Bass Management
Bass management is the process of removing the bass element of the signal fed to each satellite speaker, and routing it instead to one or more subwoofers. In essence this is no different to a normal crossover — it's just that the bass driver happens to be housed in a separate enclosure, and there needs to be some sort of mixing facility included to combine the low-frequency contributions from at least two channels.
In the case of a simple 2.1 stereo system, this bass management or crossover filtering is usually built into the subwoofer, and may be active or passive (most systems are active these days). There are various approaches to wiring, but most route line-level signals from the controller or preamp to the subwoofer first, which filters the signals and outputs them for the satellites. Some systems work the other way around, connecting the signal to the satellite first, and then down to the subwoofer. Systems intended for domestic use often work with speaker-level signals.
For 5.1 surround systems, the bass management is normally performed in the surround sound controller or monitoring controller, rather than in the subwoofer itself. The diagram on the previous page shows such a system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.
All five channels are also summed and passed through a low-pass filter to remove the mid- and high-frequency content. This signal is then combined with the dedicated LFE signal (which is also low-pass-filtered and boosted in gain, according to the appropriate specifications), and routed to the subwoofer speaker. It is worth bearing in mind that, since each of the five channels in a 5.1 system is a full-bandwidth channel, the subwoofer has to be able to cope with the bass contribution of five full channels, plus whatever might be conveyed on the LFE channel — and that could be a lot of bass! So you shouldn't really expect a very small box to be able to cope if you like listening at serious levels.
Of course, different systems implement bass management in slightly different ways. Some employ active filtering everywhere, whereas some only low-pass-filter the signal feeding the sub, relying on the satellite speakers' natural roll-off for mechanical high-pass filtering. Some will allow the filter turnover frequencies and slopes to be adjusted. Professional units usually do this with meaningful technical parameters, while domestic controllers tend to have simpler 'large' or 'small' speaker descriptions.
The better systems often include some sort of limiting or overload protection for the subwoofer, and some also include facilities for delaying the sound to each speaker, in order to compensate for less than ideal physical positions. Most domestic systems only apply bass management to digital surround inputs (Dolby Digital and DTS sound tracks) but not to discrete multi-channel analogue inputs, and this can present problems if you want to use a cheap domestic surround controller for your surround monitoring. Another common trap is that some DVD players have their own bass-management facilities built in, which means that you need to make sure you don't end up duplicating the processing
[DOUBLEPOST=1537519768][/DOUBLEPOST]Subwoofer Cabinet Types
While it is relatively easy to generate high levels of bass over very small bandwidths (and that's what most cheap subwoofers tend to do), designing something that can generate a high output over a broad bandwidth, with very low distortion, and remain a sensible size, is pretty tricky. Creating low-frequency sound at studio replay levels requires the movement of a lot of air. This requires a powerful amplifier, a very large bass driver (or several smaller ones), and a lot of diaphragm displacement.
An easy way of achieving high efficiency is to place the driver in what's called a 'band-pass cabinet'. This is essentially a resonant, tuned box, with the driver hidden inside and the sound escaping through one or more ports. You see these quite commonly on cheap home theatre systems and in car 'boom boxes.' Although efficient (for which read loud!), this kind of design always tends to sound boomy, with a one-note kind of response. This is great for film explosions and crashes, but is not much use if you want to hear which notes the bassist is playing, so it is best avoided for serious monitoring duties.
The majority of subwoofers employ some form of 'reflex' design, which combines practical efficiency with useably wide bandwidth, and in convenient sized enclosures. The design principles are thoroughly understood, with the front of the driver radiating directly and its rear contributing via the enclosed cabinet volume through one or more ports. Not all reflex designs are born equal, but most studio-quality subs will be of this kind of design.
A rather less common alternative is the 'closed box' design. The cabinet is sealed, and only the front side of the driver contributes sound to the room. Efficiency is relatively low, and significant demands are placed on the amp and driver (the latter needs to be able to cope with unusually large excursions). However, this approach has considerable benefits in terms of its phase response, timing and distortion. Another close variation on this theme is the 'transmission line' approach, which aims to combine the best elements of both sealed and reflex cabinets. These two types tend to be the most expensive, but also the easiest to align and integrate, and with the most accurate sound.
Don't be fooled by the size of the subwoofer. Bigger doesn't necessarily mean better, or even greater low frequency extension, although it does usually equate with louder; that business about needing to move a lot of air, again.
Alignment
The physical and electrical alignment of a subwoofer is a much misunderstood process, but to get it wrong is to destroy the accuracy of the monitoring system as a whole. Firstly, it is vital that the subwoofer and satellite speakers are all in the same phase as each other — by which I mean their electrical polarity and time alignment. If this is not the case, the crossover region will have an obvious bulge or dip in level. Potentially, there are a lot of sources of phase shifts that can mess up the crossover region. The subwoofer and the satellite speakers have their own mechanical phase responses to take into account, plus the electrical phase characteristics of the crossover filters themselves. There is also the time delay caused by having speakers located at different distances from the listener, and some cabinet designs introduce further acoustic delays, which often vary dramatically with frequency (and in some cases can exceed 40ms — a whole video frame!).
