What Is A Subwoofer ? – A Comprehensive Study

A subwoofer is a loudspeaker that reproduces low-pitched audio frequencies called bass. It comprises of a speaker driver, also known as woofer, which produces frequencies between 20 to 500 Hz. The woofer is housed inside a wooden or plastic enclosure. This enclosure prevents sound waves coming off the back of the woofer from interfering with the sound waves coming from the front. The woofer is powered by an integrated amplifier or by the receiver’s amplifier.

Subwoofer – A Detailed Study :

Most speakers and home theatre systems have tweeters and midrange drivers for reproducing high and midrange frequencies. But they may or may not have woofers. If they don’t, it means most of the lower frequencies below 100 Hz go unprocessed. As a result, they are unable to reproduce all the notes sent to them from the sound sources, and the original music is never fully reproduced. This is where a subwoofer comes in. It concentrates on lower end of the sound spectrum, and infuses the soundtrack with rumbling bass frequencies previously absent from the audio. It provides a foundation to the sound, strengthening it, and making it deeper and fuller, something the tweeters and midrange speakers are incapable of. Overall, it gives the sound a three-dimensional feel, and radically improves the quality of stereo sound.

Low frequencies consume a lot more power than high frequencies. Therefore, adding a subwoofer improves the power handling capacity of an audio system. The sub takes much of the burden off of the satellite speakers by taking care of the power-hungry low frequencies. This makes the system’s power handling more balanced and the sound more intelligible.

How does a subwoofer produce sound ?

A subwoofer is essentially a speaker. It converts an electrical signal into sound. The variations of the signal cause the cone in the speaker to throb in sync with it. This produces sound waves which are then propagated through air and water.

A subwoofer is always connected to an audio receiver or preamplifier which feeds the sub with low frequency signals via electric current. The subwoofer amplifies the signal and converts it to sound waves with the help of a magnetic coil. This coil produces vibrations in the woofer cone which results in sound waves being pumped into the medium (air or water) immediately in contact with the cone.

A number of factors influence the type of sound produced by a subwoofer. These include, but are not limited to, enclosure type, cone size, number of woofers, passive radiator, power handling capacity, amplifier power, sensitivity, frequency response, impedance, and single/dual voice coil.

Frequency Response of A Subwoofer :

Frequency response of speaker or subwoofer indicates the range of frequencies it can reproduce. The frequencies are measured in Hertz and written in abbreviated form as Hz. A standard subwoofer for home stereo system can handle frequencies between 20 to 200 Hz. High-power subs used in concerts have frequency responses below 100 Hz and high-end THX-approved subs operate below 80 Hz.

Active And Passive Subwoofer :

A subwoofer may be active or passive. An active subwoofer includes a built-in amplifier apart from the woofer and the enclosure. The amp boosts the audio signal and makes it powerful enough to drive the woofer. It might include a user-adjustable equalization function for boosting or reducing the output at different frequencies. It could have a single basic boost switch or sophisticated parametric equalizers for finer speaker and room corrections. An active sub must be connected to an electrical source to power the amp, and to a receiver for audio signal.

A passive subwoofer, on the other hand, comprises of a woofer and a cabinet, but does not have a built-in amp. It derives power from a receiver or external amplifier, and does not require a dedicated power source. It must, however, be connected to a receiver for signal. A passive subwoofer might have an integrated passive crossover. Entry-level home theatre systems often include passive subs while high-end models have powered subs.

Power Handling :

Power of a subwoofer is represented in peak power and RMS power. Peak power is the highest amount of power a subwoofer is capable of producing at a given point. RMS power, also known as continuous power, is the average power the device is able to put out over a period of time. Peak power obviously is higher than RMS power. RMS power is more reliable than peak power as it gives a more realistic picture of the sub’s power handling capacity. When matching a subwoofer with an amplifier/receiver, make sure their powers are compatible. Normally, you would want the subwoofer to be able to handle the same amount of power or more power than what the amplifier/receiver can provide.

