65mm Diameter Loudspeaker, 8 Ohm Voice Coil.



LS1 is a 65mm diameter paper coned speaker with an 8 Ohm impedance voice coil.  As the analog obsessive Bob Pease of National Semiconductor might once have said, "What's all this impedance stuff anyway?" 

If you put a multimeter on the low Ohms scale across the terminals of a resistor, you will measure 8 Ohms.  If you put a multimeter across the terminals of this speaker you might measure about 8 Ohms, but it will probably be a bit less, say 7 Ohms.  If you use an impedance tester working at an alternating current (A.C.) frequency of 1kHz, you should measure 8 Ohms total magnitude impedance.  This includes the 7 Ohms pure resistance in the wire of the speaker coil, and maybe 1 Ohm of inductance measured at 1kHz.*  The 1 Ohm of inductance is caused by the reluctance of the coil inside the magnet gap to pass the A.C. current - in fact you'll see "reluctance" and "inductance" used interchangeably in old texts.

1kHz is nothing special, it's just a standard test frequency for audio devices, being as it is, more or less in the middle of the low range of human hearing.  Our circuit "sees" something like 8 Ohms impedance because it's working across the normal AM radio audio bandwidth of about 100Hz to 6kHz.

In other situations where you have a 64 Ohm speaker, or even more so, with old style high impedance headphones, this difference becomes ever more important.  In those cases, the reactive (inductive) impedance of the device can be much higher than the D.C. resistance measured with a multi-meter. 

* It's a bit more complicated than simple addition, because the magnitude of the resistive and inductive impedances add-up as vectors, but a magnitude reading on an impedance tester will say 8 Ohms. 

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