B&O Tech: What are subwoofers REALLY for?

#6 in a series of articles about the technology behind Bang & Olufsen loudspeakers

 

The Setup

Back in a previous posting, I said something that could be perceived as interesting… The short version of what I said there was that, if you’re making a DSP-based active loudspeaker (like all of the new loudspeakers in the B&O portfolio), you can essentially make it sound like whatever you want. You do this by adding filters in the digital signal processing (DSP). (Let’s assume for this article that we’re only talking about the on-axis magnitude response of the loudspeaker, and we’re working in an anechoic environment (aka a “free field” situation), since that will keep things simple.) This means that, if I can apply enough boosts and cuts, I can get any magnitude response I want out of the loudspeaker. In other words, I can have a 1″ tweeter that plays with a perfectly flat response from 20 Hz to 20 kHz.

However, there are some serious restrictions on this statement. As a minor example, if there is a problem with diffraction the only way to change that is to modify the shape of the loudspeaker cabinet (if you don’t know what diffraction is, don’t worry – it will not be mentioned again in this article).

However, there is one GIANT restriction on the statement that we’ll look at this week. This is a question of how loudly you want to play. So let’s look at that.

In order to make sound, a loudspeaker driver has to move in and out – this pushes and pulls the air molecules in front of it, creating small areas of higher pressure and lower pressure (relative to today’s natural barometric pressure) that radiate outwards, away from the driver. Those variations in pressure push and pull your eardrum in and out of your head which, in turn cause stuff to happen in your inner ear which, in turn causes stuff to happen in your brain – but that is all outside the scope of this discussion.

Back to the loudspeaker – it has to move in and out. The louder you want to play (more accurately, the higher the Sound Pressure Level (SPL), the more it has to move in and out. Also, the lower in frequency you want to play, he more it has to move in and out (to keep the same SPL).

The red arrow shows the direction of movement of the loudspeaker driver required to make a positive (or high) pressure. The driver has to go the other way (into the cabinet) to make a negative (low) pressure.
The red arrow shows the direction of movement of the loudspeaker driver required to make a positive (or high) pressure. The driver has to go the other way (into the cabinet) to make a negative (low) pressure.

 

The real problem is the second of these, since the rule of thumb is that, every time you go down one octave (in other words, you divide the frequency by 2) you need to quadruple the excursion of the driver (the amount it moves in and out).

Let’s look at an example. The figure below illustrates the excursion required for different sizes of loudspeaker drivers in order to create a sound pressure level of 60 dB SPL (which is not very loud – but is a typical sort of listening level) at 1 m (which is a good approximation for how loud it will be all over your living room due to something called the room’s “critical distance” – we’ll talk about that in the future).

Notice that, for the 15″ woofer, it only has to move 0.08 mm out of the box (and 0.08 mm into the box) to produce a 20 Hz signal at 60 dB SPL. This is not very much movement. By comparison, the 4″ woofer has to move 1.2 mm which is much more than 0.08 mm, but still not much.

To bring this into the real world, this means that a woofer taken out of a BeoLab 3 (which is 4″ in diameter) would have to move 14 times farther than a woofer from a BeoLab 5 (15″ woofer) to produce the same output. This is because the 4″ woofer is smaller than the 15″, so to move the same number of air molecules, we have to move it more. (actually, what we’re really thinking about here is how many litres of air we’re moving, but that might be too much detail…)

 

The excursion of a driver (of different diameters) required to generate a signal of 60 dB SPL at 1 m from the front of the driver. Note that this assumes that your driver is mounted in a hole in the wall, not a real loudspeaker box (see text for the implications of this).
The excursion of a driver (of different diameters) required to generate a signal of 60 dB SPL at 1 m from the front of the driver. Note that this assumes that your driver is mounted in a hole in the wall, not a real loudspeaker box (see text for the implications of this).

 

Let’s consider the practical implications of this graph. Since a BeoLab 3 woofer can move 1.2 mm in and out (and, of course, a BeoLab 5 woofer can move 0.08 mm). Both loudspeakers are able to produce a 20 Hz tone at 60 dB SPL. Therefore, if we choose to do so, we can make both loudspeakers have a magnitude response that was flat from 20 Hz to 20 kHz at this listening level (or quieter).

