Help with replacing woofer 4ohm or 8ohm

[Big Easy]

Surround foam has desintegrated on my infinity wall speakers. Rear surround wall speakers. Trying to avoid cutting sheet rock crawling into attic to replace.

Speakers are infinity ERS 640. About 10 yrs old. Did a search and couldn't find many specs. Infinity tech support was dissappointing to say the least.

I think the overall impedance rating of the speaker is 8ohms. Took the woofer out by itself and it read 3.7 on ohm meter. A local av shop said that the ohm meter is not a true reading of impedance.

Parts express has woofers that look like they would fit right in. Both 4 and 8ohm.

Don't want to blow up the amp (pioneer elite vsx33).

Although the overall impedance rating could be eight for the entire speaker if the ohm meter for the driver is 3.7 should I get 4 ohm or 8 ohm?

Any insight would be greatly appreciated

 

[Harvey]

Product specifications for Infinity ERS 640.

Frequency Response: 60Hz - 22kHz
Crossover Frequency: 3.8kHz
Sensitivity: 85dB (1 watt@1 meter)
Nominal Impedance: compatible with 8 ohms
Power Rating: 15-60 watts RMS
Woofer: 6" IMG
Tweeter: 3/4" polycell
Cutout Diameter: (HxW) 11-3/8" x 7-5/8"

The DC resistance of the voice coil is not the same as the rated impedance of a dynamic speaker. The voice coil has a finite DC resistance, which you measured, but it is an inductor, which means that its resistance varies with frequency. The rated impedance of a speaker is a nominal value indicating the approximate resistance of the speaker across the specified frequency band.

The actual impedance is also affected by the way the speaker is mechanically loaded, which includes the resonant frequency of the speaker and the size, the resonance and other design parameters of both the speaker and the enclosure.

In replacing it, there are other considerations. This speaker has a molded graphite woofer cone with a ferrofluid cooled voice coil, making it a more expensive, technically advanced design so you can't expect similar performance from a random replacement driver.

Furthermore, in a system that doesn't use active electronic crossovers (biamped, triamped, etc.), the impedance of the speaker is part of the calculation of the components in the crossover network so substituting a speaker with a different impedance will change the crossover frequency in the system.

The bottom line is, if you want the system to sound the same, replace the old speakers with the same make and model.

Infinity is one of the brands of Harman International. If you want your system to work exactly as designed, you can order a replacement part from Harman. Thier customer service number is (877) 871-6755. They gave me this info:

Part Number: B16D80PR-06DW-EP
Price: $30.75 + applicable taxes and shipping.

This part is currently on back order. Product is expected to arrive next month. Confirm this with them when you call to order.

 

[LoveMachine]

The AV guy was correct about impedance. It varies depending on the frequency being output, so the listed ohm value is "nominal" impedance. No matter what, if you swap in a different driver, the final sound will be different. The crossover in the unit was designed with the stock drivers in mind, and nothing else. You can put in a PE woofer, and it will make sound and wouldn't damage your amp unless you are going crazy with the volume (not much usually comes out of the rears in terms of low frequency), but I would go for the 8ohm driver, and don't spend too much on it. A $400 Morel driver can sound worse than a $20 Dayton in a non-optimized crossover design.