B&O Tech: What is “ABL”?

Header info #1 for full disclosure: I’ve been given the green light from the communications department at Bang & Olufsen to write some articles describing some of the more technical aspects of B&O loudspeakers here on my own blog site. This is the first posting in what will be a series of articles.

Header info #2 for fuller disclosure: This particular posting will look familiar to some forum people at www.beoworld.org, since I wrote the original version of this as a response to one of the questions on their site. However, I’ve beefed up the response a little – so if you’ve come here from beoworld, there is only a little new information in here.

Almost all loudspeakers made by Bang & Olufsen include Adaptive Bass Linearisation or ABL. This includes not only our “stand alone” loudspeakers (the BeoLab series) but also our iPod docks and our televisions. The only exceptions at the moment are our passive loudspeakers, headphones, and the BeoLab 5.

There is no one technical definition for ABL, since it is in continual evolution – in fact it (almost) changes from product to product, as we learn more and as different products require different algorithms. Speaking very broadly, however, we could say that it reduces the low frequency content sent to the loudspeaker driver(s) (i.e. the woofer) when the loudspeaker is asked to play loudly – but even this is partially inaccurate.

It is important to note that it is not the case that this replaces a “loudness function” which may (or may not) be equalising for Equal Loudness Contours (sometimes called “Fletcher-Munson Curves”). However, since (generally) the bass is pulled back when things get loud, it is easy to assume this to be true.

When we are doing the sound design for a loudspeaker (which is based both on measurements and listening), we make sure that we are operating at a listening level that is well within the linear behaviour of the loudspeaker and its components. (To be more precise, when I’m doing the sound design, I typically use a standard-ish playback level where -20 dB FS full-band pink noise results in something like 70 dB (C) at the listening position (sometimes I use 75 dB (A) – but, depending on the amount of low end in the loudspeaker, this might result in the same volume setting).)

This means that

  • the drivers (i.e. the woofer and tweeter) aren’t being asked to move too far (in and out)
  • the amplifier is nowhere near clipping
  • the power supply is well within its limits, and
  • nothing (not the power supply, the amplifiers, or the voice coils) is getting so hot that the loudspeaker’s behaviour is altered.

This is what is meant by “linear” – it’s fancy word for “predictable”, (Not to mention the fact that if we were listening to loudspeakers at high levels all the time, we would get increasingly bad at our jobs due to hearing loss.)

So, we do the tuning at that low-ish listening level where we know things are behaving – remember that we always do it at the same calibrated level every time for every loudspeaker so that we don’t change sound design balance due to shifts associated with equal loudness contours. (If you tune a loudspeaker when it’s playing loudly, you’ll wind up with a loudspeaker with less bass than if you tuned it quietly. This is because you’re automatically compensating for differences in your own hearing at different listening levels.)

Once that tuning is done, then we go back to the measurements to see where things will fall apart. For example, in order to compensate for the relatively small cabinet behind the woofer(s) in the BeoSound 8 / BeoPlay A8, we increase the amount of bass that we send to the amplifiers for the woofers as part of the sound design. If we just left that bass boost in when you turn up the volume, the poor speaker would go up in smoke – or at least sound very bad. This could be because

  • the woofer is being pushed/pulled beyond its limits, or
  • because the amplifier clips or
  • the power supply runs out of steam or
  • something else.

(Note that BeoSound 8’s do not actually run on steam – but they do contain the magic smoke that keeps all audio gear functioning properly.) So, we put the loudspeaker in a small torture chamber (it’s about the size of a medium-sized clothes closet), put on some dance music (or some slightly more-boring modified pink noise) and turn up the volume… While that’s playing, we’re continually monitoring the signal that we’re sending to the loudspeaker, the driver excursion, the demands on the electronics (i.e. the amp’s, DAC’s, power supply, etc) and the temperature of various components in the loudspeaker, along with a bunch of other parameters…

beosound_8_last_prototype
One of the last BeoSound 8 prototypes. The orange/black wires connect directly to the woofers. The purple/white wires connect directly to the tweeters (at this stage of development, we are still using external amplifiers). Most of the other wires go into thermal sensors inside the device to see how hot things are getting inside. Some of these thermal sensors are actually in the final product that the customer buys. Some are just for development purposes and are not in the final product.

