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

 

This week we’ll look at how most loudspeakers in the Bang & Olufsen go from the original idea through to the final product. I’ll use the BeoSound 8 (nowadays called the BeoPlay A8) as an example of this development process. However, the process itself is almost identical for almost all of our products.

 

The concept

The first step with most (but certainly not all) of our loudspeakers is an idea from either a designer or someone from our product definition department. They’ll come to the acoustics department with an idea of the product concept. This includes things like the following

• what kind of loudspeaker is it? (i.e. a docking station, a “bookshelf” loudspeaker, a floor-standing loudspeaker, etc.)
• the target customer and usage
• the target price
• a rough idea of the size and shape

 

From there, the acoustics engineer for the project can start looking into what kind of hardware we should use for the project. For example, this means things like:

• how many loudspeaker drivers (i.e. is it a single “full range” driver, a 2-way, a 3-way or something else?)
• loudspeaker driver dimensions (i.e. diameters and depths)
• how much volume we have in the enclosure behind the driver(s)

Based on this, we get a “best guess” of what kind of  system we’re looking at – at least with respect to the acoustics. At this point, if the acoustic engineer thinks that it’s a feasible concept, then we’ll move on to building a first prototype. If not, then we’ll enter into meetings with the product definition and design people to start working out the issues. However, for this story, let’s assume that all is well, and we can keep moving on.

 

Prototype #1

In order to get some idea of the acoustic performance of the system (basically meaning “can it play bass loudly enough?”) a first prototype is constructed. This is almost always a box made of MDF with a reasonable guess of the internal enclosure volume. Typically, at this point, we’ll use some off-the-shelf loudspeaker drivers that have roughly the same size and characteristics as what we’ll need in the product. In the case of the BeoSound 8, that first prototype looks like the one shown in the photo below. This prototype looks like it’s one box, but there is a bulkhead separating the two volumes behind the woofers.

 

 

Note that, at this point, we are only considering the acoustic capabilities of the prototype. So, we won’t spend a lot of time tuning it, since there won’t be a lot of listening done to it. A rough tuning is done to clean up the serious problems, but the question being asked at this point is something like “do we have the hardware that can deliver a sound performance that we can work with?” If we were building a car, this would be like having the engine on a test block, checking to see if we are going to get the necessary horsepower out of it – we wouldn’t be taking it out for a drive yet.

So, we do a rough tuning of the prototype, have a quick listen, do some measurements and see if we’re in the ballpark – do we have a “go” or a “no go”? If it’s a “go” then we move on.

One of the big problems with Prototype #1 is that it doesn’t have the same shape as the final product. So, although we can use filtering to make this loudspeaker have the magnitude response we want in one direction – typically on-axis (which is usually, but not always, directly in front of the loudspeaker), it will not have the same off-axis or power response of the final product. This is because the off-axis and power responses of a loudspeaker are primarily determined by the physical shape of the loudspeaker itself. (For a slightly more detailed discussion of this, read this.) If the final loudspeaker is going to have a circular face, and the prototype is a rectangle, then we have no idea how the final product will behave. This is one of the big reasons why we don’t bother tuning Prototype #1 very carefully, since the off-axis and power responses are significant components in the overall “sound” of a loudspeaker. So, we have to build Prototype #2 which is shaped a little more like the final product.

 

Prototype #2

The second prototype, shown below, looks more like the final product – particularly in the shape of the “baffle” – an acoustical word meaning “the face of the loudspeaker where the drivers are mounted”. You can see that this prototype now has circular faces with a sharp angle between the front and the side/back of the enclosure. This shape has a very different acoustic effect (to be more precise, “diffraction” – but that’s a topic for a future posting) than the smoothed right angle in the MDF box in Prototype #1. So, with this prototype, we can get a much better idea of the off-axis and power responses of the final product. If we see something really problematic at this point, we enter into negotiations with the designer, since it means we are going to ask him or her to change the shape of the loudspeaker.

 

You’ll also notice in this photograph that the tweeter and the woofer have changed since Prototype #1. This may be either because we found out that there is another off-the-shelf driver available that better suits the requirements of the product – or it’s because we have gone to the manufacturer of the driver to get changes made to the device to make it better suited to the application. (This wouldn’t be surprising, since most drivers are not designed to be put in enclosures as small as the ones we use. In fact, most of our loudspeakers have loudspeaker drivers that have been customised for us specifically for the requirements of the finished products.)

