nominal impedence vs minimum impedence?

[Erman_94]

im looking at some speakers and im confused

it says they are 8 ohms nominal and 4 ohms minimum...what does that mean?


i have a pioneer 1018 receiver and im looking for some 8 ohm speakers to connect them to but this "minimum" is throwing me off

any help is appreciated,

thanks

[crackinhedz]

Nominal impedance

...nominal would be like saying "preferred"

[Big Daddy]

A loudspeakers impedance varies across a frequency range. Nominal means, somewhere around average. Look at the diagram below for an 8 ohm (nominal) speaker. Note that the highest point normally occurs at resonance. It is approximately 17.5 ohms in this example, while the minimum impedance is almost 5 ohms. Speakers like all suspending objects have a resonant frequency (denoted by Fs) at which they will vibrate most freely. At this frequency, the voice coil is vibrating with the maximum amplitude and velocity.

Impedance.jpg

At resonance frequency, the speaker is very resistive and creates no problem. The minimum impedance is normally the problem. The loudspeaker in the above diagram was listed as an 8 ohm speaker. You will note that the impedance value is below 8 ohms for all frequencies between 100 to 3,500Hz. The amplifier will see less than 6 ohms between 120 and 1,000Hz. What is important to note is that the actual load at a particular frequency represented to the amplifier is dependent on the impedance at that frequency.

Using a 4 ohm loudspeaker on an amplifier rated 6 ohms or greater could lead to overheating and amplifier failure. Most receivers cannot handle 4 ohm speakers. If you have a 4 ohm speaker, you need a separate amp to drive it.