How do you feel when a car speeds by you giving off that deep thudding bass hits of a song you can barely decipher? Many times, it’s the rich sound produced that attracts your attention more than the recognition of the beat exuded.
How about the movie theater and the thunderous sounds that come from the background when a movie is showing? What kind of speaker is used to produce these deep bass sounds?
Surprisingly, it’s a subwoofer. And just like the name, it’s not one of the main speakers yet it greatly enhances the output of the main speakers. Sometimes, it is also a part of the home theatre and home stereo.
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What is a Subwoofer?
A bass sound has low range that bothers around 16Hz and above. This range is usually picked and played by a woofer (a kind of speaker). However, the woofer tends to play from a range of 40Hz to about 500Hz. What then happens to the bass frequencies below 40Hz produced from the music being played?
This is where the subwoofer comes in. It is a special woofer that picks those ranges below 40Hz going as low as 20Hz. It plays the bass frequencies that are very low in a piece thus intensifying the low range of a regular loudspeaker and extending over higher frequencies.
The frequency range for a subwoofer may vary differently according to the brand it represents, and its use. Subwoofers used in specialized recording may come with a normal frequency range that is lower than 100Hz while that of a THX-approved system may be lower than 80Hz. The normal frequency range for consumer items may be between 20 to 200Hz.
You’d find a subwoofer set between two stereo speakers (to the left and the right). This design is known as 2.1 – that is, two chief channels and a subwoofer responsible for the low frequency effects (LFE) channel. When the subwoofer is surrounded by 5 main channels, then the system is known as 5.1.
A subwoofer is sometimes built with wood such that it is able to withstand distortion and air pressure. However, most subwoofers are built into a big box which helps to ensure that only the right quantity of air gets inside. An active subwoofer comes with a built-in amplifier while a passive subwoofer comes with an external amplifier that supplies it with power and audio signal.
Subwoofers are an important part of the set of speakers that make up a home theater audio system. However, the acoustic makeup of a musical room plays a vital role in how well a subwoofer performs. Therefore, you need to make sure that standing wave in your room is properly sorted to enable your subwoofer fulfills its role excellently.
How does a Subwoofer Work?
The subwoofer is meant to reproduce the very low frequency sounds released in a beat or sound that would, otherwise, be lost to the other speakers in the setting. Low frequencies do not move in a particular direction and it is this quality that gives the subwoofer an advantage for picking up these sounds. You can decide the best position for the subwoofer by testing out the reverberations it produces at different points from a central position.
A subwoofer receives signals from a sound system sent to it as electric current. The signals are called low frequency effects (LFE). The subwoofer contains a magnetic field through which it boosts the amplitude of the electrical current causing the diaphragm to vibrate. It is this vibration that produces the bass sound waves which fill the air.
When electricity flows through the speaker, it creates a magnetic field that pushes through the magnetic system to produce some energy. The amount of energy the subwoofer can take on is determined by the size of the voice coil.
What are the Components of a Subwoofer?
All the components of a subwoofer will vary slightly depending on the maker and the choice of material or design. However, there are basic elements that every subwoofer must have, albeit, in different designs.
The Suspension System.
This is the mechanism that keeps the voice coil at the center of the magnetic system.
The Basket. This is the part of the subwoofer that houses the entire components of the speaker. It is made from a strong but light material that allows it to fend off unwanted resonance from the speaker. The basket can be made from steel or aluminum.
The Cone. Also called the diaphragm, this part creates excitement in the air around it to produce sound through its back and forth movement. The cone has a midsection (can also be called a dust cap) whose purpose is to protect the voice coil from the encroachment of dust.
Spider. This part is fixed to the cone and the voice coil. Its job is to regulate the upward and downward movement of the cone assembly. It also helps to secure the voice coil at the center of the magnetic gap and to keep away all foreign bits from it. The spider will easily sag if it is not made from quality material.
Tinsel Leads. They are connected to the terminals behind the speaker.
Motor System
These are the parts that work together to create movement in the suspension system.
Voice Coil. This is made of a coil of wire that is wound round a cylinder and another part called former
Magnet. The magnet is connected to both the top plate and the back plate
Top plate. This part controls the magnetic flux near the voice coil thereby creating movement as an alternating current flow through the coil
Back Plate. It is connected to the poll piece (a part at the base of the speaker) and conducts magnetic flux up, inside the voice coil, and on the poll piece
Outside the suspension and the motor system, the subwoofer has other components that make up the functional mechanism of each system. Some of these functions are tangible while some are not tangible. However, they work together to produce an efficient subwoofer system. One interesting thing is that they all can be measured mathematically.
Crossover – Active or Passive
The job of crossover is to filter frequency ranges that are greater than the range for which the woofer system has been designed. The crossover program can also be called “high cut filter” or “low pass filter”.
