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impedance rating of speakers

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samcheetah

New Member
today i was reading about transformer coupled amplifiers. i read about audio transformers that had ratings in impedances. till that it was fine but when the author said that speakers have impedances in the range of 20ohms to 30ohms i got confused

from what i know XL=2*PI*f*L

so the impedance of a certain inductance(in this case a speaker) would be at a specific frequency. so if i would say that this speaker has an inductance of 16 ohms i would have to say that it is at X frequency. but speakers operate at thousands of different frequencies because the sound is constantly of varying frequencies. so how can someone say that this speaker has an impedance of X ohms when the frequency is not constant.

for transformers used in power supplies one can specify the impedance because it is at a fixed frequency (50 or 60 Hz) but for audio transformers and speakers it shouldnt be like that.

plz can somebody clear my confusion. any help will be appreciated
 

stevez

Active Member
I recall the 1 kHz "standard" as well. Also read a well written and researched technical article that described the behavior, including impendance, of speakers. Change in impedance was far from linear and was influenced by the enclosure, among other things.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Why are you wanting to know?, basically you just buy a speaker with it's impedance printed on it - usually 8 ohm, but 16 and 4 are available as well.

A transistor amplifier will have a specification which gives the lowest impedance it's suitable for - usually 4 or 8 ohms, but sometimes as low as 2 ohms. Using a higher impedance speaker will just reduce the power output, using a lower one will probably damage the amplifier.

A valve (tube) amplifier will have an output transformer, this will usually have different taps for different speaker impedances - you simply select the correct tap for the value printed on the speaker. On these sorts of amplifiers it's fairly important to use the correct transformer tap, running into the wrong impedance can damage the amplifier.
 

k7elp60

Active Member
Speaker impedance

I have seen speakers with the impedance as high as 100 ohms. Years ago I used to repair an intercom system the used 32 ohm speakers. I remember before the high input impedance digital volt meters I could check the speaker for continuity, to see if the voice coil was open. After the advent of solid state DVM with high input impedance I could measure the voice coil with the DVM and it would read very close to the speakers impedance.

Perhaps the author of the article was referring to a particular speaker or even headphones, as a lot of them these days are near 30 ohms
 

samcheetah

New Member
Why are you wanting to know?

as i said previously i was reading about transformer coupled amplifiers in a book and it said that audio transformers and speakers are rated according to impedances. it didnt state the 1kHz specification at which the impedances are measured. so i was confused at this. thanx to u people the confusion is now cleared.

k7elp60 said about measuring the impedance with a DMM. i have always been inconfident in doing that. what i know is that a speaker is made up of a coil (an inductance) which is true (i guess :) )

can i measure the impedance of a speaker with a DMM. i dont think so. how do u measure the impedance with a DMM
 

Nigel Goodwin

Super Moderator
Most Helpful Member
samcheetah said:
can i measure the impedance of a speaker with a DMM. i dont think so. how do u measure the impedance with a DMM

You can measure it 'near enough', measuring the resistance, rather than the impedance will usually make it read lower than it is. So measuring an 8 ohm speaker may read 6 ohms - but because you know it's going to either 4, 8, or 16 - you know it's 8 - with the few rare exceptions (like 32 ohm) of course :lol:

Using a similar method to that above, you can feed it from a 1KHz sinewave source through a known value resistor - by measuring the signal voltage at either end of the resistor you can calculate the impedance. If you use an 8 ohm resistor, and an 8 ohm speaker, the voltage at the input should be double that on the speaker.
 

k7elp60

Active Member
speaker impedance

I guess I should have been a little more specific. What I meant to say was the measurement in DC ohms on a high impedance DMM will show
very close to the impedance of the speaker. This is true of most of the smaller low power speakers, however the higher power speakers this is
not the case in my opinion.
 
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