Armed with that information, we are able to “know” how those parameters behave with respect to the characteristics of the music that is being played (i.e. how loud it is, in various frequency bands, for how long, in both the short term and the long term). This means that, when you play music on the loudspeaker, it “knows”

  • how hot it is at various locations inside,
  • what the velocity of the air in the port is (if it has a port),
  • the loudspeaker drivers’ excursions,
  • amplifier demands,
  • power supply demands,
  • and so on. (The actual list varies according to product – these are just some typical examples…)

So, when something gets close to a maximum (i.e. the amplifier starts to get too hot, or the woofer is nearing maximum allowable excursion, or the air velocity in the port is nearing an unacceptable level) then SOMETHING will be pulled back.

WHAT is pulled back? It depends on the product and the conditions at the time you’re playing the music. It could be a band of frequencies in the bass region, it could be the level of the woofer. In a worst-case-last-ditch situation, the loudspeaker might even be required to shut itself down to protect itself from you. Of course, there is no guarantee that you cannot destroy the loudspeaker somehow – but we do our best to build in enough protection to cover as many conditions as we can.

HOW is it pulled back (i.e. how quickly and by how much)? That also depends on the product and some decisions we made during the sound design process, as well as what kind of state-of-emergency your loudspeaker is in (some people are very mean to loudspeakers…).

Note that all this is done based on the signals that the loudspeaker is being asked to produce. So it doesn’t know whether you’ve turned up the bass or the volume – it just knows you’re asking it to play this signal right now and what the implications of that demand are on the current conditions (voice coil temperature, for example) This is similar to the fact that the seat belts in my car don’t know why the car is stopping quickly – maybe it’s because I hit the brakes, maybe it’s because I hit a concrete wall – the seat belts just lock up when they’re asked to move too quickly. Your woofer’s voice coil doesn’t know the difference between Eminem and Stravinsky with a bass boost – it just knows it’s hot and it doesn’t want to get hotter.

It’s important to note that some of what I’ve said here is not true for some products. Bang & Olufsen’s analogue loudspeakers cannot have the same amount of “self-knowledge” as the digital loudspeakers because they don’t have the same “processing power”.  However, we make every effort to ensure that you get as much as is possible out of your loudspeaker while still ensuring that you can’t do any permanent damage to it. However, it’s fair to say that, the more recent the model, the closer we are able to get to the maximum limits of the total system for a longer listening period.

  1. Congratulations on the new blog! Interesting description of ABL and the kinds of things that can be accomplished with DSP. Too often in audiophile circles, a speaker gets it’s audiophile approved credentials by touting the fact they use whatever trendy wire, capacitor type, or driver material is in vogue at present. B&O seems more focused on realizing fully the benefits of active speakers over passive ones, in all areas of performance. Back in the day, 30+ years ago, active speakers offered significant advantages to passives, but never were well accepted, in part because audiophiles couldn’t play swap amps every month, and dealers couldn’t sell said amps, etc. B&O, by moving their entire lineup to active speakers, has created an environment where active speaker technology can be used to maximum advantage.

    I think too many people underestimate the amount of acoustic and psychoacoustic knowledge and expertise B&O has and think of them as only a pretty face. Hopefully this blog will help people learn more about the seriousness of your engineering.

  2. Simon Payne says:

    Many thanks for this information. Someone on BeoWorld sent me this link when I created a thread asking about the maximum volume I could turn my recently bought BeoLab 9s up to with my BeoVision 9. I play music through them on Speaker 2 at a volume of 60 using Apple TV connected to my iMac. I was curious how much further than 60 I could go without blowing anything up! Do newer speakers including the BeoLab 9s still have foams? I have seen many posts on BeoWorld about older speakers (BeoLab 1s for example) needing foams replacing after a while. I love my new BeoLab 9s so much and want to be armed with the best knowledge on how to look after them as I can’t see myself ever selling them. The look is fantastic, not to mention the sound. Thanks to you guys at B&O for doing what you do. The only company that gives you great designs, great sound and picture to match and so important – a solid metallic build quality as a finish. I don’t want cheap plastic products! All the best and keep up the great work :)

  3. Hi,

    Thanks for your kind words!

    The ABL and Thermal Protection in the BeoLAb 9’s should protect them from almost any abuse you can throw at them. As part of our final testing, we play a sequence of music that has been mastered very loudly at volume step 90 for at least 48 hours (we call it a “party test”) to see if they survive. They do.