Many times, you might find that a crossover program is designed with another filter (i.e. a 2nd filter) that filters those low end frequencies that come from speakers with high ranges. This 2nd filter is called a low cut or high pass filter.
The concept of subwoofer filter can be applied in two different ways irrespective of the bandpass you make. One way to apply it is as a passive network and the second way is as an active network.
The passive network is designed by placing parts that have the capacity for high power (filter) handling between the speaker load and the amplifier where the amplifier is in front and the speaker load is behind. The disadvantage of this system is that the loss from insertion coming from inductive components reduces the performance of the woofer. Also, the circuits for the filter have the tendency to impact damping and response from phase.
On the good side, this network accommodates a lot of flexibility so that you can tweak the design to your taste. It is also more budget-friendly.
In the active network, the filter is placed in front of the amplifier so that the components work in line. The advantage of this network is that it carries lower current and voltage than the one contained in the passive network. As a result, the distortion experienced in this network, as well as the loss, is a lot less compared to that of the passive network.
An active filter is designed to sift out sound before sending it to the correct amplifier that handles each range of frequency. It’s a lot easier to control input and output in this system.
The downside this system displays is the high cost of making it and the volume of space that it takes up in its design. It’s also a complex system that requires the installation of more than one amp.
The subwoofer speaker or driver is enclosed in a cabinet.
The speaker comes in varying sizes depending on where it is meant to fit. While small drivers serve multimedia systems like the PC, large drivers are used in theaters, clubs, and some other entertainment rooms.
Voice Coils – Single or Dual
The normal design for a subwoofer voice coils is one that is built around the magnets which provide the speaker cone with a motive force when it vibrates. When there is a second voice coil, the coil shares a common space with the first one but terminates at another end.
The purpose of the second coil is to enable one driver to carry double outputs for its amplifier – right and left. The impact of this design is easily noticed when the subwoofer is a low budget box basically because bass has no direction on its own.
A second design that comes with a speaker that has dual coil is that you can decide the best way to link the amplifier with the subwoofer. What this means is that if you have a 4 ohms dual coil, you can either create a series connection to get 8 ohms or run a parallel connection to get 2 ohms. Consider that you can introduce other variations since you can connect the dual coil to other pairs depending on the level of impedance you want to achieve.
Power Handling
The trick with deciding the power handling capacity of your driver is to be sure of the range you want for your subwoofer and then the making sure that the RMS number listed on the label s within the range that you want. If the power handling capacity of a driver is high but the frequency is higher than your estimated range, then the driver is not suitable for your need.
Using the formula RMS = peak measured power x 0.707, you can find the value of the amount of power the driver can handle. You can begin to understand the value of RMS by considering a subwoofer that is rated as 650 watts/RMS. If you choose a range between 30 and 130Hz and your frequency of 1100 Hz is more than your range, then the RMS value will not have so much effect on your subwoofer.
What the value above shows you is how much thermal power your system can handle but it tells you nothing about the subwoofer’s limits mechanically, especially at the points of low frequency. Generally, it’s a bit tricky determining the values that define the accurate capacity of a driver to handle power this it’s safer to work with producers that have been certified to have good products since you’ll be working with their parameters. A good supplier can help you identify them.
When you play music through a speaker, you place some level of stress on the speaker. This stress is usually twice the value of RMS of the driver. As music plays through the speaker, it displaces and returns (rest) to its original position. A subwoofer that has a 200watts/RMS is the same as one that has a continuous program power of 400watts whose amp has a rating of 250 watts/RMS. Just as long as the amp isn’t overdriven since that can cause it to produce more power which can distort the system badly.
Another equally important rating for drivers is their peak power, although this is the last thing to consider. Generally, if signals are not distorted, any driver can execute high peaks for milli- or microseconds. Whatever number the producer puts on the label is simply a way to buy into the heart of the consumers but in reality, it is not very significant.
Impedance
The impedance of a speaker is channeled to an amplifier. It puts together the capacitance, inductance, and resistance of the speaker and sends it to the amplifier. The value of impedance in ohms tends to vary often depending on what range the impedance is acting on thus it is also called nominal impedance.
The performance of a subwoofer is not determined by nominal impedance values of 2, 4, 6, 8, or 16 ohms. However, you need to know this value so that you can accurately match your load to the correct amplifier. If an amplifier has low impedance, it will allow greater passage of power and vice verse. Think of the law of inverse proportion.
Imagine that you have two speaker – 4 ohm and 8 ohm each and you connect both to one amplifier. The speaker that is 4 ohm, a lower impedance, will get double the amount of power the 8-ohm speaker will get. This means that when connecting systems with similar levels of impedance in a certain range of frequency, wiring is important in setting up sound. Wiring is a trick in setting up multiple drivers as it can alter the amount of impedance that your amplifier decodes. You can only apply this trick if you know the impedance value of each driver you are using.