Let’s turn up the volume knob. We’ll go up to 80 dB SPL which is a bit loud, but certainly not enough to get the party going… Now we need to move the 15″ woofer 0.8 mm (still not very much…) and the 4″ woofer 11.6 mm to produce 20 Hz at 80 dB SPL. Of course, the BeoLab 5 woofer can easily move 0.8 mm, but 11.6 mm is too far to go for the BeoLab 3 woofer. So, if we didn’t have ABL to protect things from moving too far, we would not be able to tune the BeoLab 3 to be flat down to 20 Hz – we would have to “roll off” the low frequencies so that 20 Hz was not as loud as the frequencies above 20 Hz in order to prevent it from causing the woofer to move to far when you turn up the volume. (For example, we could tune it to be flat down to 40 Hz instead of all the way to 20 Hz.)

 

80dBspl
The excursion of a driver (of different diameters) required to generate a signal of 80 dB SPL at 1 m from the front of the driver. Note that this assumes that your driver is mounted in a hole in the wall, not a real loudspeaker box (see text for the implications of this).

 

Let’s go further, just to make things really obvious. We’ll turn up the volume to 110 dB SPL (which is very loud). Now, to get a 20 Hz tone out at this level, the 15″ driver will have to move 2.6 cm and the BeoLab 3 woofer would have to move 36.6 cm (which is silly). So, here it is obvious that, if we want to build the BeoLab 3 to play 110 dB SPL, we will have to use ABL or limit its low frequency content (or use some balance of those two things – a little ABL and a little higher low-frequency limit).

 

110dBspl
The excursion of a driver (of different diameters) required to generate a signal of 110 dB SPL at 1 m from the front of the driver. Note that this assumes that your driver is mounted in a hole in the wall, not a real loudspeaker box (see text for the implications of this).

 

Let’s look at this in another, more intuitive way. If we wanted a BeoLab 3 woofer to play as loudly as a BeoLab 5 woofer can play, at its peak excursion in and out of the cabinet, it would look like the figure below.

 

A to-scale representation of how much the woofer on a BeoLab 3 would have to move to play as loudly as the woofer on a BeoLab 5.
A to-scale representation of how much the woofer on a BeoLab 3 would have to move to play as loudly as the woofer on a BeoLab 5.

 

The Implications

So, what does this mean? Well, it means two things:

  • for normal listening levels, we can use our DSP to make our loudspeakers have as much bass as we choose
  • however, this means that we need ABL to reduce the bass at higher listening levels

But, what happens if you want to buy BeoLab 3’s (or another “small” loudspeaker in the B&O portfolio), but you don’t want to lose bass output at high listening levels? Well, you have two choices:

  • buy bigger loudspeakers
  • buy a “subwoofer”

“What’s a subwoofer?” I hear you cry. Well, let’s be honest to start. In theory, a subwoofer is a loudspeaker that should play frequencies that are below the limits of the woofer. (In a system with passive loudspeakers, this would actually be true.) However, in a DSP-based, fully-active loudspeaker system, a subwoofer has a slightly different role. In the case of a Bang & Olufsen system, a subwoofer behaves more like a woofer with more ability to play loudly than the main loudspeakers.

For example, if you have a pair of small loudspeakers (let’s say, the built-in loudspeakers in a BeoVision 11, for example) and you add an external subwoofer (say, a BeoLab 19), and you’re listening at normal listening levels, then (all other things being equal) turning the subwoofer on and off should not produce a noticeable change in the bass level. In fact, if you turn on the subwoofer and hear a difference, it means that the subwoofer is too loud.

However, if you turn up the volume, you will get to a point where the “small” loudspeakers cannot produce enough output at low frequencies, so the ABL starts turning down the bass to protect the loudspeakers from distorting. Now, since the subwoofer can play louder at low frequencies, you will notice the difference.

Of course, this assumes that you’re using something called “bass management” which is an algorithm that removes the bass from the signals sent to your small loudspeakers and re-directs it to the more capable subwoofer. So, in the example above, where I was suggesting that you were turning your subwoofer on and off, I should have been more specific, since turning your subwoofer on implies that you’ve removed bass from the small loudspeakers at the same time.