If there is a phase control on your subwoofer, you will find that small phase adjustments can often make surprisingly large differences to the smoothness of the crossover region, especially if the subwoofer and satellites are from different manufacturers.
A lot of the better specified subwoofers include a phase-adjustment facility (either switched or continuously variable), which can help to correct for the mechanical and electrical phase differences between the satellites and the subwoofer. Unfortunately, though, not all designs are equally effective and, more importantly, phase correction is not the same as delay compensation. If the subwoofer is located nearer to, or farther from, the listener than the satellites, some delay compensation will be required to achieve the correct time alignment. Though some bass-management or surround sound monitoring systems incorporate this function, not all do so.
When it comes to placing a subwoofer, there are several things to consider. Although a high-quality subwoofer should not output the higher frequencies that allow its position to be determined by listening, that doesn't mean it can be put down just anywhere. Firstly, the subwoofer's location in the room — especially its proximity to walls — will have a significant effect on its frequency and time-domain responses. In a typical room, there will be a few 'best' places amongst many more that are unacceptable. Secondly, unless delay compensation is available, the sub should ideally be located at the same distance from the listener as the satellite speakers.
It makes sense to place a single subwoofer in front of the listener, rather than behind, and directly facing the listening position too. It should be well away from corners, but you should also avoid placing it at the centre of the room width, in order to minimise excitation of standing waves. The closer the subwoofer is placed to the wall, the greater the bass boost will become. Some models are designed to be placed close to a wall, specifically to benefit from this, but some are not — which means you should always check the manufacturer's recommendations. Often, small changes of distance in relation to a wall can make a big difference in the balance of deep bass, so don't be afraid to experiment.
For the subwoofer to work properly and not be locatable, the crossover between satellite and subwoofer should be set below about 90Hz, and that means the satellite ideally needs to have a decent response down to 70Hz or so. Anything above that starts to intrude into the mid-range and the subwoofer will become locatable. The THX organisation recommends crossing over at 85Hz, and I've found that to be a good starting point in most cases.
Bass management is the process of removing the bass element of the signal fed to each satellite speaker, and routing it instead to one or more subwoofers. In essence this is no different to a normal crossover — it's just that the bass driver happens to be housed in a separate enclosure, and there needs to be some sort of mixing facility included to combine the low-frequency contributions from at least two channels.
In the case of a simple 2.1 stereo system, this bass management or crossover filtering is usually built into the subwoofer, and may be active or passive (most systems are active these days). There are various approaches to wiring, but most route line-level signals from the controller or preamp to the subwoofer first, which filters the signals and outputs them for the satellites. Some systems work the other way around, connecting the signal to the satellite first, and then down to the subwoofer. Systems intended for domestic use often work with speaker-level signals.
For 5.1 surround systems, the bass management is normally performed in the surround sound controller or monitoring controller, rather than in the subwoofer itself. The diagram on the previous page shows such a system. Each of the five main channels goes through a high-pass filter to remove the low-frequency element of the signal, before being passed on to the appropriate amplifier and speaker.
All five channels are also summed and passed through a low-pass filter to remove the mid- and high-frequency content. This signal is then combined with the dedicated LFE signal (which is also low-pass-filtered and boosted in gain, according to the appropriate specifications), and routed to the subwoofer speaker. It is worth bearing in mind that, since each of the five channels in a 5.1 system is a full-bandwidth channel, the subwoofer has to be able to cope with the bass contribution of five full channels, plus whatever might be conveyed on the LFE channel — and that could be a lot of bass! So you shouldn't really expect a very small box to be able to cope if you like listening at serious levels.
Of course, different systems implement bass management in slightly different ways. Some employ active filtering everywhere, whereas some only low-pass-filter the signal feeding the sub, relying on the satellite speakers' natural roll-off for mechanical high-pass filtering. Some will allow the filter turnover frequencies and slopes to be adjusted. Professional units usually do this with meaningful technical parameters, while domestic controllers tend to have simpler 'large' or 'small' speaker descriptions.
