Thursday, December 27, 2012
Fundamentals of Car Speakers -- Part 2
Speaker placement is not governed by hard and fast rules. Nevertheless, you can't go wrong if you follow certain guidelines. Coaxials are often mounted in the kick panels in front of the car. If your audio system consist of separate speakers, put the tweeters in front in the kick panels. Aim the right kick-panel tweeter at the driver; the left kick-panel tweeter at the passenger. Another location for tweeters is in the dashboard. Midbases are commonly mounted in the doors. Horns are often installed under the dashboard and the best place for center channels is in the middle of the dashboard.
Subwoofers (with the exception of free- air subwoofers) have to be enclosed in a box. Free air-subwoofers are normally mounted in the boot against the rear seats or under the rear deck. Since the boot acts as an enclosure for the subwoofers, the former has to be sealed. This is achieved by putting particle boards under the rear deck and behind the rear seats.
Several types of enclosures are possible for subwoofers. A sealed box features the subwoofer firing into the car with the back-wave suppressed inside the box. This box gives tight, accurate bass. A ported box features a vent (or port) that lets some air out of the box. Compared to a sealed box, a ported box delivers more output at the same wattage. For forceful bass found in rock and heavy metal music, some audiophiles prefer ported boxes. A bandpass box consists of a subwoofer between a sealed and a ported box. The sound that comes from the ported box carries extra thump, but over a narower frequency range. Bandpass boxes are thus super for rap and hard rock.
Some car audio freaks like to build their own subwoofer boxes. The most common materials used are five eight-inch (or thicker) particle board or medium density fibreboard; sometimes fibreglass is used to mould complex shapes. Subwoofer boxes usually have to be of the appropriate volume and design. Instructions for building a subwoofer enclosure can be found in car audio books. Speakers are wired either in series or in parallel. Each method has advantages and disadvantages.
The different ways are used to play with resistances so as to get more power out of amplifiers. Speakers wired in series means the positive of the first speaker is connected to the positive of the amplifier, and the negative of the first speaker is connected to the positive of the second speaker; then the negative of the second speaker is connected to the positive of the third speaker and so forth. Finally, the negative of the last speaker is connected to the positive of the amplifier.
The series total resistance is calculated by the following formula: R(t) = R(1)+ R(2) + R(3)+...R(n). Therefore if six 4-ohm speakers are connected in a series, their total resistance is 4+4+4+4+4+4= 24 ohms. One advantage of series connection is that if one speaker shorts, the others are not affected; on the other hand, if one speaker burns up, the others stop working.
Parallel connection of speakers means all speakers' positive terminals are connected to the positive terminal of the amplifier; likewise, all the negative terminals of the speakers are connected to the negative terminal of the amplifier. The formula for total resistance in parallel connection is l/R(t) = l/R(l)+ l/R(2) + l/R(3) +... l/R(n). So with six 4-ohm speakers connected in parallel, the total resistance is l/(l/4 +l/4+l/4+1/4+l/4+l/4) = l/3.5 = 0.28 ohms.
In parallel connection, if one speaker bums up, the others continue to play. Many amateur car audio enthusiasts hold he misconception that too little power can't harm speakers while too much power is bad for them. Actually, when speakers are over- powered, they can be destroyed thermally only when they don't have crossovers blocking off the frequencies they were not intended to play.
What is more dangerous to speakers is distortion, caused by turning the volume up to "make good" the lack of power. Distortion damages speakers in two ways. First, it raises the level of high frequency energy and it also raises the average power of the music signals. Unfortunately, for long term signals most speakers can handle lower power. It is therefore better to have more power than what you need. For front and rear speakers, you would need at least 30-50 watts each and for each subwoofer, at least 100-150 watts.
When you upgrade your speakers, bear in mind that it is also necessary to upgrade the speaker cables. [n most cases, however, 16 to 18 gauge wiring should be sufficient, with the exception of high-powered subwoofers.
Now how do you evaluate a speaker? The best way is to listen to recordings of music that you are familiar with. This means bringing your favourite CDs to the car audio shop. Good salespeople will play particular recordings to highlight the strengths of a speaker.
Another recommendation is to listen to light, simple music first such as sentimentals, blues and jazz, and then followed by more complex music with plenty of instruments (such as orchestra). This ensures that the speaker doesn't get hazy when things get complicated.
Two technical specifications that also warrant consideration are the speaker's continuous poer handling capability and its sensitivity. Remember that continuous power handling capability is more important than its peak power handling figure. For tweeters and midranges, power handling is not so important. For a subwoofer, its power capability must match that of the amplifier driving it. Sensitivity refers to how loud a speaker plays. At a given wattage, a highly sensitive speaker will play louder than a less sensitive speaker at the same wattage.
Is there a "best speaker" in the market? Generally, no particular brand of speaker can claim to be the best. The best speaker is the one which suits the listener's application best. May the best sounds be yours.
Published by Monsoon Books, Singapore. Sold in Malaysia from Kinokuniya, Popular Book Store, Borders and MPH nationwide. An e-book edition is also available.