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Magnets. They're pretty cool. If you connect a magnet to some thin cone, and run just the right currents through a wire somewhere in the mix, you can even get them to make weird noises. And that's what we call a speaker! But, for an class of Physics C students, that's probably fairly common knowledge, right? After all, I'm sure we've all seen the speaker contraption floating somewhere around the room. But wait - there's more! As a matter of fact, some speakers and headphones (and microphones for that matter) don't actually use magnets to make their funny sounds! "What?! How can you make something move with electricity without magnets?", you may ask. Well, I'll answer your question with a question: do you remember that whole unit we did about charges that don't move? The whole one that makes your hair stand up and lets you shock your little sibling when you're wearing slippers? Well, that's just about all you need to know, really. It's only a few steps: Take a stupidly thin film of something flexible, and make it nice and charged Place that thin sheet between two more thin, conducting sheets Use your audio signal to change the voltage between the two sheets Bam! The thin film moves with the sick beat from your mixtape! "But wait a second, I can't hear a thing! What gives?" Well, things get a bit complicated with that third step. You see, headphones have a little number called impedance attached to them. For the simplicity of this post, and so I don't have to actually do extensive research into it myself, we'll just say that higher impedance speakers/headphones are more difficult to adequately provide clean, powerful audio signals to than lower impedance ones. So anyway. Those little white earbuds you have in your pocket? Those probably have an impedance around 50 Ohms, depending on which brand they are, etc. Headphones driven by the method given above? Those have an impedance upwards of 150000 Ohms! That's quite a bit higher! So, in order to properly use these headphones and speakers, you'll also need to get a hold of a really powerful amplifier. Then, you'll be able to hear the sweet sound of electrons being lazy and not liking one-another. So, that's about it. Magnets: they're pretty cool, but they're not the only cool kid in town.
Nowadays, almost every kid has a computer, whether it's their own or not. And, with that computer, nearly every kid listens to some type of video, music, or even alert messages. Without sound, computers just wouldn't be as useful. But how do computers make sound? The answer is quite complicated. Older computers used the common magnetic speaker, which included some array of copper wires and an iron magnet. The original design was a simple iron magnet inside a copper coil, vibrated by the electric field induced inside. Nowadays almost all computers use piezoelectric speakers. This type of speaker is named after the piezoelectric effect, which describes the quality of certain materials to create an internal electric charge due to mechanical stress. A voltage is supplied to a resonator or diaphragm, which in turn begins to vibrate as the electricity causes stress in the object, reversing this effect. The sound of the speaker is controlled by a process called pulse-width modulation. This means that the power supplied to the speaker is digital, either 0V or 5V. However, by adjusting the duration of the duty cycle (the amount of time it gets 5V) different sounds can be created. Lots of ingenuity went into those tiny little buzzers you listen to every day. So take some time and really enjoy them. Some robot is slaving away making them right now.
Sometimes I like to sit back and pump some jams. Before the invention of all this modern technology such as speakers and cds and digital audio, such things just weren't possible. Music had to be performed. But with the invention of electrical speakers that all changing. People were able to finally jam out. The common speaker relies on the principles of electromagnetism. In the center is a magnet (attached to a speaker cone), surrounded by a coil. As the current through the coil fluctuates, the magnet and cone move, vibrating to reproduce the encoded sound. However, all things have inertia, so it can take time to reverse the momentum of the cone, creating a loss in audio quality in the event that the speaker cone is too heavy. Similarly, if the cone isn't stiff, it will delay its movement and creating quality losses that way as well. These losses are most noticeable with "harsher" waveforms (such as squarewave, which, as the name implies changes position very quickly at wave boundaries), or with more complex sounds, such as violin or saxophone. Because of these drawbacks good sound systems often have multiple speakers, all tuned to a different frequency. Subwoofers are typically larger because lower frequencies are less audible, and lower frequency waveforms are easier to reproduce in terms of speaker design. Tweeters are smaller for the opposite reasons - they need better accuracy because higher pitches involving larger shifts in momentum with respect to time, so they are typically smaller to achieve this. Also, because every material has a resonant frequency (where it will absorb a lot of energy), the materials in each are tailored to avoid this. Next time you're cruisin', bumpin' along to your favorite song, remember this. And invest in a better sound system.