<s>
Loudspeaker	O
time-alignment	B-Device
usually	O
simply	O
referred	O
to	O
as	O
"	O
time-alignment	B-Device
"	O
or	O
"	O
Time-Align	O
"	O
is	O
a	O
term	O
applied	O
in	O
loudspeaker	O
systems	O
which	O
use	O
multiple	O
drivers	B-Device
(	O
like	O
woofer	B-Device
,	O
mid-range	B-Device
and	O
tweeter	B-Device
)	O
to	O
cover	O
a	O
wide	O
audio	O
range	O
.	O
</s>
<s>
It	O
involves	O
delaying	O
the	O
sound	O
emanating	O
from	O
one	O
or	O
more	O
drivers	B-Device
(	O
greater	O
than	O
2-way	O
)	O
to	O
correct	O
the	O
transient	O
response	O
,	O
improve	O
accuracy	O
and	O
,	O
in	O
non-coaxial	O
drivers	B-Device
,	O
improve	O
the	O
directivity	O
or	O
lobe	O
tilting	O
at	O
the	O
crossover	B-Algorithm
frequencies	I-Algorithm
.	O
</s>
<s>
It	O
employs	O
adjusting	O
the	O
front-to	O
back	O
spacing	O
of	O
the	O
individual	O
drivers	B-Device
so	O
that	O
the	O
sound	O
output	O
is	O
truly	O
simultaneous	O
.	O
</s>
<s>
In	O
1976	O
Long	O
presented	O
"	O
A	O
Time-Align	O
Technique	O
for	O
Loudspeakers	O
System	O
Design	O
"	O
at	O
the	O
54th	O
AES	O
convention	O
demonstrating	O
the	O
use	O
of	O
the	O
Time-Align	O
generator	O
to	O
design	O
improved	O
crossover	B-Algorithm
networks	O
for	O
multi-way	O
loudspeakers	O
systems	O
.	O
</s>
<s>
This	O
technique	O
relied	O
on	O
subjective	O
evaluation	O
of	O
various	O
square	O
pulses	O
swept	O
through	O
the	O
crossover	B-Algorithm
frequencies	I-Algorithm
.	O
</s>
<s>
In	O
1977	O
Long	O
trademarked	O
Time-Align	O
and	O
later	O
trademarked	O
its	O
derivatives	O
,	O
Time-Aligned	O
and	O
Time-Alignment	B-Device
.	O
</s>
<s>
Therefore	O
,	O
most	O
hi-fi	O
loudspeakers	O
employ	O
multiple	O
drivers	B-Device
to	O
cover	O
the	O
audio	O
spectrum	O
satisfactorily	O
.	O
</s>
<s>
At	O
the	O
very	O
least	O
,	O
such	O
a	O
loudspeaker	O
may	O
be	O
2-way	O
employing	O
a	O
woofer	B-Device
(	O
or	O
mid-range/mid	B-Device
-woofer	I-Device
)	O
and	O
a	O
tweeter	B-Device
.	O
</s>
<s>
For	O
the	O
sake	O
of	O
this	O
article	O
and	O
simplicity	O
,	O
a	O
2-way	O
speaker	O
system	O
will	O
be	O
assumed	O
-	O
consisting	O
of	O
a	O
woofer	B-Device
and	O
a	O
tweeter	B-Device
.	O
</s>
<s>
Since	O
the	O
woofer	B-Device
covers	O
the	O
lower-end	O
of	O
the	O
audio	O
spectrum	O
and	O
the	O
tweeter	B-Device
covers	O
the	O
upper-end	O
,	O
the	O
dividing	O
point	O
between	O
the	O
two	O
being	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
,	O
it	O
is	O
of	O
utmost	O
importance	O
that	O
,	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
the	O
outputs	O
of	O
both	O
drivers	B-Device
should	O
acoustically	O
sum	O
so	O
as	O
to	O
be	O
seamless	O
,	O
without	O
any	O
peaks	O
or	O
dips	O
,	O
otherwise	O
the	O
loudspeaker	O
is	O
said	O
to	O
colour	O
the	O
sound	O
.	O
</s>
<s>
A	O
typical	O
characteristic	O
of	O
a	O
2-way	O
speaker	O
is	O
that	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
,	O
due	O
to	O
the	O
physical	O
distance	O
between	O
the	O
centres	O
of	O
the	O
woofer	B-Device
and	O
tweeter	B-Device
,	O
the	O
sound	O
that	O
emanates	O
from	O
the	O
combination	O
is	O
not	O
omni-directional	O
,	O
but	O
lobed	B-Device
.	O
</s>
