|
This month is the sixth and last installment
in my series about speakers. We are
going to be covering speakers used in sound reinforcement.
This type of speaker
can be divided into two categories, passive (with or out
a crossover) and powered
speakers.
PASSIVE SPEAKERS
The most common type of sound reinforcement speaker at
least until recently has
been a woofer and tweeter in a box hooked to a passive
crossover network. This
network is a circuit comprised of resistors, capacitors
and inductors that send
low frequencies to the woofer and high frequencies to the
tweeter. Sometimes
a third midrange speaker is employed also. The type of
enclosures used can be any of the
ported or horn loaded cabinets we discussed in earlier
articles in this series.
But since we need to reproduce the higher frequencies in
balance with the lower
frequencies a horn is employed to handle the highs.
The passive crossover used in this type of box can vary
widely in quality. The selection
of components used in the design will determine the overall
performance level of the speaker.
Using air core inductors instead of iron core inductors
will decrease distortion and using high
voltage Mylar caps can increase power handling capability.
Passive crossovers vary in the
complexity of their design from a simple one pole filter
to a multi-pole filter.
The more poles a filter has the more its ability to keep
lows out of the tweeter
and highs out of the woofer. A simple one pole crossover
may only give a roll off
slope of 6 db per octave. This is not very much. For example
if you have a horn
that you want to feed 1k and above into and your passive
crossover is 6 db at 1k,
you are still going to have a lot of power going into the
horn at 500 HZ (an octave down from 1K). Passive crossovers
with more poles will yield higher roll off rates as in
12 or 18 db per octave yielding more separation between
the lows and highs.
Regardless of the quality of the
passive crossover used there is one problem that can
not be gotten around- insertion loss. Some of the power
being input into the speaker cabinet is lost by resistance
in the inductors and resistors of the crossover. You
might think if you have a high power amplifier, that
giving up 20% of a thousand watts may not be that big
a deal. But what most people do not realize is that audio
power works in logarithmic fashion (powers of 10). Therefore
200 watts is not twice as loud as a hundred watts,1000
watts is twice as loud as a hundred watts. Additionally
an audio signals dynamic range (difference between loudest
and quietest parts of the signal) is 10 db. Therefore if
you want to pass your audio signal cleanly you need to
have 10 db's of headroom above the quietest part of the
signal. So if you did have a 1000 watt amplifier, you only
will get to use a hundred watts before the peaks start
to clip. If you think about how wattage translates into
sound pressure level (SPL) we can see how a hundred watts
can be marginal in its ability to create volume. Using
the example that most speakers can produce about 90 db
at 1 watt at 1 meter, this means that every time you double
the power you gain 3 db of SPL. Therefore 2 watts is 93
db, 4 watts is 96 db etc. In the 100 watt area we are at
about 108-110 db but that is at one meter and falls off
as you move further away. So you can see that in a large
room filled with people this can be a marginal amount of
power. Adding an insertion loss of 1-2 db from a passive
crossover is not a help.
You can probably now realize also that
for significant increases in volume and headroom the requirements
are for you to double the power or amount of speakers to
go up just 3 db. This gets expensive real fast and explains
why few systems at the nightclub level are loud enough
without being run into clipping. It is also always a source
of amusement to me when I see people shopping for items
such as a bass guitar amp and they are trying to choose
between a 400 watt amp and a 600 watt amp. This will make
very little difference in their volume.
PASSIVE SPEAKERS WITHOUT A CROSSOVER
If we remove the passive crossover from the box we can
then drive the speakers directly from an amplifier. We
obviously need to do something to keep the lows out of
the horn or it will blow. The solution here is to use an
electronic crossover. Here we have a crossover that operates
at line level not speaker level and is used prior to the
amplifier input instead of after the amp's speaker output.
The electronic crossover takes a full frequency input signal
and divides it into two or more parts. In the case of a
two way crossover, it sends the lows to an amplifier connected
to the woofer and the highs to a second amp connected to
the horn. This eliminates the insertion loss of the passive
crossover but also has several other benefits. The crossover
points and relative level of the woofer and tweeter can
be adjusted on an electronic crossover giving more flexibility
than a non adjustable passive crossover. Also, the slope
in an electronic crossover can be as high as 24 db as opposed
to maybe 12 db in a passive crossover, giving the ability
to keep more lows out of the horn and opening up its power
capacity to be driven harder with frequencies it can handle.
You may look at the extra complexity of wiring and more
amplifiers as a shortcoming but the efficiency boost is
worth it in my opinion
ACTIVE SPEAKERS
A more recent development has been the active loudspeaker.
Here an amplifier assembly is designed right into the speaker
box enabling it to be driven directly from a line level
source or even a microphone. The immediate downside that
came to mind when I first saw this was that speaker cabinet
vibration plus an amplifier equals a broken amplifier.
