When you turn down or up the volume on devices with either a wheel or button, what actually happens that allows it to sound quieter or louder?
In: Technology
(preface: while only a hobbyist, I’m a self-taught electrical engineer focusing on audio amplifiers, and specifically guitar amplifiers, and specifically vacuum tube based audio and guitar amplifiers.)
Someone once told me “an amplifier is just a power supply that is modulated by its input.” So when you think of a power supply, think of the simplest one for a second: a battery. You have a positive voltage on one side, and a negative on the other (usually grounded, so the negative voltage is only relative to the positive voltage).
If you hook up a 9V battery to a speaker, guess what? The speaker moves. You don’t ‘hear’ the sound, but you’re able to visually SEE what a 9V signal ‘looks’ like. It doesn’t look like the sine-wave you think of when you think about ‘soundwaves’, right? Well, that flat-line can be thought of as a wave with a frequency and/or amplitude of 0. It’s useful to note that if you used a 4.5V battery, the speaker would move the same direction, but not with the same force. If you quickly switched the batteries back and forth, say, 60 times per second, you’d be creating an electrical soundwave. 60hz is (in North America anyway), the frequency that you hear humming from electronic devices. (and fluorescent lights? those are 120hz ‘buzz’. the why 60/120 is another topic for another time!)
If, that signal were wiggled (modulated) at a specific frequency (think pitch) and given an amplitude (this is effectively what becomes volume), you’d hear it out of the speaker. When you listen to a song, it’s not just one tone, it’s a whole lot of different tones being produced–but lets just focus on a single, simple, soundwave. I want you to visualize a sine wave for a moment. The closeness of the peaks/troughs is your frequency (the pitch), the HEIGHT of those peaks and troughs is your amplitude.
The specifics of the circuit will dictate how this is all achieved, but the volume control will either expand or restrict the maximum amplitude (the ‘height’ of that electric signal). Some amplifiers actually have fixed amplification (lets say 100x for a nice round number), but will allow you to limit the input signal on them through a simple resistive divider (a potentiometer, a “knob”). (note: the following is for illustrative purpose and intentionally simplified). So it may take up to 1V of signal coming in, and will shoot out 100V of signal on the other end. The potentiometer here divides the input signal, so say that 1v signal is dropped down to .25V, then that is still amplified 100 times, and comes out as a 25V signal. Other, newer, amplifiers work by raising or lowering the whole working voltage (or power) of the amplifier. So it may take a 1v signal, but the overall level of amplification of the amp is not fixed at 100x. (most do this through some type of power regulating circuitry which can be as simple as a very, very big potentiometer (known as a passive component, similarly a rheostat), or very elaborate involving digital and solid state active components (think transistors, IC chips, etc).
Whether done by active or passive components, this restriction, in turns, limits the electrical signal that is sent to your speakers. Speakers are an electro-motive device. They convert electromagnetic energy (your electric sine wave) into mechanical energy (by vibrating the air, they ‘wiggle’ the molecules in the air; the concept for overall volume is called ‘sound pressure level” or SPL and is measured in decibels, or dB. this last bit is unimportant, but thinking of sound waves you hear as literal pressure waves is neat).
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