<s>
The	O
midwoofer-tweeter-midwoofer	B-Device
loudspeaker	O
configuration	O
(	O
called	O
MTM	O
,	O
for	O
short	O
)	O
was	O
a	O
design	O
arrangement	O
from	O
the	O
late	O
1960s	O
that	O
suffered	O
from	O
serious	O
lobing	O
issues	O
that	O
prevented	O
its	O
popularity	O
until	O
it	O
was	O
perfected	O
by	O
Joseph	O
D'Appolito	O
as	O
a	O
way	O
of	O
correcting	O
the	O
inherent	O
lobe	B-Device
tilting	O
of	O
a	O
typical	O
mid-tweeter	O
(	O
MT	O
)	O
configuration	O
,	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
,	O
unless	O
time-aligned	B-Device
.	O
</s>
<s>
In	O
the	O
MTM	O
arrangement	O
the	O
loudspeaker	O
uses	O
three	O
drivers	B-Device
:	O
Two	O
mid-range	B-Device
(	O
or	O
mid-woofer/woofer	O
)	O
for	O
the	O
low	O
frequencies	O
and	O
a	O
tweeter	B-Device
for	O
the	O
higher	O
frequencies	O
,	O
with	O
the	O
tweeter	B-Device
being	O
placed	O
between	O
the	O
mid-range	B-Device
drivers	I-Device
(	O
as	O
shown	O
in	O
the	O
second	O
image	O
below	O
)	O
.	O
</s>
<s>
D'Appolito	O
initially	O
configured	O
his	O
design	O
using	O
a	O
3rd	O
order	O
(	O
18	O
dB/oct	O
or	O
60	O
dB/dec	O
)	O
crossover	B-Algorithm
,	O
D'Appolito	O
has	O
since	O
amended	O
this	O
original	O
recommendation	O
in	O
favor	O
of	O
4th	O
order	O
topology	O
.	O
</s>
<s>
There	O
are	O
speakers	O
where	O
the	O
tweeter	B-Device
appears	O
below	O
the	O
larger	O
driver	O
and	O
though	O
electrically	O
it	O
is	O
identical	O
to	O
the	O
MT	O
configuration	O
,	O
it	O
is	O
customarily	O
denoted	O
as	O
"	O
TM	O
"	O
.	O
</s>
<s>
The	O
only	O
other	O
difference	O
is	O
the	O
lobe	B-Device
tilt	O
which	O
is	O
exactly	O
the	O
opposite	O
of	O
the	O
MT	O
configuration	O
.	O
</s>
<s>
The	O
direction	O
in	O
which	O
the	O
lobe	B-Device
tilts	O
(	O
i.e.	O
,	O
the	O
vertical	O
orientation	O
or	O
angle	O
of	O
the	O
lobe	B-Device
)	O
is	O
a	O
function	O
of	O
the	O
difference	O
or	O
offset	O
between	O
the	O
acoustic	O
centers	O
of	O
the	O
two	O
drivers	B-Device
.	O
</s>
<s>
Since	O
it	O
is	O
rare	O
for	O
a	O
tweeter	B-Device
and	O
mid-range	B-Device
(	O
or	O
woofer	B-Device
)	O
to	O
have	O
their	O
diaphragms	O
or	O
acoustic	O
centers	O
in	O
the	O
same	O
physical	O
plane	O
,	O
it	O
follows	O
that	O
sound	O
waves	O
emitted	O
by	O
them	O
(	O
of	O
the	O
same	O
frequency	O
)	O
will	O
not	O
reach	O
a	O
particular	O
listening	O
position	O
in	O
the	O
on-axis	O
plane	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
overall	O
effect	O
is	O
that	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
(	O
when	O
both	O
drivers	B-Device
are	O
reproducing	O
the	O
same	O
frequency	O
)	O
the	O
sound	O
from	O
both	O
drivers	B-Device
does	O
not	O
acoustically	O
sum	O
ideally	O
at	O
the	O
on-axis	O
listening	O
position	O
.	O
</s>
<s>
Thus	O
,	O
with	O
a	O
typical	O
TM	O
or	O
MT	O
loudspeaker	O
where	O
the	O
drivers	B-Device
are	O
not	O
time-aligned	B-Device
,	O
the	O
main	O
lobe	B-Device
is	O
tilted	O
away	O
from	O
the	O
horizontal	O
.	O
</s>
<s>
As	O
can	O
be	O
seen	O
,	O
the	O
lobe	B-Device
tilts	O
downwards	O
towards	O
P	O
 '	O
which	O
is	O
not	O
the	O
same	O
as	O
the	O
on-axis	O
listening	O
position	O
P	O
.	O
</s>
<s>
This	O
is	O
one	O
of	O
the	O
reasons	O
for	O
physically	O
offsetting	O
the	O
mid-range	B-Device
and	O
tweeter	B-Device
such	O
that	O
the	O
tweeter	B-Device
is	O
physically	O
behind	O
the	O
mid-range	B-Device
,	O
or	O
tilting	O
the	O
speaker	O
upwards	O
,	O
to	O
achieve	O
an	O
on-axis	O
response	O
that	O
is	O
coincident	O
with	O
the	O
physical	O
on-axis	O
plane	O
.	O
</s>
<s>
This	O
process	O
is	O
known	O
as	O
time-alignment	B-Device
.	O
</s>
<s>
Other	O
ways	O
to	O
time-align	O
are	O
to	O
introduce	O
a	O
phase	O
shift	O
in	O
the	O
tweeter	B-Device
signal	O
(	O
lag	O
