Randy Bey Posted August 29, 2001 Share Posted August 29, 2001 Well, I got into a discussion regarding impedance, and managed to make a fool of myself. Not that I usually need help making a fool of myself, but the reason I'm asking is because I thought I understood what you had said a (long) while back regarding impedance. I thought you said "high capacitance speaker cables are practially the definition of high impedance". Now I am told, by an audio engineer, no less, that impedance is the square root of the ratio of inductance over capacitance. This would imply that as capacitance increases, impedance decreases. Did I just remember it bass ackward? Quote Link to comment Share on other sites More sharing options...
WMcD Posted August 29, 2001 Share Posted August 29, 2001 Al.K. can comment. I'm jumping in, none the less. I'm gonna tell you somethings which are true. But are difficult to understand. Or calculate. Let me begin by stating that understanding "impedance" or "resistance" is not going to answer some bottom line questions. The bottom line questions are, typically: are the speakers, cables, and amplifer working well together? If there is a problem, what to do about it? We first have to start with "resistance" and Ohms law. And voltage and current. Mr. Ohm discovered that if you place a voltage across a resistance, some current flows. More voltage, more current. A direct relation, depending on the resistance. And it is true in REAL TIME. Ohms (resistance) = Voltage / Current. So, really he is setting down a definition of resistance. We can only measure voltage and current, but there seems to always be a relation. Put 8 volts across a passive resistance and we get 1 amp of current flow. Put 16 volts across it, and we get 2 amps. What is the resistance? Gee, we see a relation of 8. Actually, resistance can't be measured directly. It is just describing the voltage/current relation of that component. There are a couple of assumptions here, unrelated to the resistance. Do we have a source of voltage, which can deliver that much current? Maybe not. Our amplifer might not be able to deliver that current, at high voltage levels. But maybe it can at low voltage levels. One critical thing is that the poor little old resistor can not create any electical power on its own. Short term, or long term. The next problem comes up with alternating voltage and current. Like music. And also with real world speakers. Impedance is the a.c. analogy to resistance, telling us the relation between voltage across the speaker and the current through it. Above I mentioned "real time" and we're gonna stick with it. If an a.c. voltage is applied to a pure resistance, the current changes in lock step, in time. And it is always the same, regardless of frequency of the input. Do speakers act like pure resistances? No. There are little energy storage units in there. This screws up the behavior of voltage and current tremendously. Like it or not, they are little calculus machines. These are not quite nicad batteries, long term. Rather energy is stored sort term in things we can understand. Consider that the diaphragm has mass and is moving, soon to slow down, and release energy. There is air in the box which is compressed, soon to be uncompressed, and release energy. The coils in the cross over create magnetic fields, which will collape, and put energy back into the coils. The capicitors are storing an electric field, which can be discharged. The next level of complication is that the speaker itself is a motor and a generator. As a motor, it puts energy from the amp into the mass of the diaphragm and compresses the air in the chamber. But all that energy will return to drive the speaker like a super microphone. It self a source of electrical energy. Take pause here. Above I've said that the speaker has, in the right circumstances, become a generator of voltage and current. Short term. Unlike the resistor, it is pushing back. There is an analogy we're familiar with. A flashlight. When the bulb is connected to the battery, the lamp consumes power. Ohms law (resistance). However, suppose we replace the bulb with a battery of the same voltage in parallel. If the source battery has the same voltage as the "load" one, we have the same voltage, and no current flowing. No one thinks that batteries in parallel consume power from each other. There is voltage, and no current. So, at some frequecies, the speaker is acting like an auxilliary battery. We can't force current into it even though the amp is a voltage source. The load has the same voltage. (In truth, it is a lower voltage.) This is odd, but true. It is a magnificent mess. We started off with a resistance which just burns up electrical energy with no reverse voltage. Now the speaker is presenting a voltage at the terminals. We have to go back to our definition of resistance. If the speaker terminals have a reverse voltage at or near that of the generator, we can't push current into it. Gee. High voltage, no current. That is high resistance, by definition. But, there is no little old radio shack resistor in there. All of the energy storage units have conspired to present a voltage at the speaker terminals which resist current flow. This is part of the "magic". I'm running out of steam. More later. Transmission line theory. Gil This message has been edited by William F. Gil McDermott on 08-29-2001 at 10:55 PM Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted August 30, 2001 Share Posted August 30, 2001 Randy, Here's a cool link that gives a brief definition of impedance. Note - this thing may ask you to download the most recent versino of Shockwave. You don't need it unless you want to see an annotated graph move around as you change resistance, capacitance and inductance values. If you click the "Yeah, sure, download shockwave" button, they will helpfully close every single one of your browser windows for you. Here's the link: http://users.erols.com/renau/impedance.html ------------------ Music is art Audio is engineering Quote Link to comment Share on other sites More sharing options...
Colin Posted August 30, 2001 Share Posted August 30, 2001 oh god! is there going to be a test? Nobody told me! :0 ------------------ Cornwalls, Bottlehead 2A3 Paramours, Dynaco II, Rotel, KSW200 & LF10 Quote Link to comment Share on other sites More sharing options...
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