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What Does a Car Subwoofer Do?

A subwoofer is a speaker in a car audio system that projects the low frequencies, also known as bass. Even if you don’t know what a subwoofer is, you’ve heard its effect. If you’ve ever heard the low rumble in a movie theater, you’ve heard a subwoofer in action. If you’ve ever been sitting at a stoplight and felt your car vibrating from a car’s sound system behind you, you’ve heard a car sub in action.

Subwoofers are an important part of any car stereo’s sound quality. Without a sub, you’ll be missing the low-end frequencies that were present in the studio where the music was recorded originally or that the engineer was listening to while mixing the song (typically the bottom two octaves, from 200Hz to 20Hz). Your vehicle’s sub can be set to create window shaking thumps for rock and rap music or set to produce a more subtle bass response for classical music.

How Does a Subwoofer Work?

A subwoofer, like any speaker, makes sound by vibrating. This vibration moves the air, which then arrives at our ears as sound waves. A subwoofer’s speaker cone is larger than a mid-range speaker or tweeter. If you’ve ever looked inside a piano, you’ll notice the bass strings are longer and thicker than the high treble strings. The longer strings can vibrate at a rate that generates a lower frequency. The same physics of sound concept applies to a vehicle sub. Because a sub’s cone is bigger around (typically 10″, 12″ 15″, or even 19”) than a mid-range speaker, it can more naturally produce the lower frequency range.

What are the Components of a Subwoofer?

A Subwoofer Speaker has Several Main Parts

The audio’s electrical signal passes into a speaker’s voice coil which is a wire wound on a cylinder. The cylinder sits between two plates (pole piece) and is surrounded by a magnet. The plates act as attraction points for the magnet. Think of it as the refrigerator door you stick a magnet on, except there are two of them, one on each side.

The magnet receives electronic signals from the car stereo amplifier. These signals are charged positively and negatively, causing the voice coil to move back and forth between the plates. The voice coil moves up and down inside the magnetic gap, moving up on the positive part of the audio signal, and down during the negative part of the audio signal. The voice coil then transfers this energy into the sub speaker’s cone.

As the large round cone vibrates synchronously with the voice coil, it creates air pressure waves that our human ears perceive as sound. This cone is typically made of treated paper, polypropylene, or even light-weight metal. Rockford Fosgate uses polypropylene and aluminum materials for improved sound quality and durability. All Rockford Fosgate subwoofers are designed for sound quality.

Other Parts of a Subwoofer

A spider and basket hold the cone in place and keep the voice coil properly aligned inside the magnetic gap. A dust cap covers the hole in the center of the cone and keeps dust and other debris from entering the speaker.

The enclosure is the box in which the subwoofer is mounted. The enclosure affects the sound a subwoofer produces. Some subwoofers are designed to be installed without enclosures, allowing the trunk of the car to act as the enclosure. The best performance typically comes from subwoofers loaded in some type of enclosure.

Does Sub Size Matter?

A general rule is that the bigger the size of the subwoofer, the lower the frequencies it can produce. However, keep in mind that you’ll need space in your vehicle for that giant 15” or 16″ subwoofer. Also, two smaller subs can sometimes provide equivalent bass to a single larger sub. If you are installing a subwoofer enclosure, you’ll want to make sure you carefully measure to ensure you have the needed space requirements. Power, Impedance, Sensitivity, and Frequency?

Power, Frequency, Sensitivity, and Impededance

What is subwoofer power?

The size of a subwoofer isn’t the only thing that contributes to the sound quality it generates. Power matters. Every sub will have a “Watts RMS” value displayed which is the continuous power the speaker can accept over an extended period of time. This is different than peak power or max power. The continuous power a speaker can handle without being overpowered is lower than its peak power. A sub might be able to handle 400 watts peak, but only 300 watts RMS.

You want to make sure you don’t overpower your speaker in terms of the wattage the amp delivers to it. A speaker with a high RMS value paired with a low watt amp won’t deliver near the amount of bass it potentially could. The general rule of thumb is to use the RMS power ratings when matching speakers to an amplifier. Never exceed the RMS power ratings to prevent blown speakers. The most common cause of blowing a subwoofer is over-powering it and causing the voice coil to burn.

What is subwoofer frequency?

Frequency refers to the spectrum of sound the subwoofer can produce. This number is given in Hz ratings on subs. Most humans can hear frequencies between 20 Hz – 20,000 Hz. The lower the number, the lower the sound. Typically, a subwoofer has a crossover that keeps the higher frequencies from being sent to the subwoofer. As frequencies drop below 20 Hz, few people can hear the sound. Typically, frequencies below 20 Hz are felt but not heard. A subwoofer capable of producing such low pitches contributes physical vibrations, rather than audible sound, to the experience.

What is subwoofer sensitivity?

Sensitivity is a measure of how much power the sub needs in order to produce output or sound pressure level, known as SPL. Speakers with higher sensitivity will require less power to produce a higher SPL. The most common measurement is the “1 watt / 1 meter” specification that states, for 1 watt of power input, how loud will the subwoofer be when the microphone is placed 1 meter away. The lower the SPL, the more “power hungry” the subwoofer will be.

What is subwoofer impedance?

Impedance is the amount of electrical resistance the speaker has. It is measured in ohms. The lower the impedance in ohms, the more power the speaker will draw from the amplifier. A 2-ohm speaker will tax your amp more than a 4-ohm or 8-ohm speaker. In car audio, 4-ohm speakers are the norm and 2-ohm, or 4-ohm dual voice coil subwoofers are commonly used. You want to match the impedance of your subwoofer to the impedance your amplifier delivers optimal output power. Be careful not to connect a speaker impedance lower than what your amplifier is rated to handle.

What is the Difference Between a Passive and Active Car Subwoofer?

An active subwoofer has a built-in internal amplifier. You’ll sometimes hear this referred to as a powered or amplified subwoofer. A passive subwoofer requires an external amplifier to power the speaker. Because an active unit has the amp built-in, you know that the amplifier and subwoofer are properly matched for optimal output, but the enclosure is already built so choose the one that fits into your vehicle. A passive subwoofer will need an amplifier but does allow for flexibility of placement since the components are not connected.

Do I Need a Subwoofer in my Car?

Having a subwoofer helps a car stereo system sound better.

If you have a car stereo system without a subwoofer and you push up the bass on your car’s equalizer to the highest levels, you’ll hear the mid-range speakers begin to overload and break up. Why? Because the speakers are at their limit on what they are capable of reproducing.

A car subwoofer system is designed only to produce the low frequencies, allowing your midrange speakers to do what they were designed to do best–deliver the middle frequencies. The midrange becomes clearer because the speaker cone doesn’t have to move furiously back and forth, trying to reproduce low bass. Removing the low frequencies from the main loudspeakers lets them play louder because they don’t have to reproduce low frequencies.

When you move the load responsibility of low frequencies to a subwoofer, your whole car sound system will improve, being more balanced and having better sound clarity. Your musical listening experience will be improved in terms of clarity, and you’ll hear the difference. Of course, if you want massive, window shaking bass, you can set up your subs to hit hard and ignore the flat response curve.

See the next car subwoofer article: What is the best position for a subwoofer in a car?