<s>
Within	O
the	O
region	O
of	O
the	O
lobe	O
the	O
sound	O
level	O
,	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
is	O
much	O
higher	O
as	O
compared	O
to	O
outside	O
the	O
lobe	O
.	O
</s>
<s>
Therefore	O
loudspeaker	O
designers	O
try	O
to	O
make	O
the	O
main	O
lobe	O
as	O
fat	O
as	O
possible	O
,	O
by	O
using	O
drivers	B-Device
that	O
are	O
as	O
small	O
as	O
possible	O
in	O
diameter	O
so	O
as	O
to	O
allow	O
closer	O
spacing	O
between	O
them	O
.	O
</s>
<s>
However	O
,	O
the	O
lowest	O
frequency	O
of	O
interest	O
(	O
bass	O
)	O
puts	O
a	O
lower	O
limit	O
to	O
the	O
woofer	B-Device
's	O
diameter	O
.	O
</s>
<s>
A	O
typical	O
2-way	O
speaker	O
uses	O
a	O
woofer	B-Device
and	O
a	O
tweeter	B-Device
as	O
mentioned	O
earlier	O
.	O
</s>
<s>
Usually	O
the	O
tweeter	B-Device
is	O
much	O
smaller	O
and	O
thinner	O
than	O
a	O
woofer	B-Device
.	O
</s>
<s>
What	O
this	O
means	O
is	O
that	O
the	O
radiating	O
surfaces	O
(	O
at	O
the	O
voice-coil	O
or	O
the	O
dome/dust	O
-cap	O
depending	O
on	O
design	O
,	O
also	O
known	O
as	O
the	O
"	O
acoustic	O
centre	O
"	O
)	O
of	O
the	O
drivers	B-Device
are	O
not	O
in	O
the	O
same	O
plane	O
-	O
the	O
tweeter	B-Device
's	O
radiating	O
surface	O
is	O
usually	O
much	O
forward	O
of	O
the	O
woofer	B-Device
when	O
both	O
are	O
mounted	O
to	O
the	O
same	O
flat	O
panel	O
.	O
</s>
<s>
Even	O
though	O
this	O
physical	O
offset	O
may	O
be	O
of	O
the	O
order	O
of	O
20-40mm	O
,	O
at	O
typical	O
crossover	B-Algorithm
frequencies	I-Algorithm
( ≥	O
1kHz	O
)	O
,	O
this	O
offset	O
is	O
enough	O
to	O
cause	O
tilting	O
of	O
the	O
main	O
lobe	O
.	O
</s>
<s>
The	O
woofer	B-Device
is	O
the	O
larger	O
of	O
the	O
two	O
,	O
located	O
below	O
the	O
tweeter	B-Device
.	O
</s>
<s>
As	O
can	O
be	O
seen	O
the	O
tweeter	B-Device
is	O
much	O
thinner	O
and	O
its	O
acoustic	O
centre	O
is	O
ahead	O
of	O
the	O
woofer	B-Device
's	O
acoustic	O
centre	O
.	O
</s>
<s>
Due	O
to	O
this	O
,	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
since	O
both	O
drivers	B-Device
are	O
producing	O
the	O
same	O
frequency	O
,	O
the	O
sound	O
waves	O
from	O
the	O
tweeter	B-Device
arrive	O
at	O
the	O
listening	O
position	O
P	O
earlier	O
than	O
those	O
from	O
the	O
woofer	B-Device
.	O
</s>
<s>
For	O
most	O
purposes	O
the	O
tilted	O
lobe	O
poses	O
no	O
problems	O
and	O
indeed	O
many	O
speaker	O
systems	O
do	O
not	O
use	O
time	B-Device
alignment	I-Device
.	O
</s>
<s>
However	O
,	O
there	O
is	O
a	O
type	O
of	O
crossover	B-Algorithm
called	O
the	O
LR4	O
or	O
LR2	O
crossover	B-Algorithm
,	O
which	O
has	O
certain	O
unique	O
features	O
that	O
make	O
time-alignment	B-Device
worthwhile	O
for	O
speakers	O
that	O
use	O
it	O
.	O
</s>
<s>
This	O
particular	O
crossover	B-Algorithm
has	O
the	O
property	O
that	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
the	O
electrical	O
summing	O
is	O
flat	O
(	O
i.e.	O
,	O
there	O
is	O
no	O
peak	O
or	O
dip	O
)	O
and	O
the	O
signals	O
being	O
sent	O
to	O
the	O
woofer	B-Device
and	O
tweeter	B-Device
are	O
always	O
in	O
phase	O
(	O
180°	O
out	O
of	O
phase	O
in	O
the	O
LR2	O
case	O
,	O
which	O
is	O
corrected	O
by	O
simply	O