    Of course, it is possible that you can come up with some signal that may damage your loudspeakers – but it is highly unlikely with your configuration. I would certainly not worry about playing BeoLab 9’s at any volume from your BV9.

    Regarding foam: the BL9 woofers have rubber surrounds.

    Cheers
    -geoff

  4. Michael Okeke says:

    Hi love your post, I’m just about to place an order for beolab 20, is it best to use av processor or av receiver with it ? If you can thanks for your co operation.

  5. Hi Michael,

    Thanks!

    The BeoLab 20 is designed to use a Line Output from your source. So, if you buy an AV Receiver, you should make sure that it has a Line Output or “Pre Out” (with volume control). Otherwise, you should buy a surround processor or preamp (depending on the number of channels you need…)

    Cheers
    -geoff

  6. Thanks for your reply, I’m planing on buying (Classe ssp 800 ) what do you thank ? I’m just trying to put the best sound in my new home I’m new at these and finally when you said volume control av what does that entell ?

  7. Hi Michael,

    Of course, as a Bang & Olufsen employee, I can’t comment on the Classé preamp – even through I’m a Canadian… :-)

    However, since you have already said that you are looking for an AVR or Surround Processor (as a B&O employee, I would say that you might want to consider the BeoSystem 4…) for your BeoLab 20’s, I can only recommend that you look for a device that has:
    – the appropriate inputs for all of your source devices
    – line outputs with good specifications both in terms of frequency response (<20 Hz to >20 kHz with a deviation of less than 0.2 dB would be a good start) and a wide signal-to-noise ratio (say, > 100 dB)

    – the DSP that meets your needs. For example, you should consider CODEC support (if your sources cannot decode), downmixing/upmixing capabilities, and customisation (i.e. alignment – although this is fairly standard).

    Hope this helps.

    Cheers
    -geoff

  8. Thanks for your help I didn’t even know you guys have that kind of system until you mentioned it, how ever I have read the specification, It does’t support many of the audio format like dts plus Hd etc.

  9. Hi,

    It’s true that the BeoSystem 4 does not support the formats you mention, however, I posted a blog article about the actual implications of this. It may not be as important as consumers are led to believe. There are, of course, specific examples where the AVR or Surround Processor must do the decoding, for example, when you have a very low-end Blu-ray player that doesn’t support all CODEC’s. However, this is an exception rather than a rule.

    The flip-side of the BeoSystem 4 is the extremely comprehensive loudspeaker management and bass management control. You can see this in the Technical Sound Guide manual for the device available here. (Note that the link goes to the BeoVision 11 page, but the software is the same.)

    Cheers
    -geoff

  10. Hi
    So you are saying if I have blu ray player that support does codec I wouldn’t need to bother ?, I don’t know how much it cost I have serched for it online I could’t find it. Can you help!

  11. Hi,

    Yes – and no.

    For example, if your player can convert the CODEC to PCM, then you don’t need to do it (the conversion) in the AVR or Surround Processor.

    However, you also need to consider other things. For example, the BeoSystem 4 does not support 88.2 or 176.4 kHz sampling rates. If you have an SACD player that outputs PCM on an HDMI at 88.2 kHz, then this will not work on a BeoSystem 4.

    Note that these are just two examples. If you’re spending the kind of money you’re talking about (based on your mention of the Classé product), then you want to make sure that it’s going to do exactly what you want it to do. I would certainly not want you to buy a B&O product if it’s going to be a disappointment due to your specific set of requirements.

    Cheers
    -geoff

  12. Thanks for your help so far, I have heard all you said and I’m very happy with it , please can you help tell me if I use it with Oppo BDP-103D or 105D will it work ?

  13. Hi,

    The only incompatibility I know of with the Oppo players and the BeoSystem 4 is that the DSD-to-PCM conversion outputs signals at 88.2 kHz. This will result in no audio from the BeoSystem 4. So, if you are not concerned about SACD or DSD files, then, as far as I know, you will have no problems.

    There are other multi-disc players (such as the Cambridge Azur Blu-ray player) that convert DSD to 96 kHz (actually, this sampling rate is user-selected in the menus of the Blu-ray player). This works fine.