Sensitivity :

The sensitivity of the woofer indicates the output it is capable of generating. A high-sensitivity subwoofer requires less power to produce the same amount of bass as a low-sensitivity subwoofer, all things remaining constant.

Woofer Size :

Different applications have woofers of different sizes. Home sound systems often have woofers with diameters between 8 and 21 inches. Small computer soundbars and compact home theater subs have woofers as small as 4 inches. Large subwoofer installations and single prototype subwoofers have drivers as wide as 60 inches. Subwoofers used in sound reinforcement systems in discotheques and musical gigs range from 10 to 18 inches.

The size of the driver and the number of drivers in the subwoofer depends on:

  • size of the cabinet
  • design of the cabinet
  • desired sound pressure level
  • targeted lowest frequency
  • level of permitted distortion

Placement :

Low frequencies are more or less omni-directional. They travel from the source in all directions, and are generally larger than the sound source. They bounce off walls and travel back to the listener with a multiplicity of phase variances due to path-length differences. This confuses the human ear and brain, which find it difficult to make out any phase differences. Consequently, the listener is unable to tell where the bass is coming from or where the subwoofer is located.

This property of bass allows you to place the sub anywhere inside an enclosed area. You don’t necessarily have to point it at the listening position. As long as you feel its effect, it doesn’t matter where the bass is coming from. You can, of course, experiment with different locations till you find the spot that produces the best sound. This, however, is not the case with higher treble frequencies. Stereo speakers are highly sensitive to placement and need to be aimed at the ears of the listener. If they are even a little off-center, it can cause a drastic drop in sound quality and listening experience.

One factor that greatly impacts the performance of a subwoofer is the room acoustics. If the acoustics of the room are poor, the subwoofer will also perform poorly. Most rooms have standing-wave problems and it is important to solve these before investing in a subwoofer.

Equalization :

Equalizers are used for adjusting the in-room response of a subwoofer. They enhance sound in different settings like rooms, cars, open-air concerts and studios. A sub might include an adjustable low pass filter for preventing higher frequencies from getting to the woofer. For instance, if the stereo speakers process sound down to 80 Hz, the sub’s low-pass filter could be adjusted to allow only frequencies lower than 80 Hz to reach the driver.

A sub might also have a subsonic high-pass filter to prevent the woofer from reproducing frequencies lower than the safe limit. This is especially true of bass reflex subwoofers where the cones are at a greater risk of over-excursion at frequencies lower than the predefined limit. For example, in a ported sub tuned to 20 Hz, a subsonic filter eliminates frequencies lower than 20 Hz, thus safeguarding the speaker driver from mechanical damage. Some subwoofers come with parametric equalizers. These equalizer settings offer greater control over the frequency content of the audio signal. They also correct irregularities in room frequency response.

Passive Radiator :

A passive radiator is an un-powered cone within a subwoofer enclosure that moves in sync with the powered cone. Its purpose is to add more oomph to the bass and boost the final bass output without consuming more power. With a passive radiator, the sub is able to generate more output without increasing the size or power of the internal amplifier. It optimizes the performance of the powered driver and increases the slam.

Types of Subwoofers :

Subwoofers can be differentiated based on the manner in which the woofers are arranged inside the cabinet. They can also be classified based on their efficiency, power handling, construction, cone size and distortion properties.

Here are 5 main types of subwoofers :

  • Bass reflex
  • Acoustic suspension
  • Horn loaded
  • Infinite baffle
  • Bandpass

Bass reflex subwoofer :

A bass reflex subwoofer, also known as a ported or vented subwoofer, has a hole or vent cut into the cabinet of the subwoofer to facilitate air circulation in and out of the box. A pipe or a tube section is inserted into the hole, and there can be more than one hole, either in the front or rear face of the cabinet. The hole allows sound from the back of the diaphragm to boost the reproduction of low frequencies and improve efficiency of the woofer. Routing the cone’s rear sound wave through the vent increases the output volume and lowers distortion.  It improves the bass response and extension of the device, giving it an edge over sealed subwoofer cabinets.

