Alesis M1 Active Mk2 Reference Manual page 26

Speaker Installation
P
LUGGING THE PORTS TO ADJUST FOR ROOM ACOUSTICS
The low frequency response of the M1 Active Mk2s can be custom tailored by
plugging the ports. If the speakers must be placed less than one foot from a
front wall, or if they are too close to a corner, then the bass may become
exaggerated and inaccurate.
follows:
a) less than 1 foot from a front wall but away from any corner:
b) less than 1 foot from a front wall and a corner:
The "plug" can be wad of cloth to completely block the port(s) or a more
porous material like fiberfill which will give less bass attenuation. The bass
output of the M1 Active Mk2 is thus easily variable depending on in-room
positioning and/or personal preference.
If the M1 Active Mk2s are to be used in a system with a subwoofer, the
proper blend of the two systems is critical if low-bass accuracy is to be
maintained. Here again, your ear is your most important piece of test
equipment. If the subwoofer (such as the Alesis Point One) or preamplifier
has 80Hz high-pass filters built-in for connection to the other amplifiers, then
no port plugging is necessary. The proper match will have already been
taken care of electronically. In the absence of an 80Hz high-pass filter feeding
the M1 Active Mk2s, you'll need to experiment with various degrees of port
plugging to minimize the cancellations and nodes that can occur from bass
sources placed several feet apart in a room. In practice we've found that a
single port plugged with cloth is usually the best compromise. See Figure 5
below. It shows the approximate bass attenuation that can be expected with
one port plugged.
24
plug one port
plug both ports
The rules of thumb for these situations are as
M1 Active Reference Manual