The better systems often include some sort of limiting or overload protection for the subwoofer, and some also include facilities for delaying the sound to each speaker, in order to compensate for less than ideal physical positions. Most domestic systems only apply bass management to digital surround inputs (Dolby Digital and DTS sound tracks) but not to discrete multi-channel analogue inputs, and this can present problems if you want to use a cheap domestic surround controller for your surround monitoring. Another common trap is that some DVD players have their own bass-management facilities built in, which means that you need to make sure you don't end up duplicating the processing
[DOUBLEPOST=1537519768][/DOUBLEPOST]Subwoofer Cabinet Types
While it is relatively easy to generate high levels of bass over very small bandwidths (and that's what most cheap subwoofers tend to do), designing something that can generate a high output over a broad bandwidth, with very low distortion, and remain a sensible size, is pretty tricky. Creating low-frequency sound at studio replay levels requires the movement of a lot of air. This requires a powerful amplifier, a very large bass driver (or several smaller ones), and a lot of diaphragm displacement.
An easy way of achieving high efficiency is to place the driver in what's called a 'band-pass cabinet'. This is essentially a resonant, tuned box, with the driver hidden inside and the sound escaping through one or more ports. You see these quite commonly on cheap home theatre systems and in car 'boom boxes.' Although efficient (for which read loud!), this kind of design always tends to sound boomy, with a one-note kind of response. This is great for film explosions and crashes, but is not much use if you want to hear which notes the bassist is playing, so it is best avoided for serious monitoring duties.
The majority of subwoofers employ some form of 'reflex' design, which combines practical efficiency with useably wide bandwidth, and in convenient sized enclosures. The design principles are thoroughly understood, with the front of the driver radiating directly and its rear contributing via the enclosed cabinet volume through one or more ports. Not all reflex designs are born equal, but most studio-quality subs will be of this kind of design.
A rather less common alternative is the 'closed box' design. The cabinet is sealed, and only the front side of the driver contributes sound to the room. Efficiency is relatively low, and significant demands are placed on the amp and driver (the latter needs to be able to cope with unusually large excursions). However, this approach has considerable benefits in terms of its phase response, timing and distortion. Another close variation on this theme is the 'transmission line' approach, which aims to combine the best elements of both sealed and reflex cabinets. These two types tend to be the most expensive, but also the easiest to align and integrate, and with the most accurate sound.
Don't be fooled by the size of the subwoofer. Bigger doesn't necessarily mean better, or even greater low frequency extension, although it does usually equate with louder; that business about needing to move a lot of air, again.
Alignment
The physical and electrical alignment of a subwoofer is a much misunderstood process, but to get it wrong is to destroy the accuracy of the monitoring system as a whole. Firstly, it is vital that the subwoofer and satellite speakers are all in the same phase as each other — by which I mean their electrical polarity and time alignment. If this is not the case, the crossover region will have an obvious bulge or dip in level. Potentially, there are a lot of sources of phase shifts that can mess up the crossover region. The subwoofer and the satellite speakers have their own mechanical phase responses to take into account, plus the electrical phase characteristics of the crossover filters themselves. There is also the time delay caused by having speakers located at different distances from the listener, and some cabinet designs introduce further acoustic delays, which often vary dramatically with frequency (and in some cases can exceed 40ms — a whole video frame!).
If there is a phase control on your subwoofer, you will find that small phase adjustments can often make surprisingly large differences to the smoothness of the crossover region, especially if the subwoofer and satellites are from different manufacturers.
A lot of the better specified subwoofers include a phase-adjustment facility (either switched or continuously variable), which can help to correct for the mechanical and electrical phase differences between the satellites and the subwoofer. Unfortunately, though, not all designs are equally effective and, more importantly, phase correction is not the same as delay compensation. If the subwoofer is located nearer to, or farther from, the listener than the satellites, some delay compensation will be required to achieve the correct time alignment. Though some bass-management or surround sound monitoring systems incorporate this function, not all do so.
When it comes to placing a subwoofer, there are several things to consider. Although a high-quality subwoofer should not output the higher frequencies that allow its position to be determined by listening, that doesn't mean it can be put down just anywhere. Firstly, the subwoofer's location in the room — especially its proximity to walls — will have a significant effect on its frequency and time-domain responses. In a typical room, there will be a few 'best' places amongst many more that are unacceptable. Secondly, unless delay compensation is available, the sub should ideally be located at the same distance from the listener as the satellite speakers.
It makes sense to place a single subwoofer in front of the listener, rather than behind, and directly facing the listening position too. It should be well away from corners, but you should also avoid placing it at the centre of the room width, in order to minimise excitation of standing waves. The closer the subwoofer is placed to the wall, the greater the bass boost will become. Some models are designed to be placed close to a wall, specifically to benefit from this, but some are not — which means you should always check the manufacturer's recommendations. Often, small changes of distance in relation to a wall can make a big difference in the balance of deep bass, so don't be afraid to experiment.
For the subwoofer to work properly and not be locatable, the crossover between satellite and subwoofer should be set below about 90Hz, and that means the satellite ideally needs to have a decent response down to 70Hz or so. Anything above that starts to intrude into the mid-range and the subwoofer will become locatable. The THX organisation recommends crossing over at 85Hz, and I've found that to be a good starting point in most cases.
