A quick rant about one of my biggest crusades: centre speakers.
Too often does it seem that speaker manufacturers forget about adding a centre speaker to a product line, and then quickly cobble something together at the last minute. Even with mid-level gear these speakers are seriously lacking, and would be quite at home in Hi-Fi Corp had they been used as fronts. The cabinets are pathetically weakly constructed, the woofers have no midrange and precious little bass and quite often a different tweeter is also used to cut costs even further I suppose. The cabinet is also usually too small to make it visually appealling, and to create an apparent low bass extension it's tuned for a large peak at e.g. 60Hz. This is a real pity since the centre speaker is the one that is used the most of all 5! Changing the centre on a home cinema system has a more profound effect than changing any other speaker.
In the design process two factors are also often overlooked: the speaker is very likely to be seated on a broad boundary and not a stand, or even by two or even four if used in a wall unit. This will obviously affect the bass response. Some of the better speakers have a setting where this can be configured. The second factor is that it's unlikely for the centre to be on the same height of the fronts. Further or closer is no problem, this can be fixed by the delay setting on the processor. Higher or lower is a different matter. Aiming the speaker to fire at the listener helps a lot, but is still not perfect.
Another problem is that of polar lobing: a speaker on its side will sound different than when it's standing upright. This has to do with horizontal and vertical dispersion, and if the woofers are suddenly flanking the tweeter it gets all messed up. This will ALWAYS sound worse than an upright version irrespective of what is done to help it; provided it's the conventional MTM configuration.
So what are and can be done to address these issues? Well for most companies: nothing, nada, zilch. They usually do it better on their more expensive series though, but I feel that the centres provided with the cheaper series are often not nearly in the same league. B&W is the biggest culprit with their 600 series, and to a certain extent Boston?s CR's. The B&W DM600's, especially the DM602 and DM603, are very good, especially at the price. The CC6 is crap on the other hand. Those small Kevlar drivers sound horrible and not much has been done about the cone breakup. Listen to it with the other speakers muted, and you'll be amazed by the woolly, tubby bass, very muddled dialogue and so-so top end due to the same tweeter used. That's why many people rather buy the LCR6 which is a little more competent.
The methods used to combat these problems are the following:
1) The simplest is as B&W uses on their HTM7, CDMC-SE, CDMC-NT and Matrix centres: place the tweeter on top of the woofer. Of course this make it a bit tall and impractical to use in a wall unit, but that's also a good thing since no speaker should have two enclosures
2) Offset the tweeter a bit higher or lower to produce uneven diffraction. This is also often done with stereo speaker to improve imaging.
3) Adapt the crossover. This can help a lot if done right but is seldom implemented.
4) Cross extremely low so that the tweeter produces as much as possible. This is of course not too practical since it will load the tweeter too much, especially the cheaper ones.
5) Use a 3-way configuration, by placing the tweeter on top of the midrange and the two woofers at the sides. This will achieve the same as crossing a 2-way very low, by keeping most of the high frequencies in the same vertical line. The crossover to the side woofers will now be in the region of 300Hz that is low enough to prevent any lobing issues. This will render the cabinet a bit tall since the tweeter is on top of the mid, unless a neodymium tweeter is used or a very small midrange. Most top-level centre speakers use this approach. The B&W Nautilus is a good example, even though the midrange is a bit larger than most. KEF doesn't have that problem with their Uni-Q tweeter of course. Naturally the best is to use 5 identical speakers to cure all these problems, but for anything but a perforated projector screen this is impractical.
