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In Reply to: RE: How do amps drive speakers? posted by George S. Roland on September 22, 2024 at 21:39:39
Your last sentence has the secret!Looking at its simplest where the amp is delivering a single frequency, say 100 Hz, the amp delivers an alternating current that fluctuates 100 times per second. The magnet within the speaker reflects this by changing its polarity 100 times per second and this fluctuating signal induces movement of the voice coil and therefore the cone - also 100 times per second. Thus the amp's alternating signal will result in movement of the cone at the same frequency. The cone will stop moving in and out the instant the signal from the amp ceases.
The clever bit is that the amp can deliver hundreds of these AC frequencies simultaneously and at different intensities (strengths) and this results in "music". Because large cones cannot be expected to move 15,000 times per second. a crossover circuit is used to split high and low frequencies that are sent to 2 or 3 drivers that are each designed to handle a smaller range of frequencies.
The other way to answer your basic question of how the amp's signal can be made into music is simply "by magic". It is pretty remarkable that music from an orchestra or band can be reasonably well reproduced by an amp and a couple of paper cones. I quite like the "magic" theory myself and leave it at that!
Edits: 09/23/24Follow Ups:
"The magnet within the speaker reflects this by changing its polarity 100 times per second and this fluctuating signal induces movement of the voice coil and therefore the cone "
You've got it backwards, maybe accidentally. I'll spot you that.
The magnet in a dynamic loudspeaker is a permanent magnet. It's polarity doesn't change. Whan curent is passed through the voice coil (a coil of wire), the coil generates its own magnetic field. This field interacts with the magnetic field of the permanent magnet, causing the coil to move. Since the curent running through the coil alternates from positive to negative repeatedly, and this movement is controlled by the shape of the waveform of the signal, the movement of the coil is not strictly just back-and-forth, but rather it has many undulations along its travel, which are (ideally) a mirror of the current's waveform.
But, since a speaker is a transducer - converting an electrical signal into physical movement, there are "issues" with regard to how well the moving assembly follows the waveform. So, it's the alternating current running through the voice coil which makes the voice coil-spider-diaphragm-surround assembly make sound, which often is a reasonable facsimile of the electrical signal.
Many years ago, I wrote a "white paper" on the fundamental failure modes of dynamic loudspeakers. Basically, they come in two flavors: Heat-related failures, and mechanical failures. I'll try to find the diagram I drew regarding basic speaker design components.
*********
We are inclusive and diverse, but dissent will not be tolerated.
> You've got it backwards, maybe accidentally.
Thanks for correcting my schoolboy error!
Thank you for this clear, comprehensible explanation. I agree with you that it's magical. Transducers are amazing. How the stylus in an LP groove, wiggling around in that spinning ditch can reveal choruses singing behind a symphony orchestra with the degree of detail it does is astonishing. The speaker/amp interplay is equally astonishing. It's easy to grasp the principle when thinking about sine waves, but music? Magical indeed!
Cheers!
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