In a home-theater sound quality is crucial, but there is also a minimum acceptable volume level. After all, an essential part of mesmerizing sound is precise, powerful bass, and you'll struggle to get the "oomph" you need without a certain degree of power from the amplifier.

If you're disappointed with what you're getting from your current setup, don't run straight out and buy a more powerful system - there might be a way to get more out of your current AV receiver.

The human sense of hearing does not work linearly. In fact, for us to perceive one sound as being twice as loud as another, it must in fact be ten times more powerful. When you think of it this way, a powerful amplifier is far from a luxury, and actually more like a necessity.

Last but not least, large amplifiers have high power consumption, but also, therefore, an efficient power supply unit with low internal resistance. This can mean you get better sound even at low volumes, since a low internal resistance will manifest itself as good pulse response.

Modern receivers often have as many as seven amplifiers packed into a small space, producing significant amounts of heat. Manufacturers reduce the heat output by limiting the power, so that the device will meet safety standards. In fact, higher power output poses no danger whatsoever, so long as the device is properly ventilated.

This is the age-old question for the developers of AV receivers. Nevertheless, they have been limiting the output of some models drastically for years, especially the cheaper ones. The reason for this: European Community (EC) regulations state that the surface temperature of devices must, under no circumstances, become painful to the touch.

They test this - somewhat unrealistically - at maximum performance, and with the lowest terminating impedance that the manufacturer specifies. This is why many receivers specify just six or even eight ohms minimum, as these impedances allow significantly less current to flow than with a 4-ohm impedance. The direct result: less wasted power and, therefore, also less heat.

This is an attractive solution for the manufacturers, since efficient cooling systems are quite costly for today's receivers. Generally, you'll find seven highly powerful amplifiers squeezed into a very small space. Every additional watt is simply more wasted power that the device must reliably dissipate without allowing its casing to become too hot. That's no mean feat.

Receivers often specify minimum speaker impedance, but connecting lower-ohm units does no harm to either component. You do, however, get a much more powerful output. Manufacturers have largely responded to criticism of their lack of low-impedance models, for example, by providing an impedance switch that includes a four-ohm setting.

The first generation of these devices also switched the amplifier's power supply unit to a low-voltage transformer coil, reducing the power output. These devices were able to produce a similar amount of power at four ohms to what they could at eight ohms, which was largely satisfactory. As the prices of devices dropped, however, this solution became too expensive, causing the manufacturers to switch over to so-called "limiters". These limiters moderate the power output of the 4-ohm setting by reducing the strength of the input signal.

These bar graphs show what a difference the receiver's impedance setting can make. We measured the power output for 4-ohm and 8-ohm loads, each at both the receiver's highest and lowest impedance settings (right and left respectively).

The 8-ohm load produced the same output at both settings, but the 4-ohm load gave three times as much power at the high setting than it did at the low one. This is a particularly extreme example, but 1.5 or 2 times the output is fairly common.

Often, this approach simply goes way too far, producing significantly less power at four ohms than at eight. We've discussed this problem with the developers at length, and it has become clear that they are powerless - if you'll excuse the pun - to change the devices, since they would then no longer meet the EC requirements.

It was also clear that the devices would actually have no problems if you - going against the rules - use them with 4-ohm speakers, even with the impedance setting left at "8 ohms". The testers have been doing this for years without seeing a single defect. Even with devices that have no impedance setting, and that produce a terminating impedance of four ohms, disaster has never struck yet during testing, despite us pushing them pretty hard.

So far, however, we can only recommend this kind of operation with one important proviso: If you use the device in a way that the manufacturers did not intend, your guarantee may no longer be valid.

Luckily, most receiver manufacturers have now realized that it's not a problem, and many have issued statements permitting the use of their devices at 4 ohms. If the manufacturer of your receiver has indicated as such, there's no reason not to crank it up to the highest setting, even at low impedance.

Impedance is not constant for all frequencies. The standardized "nominal impedance" relates to the minimum impedance that the speaker exhibits.