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SPEAKER
FAQs and ABC's...
This page will provide
information about speakers and answers to FAQs. It is not
particularly well organized at this time. The information presented
refers to professional type speakers that DJ's typically use
(although much of the info applies to home and car speakers as
well). Hopefully it will be useful and will help clear up some
mysteries and misconceptions...
General
Speakers are highly
complicated electromechanical devices. The primary purpose of a
speaker is to convert the electrical
energy from an amplifier to sound (acoustic) energy. Speakers
currently represent the weakest technological link in an audio
system. Although many speakers of today are very good or even
outstanding, there remains considerable room for improvement
(compared to other audio equipment).
Wattage
Ratings
Often, consumers are
attracted to and impressed by speakers that can handle the largest
amount of wattage. However,
wattage is just one factor to consider
among many. Just because a speaker can
handle more wattage does not
necessarily mean that it will play louder or sound better.
To find out the real
story, one needs to consider the sensitivity
specification together with the wattage
rating. The sensitivity specification basically states how much
sound comes out for a given input (analogous to a miles-per-gallon
rating for a car). The sensitivity spec is determined by the
manufacturer, and often takes the following form: "x" dB / 1 watt /
1 meter. "x" is the sound level in dB, and may be anywhere from
about 90 to 105 dB, depending upon the speaker
model. The spec is determined by driving the
speaker with 1 watt of electrical energy and measuring the
acoustic output at a distance of one meter. There are a number of
variations in measurement procedures due to the variety of
speaker models, too many details to
cover at this time.
So, what does this all
mean? Consider two speakers, Model A and Model B. Model A has a
wattage rating of 200 watts and a
sensitivity rating of 101dB/W/M. Model B has a
wattage rating of 600 watts and has a sensitivity rating of
97 dB/W/M. The maximum output from Model A is 124 dB; the maximum
output of Model B is 124.8 dB. So, as can be seen from the numbers,
Model A and Model B have nearly identical maximum output, even
though Model B can handle three times as much power! (I haven't gone
into mathematical details of how I came up with the maximum
numbers... a topic for another day).
CONCLUSION: DON"T SELECT
A SPEAKER BASED ON
WATTAGE RATING ALONE! As we saw in the
above example, two speakers that (to the unknowing) might seem much
different actually put out nearly the same amount of sound. Model B
requires a much more powerful amplifier to get the same maximum
output; powerful amplifiers are certainly available but weigh more
and cost more!
Are 4 Ohm Speakers
better than 8 Ohm Speakers?
NO!!!
The Ohm rating of a speaker has nothing
to do with the quality of the speaker!
What is an "ohm"
rating anyway?
All speakers have a
characteristic known as impedance which is measured
in units called ohms. The most common values for
speakers are 8 ohms and 4 ohms. Many older speakers have ohm
ratings of 16 and even 32 ohms (this is
because in the old days amplifiers used vacuum tubes, and higher
impedance speakers were more compatible with vacuum tube
amplifiers). A speaker with a lower
ohm rating represents a more demanding load for an amplifier to
drive. As stated before, the impedance of a
speaker has no relation to the quality of the
speaker.
Why do speakers come
in 8 ohm and 4 ohm versions?
Many manufacturers of
speakers (especially raw drivers) offer a choice of impedance... 8
ohms or 4 ohms. Often, the remaining specifications are very
similar. The reason for offering two versions is for special
applications. For example, in a speaker
system with dual woofers, two 4 ohm woofers can be wired up to form
an 8 ohm system.
Some musicians prefer
the "sound" of a 4 ohm speaker as
compared to an 8 ohm version. I believe that this "sound" is not so
much a result of the speaker itself,
but a result of the heavier loading that a 4 ohm
speaker places on an amplifier.
In the car stereo
market, virtually all speakers are 4 ohms. The reason is due to
voltage limitations available in cars (namely the 12 volt battery).
More power can be driven into a 4 ohm speaker
as compared to an 8 ohm speaker
(assuming the same driving signal!).
What should I know
about using 4 ohm speakers?
4 ohm speakers place a
much heavier demand on power amplifiers as compared to 8 ohm
speakers. Because of their lower impedance, twice as much current
will flow through a 4 ohm speaker (as
compared to an 8 ohm speaker) for a
given volume control setting. This translates to amplifiers getting
MUCH hotter (and heat is among the top enemies of electronic
devices!).
If you use 4 ohm
speakers, your speaker wiring will have
to be (or should be) larger. This is because the resistance
of the speaker wires becomes more
significant with respect to that of the
speaker. The result is that more power is "wasted" (in the
form of heat) in the wires leading to the speakers!
If you run two sets of 4
ohm speakers from a single amplifier, you are asking for trouble.
