How Do Speakers Work?
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, Staging and Directivity
Imaging – is being able to pick certain sounds from different places. The singer would normally be located towards the middle of the car, guitars, trumpets, and other instruments towards the sides of the car. If you scatter speakers all around the car 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. 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 stereo. Tweeters are the most important part of getting good staging. They should be aimed towards the middle of the car. A way to “bring” the bass to the front of the car is to fool our ears by overlapping frequencies played by midbases and subs, so that your midbases actually “pull” the bass to the front, since lower bass in not too directional. You should crossover your midbases as low as you can (without getting distortion). Then cut your subs at a bit higher frequency (preferably 60 HZ or less). 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.
Types of Speakers
Coaxials – 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 coaxials 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, you will not have good staging or imaging. Some manufacturers try to compensate for this by making adjustable tweeters. You should usually consider coaxial speakers for the back of the car, and separates for the front, unless you only have one speaker hole and don’t plan to cut any more holes in the car.
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.
Horns – 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. Horns cost more than conventional speakers and require customization. In many installations a good equalizer is required to compensate for their high sensitivity.
Horns are not for everyone though. Many audiophiles complain of unnatural sound. It is very hard to properly setup a set of horns.
Midbases – Midbases 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. Midbases are most commonly mounted in the doors.
Subwoofers – Subwoofers add lower frequencies to the system. They have to be enclosed in a box, with the exception of free air subwoofers, which use the trunk as an enclosure. There are many different types of boxes and implementations discussed in the “subwoofers” section.
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, 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. You should be able to barely hear the rear speakers. If you are using rear speakers to add more bass/midbass to the system, at least use a crossover to cut off higher frequencies. A lot of hi-end systems don’t even have any rear speakers. Tweeters are not necessary for the rear, a set of coaxials will work good for rear fill.
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, and not in the door panels. Center channels are hard to implement: First, a bandpass 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.
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 inchers) play music from around 300, 500 Hz, to where the tweeters start in the upper level. Midbases (5, 6, 8 inches) play from around 50 Hz to 500 (and even 1000) Hz. Subs handle frequencies below 120-60 Hertz.
Do round speakers sound better than oval-shaped speakers (i.e. 6×9’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 6×9 speaker over a 6-inch speaker is that it has a bigger area, so it will move higher air volume, producing more bass.
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? As much as you can afford. At a minimum, 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.