and Systems -- The Basics
makes a good vented enclosure?
The basic purpose of any loudspeaker enclosure is to partition
the front and rear of the driver's cone. This prevents the opposing
air pressure changes produced by cone motion from combining and
canceling. Vented enclosures allow the compressibility of the
air inside the enclosure to work as a more active part of the
"system" consisting of driver and enclosure. A low frequency
loudspeaker enclosure should do absolutely nothing, that is, it
should add no effects of its own -- no vibration, no tonality,
no motion -- nothing to interfere with or absorb acoustic energy
produced by the driver inside.
it possible to get deep, punchy bass from a small enclosure?
Only if the driver selected is designed for "deep" bass
operation in a small enclosure. Unfortunately, it's usually a
relatively small driver that can work properly in a small enclosure.
Since a small driver can only move a small amount of air, lower
sound pressure levels (output) will result. In general, larger
boxes with larger bass drivers produce more bass; smaller boxes
produce less bass. "Deep" bass production simply requires
that more air be moved.
a bigger driver give me more bass from my enclosure?
enclosure will not automatically produce more bass when a larger
driver is installed; in fact, the opposite is often the result.
does "tuning" of an enclosure or port mean?
Have you ever blown over the top of a bottle and heard the note
it produces? Have you ever changed that note by adding or pouring
out liquid? A vented loudspeaker enclosure works under a similar
principle. When liquid is added to or poured out of a bottle,
its internal air volume is changed and the resonant note is raised
or lowered. Enclosure tuning is affected by the ratio of air volume
in the duct/port (equivalent to a bottle's bottleneck) to the
enclosure's interior volume. Tuning of a loudspeaker enclosure
is the result of manipulating the difference in effective air
mass between the enclosure interior and the air in the duct. The
bottle-like nature of a vented enclosure is known as a "Helmholtz
Resonator". The vents/ducts in a vented enclosure work only
over a narrow band of frequencies near the chosen tuned frequency,
producing the same effect as when blowing across a bottleneck.
are Balanced and Unbalanced Lines?
- A balanced
line is a shielded, three-conductor system in which the two
signal wires carry identical signals of opposite polarity. The
third conductor (a braided or foil sheath around the two signal
wires) acts only as a shield and does not carry an audio signal.
Both signal wires will pick up any external electrical noise
that gets past the shield. The receiving input will invert the
polarity of one of the signals and combine it with the other.
When the inverted noise is added to the non-inverted noise it
is canceled. This is known as "common mode rejection".
Note that both the output of the sending equipment and the input
of the receiving equipment must be balanced in order for the
line connecting them to be considered balanced.
- An unbalanced
line is a two-wire system in which both the shield (ground wire)
and the center conductor carry the audio signal. Unbalanced
inputs and outputs are typically found in guitar amplification,
effects processors and hi-fi and home theater systems. Unbalanced
systems will perform acceptably as long as cable length is kept
relatively short, the system is not subject to excessive RF
noise, and the AC ground of the various system components is
at the same potential.
is a crossover network and what is "bi-amplification"?
A speaker system that uses high and low frequency drivers requires
a crossover network. This network is an electronic circuit that
acts as a sort of traffic cop, directing the high-frequency signals
to the high-frequency drivers and the low-frequency signals to
the low-frequency drivers. The crossover network may be placed
after the amplifier and is usually mounted in the speaker
cabinet. This is called a "passive" or "high-level"
network (there are also passive "low-level"
networks but these are uncommon and will mostly be found in older
systems). On the other hand, the crossover network may be placed
before the amplifiers. This is called an "active
crossover" or "electronic crossover". Systems with
active crossovers will have one amplifier (or amplifier channel)
for the lows and a separate amplifier (or amplifier channel) for
the highs. A system using an electronic crossover is said to be
"bi-amplified" (2 separate amplifier channels) if it
is a 2-way system or "tri-amplified" (3 separate amplifier
channels) if it is a 3-way system.
are some advantages of bi-amplification?
- A "high-level"
(passive) crossover can degrade the audio signal from the amplifier
and can reduce system sensitivity. By eliminating these components
from the signal path, system performance can be improved. This
statement is less true today than in the past due to improvements
in passive crossover design.
- In extremely
high-powered systems, a passive crossover can become quite expensive,
large, and heavy.
are intended to handle very low frequencies - usually below
150Hz. The inductors (a coil of wire sometimes wrapped around
a metal core) needed to cross-over at these low frequencies
are prohibitively large, heavy and expensive, even for moderately
powered systems. Bi-amplification of subwoofers is usually a
crossovers can be less expensive in large systems consisting
of many speaker boxes. If these multi-box systems used passive
networks, a separate network would be needed for every enclosure.
A single active crossover can feed power amplifiers for many
active crossovers (such as the JBL DSC260) can be extremely
powerful signal processing devices that may incorporate advanced
features, such as equalization, time/phase correction, individual
band limiting and signal delays. By precisely tailoring the
parameters of these crossover networks to the speaker system,
it's possible to get the best possible performance from the
what happens when a very strong low-frequency signal such as
a kick-drum drives an amplifier into clipping. The clipping
creates harmonics. In a passive system, the crossover simply
sends these harmonics to the high-frequency driver, where they
will be quite audible. This is called "intermodulation
distortion". In a bi-amplified system, only the low-frequency
amplifier will be driven into clipping with a strong kick drum
signal. The high frequency amplifier will continue to produce
clean, undistorted power. As a result, the clipping distortion
is less audible.
are some disadvantages of bi-amplification?
is generally more expensive and requires more amplifiers. Also,
electronic crossovers are typically more costly than the passive
networks in the enclosures.
- A bi-amplified
system is more complex, although properly fabricated electronics
racks and cabling can simplify the set-up of a bi-amp'ed system.
Some expertise is required to configure the system.
- There are
more things to go wrong -- more components to fail, more connections
to be accidentally disconnected or mis-connected and more controls
to be incorrectly adjusted. Some improper connections (such
as hooking the low-frequency amplifier output to a high-frequency
driver) can destroy a driver.
if I want the best of both worlds?
One way to get the best of both worlds is with JBL's EON Series
powered speakers. Since EON powered speakers are bi-amplified,
they give you all the performance advantages of bi-amplification.
On the other hand, since the power amplifiers are built-in, they
are perfectly matched to the drivers and the cost is comparable
to that of a similar passive speaker. EON powered speakers
are faster and easier to set-up than a passive speaker and its