The relationship between perceived volume (loudness) and power output in watts of an amplifier is not a linear relationship: The human ear perceives a 50-watt amplifier as only twice as loud as a five-watt amplifier, despite a tenfold increase in power in watts. Doubling the power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is only slightly louder than a 50-watt amplifier. In addition is the human ear's tendency to behave as a natural audio compressor at high volumes. In a band, the bassist will typically need three or four times the wattage of the electric guitarist. While an electric guitarist will often find that a 50 watt amp will be adequate for rehearsals and mid-size performance venues, a bass player performing alongside them will typically need at least a 300 watt bass amp, six times the power of the electric guitar amp, to get a good bass volume. "More advanced players who regularly gig in small to medium sized venues… typically [use amps that] produce 300-700 watts of output." Some bassists believe a tube bass amp will sound louder than a solid-state bass amp of the same wattage. Impedance The power handling capabilities of a speaker cabinet or individual speaker are always given in relation to a specific impedance (a measure of electrical resistance); the most common impedance ratings in bass speaker systems are 8 ohms and 4 ohms, although some equipment is rated down to 2 ohms or even more rarely to 1 ohm. Power supply Most modern bass amps are powered solely by AC mains power. Inexpensive practice amps may have the AC mains plug hardwired into the unit. Middle-priced to high-priced amplifiers typically have a removable cable and plug, allowing simplified replacement. Most amps are designed to work for a single voltage. A small number of expensive bass amps designed for touring professionals have user-selectable voltage, which enables a bassist to use same amp in both North America and across Europe. A small number of small combo amps can run on both AC mains power and battery power. This enables bassists to play outside where there is no access to power (e.g., for busking on the street). Amps that are battery powered may have a 12-volt input, so that it can be connected to a car battery with alligator clips. Loudspeakers The lowest note on the double bass or