The reason is that the combined impedance will be down around 2 ohms
(and could actually be much lower still...see below), a value which
few amplifiers will tolerate. There are amplifiers which claim to be
able to handle 2 ohm loads with no problems; however in my opinion
this situation should be avoided.
Incedentally, you CANNOT
make a 4 ohm speaker into an 8 ohm
speaker by wiring a 4 ohm resistor in
series with it (I saw someone trying to do just this at America's
favorite electronic store). This will make the amplifier
happier, since as far as it is concerned it has an 8 ohm load (which
is easier to drive). However, resistors dissipate energy,
they DO NOT generate sound. Wiring a 4 ohm resistor in series with a
4 ohm speaker will halve the
amount of power which reaches the speaker
(it will also seriously degrade the damping factor, which won't hurt
anything except the sound quality!). Since amplifier power is
relatively precious, it would be extremely foolish to "throw away"
half of it by wiring a resistor into the speaker's electrical path!
Again, if you have 4 ohm speakers, you cannot change them into 8 ohm
speakers... get an amplifier that is capable of driving 4 ohm
systems!
Is an 8 ohm
speaker really 8 ohms?
ABSOLUTELY NOT! Due to
the highly complicated nature of a loudspeaker, its impedance is NOT
a simple number such as 8 (or 4) ohms. When a
speaker is said to be 8 or 4 ohms, this is understood to be
its nominal impedance.
A
speaker system is an extremely complicated electromechanical
device, and its behavior is correspondingly difficult to quantify.
The impedance of a speaker system in
actuality is said to be reactive; that is, in
addition to resistance it can take on
inductive and capacitive characteristics.
An inductor is a device which stores energy in a magnetic field, and
a capacitor is a device which store energy in an electric field..
These characteristics are not "designed into" a
speaker system; they are
characteristics that are "inherent" (based on the physical laws
which govern the way things work in our world).
Why do speakers take on
reactive properties? As stated, speakers are very complicated
devices, so this answer will be highly simplified. When a signal is
first applied to a speaker, there is a
time delay which occurs before the speaker
cone starts to move. This is because the
speaker cone has a non zero mass. Then, once the
cone is moving, and the input signal suddenly says "go the other
way", the cone will resist the change in motion (again because it
has mass and is moving in a particular direction). Basically, it
takes effort to start a mass moving, and once in motion, it takes
effort to stop it! An amplifier must deliver more current in order
to start a speaker cone moving, and
when the amplifier signal tells the speaker
"now go in the other direction", the speaker
responds by generating an electrical signal which "fights" the
amplifier! Again, this explanation is highly simplified. The best
way to handle a difficult speaker load
is to use an amplifier which has the ability to deliver tremendous
current. This will essentially keep a wild
speaker "in line".
So, why do I need to
know or care about this? It is important to realize that even though
a speaker may have a rating of 8 ohms,
the actual value can vary greatly. The impedance of a
speaker varies with input signal
frequency. As stated above, speaker
impedance is in general reactive. This means that the impedance
consists of a resistive part and either an inductive or capacitive
portion (inductive and capacitive impedances cannot exist at the
same time). The actual impedance (for a typical 8 ohm system) can
vary from around 2 or 3 ohms up to as high as 50 or 60 ohms! The
high numbers (50 or 60) are not so much the issue; it's the low
numbers that cause trouble. Most amplifiers can easily handle the
load of a "typical" 8 ohm speaker; the
trouble comes in when one tries to drive two systems (in parallel)
from the same amplifier. If you have an amplifier that is not too
good with low impedance loads AND you have two "difficult" 8 ohms
systems connected to it, you are inviting trouble. In general,
amplifiers do not like to drive reactive loads (which all speakers
are of course). The best way to combat this is to use an amplifier
that can dish out substantial current (i.e. one that claims it is
good with 2 ohm loads). Lesser amplifier designs will be the first
to "quit" whenever difficult speaker
loads are connected to it.
Advanced
topics... As stated, the impedance of
a speaker varies with frequency. If one
looks at a complex impedance plot (generated with a variable
frequency sine wave input), it can be seen that the impedance makes
several "loops" on the plot. There will be several frequencies where
the impedance is "real" (purely resistive). The first occurrence
(starting from the low frequency end of the plot) of a purely real
impedance occurs at the resonance frequency of the
system (technically DC is the first "real" impedance, but is of
little interest for obvious reasons). For most systems, this value
will be somewhere between 40 and 80 Hz. The impedance at resonance
is high, perhaps the highest value for the system (it might be
anywhere from 20 - 70 ohms). A speaker
system is very efficient (relatively speaking) at resonance. For
bass-reflex speaker systems (which most
DJ speakers are), the vibration of the woofer cone at resonance will
be very small (however the amount of air coming out of the vent will
be tremendous). The speaker system
presents an "easy" load to the amplifier at resonance because (a)
the impedance is purely resistive and (b) the impedance is
relatively high.