By now I'm sure most people will say that it is all good and well, but I cannot just go out and buy a different centre speaker - it will mess with my tonal matching/voicing/whatever the buzzword is nowadays. Well here's the news: on the cheaper centre speakers you don't have proper tonal matching to begin with! Well that is true, particularly with more exotic designs. However, I will rather have a proper centre that is in the same league quality-wise than a crappy one that is supposedly a tonal match. It's quite logical if you think about it. Firstly, the cabinet is much smaller and tuned completely different to the main speakers. So there goes any bass matching, especially since the TV or wall unit will add different boundary gain than with the main speakers. On to the midrange. Well, since we rarely have the same drivers as used in the main speakers, these will obviously sound different. The same drivers cannot be used since the cabinet volume would have to be more or less the same as the main speakers'. Using the same cone material for different sized drivers will produce some similarities, but not too many. Of course, since the woofers are different, so should the crossover be. This will mean a different crossover frequency, slopes, conjugate networks and so on. So all that remains is the tweeter. Since the centre is likely to be crossed higher than the fronts, we can therefore only expect similar response from about 5kHz upwards, but probably even more to the point where the woofer is at least 12dB down. So much for matching! A good example was the Boston CR2. It was said to be a tonal match to the bigger CR8 bookshelf. It used the same tweeter alright, but the CR8 had an 8" woofer; the CR2 a 4" one, tuned with a passive radiator in a small plastic cabinet. Needless to say the two didn't sound even nearly the same. Even though the tweeter was the same, the whole thing sounded completely different with very distracting sound panning. The white noise used to calibrate the speaker levels sounded vastly different.
Besides to use identical speakers, there is only one proper way to achieve real tonal matching: 3-way speakers. In addition to avoiding the lobing problems, these speakers usually use the same tweeter AND midrange than used in the main speakers. Only the woofers will be smaller, but both crossover frequencies and usually the whole network between midrange and tweeter can be kept as-is. This will lead to true tonal matching from about 500Hz and up. A typical configuration would be a 3" midrange and 1" tweeter, and two 6.5" woofers on the main speakers, reflex-loaded for low response. The centre would have the same mid and tweeter, and for example two 5.5" woofers in an alignment that doesn't extend as low and thus allow a smaller cabinet, or even a sealed alignment. This can now be seen as identical satellite speakers, each with its own local subwoofer that's crossed a bit high.
To fix the bass problem there are a few methods that are quite obvious. Make the cabinet larger, tune the port more realistically or seal the box and use a subwoofer. This is what is done by M&K and is actually the best home cinema solution provided the subwoofer is up to the task. The satellites are designed so that they're sealed and critically damped i.e. traditional Q=0.707 Butterworth alignment, with its natural -3dB point at 80Hz. This will not have too much bass if used on its own of course, and will fall off at an exact 2nd-order or 12dB/octave response. Note that this is the acoustic response, not electrical. Any speaker itself is a highpass filter, otherwise any woofer will be able to produce bass down to DC! All we did now was to tune this acoustic highpass to a certain set of parameters. If we now add some electrical filters as well, we can tune this bass response even further. This is usually done with active, either analog or digital, filters within any THX processor. The filter has the same response as the acoustic one; 2nd order Butterworth lowpass at 80Hz. These two filters, one acoustic and the other electric, will now augment the acoustic response (after all, all we care about is the acoustic response at the end!) to a 4th order Linkwitz-Riley configuration at 80Hz, with a Q of 0.707x0.707 = 0.5. So we have a predetermined highpass response. We now need a lowpass response from the sub to match it. The subwoofer driver's acoustic lowpass is always far higher than 80Hz and can therefore not be used as part of this filter. The processor's subwoofer output is therefore a 4th order Q=0.5 Linkwitz-Riley lowpass filter. This filter alignment is known for excellent integration, and theoretically we have a flat frequency response. In practice this is not always the case, but for most proper listening rooms and experimentation with speaker and subwoofer placement the response is near enough. Sometimes a graphic equalizer is used to ensure flat response. This method of subwoofer integration is far better than the usual method unless the crossover frequency is very low. Normal ported loudspeakers have very unpredictable frequency response slopes and it's very hard to match it with the subwoofer's fixed-slope lowpass filters, usually either 2nd or 4th order. This steep filter at a relatively high point means that the speaker can be fairly small, and is usually only one or two 5.5" woofers. The speakers always have more than one tweeter, often three, to control vertical dispersion according to the THX spec but also to increase their powerhandling at the extreme SPL levels of home cinema.