Bass reflex subwoofers are the loudest subwoofers (along with bandpass). They are also the commonest type of subwoofer enclosure sold in the market. The bass reflex cabinet adds more punch to the low-end frequencies. It ensures a lower cut-off frequency and a lower distortion at resonance frequency.

But how does a bass reflex sub achieve this feat? Let us try and understand the science behind it. As we know, the cabinet of a subwoofer separates the waves generated from the front of the cone from those from the back. When the waves from the back meet the waves from the front, they cancel each other out because they are out of phase. But, in case of a bass reflex sub, the hole in the cabinet acts like a Helmholtz resonator. It causes the back wave to reverse phase and emerge from the port in phase with the front wave. Consequently, the back waves do not cancel out the front waves, but reinforce them. This makes the device +3dB efficient as opposed to sealed enclosures of the same volume.

This phenomenon, however, only occurs at the resonance frequency of the vent. In case of frequencies higher than the resonance frequency, the mass of air inside the port is too great to respond to the movement of the cone and the enclosure acts like it is fully sealed. In case of frequencies below that level, the vent acts like any other hole and has no effect on the back wave.

Acoustic suspension subwoofer :

An acoustic suspension subwoofer has one or more woofers mounted inside a completely sealed enclosure without any vents. The air inside the box serves as an air cushion providing linear restoring force for the diaphragm of the woofer. The air works like a spring or rubber band, controlling the driver’s excursion and helping it oscillate a greater distance.

Acoustic suspension is one of the most widely used speaker enclosure designs. It lowers bass distortion, and offers tight and accurate bass response. It also has a smaller footprint as the cabinet is small. However, a fully sealed enclosure requires more amplifier power to produce the same amount of bass output as a bass reflex subwoofer.

Interestingly, although acoustic suspension means a sealed enclosure, it is not fully airtight. A small amount of air is allowed to filter in and out of the cabinet – mostly through a semi-porous cone surround, a fabric spider, or a fabric dust cap – to enable the driver to adjust to changes in atmospheric pressure.

Horn loaded subwoofer :

A horn loaded subwoofer employs an acoustic horn to enhance the efficiency of the speaker driver. There are 2 main types of horn loaded speakers. The first consists of a compression driver which is essentially a small metal diaphragm attached to an acoustic horn, and an electromagnet. The electromagnet vibrates the diaphragm to produces sound waves which are conducted by the flaring duct of the horn to the air. The second type is called a folded horn speaker. It consists of a woofer driver mounted in a loudspeaker enclosure with special zig-zag internal partitions which behave like the flaring duct of a horn.

Horn loaded loudspeakers increase the acoustic output power of the speaker driver. They are 10 times more efficient than cone speakers with the same amplifier output. Hence, they are commonly used in public address (PA) systems and hand-held megaphones to address large gatherings in sports stadiums, theaters, political rallies, etc. The downside of horn loudspeakers is that they have an uneven frequency response.

Infinite baffle subwoofer :

The front face of a speaker is called the baffle. It is where the speaker driver is mounted, and serves to keep the waves generated from the front and back of the driver from colliding. As a general rule, all speakers need to separate the front side of the driver from the rear to prevent the waves from cancelling each other out and losing their overall output.

In an infinite baffle speaker, the baffle in ‘infinitely’ large which helps it achieve the required separation. Some infinite baffle subs have very large sealed enclosures and some have no enclosure at all. The first kind consists of a sealed cabinet which ensures minimal air spring and gives more power to the cone. The air pressure inside the enclosure is not too high and does not restrict the movement of the speaker cone at high power and low frequencies. The latter kind comprises of a woofer mounted to a flat panel or board without any covering over it. The subwoofers you see in car doors, dashboards and rear package trays are infinite baffle subwoofers. They do not have an enclosure around the woofer driver and have an open installation.