inverting	O
the	O
tweeter	B-Device
's	O
signal	O
)	O
.	O
</s>
<s>
This	O
makes	O
the	O
LR2	O
or	O
LR4	O
crossovers	B-Algorithm
ideal	O
for	O
audio	O
as	O
compared	O
to	O
the	O
Butterworth	O
type	O
.	O
</s>
<s>
Even	O
without	O
a	O
LR	O
crossover	B-Algorithm
,	O
it	O
is	O
worthwhile	O
to	O
have	O
the	O
main	O
lobe	O
point	O
forward	O
so	O
that	O
the	O
speakers	O
will	O
illuminate	O
the	O
listening	O
position	O
evenly	O
,	O
resulting	O
in	O
better	O
overall	O
system	O
performance	O
(	O
such	O
as	O
imaging	O
or	O
audibility	O
)	O
.	O
</s>
<s>
In	O
this	O
technique	O
(	O
usually	O
)	O
the	O
tweeter	B-Device
's	O
signal	O
is	O
phase-shifted	O
(	O
usually	O
a	O
delay	O
since	O
tweeter	B-Device
is	O
ahead	O
of	O
the	O
woofer	B-Device
)	O
.	O
</s>
<s>
This	O
phase	O
shift	O
introduces	O
an	O
equivalent	O
time	O
shift	O
in	O
the	O
tweeter	B-Device
's	O
sound	O
wave	O
such	O
that	O
it	O
corrects	O
the	O
time	O
difference	O
between	O
the	O
woofer	B-Device
and	O
tweeter	B-Device
due	O
to	O
the	O
physical	O
offset	O
.	O
</s>
<s>
This	O
method	O
is	O
also	O
much	O
easier	O
and	O
convenient	O
over	O
physically	O
time-aligning	O
the	O
drivers	B-Device
.	O
</s>
<s>
In	O
this	O
technique	O
the	O
drivers	B-Device
are	O
physically	O
offset	O
such	O
that	O
their	O
acoustic	O
centres	O
lie	O
in	O
the	O
same	O
physical	O
plane	O
.	O
</s>
<s>
This	O
technique	O
is	O
used	O
when	O
no	O
other	O
means	O
of	O
time	B-Device
alignment	I-Device
are	O
available	O
or	O
meant	O
to	O
be	O
used	O
.	O
</s>
<s>
It	O
simplifies	O
setup	O
for	O
the	O
end	O
user	O
in	O
that	O
they	O
do	O
not	O
need	O
any	O
special	O
electronics	O
to	O
align	O
the	O
drivers	B-Device
.	O
</s>
<s>
However	O
,	O
this	O
technique	O
requires	O
that	O
the	O
exact	O
depths	O
of	O
the	O
acoustic	O
centres	O
be	O
known	O
at	O
design	O
time	O
,	O
so	O
that	O
the	O
physical	O
offset	O
may	O
be	O
introduced	O
in	O
the	O
front	O
panel	O
of	O
the	O
speaker	O
where	O
the	O
drivers	B-Device
mount	O
.	O
</s>
<s>
A	O
common	O
way	O
to	O
do	O
this	O
is	O
so	O
that	O
the	O
front-panel	O
has	O
a	O
"	O
step	O
"	O
(	O
as	O
shown	O
in	O
the	O
above	O
image	O
)	O
where	O
the	O
tweeter	B-Device
mounts	O
at	O
some	O
distance	O
behind	O
the	O
woofer	B-Device
.	O
</s>
<s>
This	O
step	O
can	O
cause	O
more	O
errors	O
in	O
summing	O
than	O
the	O
time	O
delay	O
between	O
the	O
drivers	B-Device
due	O
to	O
the	O
diffraction	O
of	O
the	O
tweeter	B-Device
's	O
sound	O
waves	O
around	O
the	O
step	O
.	O
</s>
<s>
Also	O
,	O
the	O
more	O
gradual	O
the	O
slope	O
,	O
greater	O
is	O
the	O
vertical	O
separation	O
between	O
the	O
drivers	B-Device
,	O
which	O
in	O
turn	O
again	O
causes	O
thinning	O
of	O
the	O
lobe	O
(	O
i.e.	O
,	O
increase	O
in	O
vertical	O
directivity	O
)	O
at	O
the	O
crossover	B-Algorithm
frequency	I-Algorithm
.	O
</s>
<s>
Another	O
way	O
to	O
introduce	O
physical	O
time-alignment	B-Device
without	O
having	O
to	O
physically	O
shift	O
the	O
tweeter	B-Device
backwards	O
is	O
to	O
tilt	O
the	O
speaker	O
itself	O
upwards	O
(	O
or	O
have	O
the	O
front-panel	O
sloping	O
instead	O
of	O
vertical	O
)	O
.	O
</s>
