In-car entertainment has come a long way since 1930, when Motorola introduced the first commercially successful car radio, the Model 5T71. Over the past 80-plus years, car audio has evolved from the basic AM radio receiver with a single speaker to complex electronic systems reproducing music and other entertainment from both over-the-air signals and recorded formats. Many systems today can play music from a staggering array of audio sources: radio, CD, portable music players like the iPod, USB flash drives, SD cards, Bluetooth audio and hard-disk drives.
The overwhelming number of audio choices can make the average consumer's head spin. This five-part series, "Understanding Car Audio Systems," will give you a grasp on the basics of today's multifaceted "infotainment" systems. You may also want to check out "How To Shop for an Aftermarket Car Audio System." Even if you plan to keep your car's stock sound system intact, a good understanding of aftermarket car audio options can be helpful when you're new-car shopping — especially if you're tempted to buy a premium factory system.
In recent years, automakers have really stepped up their game with their original equipment manufacturer (OEM) systems. But no matter how complex it is, every car audio system consists of three main components. The first is the radio or "head unit" that controls the entire system and generates the audio signal. The second is an amplifier that increases the strength of the audio signal so that it can drive the third component, the speakers that reproduce the sound.
The Head Unit
A high-end home audio system uses separate electronic components — a radio tuner, CD player, preamp, amplifier — connected by various cables. Since space is at a premium in a vehicle, automakers have to cram as many of these components as possible into one device: the head unit. Because of this, the head unit performs multiple duties, but its two main functions are controlling the overall system volume and the various audio sources in a vehicle.
When you look back at automotive history, AM radio was the only audio source in a car for years. Then came FM and tape formats such as 8-track and cassette. A CD player is now standard in most vehicles, and many factory head units can also receive satellite radio from the subscription services Sirius or XM. HD Radio has recently given AM and FM better sound and extra features, such as additional channels and information.
Head units can also be used to control media players like the iPod or navigate the content of a USB flash drive that's connected to the stereo system. Head units with Bluetooth audio can also play music that's streamed wirelessly from a compatible mobile phone.
In addition to controlling the system's volume, head units usually include basic tone controls such as bass and treble to tailor the sound to the listener's taste. Many audio systems also include signal processing that automatically adjusts the volume, depending on the ambient noise in a moving vehicle. Some high-end OEM audio systems also have a separate subwoofer and subwoofer-level controls.
A car stereo system has to have an amplifier to increase the power of an audio signal so it's strong enough to move the speakers and create sound. Amplification is a two-stage process handled by a preamp and a power amplifier.
The preamp is usually housed inside the head unit and takes data from a radio, CD player or other audio source and prepares it for the power amplifier. This process includes slightly boosting the audio signal, which makes it compatible with the input of the power amplifier and ensures that it's resistant to noise that can radiate from other electronics in a vehicle. The power amplifier then takes the preamp's low-level signal and significantly boosts it so it can move the speakers and create sound.
Many head units have a small, built-in low-power amplifier that can "drive" smaller speakers. This allows the audio system to be reduced to just a head unit and a few speakers. But better sound requires more power. So higher-end systems have separate power amplifiers that are mounted away from the head unit due to their size and the heat they generate. We cover the details of "outboard" amplifiers in a separate article in this series.
Speakers take an amplified electrical signal and convert it into mechanical energy that moves the speaker cone back and forth to create sound. Sound is essentially vibrations in the air that we hear, and a speaker cone creates these vibrations. The human ear hears these vibrations in a frequency range from about 20 hertz (very low bass) to 20,000 Hz (very high notes).
The most basic automotive speakers are designed to be "full range" to cover the entire frequency range. But by trying to cover the entire frequency spectrum, bass response is generally nonexistent and higher frequencies are dull. You can get more accurate sound reproduction by using an assortment of speakers dedicated to reproducing a smaller range of sound.
Woofers and subwoofers are large speakers designed to reproduce only low-frequency bass sounds. The aptly named midrange drivers handle the middle-range frequencies. Some systems take it one step further and use a specialized midbass driver to handle the troublesome frequencies between low bass and midrange. Tweeters are the smallest of the specialized drivers and reproduce the upper treble frequencies.
In many car sound systems, two speakers of different sizes are combined on one frame to create a two-way speaker. For example, a coaxial speaker mounts a small tweeter directly above and on the same axis as a small woofer. That creates a full-range speaker from two separate drivers.
In more elaborate systems, the speaker layout might consist of one or more subwoofers matched up with left and right midbass drivers, midranges and tweeters. Many high-end systems also use a single midrange or tweeter — or both — in the middle of the dash as a center channel to reinforce the front soundstage. We cover speakers in more detail in a separate article in this series.
We've Come a Long Way
As you can see, car audio has come a long way since the first AM tube radios were shoehorned into cars in the 1930s. Now that you've had a good overview of what makes up a car audio system, we'll dive into the specifics of the car audio command center in the next installment, which discusses the head unit.
You can reference other installments in this series here: