exceeds ~1/4 wavelength. Power handling capability is also doubled when the number of drivers doubles, for a maximum realizable increase of ~6 dB in total acoustic output per doubling of mutually coupled drivers when the total amplifier power is also doubled. Mutual coupling efficiency gains become difficult to realize with multiple drivers at higher frequencies because the total size of a single driver including its diaphragm, basket, waveguide or horn may already exceed one wavelength. Sources that are much smaller than a wavelength behave like point sources that radiate omnidirectionally in free space, whereas sources larger than a wavelength act as their own 'ground plane' and beam the sound forward. This beaming tends to make high frequency dispersion problematic in larger venues, so a designer may have to cover the listening area with multiple sources aimed in various directions or placed in various locations. Likewise, speaker proximity much less than 1/4 wavelength to one or more boundaries such as floor/walls/ceiling can increase the effective sensitivity by changing free space into half space, quarter space, or eighth space. When the distance to boundaries is > 1/4 wavelength, delayed reflections can increase the perceived loudness but can also induce ambient effects such as comb filtering and reverberation that can make the frequency response uneven across a venue or make the sound diffuse and harsh, especially with smaller venues and hard reflective surfaces. Sound absorbing structures, sound diffusing structures, and digital signal processing may be employed to compensate for boundary effects within the designated listening area. Matching amplifier to loudspeaker Charles "Chuck" McGregor, while serving as senior technologist for Eastern Acoustic Works, wrote a guideline for professional audio purchasers wishing to select properly sized amplifiers for their loudspeakers. Chuck McGregor recommended a rule of thumb in which the amplifier's maximum power output rating was twice the loudspeaker's continuous (so-called "RMS") rating, give or take 20%. In his example, a loudspeaker with a continuous power rating of 250 watts would be well-matched by an amplifier with a maximum power output within the range of 400 to 625 watts. JBL, which tests and labels their loudspeakers according to the IEC 268-5 standard (called IEC 60268-5 more recently) has a more nuanced set of recommendations, depending on the usage profile of the system, which more fundamentally involves the (worst case) crest factor of the signal used to drive the loudspeakers: For "carefully monitored applications where peak