Looking at the back side of the prototype in the photo below, you can see 8 wires coming out. There are two wires connected to each driver, and there are four drivers – two woofers and two tweeters. When we’re measuring or listening to the loudspeaker, these are connected to external amplifiers. Early in the process, we’ll use large, rack-mounted professional amplifiers, but as we get further through the development we’ll start using amplifiers that are more like the the final hardware.

Prototype #3

So far so good. This time, the changes are more evolutionary than revolutionary. We get some more changes made to the drivers, and we make a model that is even more similar to the final shape of the product. If you look carefully at the difference between the second and third prototypes, you can see that the drivers have moved slightly. In Prototype #2, they were directly centred in the circular front, however, in Prototype #3, they’ve shifted slightly. Depending on the product, this might be due to acoustical reasons, but it could also be due to other reasons, such as the necessity to make space for components (like printed circuit boards) inside the enclosure.

As you can see in the photo, Prototype #3 doesn’t have any MDF parts – actually this one was milled out of a block of plastic. However, these days, we don’t do that any more, we use 3D printers. Unfortunately, we can’t start to do a detailed tuning of the loudspeaker yet, since the plastic that we used to use in the old days for milling and the plastic that comes out of a 3D printer is different from the plastic that gets used in the final product. As a result, the vibrations from the cabinet (for example) will be different in this prototype than in the final version. And, since a part of the final tuning is compensating for vibrations in the loudspeaker cabinet, there’s no point in tuning yet.

 

 

A funny side-story here. During the actual development of the BeoSound 8, we were doing a test on a prototype that looked exactly like this one (well, not exactly, it was grey…) in the Cube. It was sitting on a small platform on the crane (which hangs from the ceiling), about 6 m off the floor. The test was called a “bass capability” measurement where we put low-frequency tones into the loudspeaker at increasing levels until we reach a pre-determined amount of distortion. Then the frequency is changed and the test is repeated. Well, the test was running, and from the control room, you could hear a “boooooop … boooooop … boooooop … boooo ……… crash” Well, it turned out that the loud low frequency tones caused the prototype to slowly hop along the platform until it went over the edge and crashed on the floor. There wasn’t much left of it, so we had the black one made.

Again, as you can see in the photo below, we’re still using external amplifiers to drive the loudspeaker for measurements and listening.

 

In the next two photos below, you can see the prototype on the crane in the cube.  You’ll notice, particularly in the first photo, that it’s securely clamped to a block of aluminium, which is also clamped to the crane itself. We wouldn’t want it to fall off and crash to the floor, now, would we?

 

 

 

 

Prototype #4

At this point, we’re getting really close to the end. The production line is being set up, with the machinery being made to build the components in the product. So, we start looking at the early models that are coming off the production line. This means that we’re testing a product that is very close to being the final product, but it also means that we’re “de-bugging” the production line itself. This is why the prototype shown in the photo below looks like the final product – but it really isn’t.

 

 

If you take a look at the photo below, you can see that we still have lots of wires having out of the back of the loudspeaker. Some of these are connected to the loudspeaker drivers themselves, because we’re still driving them with external amplifiers. However, there are a lot more wires there. The extra wires are connected to thermal sensors. We’ll come back to those later.

 

 

Since this prototype is basically acoustically identical to the final product, we can start working on the sound design of the loudspeaker. This is a three-step process, consisting of listening, measuring, and listening.

Step 1 is to ensure that the loudspeaker doesn’t suffer from any problems with something called rub & buzz. When a woofer moves in and out of a loudspeaker cabinet, there is a considerable amount of vibration sent through the system, either because the woofer is mechanically connected to the rest of the system or because of the large changes in pressure inside the loudspeaker enclosure. If there are any leaks in the cabinet or if two parts can rub together inside, then these vibrations will cause buzzing (which can sound a lot like distortion) at very specific frequencies. These are usually so bad that, if they aren’t fixed, we can’t measure the acoustical response of the loudspeaker. So, these problems get fixed by hand by using stuff like glue, felt, or foam weather stripping. There are two good things about this: the first is that we get a well-performing prototype that we can work with. The second is that we learn what needs to be fixed on the production line to avoid these problems in the final products.

