Caroluna wrote:The idea that some microphones are sensitive to a wider range of frequencies -- Is that why old-fashioned cassette tape recorders make your voice sound like a chipmunk?
(Maybe you are too young to have experienced that) The microphone wasn't picking up the lower frequencies and so it changed the
timbre of the voice? The "secret sauce" of frequency blends wasn't there anymore.
Yeah, I know what you're talking about. It could either be the microphone, the tape recording head, or the preamp, but in any case you're right, it wasn't capturing the full dynamic spectrum of the voice (or was limited in which parts of the dynamic range got more power).
You can get some of the effect by doing a voice recording in Audacity and applying a High-Pass Filter (under the Effect menu).
When the phone system was developed, the designers limited the frequency range to about 4000Hz, which was enough for the human voice. But if you try to play music over the phone to another party, it will sound pretty cruddy because the frequencies > 4000Hz will be filtered out by the phone system.
I'm still having trouble understanding the dynamic mic.
Sound waves move thin membrane of PVC.
Thin PVC membrane is ?attached to? thin copper wire
Copper wire is wrapped around a core
Core is surrounded by a permanent magnet
but when you look at the wire/core thing and the outer permanent magnet, which one is physically moving? And if this set-up is so sensitive that sound waves can make it move around, then why did the prof. say that rock musicians can be really rough with these microphones?
I tried to find a good animation for dynamic mics, but the best I could do was for speakers (which is a very similar setup):
http://electronics.howstuffworks.com/speaker6.htm
In a speaker, when you put a current through the coil of wire, it generates a magnetic field in the iron core (the same law of physics that gives us electromagnets and
solenoids). The direction of the field depends on the direction of the current: touch the ends of the coiled wire to the terminals of a 9-volt battery, you might get a north-south field, swap which wire is touching which battery terminal and you get a south-north field.
The core and coil are surrounded by a permanent magnet (which is called permanent specifically because its field direction never changes). When you have a current that is causing a north-south field in the coil, the magnet will push the core one way, when the current is causing a south-north field, the magnet pulls the core the other way. This is how the core can move in a way that follows the current.
Attach the core to a cone made of some flexible material (paper, plastic, etc), and you make that cone move in the same way the core does, which makes the air move the same way, which makes sound.
Now, the reverse of the physical law that makes speakers work also is true: instead of applying current to the coil so the magnet makes the core move,
you can physically move the core and the magnetic field from the magnet
will generate an electrical current in the coil.
So, if you attached a voltmeter to the wires of a speaker and took your hand and tapped the middle of the speaker cone, you would see a tiny change in voltage on the voltmeter (and it would go from positive to negative as the direction of the cone's movement changed).
But don't actually do that because it will break your speaker's cone. They're delicate. Instead, we developed microphone diaphragms that are not as delicate and that don't have to move their core nearly so much to generate a current.
In a dynamic mic, there is a diaphragm which moves with the sound waves in air. The diaphragm moves a core which is attached to the coil of wire. So the permanent magnet holds still, the diaphragm/core/coil move as a unit inside the magnet and this causes a current in the coil. That current is really, really small, so dynamic mics have to have some preamplification.
You're intuition about the loud singer is right, though. The short answer is that materials scientists are very clever. The long answer is that there are different dynamic mics for different applications. A vocal mic will have a stronger diaphragm to handle strong pulses (and the occasional drop onto the stage), but this is often at the expense of the its ability pick up extremely soft sounds very well. There might also be more room to move before the diaphragm physically hits something else (which would be damaging). A vocalist, even while screaming, is never going to be as loud as a bass drum or a tuba.
A recording mic may be able to better able to pick up subtle sounds but might be more easily damaged by very loud impulses.
Still, a condenser mic will always be more fragile than a dynamic because it has two plates so close together that they would gladly run into each other (ouch) at the slightest provocation.