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About the M1 Active Mk2
14 M1 Active Reference Manual
WHY THE TWEETERS ARE OFFSET
The M1 Active Mk2 speakers have been developed as a mirror-imaged pair
with dual front mounted ports and an offset tweeter. In some applications,
an extremely wide dispersion tweeter, such as used in the M1 Active Mk2,
will exhibit response dips if mounted exactly on center. Wave-guide style
tweeters, used in other products, purposely limit dispersion and thus can be
center mounted. But as hi-end audiophile speaker companies have found, a
high quality dome tweeter, offset in the cabinet toward the stereo “image
center” (between the left and right speakers) will have a flatter characteristic
frequency and power response. This (flat power response) more closely
characterizes the way humans hear.
On a horizontal polar response curve of the M1 Active Mk2, with the cabinet
placed vertically, the tweeter response at one meter stays essentially
symmetrical on either side of the cabinet’s centerline. This is because the
angle subtended between the cabinet-centerline-to-tweeter-centerline (.5”),
and the nominal nearfield listening distance (1 meter or 39.375”), is only 1.27
degrees. Thus, acoustically speaking, the offset tweeter design has no
negative effects, only the positive of smoothing the frequency response in the
2KHz-4KHz range.
In the tradition of the Alesis Superport design, which uses the energy from
the back of the speaker to enhance bass performance, the M1 Active Mk2s use
dual, front baffle mounted, long folded ports. The dual ports on the M1
Active Mk2 are also non-symmetrical with relation to their distances from the
woofer. In developing the M1 Active Mk2, Alesis engineers determined that
dual ports, offset to the outside of the stereo image center gave the most
coherent and extended low bass augmentation. The ports are flared on the
front baffle to provide smooth and quiet exit airflow while inside they are cut
with a 45° angle and faced toward the amplifier’s internal heat sink. This
arrangement aids in keeping the internal amps working at a more constant
temperature. At the same time, this 45° angle increases the port’s air entry
efficiency.