Why do 4ohm speakers need a more powerful amp than 8ohm?

From AV101:

"Ohm

The measure of resistance in a circuit to the flow of an electric current. The greater the ohm value the more difficult it is for current to flow through a given circuit. Conversely, a low ohm value represents a low resistance and the easy flow of current through a circuit.

Ohms are often used in reference to loudspeakers which have a certain level of resistance associated with them. A typical speaker may have an ohm rating (rating the speaker's resistance) of 4 to 8 ohms nominal impedance (impedance being resistance to current flow). Resistance in a speaker varies, however, from low ranges often as low as 2 or 3 ohms to high ranges as high as 30 or more ohms.

As speaker impedance decreases, more current flows and the amplifier is called upon to output more power to the speaker. When shopping for speakers and amplifiers, make sure the amplifier is able to output power properly into the speakers' impedance. Many receivers are not able to operate properly with speakers having impedance loads of less than 8 ohms. While most separate amplifiers can handle lower impedance loads down to 4 ohms or less, some speakers present impedances of 1 or 2 ohms at their minimum requiring particularly robust and powerful amplifiers designed for such loads. Generally, most any separate amplifier will be able to properly power almost any speaker (with the few exceptions being in most cases highly-exotic, very expensive speaker designs) while receivers should be mated with speakers having nominal impedances (average impedances) of around 8 ohms."



This article gives a pretty good description of what is happening. In other words as the resistance (OHMs) drops, the speaker is allowing more current to flow from the amp. Underpowered amps may not be able to supply the necessary current allowed by the speaker. I often use the analogy of a water hose: Given the same amount of water pressure (Power) to a small diameter hose (say 1/2 inch diameter) the flow of water (current) will come out pretty forcefully. Given the same amount of water pressure on a 4 inch diameter hose the flow of water comes out with much less force. The larger hose has created less resistance. To get the same force with less resistance you need to increase the pressure at the source. So a speaker with less resistance (lower OHMs) needs more power to operate correctly.

One usually needs the higher output from a separate amplifier when driving low impedance speakers like the LSi line because of excessive heat build up in most receivers. Most receivers these days have built in protection devices that shut down the amplifier sections if too much heat builds up in them. Trying to drive 4 ohm speakers with most receivers will eventually result in excessive heat build up and frustrating shut-downs. One can run 4 ohm speakers with a receiver, but not reliably, and at risk of shortening the life of the receiver even if shut downs don't occur. It's especially important now that multi-channel audio is being used over 5, 6, or 7 channels. That's a heck of a load to put on most receivers. Heat is the enemy of electronics.

Above it was mentioned not to confuse power with current. Well, for the same load they are the same thing. A higher power amp has more curent capability... it's kind of by definition.

AS alluded to in the 2nd post, an ideal amp would double in power as speaker impedance is halved. Sunfire is the only amp I know of that actually doubles it's 8 ohm rating at 4 ohms, but I'm sure there are others. However, due to current limitations of most designs you rarely see this in specs.

Often you will see something like, "100 wpc into 8 ohms/ 126 wpc in to 4 ohms". This 26% increase in power is indicative of an amp with low reserve current, i.e., headroom. Even my beloved Carver M-1.5t can only boast a 33% rise (from 375 to 500 wpc).

I believe that anemic 4 ohm ratings are why many manufacturers now list only 8 and 6 ohm ratings in their spec's. It's a point where they can hang a little closer to the theoretical curve anyway...

b,

Not talking "cheap-**** amps" here... this is not the car stereo forum.

P=i^2 * R is the Equation.

Then (P2/P1)=(i2/i1)^2/(R2/R1)
so for the same impedance R2 = R1, (R2/R1)=1

So we see that doubling the Power, P2 = 2*P1, delivers 1.41 (the square root of 2) times the current.

Whether you're comparing 100 wpc to 200 wpc Adcoms, Carvers, B&K's, Outlaws or whatever, the equation holds true.

While not all 100 wpc amps are created equal, in the class of amps above, I know of no 100 wpc amp that can equal a 200 wpc in current output. To do so the 100 wpc would have to have a full 3 dB of headroom, while the 200 wpc would have 0 dB. Neither exists to my knowledge.

dc,

I know of no benefit associated with a particular impedance speaker.

The nominal impedance rating of a speaker is the result of the drivers selected in its design... as is that speaker's "sound".