This has a secondary implication. This means that, if you have a two different types of main loudspeakers (i.e. BeoLab 5 in as your front Left / Right pair and BeoLab 12 and your surround Left / Right pair) then we can do the same “trick”. So, the bass management system should “know” that the 5’s have more capability to play low frequencies louder than the 12’s and automatically direct the bass from the surround channels to the BeoLab 5’s in the front (therefore making the BeoLab 5’s the front loudspeakers and the subwoofers). And, if we were REALLY smart, the “brain” at the centre of the system would know the bass capabilities of all loudspeakers that are attached to it and be able to make intelligent decisions about who should get the bass. This is exactly what is happening in the BeoVision 11, BeoPlay V1 and BeoSystem 4. When you enter your Speaker Types (the model numbers of the loudspeakers in your configuration), the software inside the television automatically decides whether the bass should be redirected from a given loudspeaker in the configuration to another loudspeaker, based on the maximum outputs of those loudspeakers at low frequencies. (This entire lookup table is shown in the Technical Sound Guide available here – a small section of the table is shown below.)

An excerpt from the Bass Management logic table that is included in the BeoVision 11, BeoPlay V1 and BeoSystem 4. When you tell the television what loudspeakers you have connected, the software makes automatic decisions regarding where the low frequency content should be directed.
An excerpt from the Bass Management logic table that is included in the BeoVision 11, BeoPlay V1 and BeoSystem 4. When you tell the television what loudspeakers you have connected, the software makes automatic decisions using this table to best evaluate where the low frequency content should be directed.

There is one small thing that I haven’t mentioned, but some sticklers-for-detail will want that I do so… The reason you can get away with doing this whole bass-redirection-trick is that, in a normal listening room, we humans are worse at localising where low frequencies are coming from than we are for higher frequencies. This inability on our part can therefore be exploited by moving the bass to a different loudspeaker. However, there are some people who say that this inability is over-estimated (in other words, some people say that we’re better at locating subwoofers than most people think we are) however, that debate can probably be addressed by discussing the size of the room and how low a frequency is “low” – and those are just excruciating minutiae (at least, within the limits of this article…)

 

  1. Very interesting article, thank you. One thing I’m interested in is to what extent adding a “sub”woofer can affect the performance in higher frequencies. For example, for 2-way speakers some people say that the hear an improvement in the midrange – this could be e.g. when adding a BL 11 to a couple of BL 3’s. The theory would be that pushing the speaker to its limits in low frequences affects its ability to manage higher frequency.
    Do you think this is the case, or is it a “false perception”? <– Whatever that is; if the sound is perceived to be better for one person, then I guess it is. :)

  2. Hi Matthias,

    I certainly believe that this is true (because I’ve heard it – the first time was about 10 years ago when we put a BeoLab 2 with a pair of 6000’s. This resulted in the total system sounding a lot cleaner in the midrange). However, I think that the reason is slightly different. Personally, I don’t think that the midrange of a smallish loudspeaker performs better when you move its bass out to a “sub”woofer. My guess is that, when you push a small loudspeaker too hard with too much bass, you get THD causing harmonics of the low end singing along with the midrange. When you move the bass out to a subwoofer which is much more capable (and therefore farther away from its limit, and therefore behaving more linearly) you get less distortion out of the entire system. Consequently, the midrange sounds cleaner.

    Cheers
    -geoff

  3. Thank’s Goeff!

    We’re working with Bang&Olufsen for the past 20 years. Your blog is a big help for our customers, but even more so for us! Would be great to meet up in Struer next time we’re up in the north.

    Brilliant work on the new speakers! Speakers realy seam to have their own “face”. BeoLab 18 blew us away and the feedback we get from our customers is awesome.

    Keep it up.

    Merry christmas

    Dani

  4. Morten Poulsen says:

    Hi Geoff

    Very very interesting reading. Your articles are well written and understandable. Keep up the good work (and the fine work with the B&O speakers!)

    But I have a question regarding subwoofer performance:

    I own a BV10 with a pair of BL 3 front speakers. I’am thinking of making an investment to that system. Either I would like to ad an subwoofer for example BL19 or/and I would like to add a pair of BL9 speakers as new front speakers.

    But if you own a pair of BL9 would it then make any sense to add a BL19 subwoofer or would all the low-frequency signals anyway be directed to the BL9 speakers leaving the subwoofer silent? I can see that this is the case with a BV11 – if I understand the table right.

