Wednesday, March 21, 2012

Subwoofers - A quick guide and information

Subwoofers - A quick guide and information

Subwoofer facts and everything you need to know

A subwoofer (or "sub") is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products,below 100 Hz for professional live sound, and below 80 Hz in THX-approved systems.Subwoofers are intended to augment the low frequency range of loudspeakers covering higher frequency bands.

Subwoofers are made up of one or more woofers in a loudspeaker enclosure capable of withstanding air pressure while resisting deformation. Subwoofer enclosures come in a variety of designs, including bass reflex (with a port or passive radiator in the enclosure), infinite baffle, horn-loaded, and bandpass designs, representing unique tradeoffs with respect to efficiency, bandwidth, size and cost. Passive subwoofers have a subwoofer driver and enclosure and they are powered by an external amplifier. Active subwoofers include a built-in amplifier.

The first subwoofers were developed in the 1960s to add bass response to home stereo systems. Subwoofers came into greater popular consciousness in the 1970s with the introduction of Sensurround in movies such as Earthquake, which produced loud low-frequency sounds through large subwoofers. With the advent of the compact cassette and the compact disc in the 1980s, the easy reproduction of deep and loud bass was no longer limited by the ability of a phonograph record stylus to track a groove, and producers could add more low frequency content to recordings. As well, during the 1990s, DVDs were increasingly recorded with "surround sound" processes that included a Low-frequency effects (LFE) channel, which could be heard using the subwoofer in home theater systems. During the 1990s, subwoofers also became increasingly popular in home stereo systems, custom car audio installations, and in PA systems. By the 2000s, subwoofers became almost universal in sound reinforcement systems in nightclubs and concert venues.

Active Subwoofers vs Passive Subwoofers

Simply put, a passive subwoofer depends on an external amplifier, while an active subwoofer contains a built-in amplifier. This may sound like a relatively subtle difference, but it’s enough to make an impact on your purchase decision.

Advantages of Passive Subwoofers

The passive subwoofer is generally a better bet if you’re going to be using it in a small room, where space is a concern and where you don’t need ultra powerful sound. The passive subwoofer is usually smaller and less bulky than the active subwoofer, although it produces a less intense sound. Also related to space issues, passive subwoofers can be an advantageous choice when you’re going to be installing the subwoofer in a restricted, fixed area where it will be difficult to shift the subwoofer around in the future. If you plug the passive subwoofer into an external amplifier, you can still reposition the amplifier in the future to improve sound.

Advantages of Active Subwoofers

Active subwoofers still tend to be the more popular choice. They produce a deeper, more impactful sound, and don’t require the purchase or use of any extra components or parts.

Active subwoofers' include their own dedicated amplifiers within the cabinet. Some also include user-adjustable equalization that allows boosted or reduced output at particular frequencies; these vary from a simple "boost" switch, to fully parametric equalizers meant for detailed speaker and room correction. Some such systems are even supplied with a calibrated microphone to measure the subwoofer's in-room response, so the automatic equalizer can correct the combination of subwoofer, subwoofer location, and room response to minimize effects of room modes and improve low frequency performance.

> All things being equal it is still a wise decision to buy Active Subwoofers. Also more economical 

Subwoofer Designs and Charateristics

Auto on/off: This option allows the subwoofer to turn on when an input signal from the receiver or pre-amp is detected. The subwoofer will turn off in a few seconds or minutes (depending on the manufacturer) after there is no input.

Servo Feedback: Some advanced subwoofers may use a feedback signal from a device mounted on the speaker’s cone. The servo control unit compares the subwoofer’s output to the input signal and attempts to compensate for the driver’s output errors in order to smooth out distortion level. Unfortunately, this feedback circuit, no matter how quickly it works, cannot make the correction until after the error has occurred. As a result, the correction is always arriving at the subwoofer’s output with the next signal. Therefore, the correction is made on the wrong signal and it may compress the subwoofer’s transient response and remove its impact.

Down Firing: This type of subwoofer has the woofer installed in the bottom so that it fires toward the floor. Down-Firing subwoofers look like a piece of furniture (do not need a grill) and may be more efficient. It is important that these types of subwoofers are not placed in a corner too close to the walls as they may sound boomy.

Front Firing: This type of subwoofer has the woofer installed on the side and fires its output signal parallel to the floor. Front-Firing subwoofers need a grill to cover the woofer and look more like a speaker.

Sealed Enclosure: Originally this design was pioneered by companies like Acoustic Research. It consists of a driver mounted on one side of a sealed box. The air tight enclosure completely isolates the back wave of the driver from the front.

Because of excellent transient response (i.e., no boomy sound), when designed and build properly, some audiophiles prefer these type of subwoofers. There are others who completely dispute that sealed boxes have better transient response. They claim that the perception of transient is really a function of perceived sound quality and not the type of enclosure. According to these critics, what does improve transient response (or perceived quality) is usually more headroom, more drivers, larger boxes (depending on the driver), better efficiency, and very low distortion.

Ported Design: Some subwoofer enclosures may add an additional open port (sometimes called duct, vent, or tunnel) which allows the passage of air in and out of the box. At low frequencies, the port contributes up to +3dB to the output and makes the system more efficient and thus increases the bass response. A ported enclosure system consists of a driver mounted on one side of a box that has an open port (duct, vent, tunnel)

The ported subwoofers are characterized by lower distortion and higher power handling in the operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver unloads and loses damping. Due to this, ported enclosures require a low frequency filter. The transient response of a ported enclosure is usually worse than a sealed enclosure system using the same driver. Ported enclosure systems are much more sensitive to misaligned parameters than sealed enclosure systems, which makes their construction more difficult.