Too often does it seem that speaker manufacturers forget about adding a centre speaker to a product line, and then quickly cobble something together at the last minute. Even with mid-level gear these speakers are seriously lacking, and would be quite at home in Hi-Fi Corp had they been used as fronts. The cabinets are pathetically weakly constructed, the woofers have no midrange and precious little bass and quite often a different tweeter is also used to cut costs even further I suppose. The cabinet is also usually too small to make it visually appealling, and to create an apparent low bass extension it's tuned for a large peak at e.g. 60Hz. This is a real pity since the centre speaker is the one that is used the most of all 5! Changing the centre on a home cinema system has a more profound effect than changing any other speaker.
In the design process two factors are also often overlooked: the speaker is very likely to be seated on a broad boundary and not a stand, or even by two or even four if used in a wall unit. This will obviously affect the bass response. Some of the better speakers have a setting where this can be configured. The second factor is that it's unlikely for the centre to be on the same height of the fronts. Further or closer is no problem, this can be fixed by the delay setting on the processor. Higher or lower is a different matter. Aiming the speaker to fire at the listener helps a lot, but is still not perfect.
Another problem is that of polar lobing: a speaker on its side will sound different than when it's standing upright. This has to do with horizontal and vertical dispersion, and if the woofers are suddenly flanking the tweeter it gets all messed up. This will ALWAYS sound worse than an upright version irrespective of what is done to help it; provided it's the conventional MTM configuration.
So what are and can be done to address these issues? Well for most companies: nothing, nada, zilch. They usually do it better on their more expensive series though, but I feel that the centres provided with the cheaper series are often not nearly in the same league. B&W is the biggest culprit with their 600 series, and to a certain extent Boston?s CR's. The B&W DM600's, especially the DM602 and DM603, are very good, especially at the price. The CC6 is crap on the other hand. Those small Kevlar drivers sound horrible and not much has been done about the cone breakup. Listen to it with the other speakers muted, and you'll be amazed by the woolly, tubby bass, very muddled dialogue and so-so top end due to the same tweeter used. That's why many people rather buy the LCR6 which is a little more competent.
The methods used to combat these problems are the following:
1) The simplest is as B&W uses on their HTM7, CDMC-SE, CDMC-NT and Matrix centres: place the tweeter on top of the woofer. Of course this make it a bit tall and impractical to use in a wall unit, but that's also a good thing since no speaker should have two enclosures
2) Offset the tweeter a bit higher or lower to produce uneven diffraction. This is also often done with stereo speaker to improve imaging.
3) Adapt the crossover. This can help a lot if done right but is seldom implemented.
4) Cross extremely low so that the tweeter produces as much as possible. This is of course not too practical since it will load the tweeter too much, especially the cheaper ones.
5) Use a 3-way configuration, by placing the tweeter on top of the midrange and the two woofers at the sides. This will achieve the same as crossing a 2-way very low, by keeping most of the high frequencies in the same vertical line. The crossover to the side woofers will now be in the region of 300Hz that is low enough to prevent any lobing issues. This will render the cabinet a bit tall since the tweeter is on top of the mid, unless a neodymium tweeter is used or a very small midrange. Most top-level centre speakers use this approach. The B&W Nautilus is a good example, even though the midrange is a bit larger than most. KEF doesn't have that problem with their Uni-Q tweeter of course. Naturally the best is to use 5 identical speakers to cure all these problems, but for anything but a perforated projector screen this is impractical.
