Responses to questions from readers of the Speaker & Impedance articles
Q: "I was told that due to the frequency responses of speakers, that an underpowered speaker could blow out because of the lack of appropriate watts. An example being that a car speaker with a 50 watt RMS being powered by a stereo amplifier only capable of 8 watts. I was told that because the stereo would not meet the minimum nominal output required to fully power the speaker only certain frequencies would respond based on "the path of least resistance" causing the speaker to tear itself apart."
A: If an amplifier is "underpowered," it is quite common for the user to overdrive the amp (turn the volume too high) trying to get more volume from it. When this happens, the output signal is severely distorted at the limits of the power supply. (On an oscilloscope, the waveform tops are flattened, as if they had been 'clipped off'.) Clipping distortion results in a significant amount of high frequency energy. In some cases this high frequency energy can be great enough to damage a tweeter or horn in a speaker system, as they are often selected with a much lower rated power than the woofer in the cabinet, especially in consumer stereo equipment. I have actually seen tweeters that caught fire due to being run at clipping in an underpowered system. However, it is highly unlikely that a woofer rated at 200 watts will be blown by an amplifier rated at less than 100 watts unless the woofer itself is defective. JBL has a technote that describes this situation in more detail. The solution is to be sure you select an amplifier and matching speakers with enough power capacity to meet your volume requirement.
There can be, though, an issue of tonal quality, especially at very low power levels. Speakers designed to handle several hundred watts may not be very efficient at very low power levels (less than about 3-5 watts), and since the mass of the voice coil, spider and cone is greater in order to handle the maximum power, they may not respond well at these low power levels due to inertia. This may be the source of the claim that speakers need a certain amount of power or they won't work right. No one seems to know how much power is needed by these speakers, and the estimates vary widely. I suspect the 'underpower' situation described above may also add to the confusion. In my estimation, once you have increased the volume to the point where the power level from the amp becomes more than about 1-2 percent of the maximum rated speaker power, you should be within the normal operating range of the speaker. At these power levels, though, there is absolutely no danger of the amplifier damaging the speaker.
Q: "Would it be okay to hook up a 450 Watt amp (4 ohms) to a 2x10 cabinet that is rated at about 400watts? Or can I safely hook up that same 4 ohm amp to a 4x10 4 ohm cabinet along with that 2x10 cabinet, which is 8 ohms?"
A: If the amp is rated at 450 watts RMS and the speaker is rated at 400 watts (RMS? Program?) you run the risk of blowing the speaker, especially using a bass guitar. Keep in mind that the amp can put out as much as 900 watts RMS at full distorted blast, and if the speakers are rated 400 watts program, they may only be able to handle 200 watts RMS. (900 watts max feeding a 200 watt system.) Under those conditions, keep a fire extinguisher handy. And save your dough for a new cabinet.
If the amp says 'Minimum impedance 4 ohms', then connecting both a 4 ohm and an 8 ohm cabinet to the amp would present a load of 2.7 ohms to the amplifier and risk overheating and damaging the amplifier.
If the amplifier said "Minimum load 2 ohms", then you could safely connect both cabinets (at least as far as impedance is concerned). The 4 ohm cabinet will draw 2/3 of the amp's output, and the 8 ohm cabinet will draw 1/3. Use the formulas on the web page to see what the currents and power would be, and make sure the cabinet's RMS power rating is greater than that.