    I suggest that you also post the same question to the forum at beoworld.org to find out what other persons’ experiences are with this hardware combination.

    Cheers
    -geoff

  14. Henning Hansen says:

    I am still puzzled why loudness went completely out of fashion. It was brilliant in Beomaster 8000. And lousy in most other products, regardless manufacturer. I know it meant using special chips from AD then, but with all that DSP available now it should be possible to redo now.
    So ABL is adjusted to give a little extra bass boost at medium level to give a full bass volume experience and then reduce bass at higher levels. Seems somewhat like loudness, but for some reason low level does not get further bass increase as it should.
    I would really like a switch on the speaker that activates proper loudness for spoiled listeners like me.

  15. Hi,

    There are a couple of ways to respond to your comment.

    One important thing to remember is that the evaluation of an auto-loudness function (one which is variable with the setting of the volume knob) is very personal. If your particular equal loudness contours are different from average, then the auto-loudness function will not suit you.

    Also, if your system has an auto-loudness function that is designed for recordings that are mastered at a different level than yours, then it will be “uncalibrated”. For example, if I tune an auto-loudness function using only classical music recordings, and you only listen to Metallica, then your music is roughly 20 dB louder than mine at the same volume setting, so the auto-loudness function will not work. This also happens over time. Current recordings are mastered at a much higher level than older recordings, so a new recording on an “old” device will not have the correct auto-loudness calibration.

    Regarding ABL vs. Auto-loudness, the difference is that ABL is signal dependent, whereas auto-loudness has to be volume dependent. You would not want a loudspeaker compensating for equal loudness contours with level, because this would mean that your recordings would get bass-heavy when the orchestra plays quietly. This “loudness” function is already built into the recording itself, since the conductor got the bass players to play decrescendo less than the violins during the quiet sections. Since the loudspeaker doesn’t know the volume setting, it shouldn’t try to chase the signal level to suit human response characteristics.

    ABL is simply there to protect the loudspeaker from distorting. That distortion would happen at high levels since we push the bass harder – but that bass level has to remain linear at low levels.

    Cheers
    -geoff

  16. Hi geoff,
    After reading this topic I just went back to my preamp and set my centre speaker (BL10) and sorround speakers (BL8000) as large. I thought I was protecting them by setting their size as small, but apparently that not necessary as they have the ABL feature!

    There is only one thing that got me worried: I have a pair of BL5 as fronts and you mentioned at the beggining of your post that they don’t use ABL. Is that because they have a different safety approach to protect them?

    Cheers, Dante.

  17. Hi Dante,

    You don’t need to worry. The BeoLab 5’s don’t have ABL because they don’t need it. Remember that ABL is not protection against damage – it’s protection against uncontrolled distortion. The BL5’s still have thermal protection – so you can’t overheat them. This doesn’t mean that they’re indestructible – but to destroy them, you’ll have to try REALLY hard… ;-)

    Cheers
    -geoff

  18. Henning Hansen says:

    I just happened to notice the new Beolab 50. And the new approach to technical information given to prospective customers. Very impressive.

    I also noticed that the loudspeaker includes Loudness. According to the White Paper :
    “Note that, when connected to most Bang & Olufsen sources, the Loudness function in the BeoLab 50 will be disabled for the Power Link and Wireless Power Link inputs. This is because in these cases, the Loudness function is performed by the source rather than the loudspeaker.”

    I asked about the absense of a Loudness option 2 years ago ( see above ) and you responded listing various reasons why Loudness would not work. So what has changed since then ?

    And a personal preference. I think B&O has implemented the correction wrong. One needs a lot of bass enhancement ( the 12 dB is not too much ) but much less treble enhancement, where something like 3 dB would do rather than the 9 dB the figure shows.

    And thanks for your insights in this blog.

  19. Hi Henning,

    The Loudness function is in the list features that will be released if future updates of BeoLAb 90 and 50 – this has always been listed as such in the Technical Sound Guide since its initial release. However, I see, having read your comment, that, in the latest compiling of the Technical Sound Guide, I have made a mistake and accidentally removed the grey indication that shows that the feature is not yet released. Thanks for pointing this out. I will have it corrected as soon as possible – and return to grey until it is implemented.