Sensitivity, Xmax, and Efficiency
Xmax is a function of how long the voice coil is with respect to the magnetic gap (i.e. the height) of the motor of the driver. It explains a loudspeaker’s capability with regards to linear excursion. At low frequency, the voice coil moves a greater distance to produce sound while the magnet keeps it in place. The output of the subwoofer is as clear as the control of the magnet over the voice coil.
The mass of the voice coil is usually more because more wire is needed to cover the entire surface area which is longer than that of a regular driver. This mass reduces the ability of the speaker to pick up higher frequencies as well as reduces its efficiency.
Each speaker has the potential to take electrical input and change it to acoustical output. How well it is able to use this ability is its Efficiency and is measured as a percentage. However, the term high efficiency is only relative.
For Sensitivity, this value is obtained by measuring how much power 1 w (watt) of input has in decibels (dB). This value is also relative and known as the Sound Pressure Level (SPL). One has to measure the SPL of a speaker at a 1m distance.
With respect to these specifications, most subwoofers are not very efficient because most of the power sent to them go off as heat and other forms of mechanical energy. On the other hand, you can work around efficiency by choosing a driver with a stiff cone and high mass in addition to good linear excursion capacity. The power handling should be high so that it can carry the amount of power needed to make the lengthy voice coil and heavy cone move. This efficiency quality will mean a sensitivity rate that is low but if the box is well-designed with the right amplifier, then the bass that the subwoofer produces will be clean and deep.
In essence, the only point at which the sensitivity rating of a subwoofer makes sense is at a less than 125Hz sub-bass region. Also, the power handling ability matters at a driver’s pick up/cutoff range. Sometimes, a sub’s response may rise at a frequency that is beyond the intended range. If the boosted response for high frequencies is averaged with reduced output expected at the points of low frequencies, then for these high frequencies, the sound pressure will impact a sensitivity rating.
Frequency Response
It’s important to find the frequency response of a sub in your chosen box as that matters more than the value described for the driver. It’s not enough to describe the response as 20Hz to 1000Hz with stating what the SPL (which is the relative output) is. The best way to describe the frequency response will be with a graph that shows the differences in SPL with respect to a frequency level. The sub used to arrive at these values must be tested in a box for these values to have any useful relevance.
You can use modeling programs such as BassBox 6 Pro to determine the response of various designs of box. For a subwoofer, the bass response should bother below 125Hz or 150Hz.
Is the Subwoofer for a Home Theater Different from the Subwoofer for a Home Stereo?
A subwoofer with great quality can work with both a home theater and home stereo. The difference between the subwoofer for a home theater and the one for a home stereo is dependent on how you construct the subwoofer box and how you use it. The way you use your subwoofer is determined by what you like when it comes to tone, how loud you like your music, and the pick up/cutoff area (range) you want to cover.
Subwoofers used for a home theater will usually be louder than the subwoofer used for a home stereo.
Should you buy or build your own Subwoofer?
There are a lot of subwoofer brands in the market that you can get for your car or as part of your home theater system. However, these subwoofers are quite pricey and the value they offer is usually way lower than the dough you have to part with.
It’s okay to buy an already-made sub if you can’t afford the time, patience, and creativity to make one. But if you can and enjoy getting your hands greasy outdoor every now and then, then making your own sub will be a good fit. It will save you some money, not to mention that you will have the opportunity to express your maybe dormant creative prowess.
Deciding how to build your subwoofer
First, you need to learn what qualities make the best woofers and decide what you want to achieve with your own sub. Then, you’ll need to decide how much you are willing to spend on this project. The size and shape of your driver and the material you use to build it will determine how well your subwoofer performs. Therefore, you have to make sure the specifications and style of the cabinet syncs with the speaker you are using.
Here, you’ll learn the important framework to look for in selecting the parts of your subwoofer as well as the alternatives available for designing your sub. You’ll learn about the various designs of cabinet that will suit each choice you make and how you can effectively set up your sub.
Video: How to Build a Subwoofer
Here’s how a subwoofer is built
Types of Subwoofer Enclosures You Can Choose
The Sealed Enclosure
This enclosure is a delight to a beginner and a professional. The simple design of this box and the ease of construction make it a subwoofer DIY beginner’s go-to box. A professional will find the compact dimensions endearing, alongside the transient response and power handling capacity. Sealed boxes are smaller when compared with other box designs and they produce sounds that are relatively small.
The driver you choose for a sealed box should possess the following qualities: long excursion (Xmax) ability, loose suspension (to help restore displacement, and a low frequency resonance. However, the bass roll-off and sensitivity for a sealed box is mostly low compared to other types.
