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How Do Car Stereo Speakers Work?
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Car Stereo
Speakers are air pistons that move back (on the negative cycle
of the signal) and forth (on the positive cycle), creating
different degrees of air pressure at different frequencies.
The amplifier (either separate or built-in your radio), produces
electrical impulses that alternate from positive and negative
voltages (AC). This current reaches the voice coil inside
the speaker, creating an electro-magnet that will either be
repelled, or attracted by the fixed magnet at the bottom of
the speaker. The voice coil is attached to the cone, moving
it back and forth, creating sound. The surround (rubbery circle
that joins top of the cone and metal basket) and the spider
(usually yellow corrugated circle joining bottom of cone to
magnet) make the cone return to its original position.
Speaker Sensitivity, measured in dB, is how loud a speaker
plays (usually 1 Watt, 1 meter). A higher Sensitivity rating
means that the speaker will play louder using the same power
as a speaker with a lower rating.
The back and front parts of the speaker should be isolated
from each other. When the front of the cone is pushing air,
the bottom is pulling air, creating a canceling effect. Ideally
every speaker should be in an enclosure. If you are mounting
a speaker in a big hole, make sure you build a panel to isolate
the front and back of the speaker (baffle).
Imaging And Staging
Imaging is being able to pick certain sounds from certain
places. The singer would normally be located towards the middle
of the car, in a car audio installation, guitars, trumpets,
and other instruments towards the sides of the car, in a car
audio installation. If you scatter speakers all around the
car, in a car audio installation, your imaging would be very
poor, since you would be producing the same sound at different
places. If you have a system with good imaging, the sound
should seem to come from different instruments and voices,
not speakers.
Staging is the ability of a system to "fool you" into thinking
that everything (including bass) is in front of you. The sound
should be similar to a stage in a concert, where the singer
would be in the front center, and the rest of the instruments
and background vocalists would be located to the left and
right (but always on the front). Good staging and imaging
are not so easy to implement in a car audio installation.
It takes a lot experimenting with speaker location and direction.
Directivity of sound is related to frequency. At higher frequencies
it is easier to pinpoint where the sound is coming from, than
lower frequencies. This can be used to our advantage in car
audio installation. Tweeters are the most important part of
getting good staging. They should be aimed towards the middle
of the car, in a car audio installation. A way to "bring"
the bass to the front of the car is to fool our ears by overlapping
frequencies played by midbass and subs, so that your midbass
actually "pull" the bass to the front, since lower bass in
not too directional. You should crossover your midbass as
low as you can (without getting distortion). Then cut your
subs at a bit higher frequency. This will mix the bass coming
from the front and rear, making the bass seem to come from
the front. Adding a center channel also improves staging,
if it is set up correctly, in your car audio installation.
Types of Speakers
Coaxial
Coaxial speakers (or three-ways) are two (or more) speakers
built-in the same frame. They are cheaper than separate woofer
and tweeters and also easier to install. There is no need
to worry about crossovers, since they are already built-in
(you might still need to add a crossover to block bass if
you are using high-power amplifiers). A disadvantage of coaxial
is the lack of flexibility. For example, if the coaxial is
all the way in the kick panel, or door panel aiming at your
feet, not your ears. Some manufacturers try to compensate
for this by making adjustable tweeters. You should usually
consider coaxial speakers for the back of the car, in a car
audio installation, and separates for the front, unless you
only have one speaker hole and don't plan to cut any more
holes in the car, for the audio installation.
Separates
Separates consist of a tweeter and woofer, and [most of the
time] come with an external crossover. The woofer is usually
mounted in the factory hole in the door or kick panel. The
tweeters can be mounted in different places. The most common
place to install tweeters is towards the top front corner
of the door panel, aiming (if possible) between both front
seat head rests. Another popular location for tweeters is
in the dash, either surface mounted, or in factory dash holes.
Yet another location where tweeters are commonly mounted is
in the blank plastic piece on the top front side of the doors
(where the mirror is on the outside). You would have to experiment
with angle and location to achieve the best possible imaging
and staging.
Midbass
Midbass are usually 5, 6 or 8 inch speakers that are designed
to go lower in frequency and are part of a three way system
with a mid and tweeter. The problem is that 3-way arrangements
require more complicated crossovers. A well-balanced three-way
set up will give you accurate imaging and staging. Midbass
are most commonly mounted in the doors.
Subwoofers
Subwoofers add lower frequencies to the whole system. They
have to be enclosed in a box, with the exception of free air
subwoofers, which use the trunk as a box. There are many different
types of boxes and implementations discussed in the subwoofers
section.
Horns
A few high-end manufacturers are making horns for car audio
use. Horns are very good at directing sound and have high
efficiencies. Horns are usually mounted under the dash. By
doing this, difference in distance from left and right speakers
are greatly reduced over conventional mounting locations.
Since horns play mids and highs, tweeters are not needed.
Even though horns are mounted under the dash, they give great
imaging. Horns cost more than conventional speakers and require
customization. In many installations a good equalizer is required
to compensate for their high sensitivity.
