Signal / Head Units
Receivers (“deck”, “source”, “radio”, “tape deck”, “CD Tuner”, or whatever you want to call them), are simply the component that generates the signal that you are trying to hear. The “format” is always changing, and although the Compact Disc is currently dominant, in the future we may (soon) be playing decompressed digital audio files (such as the MP3 format) in our vehicles that we have downloaded from the Internet, or using some digital broadcasting system.
Of all the components in your sound system, the signal source component you choose has the least amount of affect on the acoustic performance of your sound system. You can build a fantastic sounding system around an entry-level CD deck. Of course it will sound a little bit better with a high dollar head unit, but, if, for example, you have an extra $100 to add to the budget for your system, and you want to have it help with the sound more than say, looks or ease of use, add it to your speakers or amplifier(s).
Factory Head Units
Many vehicles now feature CD players from the factory. These decks can be used as a source if a line level converter (about $25) is added since they only have speaker output leads and not “pre-amp outputs”. The performance can be satisfactory but never superb because the signal is still traveling through the IC chip amp inside the deck, which adds distortion and muddys-up the over all signal a little.
“Factory” or “stock” sound systems that come with vehicles today sound a little better than they used to. Many people, after buying a car, can be heard saying that they sound “not too bad”. That’s great that they don’t sound too bad, but shouldn’t the goal be to get it to sound GOOD? Your listening to music in your car MUCH more than at home, not that you’re in your car more, but when your home there’s TV, phone, conversation, sleeping, etc. and we just don’t get around to really listening all that carefully even if the stereo is on. Your car is the place to listen to what you want when you want, as loud as you want, singing along if you want, and it can actually be a great “room” to listen in. Although the road noise can detract somewhat from appreciating the quiet parts of the music, the small dimensions of the room and the fixed listening position are actually ideal for creating a superb acoustic demo chamber. So, ironically, at home we have a quiet noise floor (10 to 20 db) and 100 watts per channel and in our vehicles we have a loud noise floor ( 50 to 80 db) and yet people try to get away with listening to just 12 watts per channel !? The 12 watts refers to the little amp built in to your deck. All radios now a days have the same little amp in them (no matter what they claim to have for power – “40 x 4″ is just a marketing trick – read lie) There are really just two types of amp designs found in 12 volt environments.
The first type of amp is found inside radios and is commonly referred to as a “chip amp,” or “IC amplifier.” These are extremely low cost “amps” that use the 12 volts DC that is present in the car to produce a maximum of about 12 watts of usable power. This is the most they can make due to the Laws of Physics: power (watts) = current (amps) X voltage (volts). Some additional drawbacks to this type of amp design are: high distortion (typically 1% THD-”Total Harmonic Distortion”), low signal to noise ratio (slight hiss always in your speakers), and crummy stereo separation. The advantage is that it is a small integrated circuit chip and does not give off much heat and so they are included “free” inside all radios. The confusing part is that the manufacturers, who are not regulated regarding this, put numbers like “30 or even 45 watts per channel” on these units, which sounds as if it might have enough power. They do produce micro-second long spikes up to 40 watts, (if turned way up), but it is then accompanied by about 20% distortion–and when you turn it down until it sounds okay, it is then at about 12 watts again. Remember that number – 12 watts at 1% distortion.
Amplifiers that use the second type of design are called “transformered” or “discreet” since they have separate power transistors and true power supplies. These amps take the 12 volts DC and convert it to AC. Only after you have AC, can you make higher voltage. This lets you produce more than 12 watts, and at much less distortion (typically .1% distortion [a tenth of one percent]- that’s ten times cleaner than IC chip amps, and a lot more power). The disadvantages to these amps is that they are expensive (starting at $100 for a 50 w. / chan.), big and heavy (need to be mounted under seat), and they need a big, fat, wire hooked up to the battery to run them. However, it is all well worth it once you hear the difference!
Most speakers will handle more power than you think. What they can’t handle is distortion. Distortion is what an amplifier puts out when it isn’t big enough to play as loud as you would like it to. It’s that extra, fuzzy sound over the top of your music when you reach the limit. All amplifiers distort when you turn them up past their maximum output limit. Your amp tries to amplify the small signal from your deck without altering its smooth shape. Your amp has a “voltage rail” limit of how big it can do that before it chops off the top and bottom of the sine wave, causing distortion-which will damage your speakers.