Building an infinite baffle subwoofer is not very difficult although you have to take a few things into account. First, the woofer must be capable of being used in an infinite baffle set-up. Second, it must have a subsonic filter so that lower frequencies do not damage the cone at high power. Third, if you are constructing an infinite baffle board for your car, make sure you securely seal the mounting baffle edges to the vehicle so that the front waves do not cancel out the rear waves.

The plus point of an infinite baffle subwoofer is that it has a smooth frequency response and uses the speaker’s natural frequency roll-off. It is also space-efficient and is easy to construct. The flip side it that it often limits the sound output and power handling ability of the speaker driver. Also, the wiring is exposed on the rear side of the board which might give the device a rather untidy look.

Bandpass subwoofer :

A bandpass sub has a twin-chamber enclosure. The chamber behind the woofer is fully sealed while the one in front has one or more ports in it for tuning the output. The woofer does not directly fire into the listening area, but into the ported front chamber. This chamber functions as a low pass filter attenuating higher frequencies and allowing only frequencies lower than the cut-off to pass into the listening space though the vent.

Bandpass subs are of 2 types: single reflex and dual reflex. A single reflex sub features a sealed rear enclosure and a ported front enclosure. In a dual reflex sub, both the rear and front chambers are ported.

Bandpass subwoofers produce maximum boom. The sound coming out of the ported face is super loud within a small frequency range. Because of their high volume output, bandpass subwoofers are widely used in music festivals.

Subwoofer for Home Theater System :

If you are looking to add a subwoofer to your home theater system, here are a few tips to get you started.

First, choose a sub that blends well with the rest of your system. It must enhance the overall sound profile of the speakers and produce a balanced audio.

Second, don’t go by size; it is not always a reliable indicator of the device’s performance. A huge sub might look tempting, but it might not produce the best sound or be the best match for your speakers.

Third, different kinds of speakers require different subwoofers. If you have compact satellite speakers or bookshelf speakers, an 8 or 10 inch subwoofer would suffice. If you have a larger system with floor-standing speakers and powerful center channel speakers, a 12 inch subwoofer would do. If your room is huge, and you want thunderous bass, you might want to check out larger subs around 15 inches. For larger sound reinforcement systems, you might want to consider an 18 inch subwoofer or even a 20 inch one.

Fourth, if you want a clean clutter-free set-up, or if you want to place the sub far away from the stereo receiver, a wireless subwoofer could be an excellent option. A transmitter connected to the receiver, or housed in the subwoofer enclosure, sends the wireless signal to the sub. All you have to do is plug in the sub to an electrical outlet, power up and you are good to go. On a side note, any sub can be converted into a wireless sub using a wireless kit (although you still have to connect it to a power socket). If you already have a sub that is wired, this might be a cool way to make it wire-free.

Fifth, if you want to customize the subwoofer’s sound to your space, look for subs with room-correction software. It is the latest in subwoofer technology and is widely used in high-end models. It allows you to match the bass output to your room’s acoustics to produce the best chest-thumping effect. Some subwoofers also have advanced DSP (digital sound processing) systems that let you tailor the sound to your space. Some are app-controllable and can be monitored from your smart phone. The intuitive subwoofer uses the microphone on your phone to capture a near-field measurement of it bass output. It analyses how the audio sounds from your listening position, and automatically makes adjustments in the EQ settings to even out any irregularities, and give you the best sound possible.

Subwoofer for Car :

Car audio systems have compact speakers. They lack the ability to produce power-heavy low frequencies. Hence, the sound may sometimes feel tinny and metallic. The best way to counter this is to add a quality subwoofer. It will take care of the bass, and make the track deeper and warmer. Irrespective of the genre of music you listen to – soft rock, hard rock, country, pop, jazz, EDM – a subwoofer can greatly enhance your listening experience.

If you want a hassle-free ready-to-use solution, we would suggest you go for an enclosed sub. It comes pre-fitted in a sealed box which forms the housing of the woofer. You won’t have to bother with designing the sub or selecting an enclosure for the woofer, but you might have to add an external car subwoofer amplifier to the device. Sealed box subwoofers produce the most accurate bass among all types of subwoofers. They are widely used in cars, trucks and RVs.