Step 2 is to measure the loudspeaker in the Cube (a 12m x 12m x 13m room) to see how it behaves both in the frequency domain (i.e. what does its magnitude response look like) and the time domain (i.e. when you send in an impulse, are any frequencies ringing longer than others). The acoustical engineer and the DSP engineer work together at this point to look at the measurements and firstly determine whether any physical changes are needed in the loudspeaker to correct problems in its acoustical response. Once these problems are corrected, the difference between the desired response of the loudspeaker and the actual response of the loudspeaker is analysed. That analysis is used to build a filter that reduces the difference so that the we get the desired response from the loudspeaker – at least according to the measurements. For example, if the loudspeaker has too little bass and a bump in its response at 2 kHz, then we will boost the bass and put in a dip at 2 kHz. I’ll go into a lot more detail about this in a future posting.

Step 3 is to listen. The loudspeaker with its corrective filter is brought into the listening room and we start playing music through it. We don’t start fiddling with equalisation right away. The first thing to do is to listen for problems that don’t show up in the measurements. If we detect any problems in the listening room, then we go back to the measurements to see if we can find out why something sounds weird. This puts us in a loop of listen – find problem – fix problem – listen some more – etc. until we run out of problems with physical solutions. Finally, we start listening to music and equalising to get the loudspeaker to sound as we want it (whatever that means). So, we go into the listening room and do this (this usually takes between 3 and 5 days if all goes well). Then we go to a different room (like, say, my living room at home, for example) and start tuning from scratch again. This process of tuning in a room is done in 4 or 5 rooms, resulting in one tuning filter for each room (usually I wind up with between 20 and 40 equalisers for a typical loudspeaker in each room). The problem here is that some of the filters that get put in to clean up the sound of a loudspeaker in a room are actually to correct problems in the room – not the loudspeaker. This is why we do the tuning in more than one room – the different tunings are taken and only the corrections that are common to more than one room are implemented. (For example, we have a room mode at 55 Hz in the main listening room at B&O – so I’ll put in a filter at 55 Hz to reduce that problem when I’m tuning in that room. However, since your living room does not necessarily have a mode at 55 Hz, then that correction should not be part of the loudspeaker.)

 

 

 

After the sound design has been finalised, then there are three more things left to do.

Firstly, the filters for the position switch (free / wall / corner) need to be tuned (using measurements from the Cube) and verified (by listening to music in different positions in different rooms).

Secondly, the final thermal tests have to be performed. For this, we connect the outputs of the thermal sensors (seen in one of those photos above) to a computer and we start playing some techno music really loudly, and we go home for the weekend. When we get back, we have a log file on a computer that tells us how hot the various components got and how that related to the music that we were playing. This tells us how close the loudspeaker components will get to their thermal limits in real life. Using this data, we can program the DSP to not allow the loudspeaker that you purchase to get hotter than it should. This was explained (sort of) in a previous posting.

Finally, we program a bunch of early production models with the “final” software and send them home with various people in the company for “real world” testing.

Production models

Once the production starts for real, we get the first samples that come off the line so that we can measure and test them to ensure that their performance and sound matches the prototypes that we worked on. Sometimes this doesn’t just mean putting the production model in the cube – sometimes it means something a little more customised. For example, for the BeoSound 8, we had to build a custom test rig and software to ensure that the fabric on the grilles was properly attached to the plastic backing. You can see the prototype of this test setup in the photo below.

 

 

Finally, we’re done! We sign off the production models and give the go-ahead to start shipping to the dealers.

 

Of course, the story I’ve told above is sort of skipping over a lot of details – but I’ll fill in some of those holes (at least partially) in future postings.

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

 

I recently saw a posting on a website showing a “naked” BeoLab 18 – meaning one without the front grille. The enthusiasm generated by that photo made me think that there might be some interest is seeing some Bang & Olufsen loudspeakers when they’re really naked. Visitors to the acoustics department in Struer are greeted by a collection of loudspeakers that have been opened up for viewing. I’ll show some photos of these in future posts. Today, I’ll reveal just two loudspeakers – the BeoLab 3 and the BeoLab 11. Do not try this at home.