    Greetings

    Morten

  5. Hi Morten,
    If you have a BV11 and you add a pair of BeoLab 9’s and a BeoLAb 19, for example, then nothing will be sent to the BL19, since the 9’s are a more capable “subwoofer” than the 19 (or the BL2 or the BL11, for that matter…). However, to be honest I don’t know what will happen with a BV10 – although I think that its “old” bass management system in there will use the subwoofer instead of the 9.

    If you are expanding your system, I would suggest that you buy only a pair of BeoLab 9’s, and move your BL3’s back to be used as surround loudspeakers.

    Cheers
    -geoff

  6. Hi,

    I love these articles. Thank you.

    When I switch between speaker setups on my Beovision 11, I noticed it takes some time for the Beolab 9s’ subwoofer to “wake up”. Treble and midrange is available as soon as the speakers are switched on, but bass fades in a few seconds later.

    It doesn’t bother me, but I’m curious as to why this is. Is the bass fade-in done by design or a hardware limitation?

    Gabriel

  7. Hi,

    This is a “normal” behaviour of the BeoLab 9. It’s caused by the amplifier used by the woofer taking a slightly longer time to be enabled than the amps for the mid and tweeter.

    Cheers
    -geoff

  8. Hello Geoff,

    Thank you for your article! In this case, I don’t need Beolab 19 for Beolab 18?

  9. Hi,

    Adding a BeoLab 19 to a pair of BeoLab 18’s will most certainly give you more capability to play low frequency content louder. However, the amount of benefit that a 19 brings to a pair of 18’s is directly dependent on how loudly you play. The louder, the more benefit you get by having the 19 in the system.

    The best way to find out whether you would hear an improvement in your system with the 19 added is to ask your dealer for a home demo. Add the 19 to your 18’s for a week. At the end of the week, when they remove the 19, if you find yourself missing it, then you know you were using it… :-)

    Cheers
    -geoff

  10. Interesting article Geoff. I previously owned a pair of BL8000 (mk1) and replaced them recently with BL18. I heard the BL18 playing both solo and in combination with the BL19 in a huge open showroom. In that environment, they did sound flat and dull and the BL19 was a welcome addition, even at low volume levels. However, I took the advise of the store owner and first evaluated the BL18 solo at home.

    In all honesty, they blew me of my socks, completely different in terms of bass compared with my BL8000 mk1 (also in the highs). In fact, I had to reduce my bass setting from +3 to 0 because there was simply too much of it.

    I did notice that you have to sit in front of the BL18 to hear the bass. Once you move to the side of the speakers, the highs remain but the bass disappears.

    And if I listen to the album ‘Chaleur Humaine’ of ‘Christine and the Queens’ (Spotify), the bass (at a 0 setting) is simply annoying and pounding at you even at 30-40 volume levels. I’m starting to wonder if there is something wrong with either the Spotify recording of this album or if I still have a wrong setting somewhere in my V1. Perhaps I am still in an adjustment period of these new speakers :-)

  11. Hi,

    Thanks for your comments. I don’t know the “absolute” answer to your questions, but I have some observations that may shed some light on your experiences.

    Typically, I find that when I’m doing demos in larger rooms, I tend to naturally push the volume levels higher. I don’t think that this is only because the listeners are further away – although I’m sure that this has an influence. If you have 8000’s or 18’s and you push them harder, of course the ABL will kick in, resulting in a timbral change as the bass pulls back. If you add a subwoofer, then the woofers of the main loudspeakers aren’t working as hard, so you find that you not only get more bass (from the sub’s additional capability) but you also get a cleaner midrange if you have a two-way loudspeaker.

    As for the comparison between the 8000 and the 18 – although the visual design is certainly similar, these are completely different loudspeakers mechanically, acoustically, and electrically speaking. When we did the sound design of the 18’s, there was no attempt to “duplicate” the sound of the 8000’s. We did, of course, at the end of the sound design process, check to make sure that the two models were not so different that they could not be combined in a surround configuration – however, they certainly sound like different loudspeakers.

    Your comment about your listening position and the bass level variation is more likely a result of room modes, or (more generally) your position in the room, and not your position relative to the loudspeakers. For example, turn your loudspeakers around so that they’re facing away from you and move around the listening position while listening for the same variations in bass level (ignore the high’s, of course…). I suspect that you’ll experience the same behaviour.