Speaker
Efficiency
Efficiency
is typically defined as the ratio of useful power output of
a system to the power input. Most devices are less than
100% efficient; that is at least a portion of the input energy is
wasted, usually in the form of heat. As an example,
consider an incandescent light bulb. A light bulb ideally generates
only light, however anyone who has ever touched a 100 watt bulb
(which has been on for a while) knows that it gets quite hot. This
heat represents wasted energy and is a result of the bulb being less
than 100% efficient. Anyway, on to the topic at hand...
Speakers are
notoriously inefficient. A speaker
takes energy from an amplifier (electrical watts) and converts it to
sound energy (acoustical watts); however, most of the power is
wasted (in the form of heat). A typical
speaker might be about 5% efficient; this means that if 100
watts of power are being sent to the speaker,
only 5 watts of sound comes out! If
speakers were 100% efficient, the average "boom box" would be able
to fill a gymnasium with sound!
Why are speakers so
inefficient? They are not designed to be inefficient, it
just "works out that way" due to the laws of physics which govern
how things behave in our world. The biggest problem is that a
speaker cone forms a very poor
impedance match with the air it moves to create the sound.
Whenever a poor impedance match occurs, energy transfer (in this
case from the speaker cone to the air)
is also poor. Basically, air is "too thin" to work well with a
speaker. A
speaker placed underwater would be much more efficient (but
probably wouldn't work for very long!) since water is much denser
than air.
Can you
explain "impedance match" in familiar
terms? (I'll try): Consider a person with a 10 speed bike. The
person is riding on flat terrain with a slight headwind, and has the
bike in first (the "easiest") gear. The bike is very easy to pedal,
however, the distance covered by the bike is minimal. If the rider
pedals very fast, more distance is covered, but soon the rider is
out of breath from pedaling at a high speed. Now, the rider shifts
to tenth gear (the "hardest" to pedal). The bike goes much farther
with a single revolution of the pedals, but it is much
harder to pedal. Even though the rider only has to pedal at a slow
rate, the extreme force required to move the pedals eventually
causes fatigue. What is the solution? Pick a gear "somewhere in the
middle" that provides a good balance of speed and pedal effort. That
is, select a gear that "feels right". One of the middle gears will
provide a good match for the riding conditions. By selecting the
"comfortable" gear, the rider has essentially "matched" the
impedance of his/her legs to the "load" provided by the bike pedals.
Some
speaker types are much more efficient
than others: most notably, horn type systems are
the most efficient. Some narrowband horn systems have efficiencies
approaching 50% (this may still seem like a low number but for a
speaker it is extremely high). The
reason horn systems are so efficient is that the horn acts like an
impedance matching device; essentially an acoustical transformer.
Whenever you cup your hands around your mouth to increase the volume
when shouting at someone, you are forming a (crude but effective)
horn. Many high frequency and midrange speakers are of the horn
variety. Due to physics, low frequency horn speakers are very large
(if not huge); hence the reason for "horn woofers" being scarce. A
number of low frequency horn systems are available, but they are
usually of the folded horn design. Because a low frequency
horn would be excessively large (a big consideration for portable
sound systems), engineers came up with the folded horn design.
Folded horns result in some losses (as compared to a "straight"
horn), but the result is still a very efficient (again, relatively
speaking) system.
Big Magnets mean a
Great Speaker, right?
NOT NECESSARILY!!! Many
people are impressed by speakers with gigantic magnets, thinking
that they make a superior speaker. The
purpose of a magnet in any speaker is
to generate a magnetic field in the voice coil gap
of the speaker. When a
speaker driver is designed,
calculations are made as to how much magnetic flux must be
present in the voice coil gap for desired operation. A quality
speaker has a magnet size based on
these flux requirements. A larger magnet, although "looking
impressive", is a waste: it costs more, weighs more, and may even
degrade performance!
Woofers in general have
larger magnets than mid and high frequency drivers primarily due to
the necessarily larger width of the voice coil gap. Woofers have
larger voice coil gaps than high frequency drivers since the
woofer's cone (and voice coil) is required to move much larger
distances (up to an inch or so). In order to maintain clearances
under such conditions, the voice coil gap must be larger. Magnetic
flux "likes to stay" in highly permeable materials like steel, it
does not travel well in air (and the voice coil gap is full of air).
Thus, as a voice coil gap gets wider, the strength of the magnet
required to maintain the proper value of magnetic flux gets larger
still.
