dbx 4BX - About Impact Restoration, Expansion, and Dynamic Range

ABOUT
IMPACT
RESTORATION,
EXPANSION,
AND
DYNAMIC
RANGE
Music
works
largely
through
our
anticipation
of it:
the
pleasure
of
knowing,
but
not knowing
precisely,
what's
going
to happen
next.
Whether
the
form
is
fugue
or blues,
a rock
refrain
or
Haydn's
Smjmse
Symphony,
the
lasting
fun
comes
from
our
expectations
being
satisfied
-
but
not
too
predictably
so. In
a
good
piece
of
music,
over
time,
the startling
becomes
familiar,
even
as the
famil-
iar
stays
startling.
This
effect,
this
freshness,
frequently
is
enhanced
by
the attack
of a note
or
chord.
Depending
on the
percussiveness
of the
instrument
and/or
on the
loud-
ness
of
the
attack
compared
with what
came
just
before,
these
initial
transients
have
a
considerable
impact.
It's
not just
that
Haydn
put
surprises
in
his famous
symphony's
slow
movement,
it's
the nature
of
their
sound,
an
orchestral
sfor-
zando
that
comes
delicately
out
of nowhere.
This
is
what
the
4BX's
unique
impact-restoration
circuit
aims
to do:
to
increase
the
snap,
the
immediacy
of these
musical
attacks
that
enhance
our
anticipating.
Many
of
these
transients
get clipped
off
or
otherwise
dulled
and
muted
anyway
during
the
recording
or
broadcasting
process,
because
of the
compressors
and
limiters
that
are
customarily
employed.
But
even if
the
peaks
aren't
blunted
in
the
processing,
virtually
all
reproduced
music
sounds
as
though
at least
some of
its
excitement
is
missing.
Conventional
dynamic-range
expansion,
as
we
shall
see,
recovers
much
of this,
yet
cannot
restore
the
excitement
altogether.
So
with
impact
restoration
we
have
tried
to bring
back
some
of the
heart-stopping,
breathtaking
quality
of live
performance.
After
you have
experimented
with
dif-
ferent
amounts
of
impact
restoration
for
various
kinds
of music,
we
think
you
will
agree.
As for
dynamic
range
itself,
it
may
be simply
defined
as
the
difference
in
level
between
the
loudest
and
the
softest
sounds
in any
given
musical
performance
or
recording.
In
a
performance,
the
loudest
sounds
are
limited
only
by the
musician's
ability
to strike,
bow,
or
blow
into
the
instrument,
by its
own
limitations
(e.g.,
a pipe
organ),
or,
with
amplified
music,
by the
amp/speaker
system's
ability
to fill
a
given
space
loudly
without
objectionable
distortion.
True
(instantaneous)
peaks
may
be
more
than
15
dB*
above
the
average
level,
so any
measurements
of
the
"maximum"
levels
really
should
reflect
the
instantaneous
peaks
achieved.
•The
decibel
is a
unit
thnl
expresses
relative difference
in
power
or sound
level.
The
figure of
1 dlJ
is
usually
given
as
(he
smallesl
sonic change
delectable
by most
people,
although
over
a wide
spec-
trum
of music
or
sound
and
under
controlled
conditions,
a third
of this
amount
is
delectable
to
many.
The
threshold
of hearing
(the
point at
which
one
no longer
can hear
anything
even
at
the
most
sensitive
frequency
range
of 1-3
kl
\z, in the
upper
midrange/lower
treble) is
approximately
"0-dB
SPL"
(sound-pressure
level,
a
standardized
measurement),
and
the
level
at which
most
people
put their
hands
over their
ears is
about
115-120
average
SPL.
Some
can tolerate
130-dH-SPL
averages
which
are
harmful
over time;
others
leave
the
room
at
tlO; the
dynamic
range of
human
hearing
thus
is
customarily
said
to be around
120 dB.
Note
that
the
relationships
among
SPL terms
are
widely
misunderstood
and
misidenlified.
If the
true
(instantaneous)
peaks
are around
115-dB
SPL, then
the
standard
fast
readings
of
maxima are
about
103-106
-
and these
are often labeled
"peaks" even
within
the
audio
world,
especially
in
discussions
of
power-amplifier
requirements
for
reproducing
conventionally
measured
fast
maximum
SPLs
-
and
the average
(slow
readings)
level
is around
100-102
or
so.