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Introduction to Car Audio: Selecting the Equipment

March, 2003

Brian Weatherhead

 

Introduction

By creating a list of features that you want in your car audio equipment, you can significantly narrow down the eligible equipment list. This list allows you to focus on the feature set that you really need, rather than being unduly tempted by marketing brochures and flashing lights.

Our Requirements for the Project Car Audio Equipment Installation:

Head Unit

  • DVD - We want to be able to watch (passenger only) and listen to DVDs.
     

  • MP3 - We want the head unit to play CD-Rs with MP3s.
     

  • Viewing Screen - Since this is being installed in a two seat roadster, there is no back seat. So, the screen has to be able retract into the head unit.
     

  • 5.1 - The unit has to be able to output 5.1 (DTS and DD), or decode it internally.
     

  • Dolby Pro Logic II (DPL-II) - The unit has to be DPL-II capable (External Processor).
     

  • Digital Out - We plan on running a digital signal to the external processor, bypassing all the possible noise issues RCA connections can expose.
     

  • CD/DVD Changer Control - There is little room upfront for a multi-CD case, so changer control is a must.
     

  • XM - On long trips, satellite radio is a really neat feature, because you never drive out of range of the transmitter. We may not use it right away, but we need the capability.
     

  • Satellite Navigation - Being able to interface to a navigation unit is a hot feature. We may use it, so we need a head unit capable of controlling a slave navigation unit. Or at least have a dedicated Navigation input.
     

  • Quality - In general, we want the head unit to have an S/N of at least 105 dB for CD, and a THD no more than 0.01% . Now this is where reputation comes in, as some manufacturers inflate their numbers.
     

  • Sound - Listen to the unit in the store, and if possible, partnered with the amplifiers you will be using.
     

  • Usability - In the car, usability is very important, as you don't want to take 45 seconds to navigate around to find your favorite track while driving.

Amplifiers

  • Power - We want 100 rms watts - 200 rms watts of clean output, into 4 Ohms. We want to be able to push them, because power leads to a tight sound and great dynamics. Note that car audio amplifiers are usually specified as power into 4 Ohms rather than 8 Ohms, because the speakers are usually 4 Ohms due to the relatively limited voltage (12 v - 14v) coming from the car battery. Often, there will even be 2 Ohm ratings, which are used with 4 Ohm drivers wired in parallel. The lower impedance results in more current, and thus more watts delivered with the 12v supplied from the car battery.
     

  • Power Generation - Since an SQ car (term used in car audio contests) is judged with the engine off, we need an amplifier that will make its rated power at 12v. Many amplifiers make 50% less power when the car isn't running, which results in a drastic change to how your music sounds. Having an amplifier that makes the same power at 12v (engine off) vs 14v (engine on) is a significant advantage.
     

  • Quality - We want an amp that's neutral, so the frequency response should be within 0.5 dB throughout the audible range. Signal to Noise should be equal to or better than the head unit (>105 dB). THD should also match or exceed the head unit, with a maximum of 0.01% at rated output. These numbers in a spec sheet mean nothing if the company has a shady history.
     

  • Sound - Listen in the store.
     

  • Crossovers and Inputs - Plan your installation, because you may find out you need an amp with speaker level inputs, or built-in crossovers. Our 5.1 processor will be doing all that for us, so it isn't a consideration for us.

Main Speakers

  • Listen at the store. Good speakers are an investment, because good speakers cost good money. Stay with established names and listen before you buy.
     

  • Size - I recommend that you fit the largest speaker you can in the front door. This brings the mid-bass closer, and gives better clarity.
     

  • Quantity - I highly recommend a 2-way (tweeter and mid-bass) or 3-way design (tweeter, mid, and bass). This allows the individual speaker driver to focus on what it can reproduce the best, rather then having to play all the frequencies.
     

  • Placement - If you do a 2-way or 3-way system, keeping all three of the speakers physically close to each other is key. The greater the distance, the greater the chance for noding (places in the car where the sound is canceling), and staging problems.
     

  • Rear-Fill - For two-channel reproduction, you really don't want to hear the back speakers. Spend all your money on a high quality front set, and disconnect the back, or leave them connected directly to the head unit. For surround sound, of course, this is a different issue.

Subwoofer

  • Size - A single 10" or 12" driver will be more than enough for 90% of us. 12" drivers are a great compromise of size and extension. Often the space you have to dedicate to a subwoofer is a very important factor in selecting the correct one. Talk to a local shop, tell them what music you listen to, and demo a few. Show them where you want it installed. They will tell you what will fit in that spot.
     

  • Power - While you demo the sub, make sure you are listening at a power level (wattage) that you will be using in your car. The more power, the higher the SPL, and tighter the sound.
     

  • Enclosure - With today's drivers, I always recommend a sealed enclosure rather than a ported one. A sealed enclosure allows for better cone control, and a more linear playback. The tradeoff is 3 dB - 6 dB less output for the sealed
    enclosure. I recommend staying away from drivers that are geared at
    producing extreme loudness, as they usually give up some quality (more
    distortion).

Bass-reflex enclosures, also known as ported enclosures, are much more prone to output variations due to tolerance difference between drivers, which is
actually common. Every enclosure has a tuning frequency called F3 (F3 is the - 3 dB down point, and is one of the Thiel/Small parameters for characterizing speaker design). When a subwoofer plays below that tuning frequency, it's harder for the cone to maintain a linear motion. Due to the damped nature of a sealed enclosure, the sealed design lends itself to lower extension with more
cone control below that F3 point. Although the ported enclosure has a benefit of generally being 3 dB louder at that same tuning frequency, the design doesn't damp the cone very well bellow F3. This leads to leads to lower output and less extension when compared to a sealed enclosure.

For a given -3 dB point, a reflex system can be designed with roughly half
the box space. However, if you took a given driver in a sealed enclosure, you could not put it into an enclosure half the size by porting it. On the contrary, you'd need a larger box. However, you'd end up with a much lower F3 point. If the F3 point were fixed, you could also get more output (above the tuned frequency) out of the ported system with the same cabinet size, through not only the contribution of the reflex device, but greater efficiency. That's not to say that you can do any of this simply by changing one parameter. If you change the alignment, you must also change the enclosure and/or driver to get a good response.

In terms of drivers that play loud, from what I've seen of good drivers, the compromise tends to be more in terms of bandwidth. Playing very deep requires excursion. More excursion requirements mean less output capability. Generally speaking, for woofers, you want high thermal capacity, a linear motor, and a large X-max, particularly with sealed alignments. (X-max is the maximum length that a driver's voice coil can move back and forth inside the magnetic field of the driver.)

And now, see our own choices for our project car . . .

Click HERE to see the products we chose.

 

Related to the article above, we recommend the following:

Car Audio Intro - Our Project Car

The Corvette C5 Factory Audio System Evaluation

The Digital Link

What we Hear

High Fidelity

Accuracy, Distortion, and the Audiophile

 

Copyright 2003 Secrets of Home Theater & High Fidelity
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