UNDERSTANING THE
SPECS
When you begin to choose a loudspeaker
component, you will soon begin to feel a little
overwhelmed by the confusing and somewhat
contradictory numbers and words thrown at you.
For a quick overview of the specifications that
matter the most for subwoofer selection, we will
begin with the ones that are the most broadly
applicable. When we get a little further along
with discussing particular subwoofer box
configurations, we will more thoroughly examine
the parameters that affect those individual
designs.
POWER HANDLING
The industry seems to have almost convinced us
that power handling capacity is the most
important number to consider when choosing
speakers. High power handling is certainly a
factor to consider, depending on your
application, but be sure that you are looking at
the right numbers. The RMS rating is based on a
mathematical formula (RMS= .707 X peak measured
power) for determining average power that the
speaker is subjected to, and it is usually a
fairly conservative number compared to the
continuous program power or peak power ratings
that we will discuss in a moment. RMS ratings
are only as good as the "qualifiers" that
accompany them, for example, rating a
hypothetical subwoofer at 600 watts/RMS doesn't
mean much if the frequency chosen for
measurement (1000 Hz) is outside the range that
you intend for the subwoofer to be used (25 to
125 Hz). In the case of this example, the power
handling number would give us some idea of what
the subwoofer could withstand in terms of
thermal power handling capacity, but it would
not reveal much about the mechanical limits at
the lower frequencies where the subwoofer will
be used. Lower frequencies require greater cone
travel, and we will be expecting that subwoofer
cone to be moving quite a bit! For the purpose
of selecting a speaker it is useful to know the
range of frequencies tested, or the single
frequency if that is what was used. Since these
qualifiers are sometimes a little hard to nail
down, your best bet is to stick with
manufacturers that demonstrate the greatest
credibility, and suppliers that take performance
specifications seriously.
Continuous program power is generally understood
to be double a given RMS rating, and tends to
represent the kind of stress that a speaker is
subjected to when that amount of music program
is played through it. Although music can place
dynamic stresses on a speaker, it also gives the
components a chance to "rest" at intervals, even
if those intervals are only a few milliseconds
in duration. If you use a subwoofer rated at 200
watts RMS or 400 watts continuous program power,
with an amp rated at 250 watts RMS, you are
actually getting a pretty good match as long as
the amp is not driven so hard that it exceeds
it's "clean" power rating of 250 watts RMS. When
overdriven, the amp can exceed it's RMS power
rating and produce even more power, and that
extra power will be loaded with a lot of harmful
distortion.
Peak power is probably the least helpful rating
for speakers. Any speaker can handle extremely
high peaks of undistorted signal for
milliseconds or microseconds, but these
impressive numbers are rarely qualified by the
disclosure of the test conditions that were used
to determine them. In effect, peak power ratings
are an example of a "numbers game" that
marketing pressures have forced the
manufacturers to compete in because higher
numbers impress some buyers. The key to power
handling ratings is reviewing and understanding
the specifications that actually apply to the
job at hand.
IMPEDANCE
|
Help Guides |
Speaker
Connection In
Series And Parallel
The diagram
above shows the basic electrical
difference between connecting 4O
speakers in parallel and in series.
The connection point where the
speakers are attached to the amp is
shown at the right side of each
frame with the resulting impedance
that each situation would provide to
the amplifier's output. |
Speaker impedance can be defined as the combined
total of all resistance, inductance, and
capacitance that the speaker presents to an
amplifier. It is also referred to as "nominal
impedance" because the actual number measured in
ohms can vary quite a lot over the frequency
range of the speaker. The actual nominal
impedance value of 2, 4, 6, 8, or 16 ohms has no
connection whatsoever with subwoofer performance
or quality, but it is important for the proper
match of an amplifier to the intended load. The
lower the impedance value, the more power will
flow through the speaker. If a 4 ohm and an 8
ohm speaker are connected to the same amplifier,
the 4 ohm speaker will receive twice the power
of the 8 ohm speaker, so matching values within
the same frequency range can be pretty
important. Multiple driver designs will require
an awareness of the impedance of the intended
drivers as various wiring configurations can
increase or decrease the total value that the
amp will see. It is also important to correctly
identify subwoofer impedance as it is also a
factor in establishing the values used to
determine passive crossover components.