Center Channels
Center channels consist of a midrange speaker (3 or 4 inch)
mounted in the middle of the dash (usually) on the top. Center
channels play a mono (Left + Right) signal between 350 - 500
and 3500 Hertz (voice range). The purpose of the center channel
is to raise the sound stage, by creating the sensation of
the singers "being" in the front of the car, in a car audio
installation, and not in the door panels. Center channels
are hard to implement though: First, a band pass crossover
is needed. Left and right channels have to be summed up. There
are various commercially available center-channel processors
(many with built-in amplification). The volume level of the
center channel should be lower than the other speakers, since
it is only supposed to make subtle changes to the total sound
image.
Mounting Locations
Front Speakers
The best place to mount speakers in the front, in custom kick
panels. By doing this, the path between the speakers and ears
is minimized giving the best possible sound without having
to add time delay circuitry. If this is not possible, try
to point the speakers towards the center of the car, in a
car audio installation, and try to minimize the distance between
the right and left speakers to your ears. Custom kick panels
are usually built from fiberglass or molded plastic, and are
available from some manufacturers such as Ai Research.
Rear Speakers
Rear speakers should give a sense of space to the music, but
not overpower the front speakers. If you are using rear speakers
to add more bass to the system, at least use a crossover to
cut off higher frequencies. You should be able to barely hear
the rear speakers. A lot of hi-end systems don't even have
any rear speakers. Separates are not necessary for the rear,
a set of coaxials will work good for rear fill.
Sizes and Shapes
There are many speaker sizes ranging from 1-inch tweeters
to 18-inch (or bigger) subwoofers. A smaller speaker will
reproduce higher frequencies better than a bigger one. The
wavelength of a 20,000 Hz signal is very small, while the
length of a lower (bass) note moving in the air could be as
big as 40 feet. That explains why a 4-inch speaker can't really
put out bass (the lower the frequency, the more air mass that
has to be moved by the speaker). Tweeters are designed to
play frequencies from 3500, 4500 or even 6000 Hz, all the
way up to 20,000 Hertz. Midranges (3, 4 or 5 inchs) play music
from around 300, 500 Hz, to where the tweeters start in the
upper level. Midbass (5, 6, 8 inches) play from around 50
Hz to 500 (and even 1000) Hz. Subs handle frequencies below
120 Hertz.
Do round speakers sound better than oval-shaped speakers (i.e.
6x9's)? The answer is yes for most practical purposes. A round
cone is more rigid than an oval-shaped one, so at higher levels,
an oval-shaped speaker will distort more. The reason why there
are oval-shaped speakers is because of rear deck space considerations
by manufacturers. An advantage of a 6x9 speaker over a 6-inch
speaker is that it has a bigger area, so it will move higher
air volume, producing more bass.
Power Considerations
Most people think that if they use a 50 watt per channel amplifier
on their factory speakers, the speakers will be damaged. This
may be true if the speakers do not have crossovers blocking
off frequencies speakers were not designed to play. What destroys
speakers is distortion. If you turn the volume all the way
up on the radio, there will be distortion. If you start hearing
distortion, turn the volume down. A high power amplifier allows
the volume in the system to be higher, while the volume control
on the radio is down in the range where no distortion is present.
It is better to have more power than what you need to get
cleaner sound.
So how much power do you really need? 30 to 50 Watts (each)
would be OK for your front and rear speakers, while a little
bit more (100-150 Watts) should be applied to each sub. If
you are powering up your tweeters independently, they require
less power (20 - 40 Watts). Example: A four-channel set-up
with separates in the front and coaxials in the rear with
two subs will need about 40 Watts on each channel (Total=160W),
and 100W going into each sub (Total=200W). Notice that total
power going to subs is more than total power going to the
rest of the speakers. This is because our ears are less sensitive
to bass.
Wiring
A trick that professional
car audio installers use to get
more power out of amplifiers is to wire up speakers in different
ways, playing with resistances to achieve a desired total
impedance "seen" by the amplifier. Even though speakers are
active loads (resistance changes with frequency), it is accepted
to treat speakers as resistors with a fixed resistance value
(usually 4 ohms).
Parallel Resistance:
People commonly hook up two or more speakers to the same channel
out of an amplifier in parallel. This is achieved by hooking
up the negative wire from the amp to all the negative connections
of the speakers, and the positive to all the positive connections
of the speakers. By doing this, the load seen by the amplifier
is lower. For example, if two 4-ohm speakers are wired-up
in parallel, then their total resistance will be half, or
2 ohms. If three speakers are wired up in parallel, and they
all have the same resistance value, then the total load would
be a third of the value of each speaker's resistance. Here's
a formula to calculate parallel total resistance for two speakers:
For more than two speakers, use the following formula:
So what are the advantages and disadvantages of this scheme?
First, if one of the speakers burns out, then the other one
(s) keep playing. If the amplifier is not designed to receive
lower loads provided by hooking the speakers up in this fashion,
you might end up destroying your amplifier. Check your manual
or consult an expert.
Series Resistance:
Speakers are hooked up in series to decrease total load to
an amplifier. To hook up speakers in series, connect the positive
terminal of the amplifier to positive of one speaker, then
hook up negative of that speaker to positive of next speaker,
and so on. Then hook up negative of last speaker to negative
of the amp. It is a lot easier to calculate total resistance
for speakers hooked up in series. This is easily done by adding
up all the individual resistances:
The disadvantage of hooking up speakers in series other than
getting less power out of an amplifier, is that if one of
the speakers burns up, the other one (s) stop working.
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