By now I'm sure most people will say that it is all good and well, but I cannot just go out and buy a different centre speaker - it will mess with my tonal matching/voicing/whatever the buzzword is nowadays. Well here's the news: on the cheaper centre speakers you don't have proper tonal matching to begin with! Well that is true, particularly with more exotic designs. However, I will rather have a proper centre that is in the same league quality-wise than a crappy one that is supposedly a tonal match. It's quite logical if you think about it. Firstly, the cabinet is much smaller and tuned completely different to the main speakers. So there goes any bass matching, especially since the TV or wall unit will add different boundary gain than with the main speakers. On to the midrange. Well, since we rarely have the same drivers as used in the main speakers, these will obviously sound different. The same drivers cannot be used since the cabinet volume would have to be more or less the same as the main speakers'. Using the same cone material for different sized drivers will produce some similarities, but not too many. Of course, since the woofers are different, so should the crossover be. This will mean a different crossover frequency, slopes, conjugate networks and so on. So all that remains is the tweeter. Since the centre is likely to be crossed higher than the fronts, we can therefore only expect similar response from about 5kHz upwards, but probably even more to the point where the woofer is at least 12dB down. So much for matching! A good example was the Boston CR2. It was said to be a tonal match to the bigger CR8 bookshelf. It used the same tweeter alright, but the CR8 had an 8" woofer; the CR2 a 4" one, tuned with a passive radiator in a small plastic cabinet. Needless to say the two didn't sound even nearly the same. Even though the tweeter was the same, the whole thing sounded completely different with very distracting sound panning. The white noise used to calibrate the speaker levels sounded vastly different.
Besides to use identical speakers, there is only one proper way to achieve real tonal matching: 3-way speakers. In addition to avoiding the lobing problems, these speakers usually use the same tweeter AND midrange than used in the main speakers. Only the woofers will be smaller, but both crossover frequencies and usually the whole network between midrange and tweeter can be kept as-is. This will lead to true tonal matching from about 500Hz and up. A typical configuration would be a 3" midrange and 1" tweeter, and two 6.5" woofers on the main speakers, reflex-loaded for low response. The centre would have the same mid and tweeter, and for example two 5.5" woofers in an alignment that doesn't extend as low and thus allow a smaller cabinet, or even a sealed alignment. This can now be seen as identical satellite speakers, each with its own local subwoofer that's crossed a bit high.
To fix the bass problem there are a few methods that are quite obvious. Make the cabinet larger, tune the port more realistically or seal the box and use a subwoofer. This is what is done by M&K and is actually the best home cinema solution provided the subwoofer is up to the task. The satellites are designed so that they're sealed and critically damped i.e. traditional Q=0.707 Butterworth alignment, with its natural -3dB point at 80Hz. This will not have too much bass if used on its own of course, and will fall off at an exact 2nd-order or 12dB/octave response. Note that this is the acoustic response, not electrical. Any speaker itself is a highpass filter, otherwise any woofer will be able to produce bass down to DC! All we did now was to tune this acoustic highpass to a certain set of parameters. If we now add some electrical filters as well, we can tune this bass response even further. This is usually done with active, either analog or digital, filters within any THX processor. The filter has the same response as the acoustic one; 2nd order Butterworth lowpass at 80Hz. These two filters, one acoustic and the other electric, will now augment the acoustic response (after all, all we care about is the acoustic response at the end!) to a 4th order Linkwitz-Riley configuration at 80Hz, with a Q of 0.707x0.707 = 0.5. So we have a predetermined highpass response. We now need a lowpass response from the sub to match it. The subwoofer driver's acoustic lowpass is always far higher than 80Hz and can therefore not be used as part of this filter. The processor's subwoofer output is therefore a 4th order Q=0.5 Linkwitz-Riley lowpass filter. This filter alignment is known for excellent integration, and theoretically we have a flat frequency response. In practice this is not always the case, but for most proper listening rooms and experimentation with speaker and subwoofer placement the response is near enough. Sometimes a graphic equalizer is used to ensure flat response. This method of subwoofer integration is far better than the usual method unless the crossover frequency is very low. Normal ported loudspeakers have very unpredictable frequency response slopes and it's very hard to match it with the subwoofer's fixed-slope lowpass filters, usually either 2nd or 4th order. This steep filter at a relatively high point means that the speaker can be fairly small, and is usually only one or two 5.5" woofers. The speakers always have more than one tweeter, often three, to control vertical dispersion according to the THX spec but also to increase their powerhandling at the extreme SPL levels of home cinema.
