8:13

How does electricity represent sound?

There are many ways, these can be characterised as analogue or digital representations.

The most direct analogue representation has voltage proportional to air pressure.

A sound wave in air is a pressure wave where the pressure reaching your ears is alternately of higher than ambient pressure and lower than ambient pressure. The electric representation of this, as produced by a simple microphone has a voltage proportional to this pressure.

Image from Electronics-Tutorials.ws

However, when you copy an MP3 file from your PC onto an SD-card, an electrical representation of a sound wave is travelling through the wires leading to the SD-card. This digital representation is very different and much more complex.

'Sound' is just any medium oscillating in the audible frequency - 20~20000 Hz. For example, to play the standard A note to the speaker, you send electrical signal to make the speaker turn ON-OFF at 440 times per second, or Hertz. Then that vibration is carried through air into your ear. That signal through the wire is also a voltage oscillating at 440Hz, but you can't hear it since your wire doesn't vibrate enough to be transmitted through air into your ear.

• Is it really just ON and OFF? Assume it's a sine wave sound at 440Hz. Wouldn't it oscillate between +V and -V? Commented Jan 30, 2014 at 11:31
• Doesn't sound use AC? Or is it DC? Commented Jan 30, 2014 at 11:39
• It works either way. If you send a sine with positive and negative voltage to the speaker, the surface will oscillate between positive and negative displacement. If you send PWM signal, then it will oscillate between zero and positive displacement. The sound you hear is air oscillating, and that oscillation comes from the speaker vibrating, and that vibration comes from change in magnetism that comes from change in voltage. So, even if you send a sine that oscillates from +3V to +5V, you will still hear something from the speaker. Answer to the other question is AC. Commented Jan 30, 2014 at 15:33

About decibels: dB on its own represents a ratio between two quantities. In audio there are are a number of dB-based units: http://en.wikipedia.org/wiki/DBu#Voltage

The one you're most likely to encounter is dBu, which is voltage referenced to 0.776V and is the quantity shown on a VU meter.

Note that within an audio system, there is usually no absolute relationship between the voltage levels and the audible volume; the same signal will have a different level in the preamp, along "line level" signals, and in the power amplifier at the output.