I have decided to come up with an ongoing updatable post intended to be a sticky in regards to subwoofers. It is a list of definitions and concepts. There may be some terms that blend into other areas of audio but help out in general subwoofer discussion. I am putting descriptions in a more simple format so that those with limited experience will still be able to follow. Please feel free to post terms or topics you would like explained so they can be added. If you see any errors point them out. I am not afraid to admit I am wrong. I am going mostly off of memory for this. Here are the most common items I have seen lately: T/S Parameters: Shortened form of Theile Small Parameters. These are the set of electro-acoustical properties of an individual driver intended to give a perspective on how the driver will perform and gives insight into what enclosure will best suit a driver. Xmax: How far the subwoofer can move in a linear fashion one way. Low Pass Crossover/Filter: A crossover is an electro acoustical filter that allows only low frequencies to be sent to an appropriate driver beginning at a certain frequency (cut-off frequency). High Pass Crossover/Filter: Again just like the Low Pass but does the opposite. It sends only frequencies higher than the cut off frequency to speakers. Crossover Slope: This is how fast the crossover/filter rolls off frequencies from the cut-off frequency. It is usually listed in X db/oct. or decibels per octave in long form. They are also listed in an order type as well. For example one may have a 4th order crossover slope or network. (see below for further explanation on the bolded terms). Octave: The interval between any two frequencies having a ratio of 2 to 1. In layman's terms this is best described with an example. 500hz to 1000hz is one octave, 250hz to 500hz is one octave. 250hz is one octave below 500hz. 1000hz is one octave above 500hz Crossover Order: Crossover orders are as follows: 1st order (6db/oct.); 2nd order (12db/oct.); 3rd order (18db/oct.); 4th order (24db/oct.); 5th order (30db/oct.); 6th order (36db/oct.); ect. Crossover Roll off (i.e. SLOPE): This ties into the three above it. It is best explained with example. There is a theater system that has a subwoofer crossed over at 80hz lowpass with a 24db/oct. filter. The highpass crossover is also at 80hz but at 18db/oct. There is a fundamental full range signal playing through the system at 100db. This means that the subwoofer will begin cutting back sound at 80hz and will be -24db below the fundamental signal at 160hz or one octave above the crossobver point. The fron center and rear speakers will be at -18db below the fundamental at 40hz or one octave below the high pass crossover point. Infrasonic filter (incorrectly known as Subsonic filter): An Infrasonic filter is a highpass crossover that is typically employed at inaudible frequencies (i.e. below 20hz for the normal human). It is most often used with vented or bandpass enclosures to avoid speaker damage. Subsonic refers to the something traveling below the speed of sound. This is not a correct term. F3: This is a point in a sub system's response in which its response drops -3db below the reference plane or 0db on a frequency response graph. FB: Tuning frequency of a ported/vented/bass reflex enclosure. Node, Null or Dip: A dip in frequency response caused by room characteristics. these can wreak havoc on a system as they are very difficult to cure with an eq occur in every room. Careful placement is necessary to minimize them as much as possible. Mode or Peak: A boost in frequency response caused by room characteristics Not looked at with as much scorn as nulls modes can be troublesome as well. However they are easier to tame with an eq and can be advantageous with slightly underperforming gear and/or for bass heads who like boom. They have been known to muddy up response as well. Enclosure types: Sealed or Acoustic Suspension Enclosure: This type of enclosure is completely sealed with no openings into the ambient environment. It is the most forgiving enclosure to build and are usually recommended for their transient response characteristics. They typically have a 12db/oct. roll off below the sealed system's resonance point. Vented/Ported/ Bass Reflex Enclosure: This is probably the most widely used design in home audio/theater.This enclosure has a vent or port that allows air inside the enclosure to interact with the ambient environment reinforcing bass response. All vented enclosures are tuned to a specific frequency. The tuning frequency of the enclosure is dependent upon enclosure volume, vent/port length, and vent/port surface area (diameter for round ports). These enclosures, when designed correctly, can substantially lower the frequency response of a given driver vs. a sealed enclosure. When designed correctly they can also have very similar, if not undetectable, transient response characteristcs as opposed to sealed enclosures. They typically are about 25% larger than a sealed counterpart however. They also rolloff at 24db/oct. below the tuning frequency of the enclosure. Bandpass Enclosure: This type of enclosure design is all but unused in the home audio arena for the most part. It typically uses a sealed portion behind the driver and a ported portion infront of the driver to create a specific frequency pass band of operation. There are many different designs of bandpass enclosure however. This system rolls off frequencies on the high end and the low end to create a narrow frequency operation window. This allows for very high sensitivity. If done correctly (though very difficult) they can be quite impressive. They have many drawbacks though. They tend to be boomy enclosures due to the fact of peaky type response . Properly done enclosures can exceed ported designs in size by as much as 50%+! As previously mentioned they are very difficult to design and build. Their limits on low end response due to ever increasing size limits them in home theater applications. Infinite Baffle: This is not so much an enclosure. One would typically see this design in a wall, ceiling, or floor where the front of a speaker is completely isolated from the rear. This design also require (usually) many woofers and a large space such as an attic or basement to use as the enclosure for the subs. This is not very common and requires a fair bit of knowledge and DIY ability to tackle. Compound Compression Enclosures and/or Isobaric Enclosures: These types of systems are nice if slightly inefficient. The benefits are very small enclosures and extended low frequency extension. It essentially uses two woofers that act as one. They also only give you the output of one woofer. However, they typically will play much deeper and more accurate than a single woofer alone in a sealed enclosure. The accurace usually comes from the way the woofers are loaded cancelling any anomalies. The disadvantage here is having to use two woofers and get the output of one and the use of a lot of amplifier power. Given the circumstances the advantages can significantly outweigh the disadvantges. Power handling Myths related to subs: Underpowering will blow subs. This is totally false. If it were true, everytime you turned down the volume your subs would blow. It is quite silly when one thinks about it. Clipping blows subs/speakers. Cliping in and off itself does not blow speakers. It is the extra power created, along with potential odd order harmonics that cause non-linearities in the speaker to fry the coil or go beyond the mechanical limits of the driver. When an amplifier fully clips it will double its continuous output power when taken at .1% thd (total harmonic distortion). If a sub is rated at 100 watts continuous power handling and the amplifier is also 100 watts continuous output, when the amplifier fully clips it sends 200 watts into our 100 watt speaker. This is what causes the sub to fail. The extra power significantly increases heat generated in the speaker. The subwoofer cannot dissipate this extra heat and the coil soon fails. The extra power can also cause mechanical failure by pushing the sub beyond its mechanical limits and tolorances. One could have a 25 watt amplifer @.1% thd run fully clipped into our 100 watt sub and never have any damage. When fully clipped, it would only deliver 50 watts of power which are within our sub's tolorances. Sure it will sound horrible but the sub should not fail. Because there would not be enough power to exceed the mechanical limits of the woofer it should not fail there either. A 3db increase in output means it is twice as loud. This is just a bit of confusion. To gain a theoretical +3db in output the input power to the sub must double. To gain a percieved doubling in output the level must increase by 6db-10db depending upon the study evaluated. To achieve this power must increase by nearly 10x! Sensitivity and Efficiency are the same thing: This is a common misconception. Sensitivity refers to how much output a sub will do with a given amount of input voltage. Efficiency is entirely different the that it is a measure of how much power is used for motion or output and how much is wasted in heat. Most subs are only about .3% efficient. Some of the best are around .6%, the rest is wasted in heat. That means that for every watt you put in 99.7% is wasted in heat. It is no wonder that cooling on a driver is so critical.