 

BeoLab 3

The BeoLab 3 is a two-way fully active loudspeaker with analogue filtering. It has ABL, two 125 W ICEpower Class-D amplifiers driving a 3/4″ tweeter and a 4″ woofer in the front. In addition, it has two side-mounted 4″ passive radiators. If you take the front woofer off, you’ll get a look inside it as is shown below.

This gives you a direct view of the printed circuit board (PCB) with the analogue filtering and ABL circuitry which live directly behind and below the woofer.

In addition, you can see the PCB with the two power amplifiers on it.

Looking from the sides, through the holes the passive radiators normally occupy, you’ll see how little space there is behind the woofer when it’s mounted in the enclosure.

In the photo above, you can see two “potentiometers”, directly behind the woofer, attached to the vertical PCB that contains the filter circuitry (they have numbers printed on them and they look like the heads of phillips screws). These are for making gain adjustments to on the production line (or if you have to get your loudspeaker repaired) to ensure that the woofer and tweeter have the appropriate levels so that they not only match each other, but that they match the “golden sample” that we keep as a Master Reference. These are necessary to adjust for small differences in components within the circuitry as well as the exact sensitivities of the woofer and tweeter.

On the production line, this procedure is actually pretty cool. The acoustic response of the loudspeaker gets measured on the production line, then the two potentiometers are adjusted by hand to ensure that the response of the loudspeaker is correct – then the loudspeaker is measured again to make sure that the adjustment was performed correctly. This is done for each and every BeoLab 3 that we make.

Note that the PCB containing the power supply which delivers the voltage rails and current to the entire loudspeaker is on the “back” of the enclosure, behind the PCB containing the filters and ABL. The photo below shows a highlight of that circuit – although it’s hard to see from the side.

I know it’s difficult to see everything in there, so let’s take a different look at the components. The photos below show what could be considered to be an “exploded view” of the BeoLab 3. This was done for a special exhibit, so don’t ask for a similar photo of other loudspeakers in the portfolio. Sorry.

 

BeoLab 11

A block diagram of the BeoLab 11 would be surprisingly similar to the BeoLab 3. It has two 200W ICEpower Class-D amplifiers for the two 6.5″ loudspeaker drivers (each in its own sealed enclosure), filtering (although this time, the filter circuit includes a bass management system that also has a high pass filter for a pair of external loudspeakers), ABL, and a power supply.

In the posting describing ABL, I mentioned that there are thermal sensors distributed inside B&O loudspeakers to allow the device to continually “know” how hot it is. The photo below shows one of those sensors. It’s mounted on the small, green PCB that is screwed directly to the magnet assembly of the woofer (in the centre of the silver circle). This tells the circuitry the temperature of the woofer magnet. By itself, this information is not really useful, since the woofer magnet can get very hot without suffering damage. What we’re REALLY worried about is the temperature of the wire voice coil that is located inside the magnet – however, we cannot mount a temperature sensor on the coil, since this would stop the loudspeaker from working properly. So, the loudspeaker’s circuitry contains a “thermal model” of the woofer which calculates the temperature of the voice coil based on the temperature of the woofer magnet and the amount of power that has been sent into the woofer. This allows the loudspeaker to calculate the temperature of the voice coil based on the magnet temperature and the music that you’re playing.

 

 

 

You may notice that there is no thermal sensor on the opposite woofer. This is because the same signal is being sent to both woofers, so it is safe to assume that the two magnets (and therefore the two voice coils) are the same temperature.

 

 

That’s it for this week. Next week, I’ll walk through our development process – describing the steps that we take when we develop a loudspeaker starting with the first meetings with the designer, all the way through to the first products off the production line.

 

 

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

 

Part 1: The very basics

Let’s build a loudspeaker with a relatively decent frequency range. Actually, I should be more specific – I mean not only that it can play a wide range of frequencies, but it can do so adequately loudly to be useful. Chances are that you’ll want it to play down to something around 100 Hz (which is actually not that low… It’s only about an octave and a half below concert C – also known as Middle C to pianists) and up to about 15 000 Hz (which is probably still audible, depending on how old you are, how many hours you have spend clubbing,  how loudly your iThingy is usually playing, and whether or not you use ear plugs when you ought to…).

