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#1
12-09-2012, 08:41 PM
 Registered User Join Date: Nov 2011 Location: Oklahoma City, Oklahoma
If I have a set of speakers, lets say 2 pair of 16 Ohm speakers, and I test the impedance rating. I'll will get around 4 ohms correct? (Assuming that are all hooked in Parallel)
Now if I hook the 4, 16 ohm speakers wired in parallel to an amp and measure the impedance, will the measurement vary? And if so, by how much approximately?
#2
12-09-2012, 08:46 PM
 Temp Banned (TOS Violation) Join Date: Feb 2005
I'm confused. Resistance is what it is. Why would it vary when voltage is applied?
#3
12-09-2012, 08:52 PM
 Registered User Join Date: Nov 2011 Location: Oklahoma City, Oklahoma
Quote:
 Originally Posted by two fingers I'm confused. Resistance is what it is. Why would it vary when voltage is applied?
That's what I was thinking. I have a 'sound man' telling me that impedance changes when an amp is applied.
#4
12-09-2012, 09:16 PM
 Registered User Join Date: Nov 2004 Location: Lawton, OK / Ruston, LA
#5
12-09-2012, 09:27 PM
 Registered User Join Date: Jun 2012
Simply put, the speakers impedance changes with frequency. It's a MINIMUM of 4 ohms, usually.
#6
12-09-2012, 09:32 PM
 Registered User Join Date: Nov 2011 Location: Oklahoma City, Oklahoma
Quote:
 Originally Posted by bumperbass Nice link, 3506string. Simply put, the speakers impedance changes with frequency. It's a MINIMUM of 4 ohms, usually.
His hypothesis was that impedance changes when and amp is connected and just turned on.

I assume it might change, but that change is small.. In my assumption.
#7
12-09-2012, 09:52 PM
 Registered User Join Date: Oct 2007 Location: Vortex of sin and degradation
Two 16 ohm speakers in parallel will give you 8 ohms.

Are you saying you are measuring your speakers while they
are still connected to the amp? That will give you an
indeterminate and meaningless reading. There is no telling
how it will affect the measurement.

Speaker impedance is a function of frequency. The impedance
given for a speaker is generally a nominal impedance -- a
ballpark (but not exact) figure of what you can expect.
#8
12-09-2012, 10:21 PM
 Registered User Join Date: Nov 2011 Location: Oklahoma City, Oklahoma
Quote:
 Originally Posted by warnergt Two 16 ohm speakers in parallel will give you 8 ohms. Your speaker impedance is independent of your amp. Are you saying you are measuring your speakers while they are still connected to the amp? That will give you an indeterminate and meaningless reading. There is no telling how it will affect the measurement. Speaker impedance is a function of frequency. The impedance given for a speaker is generally a nominal impedance -- a ballpark (but not exact) figure of what you can expect.
Two Pairs of 16 Ohm speakers.

And thanks, this is what I was looking for.
#9
12-10-2012, 11:38 PM
 Registered User Join Date: Nov 2003 Location: Indianapolis, IN
When an amp is connected, the total impedance changes so far as the outside world is concerned, as the amp's output impedance is in parallel with the speaker impedance. But the loudspeaker impedance as seen by the amp does not change compared to what the measurement device 'saw'. The impedance does change with frequency. It also changes as more power is fed to the speaker (the coil gets hot and the impedance goes up - look up 'power compression').
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#10
12-11-2012, 12:33 AM
 keepin' the beat since the 60's Join Date: Dec 2007 Location: Studio City, SoCal, USA
It is not a hypothesis - it is the laws of physics and electrodynamics. Resistance is NOT the same as impedance. Impedance is the combined effects of resistance, the inductance of the coil and the voltage created by the coil moving in the magnetic gap.

Since speakers and amps are for AUDIO, they are spec'd at audio frequencies, not while sitting still. Using an impedance meter, you will find the speaker has say, 4 Ohms impedance at 1kHz, while the static resistance measures about 5-6 Ohms.

To answer your question- 4 16 Ohm speakers all in parallel will be 4 Ohms. You do NOT measure the resistance while they are connected to an amp. The output impedance of an amp is much lower than the speakers and will give you an inaccurate reading.
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Last edited by Bassamatic : 12-11-2012 at 12:36 AM.
#11
12-11-2012, 01:56 AM
 Total Hyper-Elite Member Independent Contractor to Bass San Diego Join Date: May 2000 Location: Groom Lake, NV
Quote:
 Originally Posted by Exxcell His hypothesis was that impedance changes when and amp is connected and just turned on. I assume it might change, but that change is small.. In my assumption.
If you measure the DC resistance across, say, two 16-ohm speakers in parallel, you'll get something around 6 ohms. The AC impedance will typically be greater when signal is applied, but not necessarily exactly 8 ohms. It varies with frequency. The DC resistance will be unchanged if the amp is connected, but no signal is passed.
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#12
12-11-2012, 01:58 AM
 Total Hyper-Elite Member Independent Contractor to Bass San Diego Join Date: May 2000 Location: Groom Lake, NV
Quote:
 Originally Posted by Bassamatic Using an impedance meter, you will find the speaker has say, 4 Ohms impedance at 1kHz, while the static resistance measures about 5-6 Ohms.
Typically the static (DC) resistance will be lower than the audio (AC) impedance.
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#13
12-12-2012, 06:03 PM
 Registered User Join Date: Aug 2006 Location: Austin, TX
Quote:
 Originally Posted by Munjibunga Typically the static (DC) resistance will be lower than the audio (AC) impedance.
Replace "typically" with "always". Impedance is resistance plus the AC effects of reactance (inductance and capacitance), and reactance cannot be subtractive.
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#14
12-12-2012, 06:19 PM
 Back. And grumpier than ever. Moderator Join Date: Feb 2003 Location: Warwick, Rhode Island, USA
And for the users benefit, there is a complete discussion of Ohms Law, resistance and related topics in the FAQ thread in the main sticky at the top of the Amps forum. It is well worth your while to peruse it, it will answer many of your basic questions on this set of topics.

Ohms FAQ

This thread should probably be linked in this forum as well, I am sure these questions come up often.
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#15
12-12-2012, 07:01 PM
 Total Hyper-Elite Member Independent Contractor to Bass San Diego Join Date: May 2000 Location: Groom Lake, NV
Quote:
 Originally Posted by ggunn Replace "typically" with "always". Impedance is resistance plus the AC effects of reactance (inductance and capacitance), and reactance cannot be subtractive.
Just trying to be safe.
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