XMAX, EFFICIENCY, AND SENSITIVITY
Xmax is the term used to describe the linear
excursion capability of a loudspeaker. It is
basically determined by the length of the voice
coil relative to the height of the magnetic gap
of the speaker "motor". The subwoofer can only
provide clear, undistorted output if the magnet
is able to maintain control of the voice coil,
and lower frequencies demand greater travel or
excursion. The voice coil tends to have greater
mass due to the increased amount of wire
required for the longer surface area covered;
this usually results in a reduction in higher
frequency response, and lower overall driver
efficiency.
Efficiency and sensitivity are very closely
related. Efficiency is usually shown as a
percentage that describes the ability of the
speaker to convert an electrical input into an
acoustical output. You will hear a lot of
advertisements for speakers that declare the
product to be "high efficiency", but in fact
that is a relative term.
Sensitivity ratings for loudspeaker products are
representations of relative loudness, using a
scale of decibels (dB), and with an input power
of 1 watt (w). The resulting Sound Pressure
Level (SPL) generated is typically measured at a
distance of 1 meter (m) from the loudspeaker, so
it would appear that we have everything we need
to compare one speaker to another, right?
Actually, it is just a start.
It is a surprising fact that loudspeakers in
general and subwoofers in particular are very
inefficient devices, electrically speaking. An
"efficient" pro sound 12" midrange speaker with
a sensitivity rating of 103 dB at one watt/one
meter might have a reference efficiency of
around 5 or 6%. This means that for all of the
amplifier power going into the speaker, as much
as 94% is wasted as heat and mechanical losses.
Wait, it gets worse! Your state-of-the-art
subwoofer loudspeaker has a sensitivity of only
87 dB 1w/1m, or a reference efficiency of only
0.27%, so what good is that? Before you get mad
at your speakers for being very expensive round
heating elements, just relax and enjoy the
benefits of modern technology! High performance
amplifier power has gotten relatively cheap and
reliable, and improvements with adhesives and
high tech design have pushed speaker power
handling to limits that could only have been
dreamed of a generation ago. It was not that
long ago that you were the king of the hill if
your (probably tube) amp was cranking out a
blazing 25 watts per channel, and your woofers
(probably not subwoofers) were rated at 50 watts
each. The high efficiency designs of that era
were actually able to provide a very dynamic hi
fi experience, but there are some alternatives
for the contemporary user.
The most popular current thinking regarding
efficiency relative to subwoofer output is to
select a driver with a fairly high mass stiff
cone, good linear excursion capability, and
power handling that is high enough to withstand
the power required to move that massive cone and
long voice coil. This driver will have a fairly
low sensitivity rating because of these design
characteristics. In a properly designed
enclosure, and driven by the appropriate
amplifier, our example subwoofer will be able to
deliver deep, clean, high impact bass.
We have established that power handling ratings
are only relevant within a driver's actual
operating range; sensitivity ratings for a
subwoofer only count in the sub-bass region
below approximately 125 Hz. A subwoofer could
actually have a response that is rising at
frequencies that are outside the range that you
intend for the sub to be used. The sound
pressure of these higher frequencies will affect
a sensitivity rating if the manufacturer has
averaged the boosted response of the higher
frequencies with the reduced output that we
would expect at lower frequencies.
FREQUENCY
RESPONSE
The frequency response of a subwoofer is
important but becomes far more meaningful when
we are able to determine the actual response in
the enclosure of our choice. As we have seen
with other key specifications, frequency
response numbers mean little without some kind
of qualification. To simply state that a driver
has a response "from 19 Hz to 1000 Hz" is
leaving out critical information. If the
relative output (remember SPL?) versus frequency
is not disclosed, we could be looking at a
response at 19 Hz that is 3, 6, or even 16 dB
below the average response for the speaker.
Similarly, there could be a 10 dB peak at 90 Hz.
Frequency response graphs can be far more
helpful, because the associated graph will
display the variations in sound pressure level
at any given frequency. Be sure to try to
determine if the woofer shown on the graph was
tested in an enclosure or free air, because free
air plots will make the bass response look very
puny indeed. Computer modeling programs like
BassBox 6 Pro can fill in a lot of information
as far as what to expect for bass response from
a given enclosure design. Keep in mind that
since we are discussing subwoofers, the response
of the driver above 125 or 150 Hz is not a
matter of great importance to us. |