But as time goes on I don't see this as much of a problem
as I thought it would be. The convenience factor of having
the amplifier in the cabinet can't be ignored. Gone are
the heavy amplifier racks with long heavy speaker cables
to run to the speaker cabinets. All you need is a line
level cable from your signal source. One benefit of using
powered speakers is modularity. A small venue can use fewer
speakers or it can be scaled up proportionally by adding
more of the same speakers. Another benefit of the powered
speaker is that it can be designed as a total system taking
into account overall voicing, impedance, power handling
,protection etc. Plug in a line level source and everything
is already in place to provide optimum sound from the components
used. Onboard limiters calibrated to the specific speaker
system reduce distortion and the chance of blowing the
woofer or tweeter.
The recent development of lightweight class
D amplifiers and switching power supplies inside speakers
cabinets containing lightweight Neodymium speakers give
a lot of volume from an almost unbelievably lightweight
package. But as I mentioned in last month's article there
are a lot of shortcomings to these type of systems, especially
when they are pushed past their design parameters.
SUBWOOFERS
A subwoofer is a speaker cabinet dedicated to the reproduction
of low frequencies only. It must be used in conjunction
with other speakers that reproduce the top end. Here it
is important to remember that it is not only speaker size
that determines the frequency range and volume of the subwoofer
but the speaker and cabinet working together. As we mentioned
in earlier articles the larger a speaker is, the more mass
the cone has and the harder it is to reverse the motion
of the cone due to inertia. This leads to muddiness in
the sound output. Coupling two smaller drivers together
can give just as much low end with less muddiness.
There are two ways to incorporate a subwoofer into a sound
system. I will use as an example powered speakers, but
this can be done with separate amps and speakers too.
The first way to do this is to use
a system that is specifically designed as a subwoofer/satellite
speaker combination. Perhaps the most well known system
of this type is the Mackie SRM450. Here we have a top
speaker containing a 12" speaker and horn combined
with a subwoofer containing a fifteen inch speaker. The
top speaker is mounted in the air for projection by a
pole that fits into the top of the subwoofer. Using the
onboard electronics in the sub and satellite speakers,
the onboard electronic crossover divides the frequencies
and you have a full range system with each speaker or
horn receiving the correct frequencies and level. In
a smaller situation, the subwoofer does not need to be
used and the top speaker can be set to reproduce a full
frequency response.
The second way a subwoofer can be utilized is to add it
to an already existing system. If the system is not using
powered speakers it may be possible to make changes to
the electronic crossover and readjust it to include the
subwoofer to handle the lowest frequencies. But if the
sub is being added to an already existing powered system
it can still be used. I recommend that the system be set
up and adjusted without the sub. Remember the cabs from
the existing system are already handling the low frequencies.
Once it is set up as well as possible, then start to fade
the subwoofer in as a reinforcement to what is already
there which will fill out the sound without being overbearing
and take the load off the other speakers.
LINE ARRAY
The last type of sound system speaker I want to mention
is the Line Array. These are the speakers you see in use
at most concerts today. Multiple speaker enclosures with
the same components are placed close enough for there to
be positive interference between them The theory behind
how they work is beyond the scope of this article, but
the result is they cover large audiences with a more accurate
sound in a controllable manner. They are also more efficient
and demand less amplifier power. This is not something
you will see in use at the nightclub level
LIMITERS
I want to touch on one more idea. Above we discussed how
the 10 db peak headroom needed to pass an audio signal
cleanly will limit the amount of average power that can
be used. If we introduce a limiter to restrict the peaks
from going to 10 DB to say 7 DB then your average level
is only 7 DB down from maximum power enabling more use
of the amp's power for the continuous signal. An alternative
to do this is to set the limiter threshold to just below
the point that the amplifier clip light will turn on. If
the compression ratio is then adjusted to infinity, any
time a signal tries to pass larger than the threshold it
is severely clamped. Now you can use any amount of continuous
power the amp can produce, but the louder the system is
played the less dynamic range it has and starts to sound
squashed and apparently lower in volume. I recommend this
type of adjustment to be used for speaker protection only
as it really keeps an amp from going into distortion. All
in all limiting is a tool that used sparingly will give
you a performance boost. If you are trying to make a way
underpowered system louder it by limiting it is not going
to work well.
Next month I am going to discuss fuses,
which according to most of my repair customers is the
only thing wrong with their non functioning equipment.
We will see why this is not true.
Steve
Weiss is the owner and main technician of Steve Weiss
Electronics Inc. He is experienced in the repair
of analog and digital musical equipment. This includes
everything from Vintage Tube Amps and Pro Audio equipment
to Digital Keyboards There is also a guitar repair
shop staffed by some of the areas top guitar repair
techs. He is authorized for warranty work on most
major brands. Steve Weiss Electronics is located
inside of Sam Ash Music at 5460 West Sample Road
Margate, FL 33073 954-975-3390 Ext 272. Steve has
also spent 25 years on the road as a performing guitarist
and is the designer of Primal Guitar amps that can
be seen at Primal
Audio.com Steve can also be reached at stevew@metromusicmayhem.com |
|
|