Passive Radiator: Another type of subwoofer enclosure may add a passive radiator, instead of a port, to increase the efficiency of the sub. Passive radiators are sometimes drivers with the voice coil and magnet removed, or like a flat diaphragm. The radiator must usually be at least as large (or larger) than the driver in the enclosure.

Advantages of the passive radiator include the absence of port noise, and some audiophiles claim the radiator provides a better sounding bass than a ported enclosure. However, the cutoff (-3dB) frequency is slightly higher than ported design using the same driver.

Bottom Line: Unless you are an extreme audiophile, it doesn't really matter if it down or front firing. As long as it placed correctly in a room then you will still get the Bass that you want to hear.

Recommended Minimum Power for Subwoofers:
8"   50 watts
10" 75 watts
12" 100 watts
15" 150 watts
18" 250 watts

Subwoofer and Room Sizes

For a smaller room, you can use subwoofers with smaller drivers. However, for bigger rooms, since there is more air volume for the subwoofer to pressurize, a 12" or 15" subwoofer is recommended. One of the biggest problems in home theater rooms is caused by standing waves. These are created when the wavelengths (or ½ or ¼ wavelengths) of certain frequencies coincide with one or more room dimensions. Standing waves cause certain frequencies to be reinforced and cancelled at different locations throughout the room. The effect of standing waves is to have areas of the room where bass is very boomy and others where there is no bass at all. An equalizer will do nothing to fix these problems. These problem frequencies are known as room modes.

Ideally, two or more subwoofers may be a better option than a super large one, and since low-frequency sounds are non-directional, a subwoofer can be placed anywhere in the room. Please refer to the subwoofer placement section below.

Subwoofer Placement Guidelines

Instructions on getting good bass from a subwoofer in any given room usually begin by suggesting corner placement of the subwoofer. And it's true: placing the subwoofer in a corner will equally energize all the room's resonances and maximize the subwoofer's output. However, one of the more bizarre aspects of how subwoofers couple with the specific dimensions of a room -- is that to hear all the bass energy from the subwoofer in the corner of your room, you would have to sit in the corner diagonally opposite the subwoofer!

Clearly, such social isolation won't endear you to family and friends, so compromise is in order. And given the understanding that no room is ideal (not even a room with asymmetrical dimensions), the trick is to combine careful subwoofer and furniture placement (with the possibility of using two subwoofers) to more evenly distribute the deep bass frequencies throughout the room. Then every listener will hear the powerful bass frequencies that bring impact to home theater and music.

As you might suppose, a subwoofer and your particular room work together. It's not just the location of the subwoofer that matters: where you place the sofa and chairs is just as important. What follows are some subwoofer room-placement basics:

The worst place for a subwoofer is in the middle of a room.

The most difficult room shape is square, so if you have the flexibility to choose which room you'll use for home theater or you are building a new home and designating a space for home theater, avoid rooms with equal dimensions.

As you move the subwoofer closer to a wall, the bass output will become stronger.

Bass output will be maximized as you push the subwoofer into a corner.

The closer you sit to a wall, the more pressure your ear will pick up and the greater the bass intensity will be, but it may become uneven-- alternately boomy or anemic as you move in either direction.

Adjusting the distance of the couch or chairs relative to the walls and/or the subwoofer relative to the corner will almost always be beneficial in helping smooth out the deep bass heard at several listening locations in the room.

Adding a second subwoofer won't cure the problem of standing waves or uneven bass, but it will result in a greater number of listeners hearing smoother overall bass in more locations.

Try placing the second subwoofer in a location near the wall opposite the first subwoofer.

Avoid rooms with concrete floors and walls. Walls where the wallboard flexes are more absorptive and produce fewer problems with "bass boom." If you can' t avoid concrete walls, add studs and one layer of wallboard to the walls of the room to further aid absorption.

The advice I supplied in a newsletter some time ago, now in the Axiom AudioFile archives is worth repeating:

Move your subwoofer as close as you can to where you sit. If it's a chair, move the chair aside and place the sub in the spot where the chair was. If it's a couch, slide the couch temporarily out of the way and put the sub about where you usually sit.

Play a DVD with lots of low-frequency effects or a CD with plenty of deep bass, the kind that really kicks your sub into serious bass output. Get out the knee pads and crawl about the room in the general area where you were thinking of locating the sub.

Go several yards in each direction--near the wall, out from the wall, towards a corner, away from the corner, and so on--while you listen for smooth and extended deep bass response, free of exaggeration and "one-note" boom. Mark the spot, then move the subwoofer into that position. Now put the furniture back. If you are using two subwoofers, mark two locations and place the subs in those two positions.

Tech Talk

All subwoofers produce acoustical pressure, and that is what your ear responds to. The only place where you will hear the bass output that your subwoofer produces exclusively is out of doors (or in an anechoic chamber). But soon as you put a subwoofer into a room, the sound waves bounce back and forth between the parallel surfaces of the room, some combining or "adding," which will emphasize those sounds, and others canceling each other out, which results in a null. If you are sitting in a null, you won't hear any deep bass at all. Conversely, if your chair is in a location where standing waves peak, you will likely hear boomy, one-note bass (you won't be able to follow the tuneful bass line of a recording, for example). Sometimes, by just moving a foot or two, the deep bass will "magically" reappear.

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