The sealed box design also has a version that is bigger than the usual. This version is called infinite Baffle and needs a driver built with a heavy mechanical suspension. The large space that comes with the infinite baffle is not suitable for most programs.
The Vented Enclosure
There are many names that describe this enclosure such as ducted, ported, or bass reflex enclosure. The vented box is more renowned now because acoustic research has modeled an effective system to find the acoustic and mechanical parameters of a driver as well as implement these parameters mathematically to suggest how a given box will behave.
In this system, the excursion of a driver reduces as the frequency reduces so that there is an increase in pressure towards the air outside from the air inside the vent. The high pressure air is designed to act as a real diaphragm built with physical materials at every frequency point. The advantages include relatively good cutoff frequency, come excursion that is reduced near the resonance frequency of the box, and generally better performance than a regular sealed enclosure will offer.
In spite of its great performance level, the vented enclosure has a relatively poor transient response. Also, the driver in this system has no loading for acoustic tuning below the frequency set by the box. This means that when the frequency is below the resonance of the box, cone motion has no control and is dependent on the mechanical suspension of the subwoofer. The “no control” feature ruins the driver by causing it to bottom out.
The transient response of this box type can be improved on if you take the time to choose your driver and tune the box. Again, you can avoid bottoming out by placing low cut filter in front of your amplifier so that subsonic frequencies do not affect the speaker at cutoff low points.
Generally, designing a vented box is more complex than designing a sealed box. You can use applications like BassBox Pro to design the right enclosure.
The Passive Radiators Enclosure
A passive radiator box is a variant of the vented enclosure where the virtual diaphragm is replaced with a real diaphragm — more like a speaker that doesn’t have a voice coil and magnet. The presence of a real diaphragm will mean the absence of windy sounds produced by a typical vent. It also is able to filter sounds from higher frequencies that echo behind the driver or from the vent. Another advantage is that you can use it when you need a vent that is longer than your enclosure for tuning.
The downside here is that the transient response is of a lower quality because the diaphragm of the passive radiator is heavy. Another downside is that it is more difficult to tune this system than it is to tune a regular vented box.
The Bandpass Enclosure
This kind of enclosure has many chambers that form a filter that is acoustic, and which pay attention to sound coming from a specified frequency range to increase the output. There are many designs of this enclosure type but the most known among them is the 4th order. This box looks like the regular sealed box with the output channeled at the 2nd chamber (with a vent) facing the speaker.
The purpose of the port is to increase how much sound the sub can pick in a narrow range while also doing the work of a low pass filter in reducing any output of higher frequency than the needed bass frequency. The output of the bandpass box is usually exposed to the noise coming from the vent of the box because all the output goes through the same opening.
The transient response of a bandpass system is akin to that of a regular sealed enclosure while the output has the tendency to surpass that of vented enclosures. The disadvantage of this type of enclosure is that they produce one note sounds because their range tends to always peak.
The Transmission Lines Enclosure
Another name for this enclosure is the acoustic labyrinth. It has a complex pathway that is long which the radiation from the back of the speaker has to pass when leaving the box. What happens here is that the box is designed to pick out those bass frequencies that are high through the long tunnel with the help of damping material. The length of the labyrinth is then tuned to match the speaker’s resonant frequency at a quarter wavelength.
The driver’s back radiation combines with radiation from the front that is made from a shift at 90° at the same time as peak of driver’s resonance dampens and controls come motion. Many types of transmission lines enclosure exist however the data to model the performance are in short supply.
You’ll need to mindfully analyze the parameters of the loudspeaker, tune and test, and check if the box can take a heavy or large not. The features of a TL are great if they are used well.
The Horn Loaded Enclosure
Amplifiers with small power have been there for a while, working with drivers that have low wattage. This kind of system needs a little boost to attain the SPL (sound pressure level) that most music players want. Even though the horn is a little outdated, it is still relevant to sound engineers.
What the horn does is to improve the output and direction of the sound sent to it — much like when you cup your mouth to help your voice become louder. It takes the high acoustical pressure from the driver and pairs it with low acoustical pressure that the air at the horn’s mouth sees. The flare rate measures how consistently sound travels along the path in the horn (linearity).
The length of the horn and the area of the mouth have a major impact on its low frequency response and direction. You can fold a large horn that has low frequency so that the size can be managed. Advantages of the horn include great transient response, high efficiency, and dispersion control. However, it’s usually big and too complex for most users of the home theatre or stereo.
The Bass Actuators Enclosure
This subwoofer is small but does not come with a box. When any audio signal is sent to this device, the input is changed to mechanical force by placing it against a solid. This device is built to produce low frequency sounds from solids. It produces just as much a similar bass sound as an enclosed sub.
One good point of this device is that it helps to conserve space while delivering the needed results. You can easily connect it to amplifiers; however, you need more than one to get the full effect of using the bass actuator.