In order to do this, you’ll probably have to use at least two loudspeaker drivers – a woofer for the low frequencies (say, below about 2000 – 3000 Hz) and a tweeter for the high frequencies. The woofer is either big in diameter (say, about 12 to 40 cm) , or it can move very far in and out, or both. The tweeter is much smaller in diameter (on the order of 20 mm or so in diameter), and doesn’t need to move in and out as much. For the purposes of this posting, let’s say that that’s enough (which is not entirely infeasible – there are many loudspeakers in the world that are based on one woofer and one tweeter. Some of them are actually good!) The reason you need a bigger loudspeaker driver for the low frequencies is because, the lower you go in frequency, the more air molecules you need to move. Unfortunately, for every time the frequency is halved (i.e. you go down one octave), you need to quadruple the volume of air that you have to move in order to get the same sound pressure level. So, when it comes to bass, physics is your enemy.

Okay, so we have a woofer and a tweeter, and each of them has to get a different portion of the audio signal. This means that we have to divide the signal using something called a “filter” which, in its most basic form, lets some frequencies through unimpeded and makes other frequencies quieter. A “high pass filter” will let high frequencies through and make lower frequencies quieter. A “low pass filter” will do the opposite. So, we put a low pass filter in the path of the signal going to the woofer, and a high pass filter in the path of the signal going to the tweeter. The combination of those two filters are what is called the crossover, since it is the circuit that allows the audio signal to cross over from the woofer to the tweeter and back again, as is necessary.

 Part 2: Amplification

Unfortunately, loudspeaker drivers are very inefficient. Typically, you should expect about 1% of the electrical power you send into a loudspeaker driver to be available as acoustical power. The other 99% is lost as heat. This means that if you want your loudspeakers to play loudly, then you’re going to have to feed them with a lot of power (because you are throwing away 99% of what you put in). Consequently, you need something called a “power amplifier” connected to the loudspeaker drivers. This is a device that has a small audio signal coming into it (typically a change in voltage with almost no current) – it makes the signal much louder, typically by increasing the voltage by some multiplication factor (say, around 20 times) and making current available as is needed. (And since voltage multiplied by current is power, we get a power amplifier.)

 

Part 3: Signal flow

Now we start getting into the interesting stuff. At this point in the process of designing our loudspeaker, we have to make a choice. Either

• we put one power amplifier at the start of the chain, and filter its output before sending the signals on to the woofer and tweeter (a passive loudspeaker design), or
• we filter the signals first and then use a separate power amplifier for each driver (an active loudspeaker design) .

To be honest, if the diagram above was all there was to it, there wouldn’t really be much point in making an active loudspeaker. If all we did was to make relatively simple low pass and high pass filters, we basically could do the same filtering to the audio signal either way. The passive filtering circuit is big, and the active filtering circuit is small (basically because the passive components have to be able to dissipate more power) but the power amps in the active design take up space, so there’s not much gained there. So what’s the point?  Some people will make the claim that the amplifier has “better control” of the loudspeaker driver if there is no circuitry (like a low-pass or a high-pass filter) between them. However, to be honest, even if that were true enough to make an audible difference in things (I won’t say whether it is or it isn’t – since this is a debate best left out of this posting), it certainly wouldn’t be the first item on your list-of-things-to-worry-about. So, what IS the point?

Well, in order to get the point, we need to know a little more about how a driver behaves when you put it in an enclosure.

Part 4: Some basic acoustics

Take a really big sealed box and cut a hole in one side that has the same diameter as a woofer. Put the woofer in the hole so that the woofer is now in a “sealed enclosure”. If you do a frequency response measurement of the output of the woofer (on-axis, meaning “directly in front of the woofer” you’ll probably see that, as you go lower and lower in frequency, you’ll reach a point where the output of the woofer drops as you go lower. In fact, it has a natural high-pass characteristic. The reasons for this are beyond the scope of this discussion – you’ll either have to trust me on this one, or go read more stuff. If you thump the woofer with your thumb when it’s in this box, it will sound a little like a kick drum – it’ll go “thump”.