The "giant magnet" hype
is primarily confined to lower quality car stereo speakers. An
example is a ten inch woofer with an 80 ounce magnet (selling for
$19.95 at a "tent sale"). Such speakers are likely to do more for
one's ego than their ears.
How many OHMS
are you running?
While setting up a sound
system for a school dance, someone came up and asked me how much
power one of my amplifiers had. I said "it's 465 wpc at 8 ohms, and
625 wpc at 4 ohms". He then said, "Run your speakers at 4 ohms!" I
sensed immediate confusion on the part of the person asking the
question. IMPORTANT: If you have 8 ohm speakers,
you cannot "run" them at 4 ohms; they are what they are. You cannot
change the impedance of your speakers. A given set of speakers with
a given amplifier will have a particular maximum output; you cannot
get more by "running" your speakers at 4 ohms. If you have 8 ohm
speakers and your amp puts out 200 watts at 8 ohms, that's what
you'll get. That same amp might put out 300 watts at 4 ohms, but to
"get it" you'll have to use a 4 ohm set of speakers.
There is a way to
"adapt" the impedance of a speaker to a
different value by using a transformer. However,
this practice is restricted to office PA systems and other
applications where audio quality is less important. Transformers
that maintain high audio quality at high power are extremely
expensive, very large and very heavy (in other
words, totally impractical for DJ use!).
Again, you cannot "run"
your speakers at 4 ohms in an attempt to get more power out of your
amp! (You can connect two sets of 8 ohm speakers (in
parallel) to your amp to get more power out of the amp.
However, under such conditions the power to any one
speaker will be less than the
amount it would receive if powered by itself at 8 ohms!
Low Frequency
Speaker Systems
The dominating factor in
a low frequency speaker system's
performance is the enclosure design. A woofer sitting in
open air (no cabinet) has low frequency response that is absolutely
terrible. Most "raw" speakers are designed with a
particular type of cabinet design in mind (for DJ speakers, the two
main types are bass reflex and folded horn). "Acoustic suspension"
or "sealed box" enclosures are not often used for DJ speakers
because they tend to be too inefficient.
Bass Reflex (sometimes
called "vented" enclosures) are designed to take advantage of the
acoustical energy inside the enclosure. The design
essentially uses the sound inside the enclosure to reinforce the
"direct" energy from the speaker. The
result is more sound (as compared to a sealed box system) for the
same wattage input. However, this
phenomenon only works at low frequencies (from resonance to about
150 Hz or so). The mid band output of a
speaker is much less dependent upon enclosure design.
Bass reflex enclosures
are not easy to design. Often, much "tweaking" and "tuning" is
necessary, requiring expensive equipment and a lot of brainpower.
Unless you love building and tinkering with speakers, you are
probably better off purchasing a system from one of the many
excellent manufacturers that offer them!
Folded horn low
frequency systems are even more complicated to design, and probably
"out of scope" for all except those with an intense interest in
speaker design (and lots of time and
money)! However, a well designed horn system can't be matched for
the amount of acoustical energy they can generate. Horns are not
noted extremely deep bass response, but the "punch" they can
generate is unmistakable.
Speaker
Placement
Where you place your
speakers in a room can have a large effect on the sound, especially
the low frequencies. It is best to keep low frequencies on (or near)
the floor, and to keep the mid and high frequencies above the
tallest person in the room (6 feet or so). This presents a dilemma
for people using single speakers (as opposed to those who run
separate subwoofers and mid/high units).
In my experience, an
excellent speaker configuration (for
most rooms anyway) is to use two subwoofers and two smaller mid/high
frequency systems. The two subwoofers are placed together (like one
big unit); the two small speakers are placed on stands (about 7 feet
up). This configuration offers a number of benefits:
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The subwoofers are on
the floor and placed together. This allows them to "couple" with
the floor creating better energy transfer. In addition, the
proximity of the woofers to each other further increases coupling
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The mid/high frequency
systems (actually a 12" two way system) are above the heads of the
tallest people, allowing the high frequencies to travel much
further (high frequency systems on the floor will sound very
muffled ten feet into a crowd).
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The system is
relatively compact and easy to handle
Note that since low
frequencies are very non-directional, the subwoofers can be placed
in an out-of -the-way place. If you are creative you can hide them
behind other items in the room. Your guests will wonder how "those
two little speakers" (the ones they actually see) can put out so
much bass!
Disclaimer
The information presented above is just that, information. Use it
at your own risk.. You (the reader) are
responsible for anything that you may regret as a result of
reading this article. You assume complete and total
responsibility for your actions. The information presented above is
believed to be technically correct. In simple terms.
The Henley Trading Company inc. cannot be held responsible.
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