    In the Bang & Olufsen televisions, the amount of boost applied by the Loudness function is user-adjustable – so if you prefer a 3 dB boost, then this is possible to set in the menus. This adjustment is not currently adjustable in either the BeoLab 90 or 50 menus.

    It should be noted, however, that the amount of boost applied in both the Bass and Treble are dependent on the output level. While =3 dB may be appropriate at one listening level, is certainly too little for very low listening levels – and too much for others.

    Cheers
    – geoff

  20. Henning Hansen says:

    I sense a kind of animosity against loudness, the concept you refer to as “auto-loudness”.

    I am spoiled, I know. Having enjoyed the Beomaster 8000 for 30+ years. This device had (*) a superior loudness implementation due to the use of the Analog Device AD7110 chip. Used with speakers like the MS150 or better, one had a balanced bass response at any volume setting.

    You are obviously younger than that HI-FI generation. But you should dig one of the old units out from the museum and listen to the loudness implementation.

    (*) After 30+ years the computers in the Beomaster 8000 are quickly detoriating. So now I have no functional unit left.

    ===

    Another subject : Frequency range and ABL. The Beolab 5 has no ABL as it don’t need it. This implies that the stated frequency range ( using the -10 dB drop concept ) is valid at any volume level, like at 100 dB SPL. As the Beolab 50 do have an ABL function then at what SPL level is the stated frequency range valid ? Is the Beolab 5 inferior to the Beolab 50 with respect to maximum SPL level, say at 30 Hz. This is a tricky subject and maybe you have covered it elsewhere.

    Regards

  21. Hi Henning,

    No animosity at all! I am a very firm believer in the necessity of a loudness function. I prefer to use the term “auto loudness” in modern implementations since the magnitude of the modification (the bass and treble boosts) vary with volume. In the pieces at the museum :-) the loudness function was a fixed boost, and was therefore not varying with volume setting.

    Of course, the problem is that it is impossible to have a “correct” loudness filter, as I’ve talked about in this posting

    ===

    All BeoLab loudspeakers are measured at 88 dB SPL (in the speakers without a volume control, this corresponds to a 125 mV RMS signal on the Power Link input, measured in a free field). As you rightly point out, the magnitude response will change at higher levels, either due to ABL, thermal protection, or thermal compression (which is true for all loudspeaker without Thermal Compression Compensation).

    For more information on the details surrounding this, you can check the following:
    Thermal compression compensation
    Loudspeaker Frequency Range Specifications

    Cheers
    -geoff

  22. Henning Hansen says:

    Something bad happened in the 90’s. All the old knowledge seems to have been killed by the silver and gold core cable terrorists ..

    You write elsewhere :
    “This compensation is called “loudness” – although in some cases it would be better termed “auto-loudness”. In the old days, a “loudness” switch was one that, when engaged, increased the bass and treble levels for quiet listening. (Of course, what most people did was hit the “loudness”switch and left it on forever.) Nowadays, however, this is usually automatically applied and has different amounts of boost for different volume settings (hence the “auto-” in “auto-loudness”).”
    You elaborated on it afterwards in the comments.

    But it is honestly wrong, what you write of the old days. The Beomaster 8000 was way ahead of its time also in terms of a proper loudness implementation which was truly “auto-” in the sense mentioned above.

    Other implementations of the time were based on a volume potentiometer with a single intermediate output where a capacitor network was attached. This would also qualify as “auto-” but often failed to work over an extended volume settings range. Simply to simple.

    Yamaha made an awkward attempt using two volume potentiometers to allow for varying the maximum “boost”. One has to try it to experience the oddness, and the correction didn’t fit anyway.

    So loudness correction fell out of fashion as even tone controls was abandoned in “serious” Hi-Fi gear.

    An the transition to AV receivers didn’t change that. Only lately has loudness been re-invented by most of the elephants in that business triggered by “Audyssey’s Dynamic EQ”.

    B&O apparently had loudness implemented in various models throughout those bad years. I didn’t know that. I also didn’t care to know as I had my Beomaster 8000’s ..

    So I truly look forward to a reborn loudness implementation.

    Regards

  23. Hi Henning,

    Interesting what you write about the “auto-ness” in the loudness in the BeoMaster 8000. I’ve dug one out of the basement here to run some measurements on it to see how it behaves… I’m looking forward to learning something that was lost in the 90’s. :-)

    Cheers
    -geoff

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