<s>
Loudspeaker	B-Application
measurement	I-Application
is	O
the	O
practice	O
of	O
determining	O
the	O
behaviour	O
of	O
loudspeakers	O
by	O
measuring	O
various	O
aspects	O
of	O
performance	O
.	O
</s>
<s>
This	O
measurement	O
is	O
especially	O
important	O
because	O
loudspeakers	O
,	O
being	O
transducers	B-Algorithm
,	O
have	O
a	O
higher	O
level	O
of	O
distortion	O
than	O
other	O
audio	O
system	O
components	O
used	O
in	O
playback	O
or	O
sound	O
reinforcement	O
.	O
</s>
<s>
The	O
measuring	O
microphone	B-Application
is	O
normally	O
mounted	O
on	O
an	O
unobtrusive	O
boom	O
(	O
to	O
avoid	O
reflections	O
)	O
and	O
positioned	O
1	O
metre	O
in	O
front	O
of	O
the	O
drive	O
units	O
on	O
the	O
axis	O
with	O
the	O
high-frequency	O
driver	O
.	O
</s>
<s>
This	O
compact	O
measurement	O
system	O
for	O
loudspeaker	B-Device
drivers	I-Device
is	O
defined	O
in	O
IEC	O
60268-21:2018	O
,	O
IEC	O
60268-22:2020	O
and	O
AES73id-2019	O
.	O
</s>
<s>
Digging	O
a	O
hole	O
and	O
burying	O
the	O
speaker	O
flush	O
with	O
the	O
ground	O
allows	O
far	O
more	O
accurate	O
half-space	O
measurement	O
,	O
creating	O
the	O
loudspeaker	O
equivalent	O
of	O
the	O
boundary	B-Algorithm
effect	I-Algorithm
microphone	I-Algorithm
(	O
all	O
reflections	O
precisely	O
in-phase	O
)	O
but	O
any	O
rear	O
port	O
must	O
remain	O
unblocked	O
,	O
and	O
any	O
rear-mounted	O
amplifier	O
must	O
be	O
allowed	O
cooling	O
air	O
.	O
</s>
<s>
At	O
low	O
frequencies	O
,	O
most	O
rooms	O
have	O
resonances	B-Application
at	O
a	O
series	O
of	O
frequencies	O
where	O
a	O
room	O
dimension	O
corresponds	O
to	O
a	O
multiple	O
of	O
half	O
wavelengths	O
.	O
</s>
<s>
Sound	O
travels	O
at	O
about	O
,	O
so	O
a	O
room	O
long	O
will	O
have	O
resonances	B-Application
from	O
27.5Hz	O
upwards	O
.	O
</s>
<s>
All	O
multi-driver	O
speakers	O
(	O
unless	O
they	O
are	O
coaxial	B-Device
)	O
are	O
difficult	O
to	O
measure	O
correctly	O
if	O
the	O
measuring	O
microphone	B-Application
is	O
placed	O
close	O
to	O
the	O
loudspeaker	O
and	O
slightly	O
above	O
or	O
below	O
the	O
optimum	O
axis	O
because	O
the	O
different	O
path	O
length	O
from	O
two	O
drivers	O
producing	O
the	O
same	O
frequency	O
leads	O
to	O
phase	O
cancellation	O
.	O
</s>
<s>
Published	O
results	O
are	O
often	O
only	O
valid	O
for	O
very	O
precise	O
positioning	O
of	O
the	O
microphone	B-Application
to	O
within	O
a	O
centimetre	O
or	O
two	O
.	O
</s>
<s>
This	O
means	O
the	O
microphone	B-Application
must	O
be	O
positioned	O
precisely	O
equidistant	O
from	O
the	O
two	O
speakers	O
if	O
'	O
comb-filter	O
'	O
effects	O
(	O
alternate	O
peaks	O
and	O
dips	O
in	O
the	O
measured	O
room	O
response	O
at	O
that	O
point	O
)	O
are	O
to	O
be	O
avoided	O
.	O
</s>
<s>
Distortion	O
measurements	O
on	O
loudspeakers	O
can	O
only	O
go	O
as	O
low	O
as	O
the	O
distortion	O
of	O
the	O
measurement	O
microphone	B-Application
itself	O
of	O
course	O
,	O
at	O
the	O
level	O
tested	O
.	O
</s>
<s>
The	O
microphone	B-Application
should	O
ideally	O
have	O
a	O
clipping	O
level	O
of	O
120	O
to	O
140	O
dB	O
SPL	O
if	O
high-level	O
distortion	O
is	O
to	O
be	O
measured	O
.	O
</s>
<s>
All	O
forms	O
of	O
resonance	B-Application
cause	O
this	O
,	O
by	O
storing	O
energy	O
,	O
and	O
resonances	B-Application
with	O
high	O
Q	O
factor	O
are	O
especially	O
audible	O
.	O
</s>