If you make the box much, much smaller in volume, you’ll see that the natural frequency response of the system changes. This is because the air in the box acts as a spring behind the woofer, and as the box gets smaller, the spring gets stiffer. The result of this in the frequency response is that you get a peak at some frequency. If you thump the woofer in this smaller box, you’ll now hear it ringing (at the frequency where you see that peak in the response) – now it goes ‘boommmmmm’, humming at one pitch – a bit like a big bell. The smaller you make the box, the higher in frequency the pitch go, and the longer it will ring. In addition, you’ll notice that there is a lot less low-frequency output below the ringing frequency.

If you take a look at the plot below, you can see examples of this. The curves show the response of the same woofer in different sized sealed enclosures. The flattest curve is the biggest box – notice that it doesn’t have a peak poking up, and it has about 40 dB (this is a LOT) more output at the very bottom end (okay, okay, it’s 1 Hz, but the absolute values aren’t important here – it’s the difference in the curves that counts). The curve with the biggest peak is the result of putting a woofer in a box that’s just too small for it. (If you’d like to know the details behind this plot, read this.)

 

Part 5:  Bringing it all together

Let’s start this section by admitting a simple fact: if the only thing criterion you use to judge a loudspeaker with is the volume of the enclosure behind the loudspeaker drivers, Bang & Olufsen loudspeakers are too small (yes – even the BeoLab 5). Take any of our loudspeakers, and you have an example of a woofer that is put in an enclosure that has too little volume for it to behave well naturally. In other words, when we look at the natural response of any of our loudspeakers, they look more like the “bad” curve than the “good” curve in the plots above. This means that we have to encourage  it to behave a little better. This means, in the simplest case (still looking at the curves above) that we have to boost the bass and remove the peak in the natural response of the system.

 

 

We do this by making a filter (in addition to the low pass filter) that overcomes the natural behaviour of the woofer in its enclosure. If we want more bass out of the system, we turn up the bass. If we want to remove a 7.3 dB peak at 143.5 Hz that has a Q of 4.6, then we put in a dip of 7.3 dB at 143.5 Hz and a Q of 4.6 (If those terms don’t make any sense, don’t worry – all that’s really important to know is that we can “undo” the effects of a peak in the natural response of the system by putting in a reciprocal dip in the signal that we feed it.)

In theory, this is possible using filters that happen after the amplifier – but it is certainly MUCH MUCH easier to make those filters (even without going to digital processing) using small resistors and capacitors and op amps before you get to the amplifiers. For example, you can see in the photo above, the SMD resistor and capacitor (which can be used in a modern active crossover) are much smaller than the power resistor and the inductor (which we would still have to use in a passive crossover).

So, even if you’re not doing anything other than trying to customise the sound of a loudspeaker using some filters (also known as equalisers) – as we do in almost all of our loudspeakers – it is smarter to make an active loudspeaker than a passive one.

 

Part 6: The beneficial side effects

So, in order to compensate for the acoustical effects of putting a woofer in too small a package, we have to make an active loudspeaker design instead of a passive one.

But this then raises the question, now that we have an active loudspeaker, what else can we do? The answer is lots of stuff!

Since we can apply filtering independently to each loudspeaker driver we can do some serious customisation of the system. To give just a few simple examples:

• You have a resonance in the woofer at a frequency that is above the crossover. You want to correct the problem in your filtering (because you can hear and/or measure it), but the problem does not exist in the midrange. So, you want to have a filter on the woofer alone – not the woofer and midrange and a passive crossover.
• You want to do some dynamic processing on a driver without affecting the others. (for example, ABL)
• You want to compensate for small differences in loudspeaker driver sensitivity on a production line by doing an automated measurement and a gain offset on a driver-by-driver, loudspeaker-by-loudspeaker basis to ensure that loudspeakers leaving the factory are better matched to the “golden sample”

 

 

An active loudspeaker design makes all of these examples MUCH easier (or perhaps even “possible”) to achieve.

Conclusion 

All of that being said,

• if your electroacoustical behaviour of every component in your audio chain was “perfect” (whatever that means) AND
• if loudspeakers behaved linearly (i.e. they gave you the same frequency response at all listening levels, and they didn’t change their behaviours when they heat up, and so on and so on) AND
• if you did everything properly (meaning that your cabinets were the right size and shape) AND
• if your production tolerances of every component in the system was +/- 0%.

Then MAYBE a passive loudspeaker design could work just as well as an active design…

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…

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,
• 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) 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.