To summarize, if your stereo is not loud enough, or if you’ve blown a speaker from distortion (which isn’t covered under any warranty) and had to get another one, buy a bigger amp. Almost every time a speaker “blows” it is due to distortion. It is very rare indeed that a manufacturing defect doesn’t show up in the first few hours or minutes of play. For best performance use an amplifier that puts out as much power as the rating of the speaker. The only other way a speaker can blow (even with clean power) is from over-excursion. However, you need to play it at over-excursion levels for a while in order to break it, and it sounds so terrible and obvious, that anyone would turn it down long before that would happen.
Bigger the Better? Yes. That is, the more power an amp can produce, the better the sound quality will be, all other things being equal. The physical size of an amp, however, will not tell you much about how much usable power it can produce. In fact it is quite common for cheaper, lower quality amps to feature a huge, over-sized heatsink to try to look big and powerful.
Power ratings. Since there are no regulations to ensure truthful, comparable power ratings of car amps, the best way to determine the output of a particular model is to look in the owners manual, and under specifications, read what is usually the SECOND line (the first is just Maximum power with any amount of distortion) that should state the “RMS” (average) continuous power at a certain distortion rate (.1% or less), per channel, at 4 ohms, while playing a full-frequency signal (20-20khz). Only then can you know how much “usable” power is available, and even then there are other aspects of a amps’ design that can make one sound better than another with the same power specs.
Factory vs Aftermarket
A discussion of speakers must always refer to the amplifier that will drive the speaker. This is very important to understanding speakers, so be sure you’ve read the discussion on “Amplifiers”. “Stock” or “factory” speakers that come with your car are often good enough to be part of a great sounding system IF you power them with a clean, discreet amp. As an additional measure of building a really great sound system, you could upgrade the door speakers and lastly the rear speakers.
Do I really need “midbass” drivers? “Midbass” drivers in a car audio system are ones that only play a narrow band of frequencies typically between 70 and 2or300hz. They usually are in the form of a pair of 6″drivers shoe-horned into the front doors. There needs to be a midrange speaker to play above 300 hz. and advanced crossovers and/or a dedicated amp for the midbass drivers as well. In other words, midbass drivers are usually only found in very elaborate systems and provide only minor enhancement to a sound system when compared with other options available with that much time, energy, and money (like a trunk e.q., for example).
In order to make the right decision on which type, size, and model of subwoofer you may need, you should ask yourself a few key questions: “What type of vehicle do I have” (if sedan, see “Cars with Trunks”) “What level of sound performance do I need?” “How much space am I willing to give up?” “What other items do I sometimes need to have in my car?” (recreational gear, etc.) “What is the approximate budget for the system or components being added at this time?”(sub, amp, etc.)
In optimizing placement of subwoofers in vehicles, one must be aware of the “transfer function” of the interior space of the vehicle, commonly referred to as “corner loading”. Bass is resonating air, and it’s easier to do that in the semi-trapped air of a corner. Therefore, the best place to locate your sub for maximum output is in and toward a corner. Unfortunately, this is not always where you would like it for aesthetics or ease of use, however, it will be loudest there and may allow you to have a smaller box and/or amp. Also, if you use a “bandpass box” (which is the best type for use in vehicles, which need small size and high output over a narrow band), it will also minimize the slight port noise inherent in all ported boxes. In home stereos, this corner loading is undesirable, producing an unnatural peak of sound energy at a low frequency, however in a car, or especially a truck, it is very welcome to do battle with the road rumble trying to mask your music’s rhythm.
Sedans (cars with trunks)
Bass is the resonating of air. In order to hear (or feel) bass, you need to resonate the air near you. If you put a subwoofer inside a closed trunk only a small portion of the bass can be heard or felt in the cab of the vehicle. However, in sedans, there is no other place for a sub, and so one theory (not a very good one), is to put in a huge sub and huge amp and hope it can be heard up front. This idea of wasting