A subwoofer is a specialised form of loudspeaker, designed to carry only the lowest frequencies of the audio spectrum. Subwoofers are used to add extra impact to a film or movie soundtrack, and the soundtracks and replay systems are specifically designed with their use in mind. While subwoofers are most commonly employed in home theatre or home cinema systems, they can also be used in conventional music replay systems, or HiFi systems, where they often find favour with enthusiasts of music genres such as heavy rock, reggae, and forms of dance music. Separating the subwoofer from the remainder of the speaker system can allow the remainder of the system to be of a smaller design, and many home theatre systems currently on the market employ small satellite speakers which are optimised to handle the upper bass, mid, and high frequencies. Reproduction of deep bass at high volumes requires the speaker to move large volumes of air, and the more of this that can be transferred to the subwoofer system, the less is required of the other speakers, and the more compact they can be. The use of subwoofers in home theatre systems is designated in numeric form, by describing the systems as 5.1, 7.1 etc. The figure before the decimal point indicates the number of conventional speakers in the system, with a 5 normally indicating four surround speakers, plus a centre front, and the figure after the decimal point indicates the number of subwoofers, hence a 5.1 system has one subwoofer, a 5.2 system has two. Subwoofers have taken their name from the colloquial term for the bass speaker in a multi-speaker system, the low-end speaker being the 'woofer'.
Size and Build Quality
As mentioned above, the size of the subwoofer system largely determines its power handling and its output. Subwoofers are available in all sizes, from compact units intended to be paired with computer sound systems, through to those intended for use with the largest home theatre installations. With this in mind, domestic and decor considerations may be a limiting factor on the choice of a subwoofer, to a greater extent than sound quality or power handling considerations. Essentially, the buyer must decide how much of the living space they are willing to give up to the subwoofer. It should be noted, though, that the positioning of a subwoofer within the listening room does not need to be specific. Low frequencies are generally non-directional, and this allows the subwoofer to be placed almost anywhere in the room, within reason, without affecting its sound quality. This may allow it to be sited remotely from the rest of the system in the interests of domestic harmony. However, many users choose to keep cabling to a minimum, and install the subwoofer to the front of the system, a short cable's length from the amplifier or source component. Since the subwoofer is routinely expected to move a lot of air in the listening room, it has to be of solid construction, and built well. This generally can't be done without building a large and heavy cabinet around the speaker, and as the size of the room and system increases, so does the optimum size of subwoofer.
Passive and Active Subwoofers
Subwoofers can be of two types, passive or active, and the choice of one or the other will depend on the equipment with which they will be paired, and the outputs available from that equipment. A passive subwoofer contains only speaker and crossover components, and must be driven by a separate amplifier. In most cases, the subwoofer will have an internal crossover component, which filters the sub-bass frequencies to the subwoofer itself, and a set of speaker outputs to feed the remainder to the conventional speakers with which it is paired. This arrangement is most likely to be used when a subwoofer is employed with a conventional stereo pair of loudspeakers in a HiFi system. Alternatively, if the passive subwoofer is being used with an amplifier or receiver which has a dedicated subwoofer speaker output, this would be connected directly to the subwoofer, and the other speakers connected separately to the receiver. An active subwoofer contains its own amplifier, and is driven from a line-level output, which could include that from an audio-visual, or AV, receiver or amplifier, a DVD or Blu-Ray player, or a preamplifier. Active subwoofers come with the added bonus that matching of amplifier and subwoofer doesn't need any consideration by the buyer, since the manufacturer has already taken care of ensuring that the internal amplifier matches the speaker, in terms of power handling and tonal quality. Both passive and active subwoofers can be found with sealed enclosures or in ported designs. Sealed enclosures are generally of smaller size than ported designs, due to the absence of the ports. Ported enclosures are designed to allow the pressure wave from the rear of the speaker to be directed outward from the subwoofer, effectively doubling the cone area, but this naturally leads to the construction of a larger and heavier cabinet to accommodate the porting arrangements.
Brand and Range Matching
Subwoofers are made by a large number of speaker manufacturers, and it's perfectly feasible to mix and match a subwoofer from one manufacturer with a speaker system from another. However, whether the buyer is acquiring a subwoofer to add to an existing system, or buying the subwoofer as part of a complete system in one transaction, they may want to consider buying a subwoofer from the same maker, and possibly within the maker's same range. The speaker manufacturer will, if promoting a specific range of speakers including a subwoofer, have taken steps to ensure that the subwoofer matches the rest of the system in tonal quality and general power handling, as well as having all the speakers being a visual match for each other.
Choosing the power rating for a subwoofer will depend on the circumstances of the purchase, as well as the size of the room in which it will be placed, and the output of the system with which it will be paired. Passive subwoofers should be selected according to the output power of the amplifier with which they will be paired. Often, the ideal amplifier output will be specified in the manufacturer's manual or data sheet for the subwoofer, but if not, the buyer should look for a close match between the power ratings for each of them. An amplifier of, say, 40 watts output should be paired with a subwoofer of at least 40 watts, possibly more. Active subwoofers, since they have their own power amplifiers, function independently of the main system amplifier, and the output power can be selected according to room size. Broadly speaking, the larger the room, the larger the subwoofer should be.
It should be determined, either from the manufacturer's manuals or data sheets, or from the seller, what connections are available on the subwoofer itself, and how these will match with the system with which it will be used.
How to Find Subwoofers on eBay
From the eBay homepage, select Shop by category, and if Sound & Vision appears on the pop-up category list, select it. Otherwise, expand the category list with the See all categories button, and from the expanded list, choose Sound & Vision. Select Home Audio & HiFi Separates, and from the subcategory list to the left-hand side, Speakers and Subwoofers. The Configuration subcategory allows the selection of standalone units - the Single Subwoofer subcategory, whilst other Configuration selections cover the inclusion of subwoofers within complete systems, whether 5.1 System or 2.1 System.
Subwoofers are available in many sizes, and with a number of connectivity options, allowing their use with HiFi and home theatre systems, whether driven from line or speaker outputs. Using the guidelines above, it should be possible to make a choice of subwoofer which will fit with any existing system in connectivity terms, and have the appropriate power output to match both the system and room size.