or	O
lead	O
,	O
depending	O
upon	O
the	O
offset	O
between	O
the	O
drivers	B-Device
)	O
so	O
that	O
it	O
mimics	O
the	O
physical	O
offset	O
.	O
</s>
<s>
When	O
another	O
mid-range	B-Device
or	O
midwoofer	O
is	O
added	O
vertically	O
and	O
symmetrically	O
opposite	O
to	O
the	O
existing	O
one	O
,	O
the	O
result	O
is	O
that	O
the	O
tilted	O
on-axis	O
plane	O
gets	O
corrected	O
such	O
that	O
the	O
differences	O
between	O
the	O
midwoofer	O
and	O
tweeter	B-Device
planes	O
become	O
immaterial	O
-	O
the	O
on-axis	O
plane	O
is	O
always	O
w.r.t.	O
</s>
<s>
the	O
tweeter	B-Device
's	O
center	O
.	O
</s>
<s>
This	O
of	O
course	O
requires	O
that	O
the	O
two	O
midwoofers	O
are	O
in	O
exactly	O
the	O
same	O
plane	O
,	O
and	O
exactly	O
the	O
same	O
distance	O
from	O
the	O
tweeter	B-Device
-	O
which	O
is	O
much	O
easier	O
to	O
do	O
physically	O
.	O
</s>
<s>
There	O
is	O
however	O
a	O
price	O
to	O
pay	O
for	O
this	O
-	O
although	O
the	O
MTM	O
's	O
on-axis	O
response	O
is	O
near	O
perfect	O
,	O
its	O
radiation	O
pattern	O
or	O
main	O
lobe	B-Device
is	O
much	O
narrower	O
than	O
that	O
of	O
the	O
MT	O
or	O
TM	O
configuration	O
's	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
off-axis	O
response	O
(	O
i.e.	O
,	O
response	O
at	O
locations	O
vertically	O
away	O
from	O
exactly	O
opposite	O
the	O
tweeter	B-Device
)	O
is	O
much	O
weaker	O
and	O
there	O
can	O
be	O
obvious	O
and	O
discernible	O
change	O
in	O
tonality	O
at	O
the	O
crossover	B-Algorithm
as	O
one	O
's	O
listening	O
height	O
relative	O
to	O
the	O
tweeter	B-Device
changes	O
,	O
and	O
the	O
effect	O
is	O
perceived	O
more	O
as	O
the	O
listener	O
moves	O
closer	O
to	O
the	O
loudspeaker	O
.	O
</s>
<s>
This	O
is	O
corrected	O
by	O
(	O
1	O
)	O
mounting	O
the	O
midwoofers	O
as	O
close	O
to	O
the	O
tweeters	B-Device
as	O
possible	O
(	O
so	O
that	O
the	O
listening	O
distance	O
is	O
much	O
larger	O
than	O
the	O
distance	O
between	O
the	O
drivers	B-Device
)	O
and	O
(	O
2	O
)	O
operating	O
the	O
midwoofers	O
and	O
tweeter	B-Device
in	O
quadrature	O
,	O
i.e.	O
,	O
achieved	O
by	O
ensuring	O
that	O
the	O
tweeter	B-Device
lags	O
the	O
midwoofer	O
by	O
90°	O
in	O
phase	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
,	O
and	O
this	O
in	O
turn	O
can	O
be	O
achieved	O
if	O
the	O
crossover	B-Algorithm
has	O
a	O
3rd	O
order	O
Butterworth	O
characteristic	O
.	O
</s>
<s>
To	O
achieve	O
the	O
smallest	O
distance	O
between	O
the	O
mids	O
and	O
tweeters	B-Device
requires	O
the	O
drivers	B-Device
to	O
have	O
the	O
smallest	O
possible	O
size	O
-	O
but	O
there	O
are	O
design	O
limitations	O
to	O
this	O
(	O
lowest	O
mid-range	B-Device
frequency	O
,	O
for	O
example	O
,	O
will	O
place	O
a	O
lower	O
limit	O
to	O
the	O
mid	O
's	O
diameter	O
)	O
.	O
</s>
<s>
At	O
any	O
off-axis	O
listening	O
position	O
,	O
although	O
both	O
midwoofers	O
operate	O
in	O
phase	O
(	O
are	O
time-aligned	B-Device
)	O
,	O
the	O
waves	O
from	O
each	O
reach	O
the	O
listening	O
position	O
at	O
different	O
times	O
(	O
and	O
therefore	O
have	O
a	O
relative	O
phase	O
difference	O
)	O
-	O
at	O
frequencies	O
where	O
the	O
time	O
displacement	O
between	O
the	O
two	O
midwoofers	O
corresponds	O
to	O
one-half	O
of	O
one	O
wavelength	O
,	O
the	O
outputs	O
of	O
the	O
two	O
midwoofers	O
will	O
null	O
.	O
</s>
<s>
Since	O
the	O
basic	O
reason	O
for	O
this	O
is	O
the	O
distance	O
between	O
the	O
midwoofers	O
,	O
again	O
the	O
solution	O
is	O
to	O
use	O
as	O
small	O
drivers	B-Device
as	O
possible	O
,	O
placed	O
as	O
close	O
as	O
possible	O
.	O
</s>
<s>
With	O
this	O
,	O
for	O
listening	O
distances	O
much	O
greater	O
than	O
the	O
distance	O
between	O
the	O
drivers	B-Device
the	O
lobing	O
effects	O
are	O
much	O
less	O
obvious	O
.	O
</s>
