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8ohm speaker question (boston acoustic)

A

antistock

Guest
ok, i'm semi new to home audio and i have been into car audio for some time now (i know i know)

oh, i just purchased a set of cr67's (boston acoustic) to use as fronts in a modest surround sound setup...now everywhere i can search says that these speakers are 8ohm...but when i meter them with a digital meter i read out 4ohm...they are brand new just came out of the package everything was factory sealed and such...

but the funny thing is that i almost purchased an open box set of c57's but i had the store meter them (circuit city) and they read 4ohm so i told the sales person that they must be blown because they are supposed to be 8ohm.

both of the c57 and c67 state on the back of the speaker that they are 8ohm...

please help me i think i'm going crazy. I have an onkyo 304 receiver and it can handle 6-16ohm so i'm hesitant to put the bostons on it until i clear things up... any help would be great thanks
 

BlueWizard

Distinguished Member
Most modern consumer amps can handle a final combined impedance of 4 ohms to 16 ohms, that has been the standard for years. There are many commercial amps that will handle load down to 2 ohms or lower, but few typcial users are ever going to encounter them.

A speaker impedance rating is the 'nominal' rating. In reality, for every speaker the dynamic impedance will go noticeably above and slightly below the rated impedance.

Here is a link to an impedance graph -
http://www.partsexpress.com/projectshowcase/tritrix/impedance.jpg

of this speaker -

http://www.partsexpress.com/projectshowcase/tritrix/index.cfm

Notice it starts out at about 7.5 ohms rises up to about 15.5 ohm, then drops to about 7 ohms before rising up to about 11 ohms.

What you are measuring is not really impedance, which implies a dynamic response to a dynamic signal. What you are measuring is the DC (direct current) resistance of the speakers voice coil. That implies that at some very low frequency, the resistance could actually drop to 4 ohms, but in response to a dynamic music signal, it is likely to average much higher.

Typically the DC ohm-meter resistance of a voice coil will be less than the rated impedance, though measuring 4 ohms for an 8 ohm speaker is unusually low, it is not unheard of.

If the speakers are clearly marked and rated 8 ohms by the manufacturer, then I would trust that.

Steve/bluewizard
 

phil t

Well-known Member
Just to echo what BlueWizard said, you’re measuring resistance with your meter not impedance, the two are different.

A coil of wire (inductor) doesn’t “like” a changing current; it will try to impede the (changing) current flow. The faster the current (in the coil) changes, the greater it will try to impede the current. You are effectively measuring the impedance at 0 Hz, whereas a speaker in use will “see” a changing current between a few Hz and tens of KHz.

:)
 
A

antistock

Guest
wow, thank you all very much for the in-depth answer...good to hear a clean and exact explanation rather then a simple yes/no. i had never known that a digi meter set to ohms isnt an accurate way of checking a speaker...you have opend my eyes and i thank you guys.
 

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