If you want to build a fully customized unit, we would suggest component subs. You can choose the woofer you want and then match it with a suitable enclosure. You can try out different subwoofer crossovers, and impedance and voice coil set-ups. You can add an external amplifier and experiment with a variety of custom subwoofer enclosures to get the best output. Using this method, you can build anywhere between an 8 inch subwoofer to a 15 inch subwoofer with monstrous bass.

If you have a small car and are looking for a mini subwoofer, you should get yourself a powered subwoofer or may be a underseat powered subwoofer. It comes with the woofer and the amplifier built into the subwoofer enclosure. All you need to do is connect the sub to the receiver and a power source to drive the built-in amp. It has a space-saving design and won’t hog space. It produces good amount of bass to rock your car audio system. However, it might not be capable of high-power bass output as the size of the cabinet limits the size of the cone and the amplifier.

If you want your subwoofer to perfectly blend in with the interiors of your car, you might want to go for a vehicle-specific subwoofer. Brands often design tailor-made subs for a wide variety of vehicles. The subs match the color of the interiors and fit seamless into discrete hollow spaces inside the automobile. However, like powered subwoofers, they may not be able to produce big booming bass due to their small size.

Difference between a speaker and subwoofer :

The fundamental difference between a speaker and a subwoofer lies in their frequency response. As a general rule, a speaker processes higher and a subwoofer processes lower frequency bands. Let us discuss this in a little more detail.

Loudspeakers can be broadly categorized into 3 kinds: tweeters, mid-ranges and woofers. Tweeters reproduce higher frequencies, mid-range drivers take care of mid-range bands, and woofers produce low-end bass frequencies.

Most speakers and soundbars produce sound with the help of tweeters and mid-range speakers mounted inside an enclosure. They might ignore woofers as the focus is more on processing higher and mid-range frequencies with limited reproduction of the lower frequencies.

On the other hand, a subwoofer, being equipped with a dedicated woofer, concentrates on low-end frequencies, mainly below 80Hz, and does not handle high and mid-range bands at all. So, a speaker is a sound system which may or may not contain a woofer, while a subwoofer is a loudspeaker built around a woofer. That being said, it is important to note that home theater systems nowadays come with their own external subwoofers to improve the sound profile of the system. So, a multi-channel speaker system might include a separate subwoofer to cover for bass.

The cone of a woofer is also larger than that of a tweeter or mid-range. Therefore, subwoofers are typically bulkier and heavier than speakers.

Can a woofer be used as a subwoofer ?

A subwoofer reproduces low-pitched power-hungry frequencies between 20 to 200Hz. It has a very narrow frequency response and cannot process higher frequency bands. On the other hand, a standard woofer is more versatile. It is a broad-spectrum low-frequency loudspeaker that can reproduce sound between 20 and 2000Hz. It produces a wide range of sounds unlike the subwoofer that only focuses on a small range at the lower end of the sound spectrum. Another type of woofer called the mid-woofer can reproduce still higher frequencies up to 5000Hz, but is not so good at low frequencies below 200Hz. It performs best between 500 and 2000Hz, and fades away on both ends.

So the answer is no. A standard woofer or mid-woofer cannot reproduce the ultra-low frequencies expected of a subwoofer. They cannot be used to build a subwoofer, but can be teamed up with tweeters and mid-range speakers to construct soundbars and stand-alone speakers.

Best subwoofer brands :

The market is flooded with subwoofers of every make and shape. We advise you to do your research – read up on product details, online reviews, etc. – before selecting a product. Here are some leading brands you can check out : ELAC, JL Audio, Rockford, Kicker, Bose Corporation, Alpine, Klipsch, Polk Audio, BIC America, Sonos and Pioneer. They make subwoofers for home theater, car audio and sound reinforcement systems.


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