    As for the question about Spotify, there is certainly a difference between the Spotify version of a track and a CD release. For example, if you compare Jennifer Warnes’s recording of “Bird on a Wire” to a track from Metallica’s “Death Magnetic”, you’ll notice that the difference in level from the CD’s is about 20 dB. The difference on Spotify is much less. This makes sense for Spotify, since you don’t want to be chasing different mastering levels by moving your volume up and down on every track – it’s more like a radio. However, this also means that there must have been some compression applied somewhere to make the tracks more alike in level. I have not analysed this, or read about it anywhere, so I can’t tell you much more than this about the dynamic range reduction – or its possible timbral consequences.

    Cheers
    -geoff

  12. Hi Geoff, wanted to give you some feedback. So I turned my loudspeakers about 80 degrees on their axis and I noticed very little change in the pounding bass effect I was hearing at my listening position. Your assumption was correct, my problem is room acoustics. In fact the more I move towards the opposite wall of where my speakers are located, the more bass I get (which unfortunately happens to be the spot where my sofa is located). The more I move away from the opposite wall (and towards the speakers), the less bass I get. Either the bass it pounding in my sofa and ok elsewhere or it’s ok in my sofa and insufficient elsewhere. I guess I’ll have to fiddle a bit more with the switches and the bass level but I haven’t found the correct combination yet. Thanks for the feedback.

  13. Hi,

    Great to hear that you’ve isolated the source of the problem! Now comes the REAL problem – fixing it…

    However, in my experience, especially in a small-ish room, it’s almost impossible to tune loudspeakers so that it’s equally good everywhere. The only way to approach this is with some significant acoustical treatment. Your first target is to reduce your room modes either with panel absorbers or slit absorbers. Neither one is terribly attractive in a living room.

    It will likely be that you have to accept that you get a good response in your primary listening position at the expense of other locations in the room.

    Cheers
    -geoff

  14. Hi Geoff,
    What a wonderful website. Since I have visited (and worked in) Struer and had a demo in the b&o equiped Audi A8 I am a big fan of B&o. This website only stresses the dedication to sound perfection to me. Since then I have bought a b&o play V1 (incredible value for money if you include media player and sound processing options) and a pair of BL3 s.
    I am very happy with sound quality in my apartment. In the table I read that adding a BL11 will direct the low frequency sound to the subwoofer. On other websites I read though that the value add of BL11 over Bl3 is limited. What is your opinion? Would you go for another subwoofer or first expand with additional surround speakers before adding a sub (e.g beolab 12-2)?

  15. Dear Geoff,

    Thank you for a very good article. What I wonder is to which sound level in db, the bass management logic table in your article refer to?

  16. Hi Sia,

    I’m sorry, but I can’t answer that question for at least a couple of reasons:
    1. The lookup table does not refer directly to a sound level in dB, but a value that is based on the overall bass capabilities of the various loudspeakers.
    2. The detailed information regarding the algorithm itself would be proprietary.

    Remember that the table is merely comparative. So, the actual values are irrelevant, more or less. The point is that the televisions “knows” that the Beolab 5 can deliver a higher SPL level across the bass region than a Beolab 4, and so the values in the bass management system are chosen accordingly.

    Cheers
    -geoff

  17. Thanks Geof for you reply. If I may raise another question, I would like to know more about the principals of the logic behind the bass management tabel.
    For instant if I read the values for Beolab 11, I can for instant see that the Lab11 would not contribute to Beolab 7-4 (I’m assuming that contribution is through low frequency register of the sound spectrum), while it would help Beolab 7-1

    Considering lowest registered frequencies for 7-4 (50Hz) and 7-1 (37Hz), it seesm that the bass logit table claims otherwise. Given the lowest frequency for Lab11 being 33 Hz, it should be able to contribute to both speakers and 7-4 more so. If that is the case, could you please explain why the table says otherwise?

  18. Hi Sia,

    The low-frequency cutoff is only true for the BeoLab active loudspeakers for low-to-normal listening levels. As you increase the level, the ABL starts to protect the loudspeaker by (in part…) reducing the output level of the lower frequency region. This is described in this posting.

    The Bass Management logic table is not based on the low-frequency cutoff – it’s based on the maximum output level of each loudspeaker over a range of frequencies in the low-end. The question is “which loudspeaker plays loudest” – not “which loudspeaker has the lowest frequency when I play quietly.”

