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 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…)
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.)
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).
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.
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.)
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…)