    As I said in the article that we’re commenting on, A B&O loudspeaker can have any frequency response we want. At an extreme example: it’s possible to push a 1″ tweeter to have a low-frequency cutoff of 20 Hz – it just can’t play that very loudly. So, if you have another loudspeaker that can play 20 Hz louder (even if that second loudspeaker has a higher low-frequency cutoff) then you should send the bass to the louder speaker instead of the heavily-equalised tweeter…

    Cheers
    -geoff

  19. Hello there

    I want to buy a pair of Beolab’s 18. Can you tell me what the frequency range is.
    Can you also tell it for the Beolab 19?

    Tnaks

    Paul (Holland)

  20. hi Paul (Holland)

    The range of the BL18 is 41 Hz – 24 kHz. For the BL19 it’s 21 Hz – 900 Hz.

    In both cases, this assumes that the dynamic protection is not engaged. For the BL19, it’s with the LPF off. The reason the high frequency cutoff is so high is to ensure that if you are using the subwoofer with an external bass management system (such as a BeoVision television) the phase response of the low pass filter in the subwoofer does not interfere with the low pass filter in the bass management processing.

    Cheers
    -geoff

    For information on how this range is measured (at least for the BL18) please read this posting.

    Cheers
    – geoff

  21. Hi Geoff

    What do you mean in the previous article by saying that THIS ASSUMES THAT THE DYNAMIC PROTECTION IS NOT ENGAGED. Do you mean that IF the dynamic protection is engaged, the frequency response is different?
    How is the dynamic protection being engaged or disabled?

    Paul (another Paul from Holland)

  22. Hi “another Paul”,

    It is not possible for a customer to decide whether the dynamic protection (better known to B&O customers as “ABL” or “Automatic Bass Linearisation” is on or off. It’s always on.

    We do our measurements of frequency range at an input (and therefore output) level that is below the point where the ABL has started protecting the loudspeaker. Therefore the dynamic protection is not engaged because the measurement is done at a the sound pressure level that lower than where the ABL would need to protect the loudspeaker.

    If the ABL is working (because you’ve turned up the level of your source to a point where something in the system needs protecting) then both the frequency response and the frequency range of the loudspeaker will be different. IF this were not true, then the ABL protection would not be working.

    For more information on ABL, see this posting.

    Cheers
    -geoff

  23. Hi Geoff,

    Very much enjoyed reading your post. I am new to B&O world and have recently ordered Beolab 18’s. My question is with regards to adding a sub woofer to the setup of beolab 18 . Are my choices limited to Beolab 19 only ? Or can I add Beolab 11 as subwoofer to work with beolab 18? You advice would be much appreciated.

  24. Hi Sam,

    Sorry it took so long to respond. I have checked the measurements and I can say that a BeoLab 11 will increase your bass capability (maximum possible output level, averaged over a frequency range) if it is added to a pair of BeoLab 18’s. Whether or not it will be enough for you is more a matter of preference.

    I would suggest that you ask the forum at http://www.beoworld.org to solicit opinions from people who have done the same.

    Cheers
    -geoff

  25. Per Ullum Hansen says:

    Hi Geoff
    I am wondering which would be the best setup sound wise when turning up the volume. Beolab 18 & Beolab 19 versus Beolab 20?

    BR.
    Per

  26. Hi Per,

    ASSUMING
    – that you are considering one BeoLab 19 vs. two Beolab 20’s AND
    – that the “target” for the maximum output is the same amount of distortion (we typically use 10% as a measure of “loud enough” at any given frequency for the measurements – although we use other values (both higher and lower) for setting the system’s limiting, depending on frequency) AND
    – looking at a range of frequencies from 20 Hz to 250 Hz (since anything higher is irrelevant for the BeoLab 19 – actually we could argue whether we should go as high as 250 Hz – but we “weigh” the 250 Hz measurement results lower, since they’re less important, and more similar across all loudspeakers)

    Then the 20’s will go significantly louder than the 19.

    Actually this would still be true (although it would be less “significant”) even if you were comparing one BL19 to one BL20 (or, obviously, two of each…)

    So, if your only parameter is maximum output level, then you should go for the 20’s. In fact, if you connect a pair of BL20’s, a pair of BL18’s and a BL19 to a BeoVision television, the TV will automatically choose to bass manage the low end out of the 18’s and 19 and send it to the 20’s.

    Cheers
    -geoff

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