{"id":2660,"date":"2015-07-28T09:45:31","date_gmt":"2015-07-28T07:45:31","guid":{"rendered":"http:\/\/www.tonmeister.ca\/wordpress\/?p=2660"},"modified":"2018-05-29T07:58:23","modified_gmt":"2018-05-29T05:58:23","slug":"bo-tech-shark-fins-and-the-birth-of-beam-width-control","status":"publish","type":"post","link":"http:\/\/www.tonmeister.ca\/wordpress\/2015\/07\/28\/bo-tech-shark-fins-and-the-birth-of-beam-width-control\/","title":{"rendered":"B&O Tech: Shark Fins and the birth of Beam Width Control"},"content":{"rendered":"

#37 in\u00a0a series of articles<\/a>\u00a0about the technology behind\u00a0Bang & Olufsen<\/a>\u00a0loudspeakers<\/strong><\/p>\n

In the last posting<\/a>, I described a little experiment that we did where we could easily be accused of asking a stupid question and getting an obvious answer. This posting is a little different, although I’ll also describe a little experiment where we asked a much-less-stupid question and got an answer we weren’t looking for… However, before I describe the experiment itself, we’ll have to start with\u00a0a little background information.<\/p>\n

Let’s say that we wanted to build a “simple” two-way loudspeaker with a tweeter and a woofer in a box like the one shown in Figure 1.<\/p>\n

\"Figure<\/a>
Figure 1. A simple two-way loudspeaker.<\/figcaption><\/figure>\n

Since we’re Bang & Olufsen<\/a>, it is implied that this will be a fully active, DSP-based loudspeaker. Let’s also, for the sake of simplicity, say that we used loudspeaker\u00a0drivers and other components with absolutely no linear distortion\u00a0artefacts (I can dream, can’t I?). This means that we can apply any kind of filter we like to each of the two loudspeaker drivers to result in whatever response we want (in a given direction in a “free field” (a space with no reflecting surfaces, extending to infinity in all directions) – for more information on this, please read this posting<\/a> and\/or this posting<\/a>).<\/p>\n

Now, it’s fairly reasonable to assume that one portion of a good starting point for a loudspeaker’s acoustic response is to have a flat magnitude response when measured on-axis in a free field. This means that, if you have the loudspeaker in a space that has no reflections, and you are directly in front of it, no frequency will be louder than any other (within some limits, of course…). In fact, this is the way a lot of studio monitor loudspeakers are tuned (which makes sense, since many studio control rooms don’t have a lot of reflections as was discussed here<\/a>).<\/p>\n

The problem is that, if you do make a loudspeaker that is flat-on-axis, you’ll probably have problems in its power response (for a description of what a loudspeaker’s “power response” is, see here<\/a>). This is dependent on a lot of things like the crossover frequency, the sizes and shapes of the drivers, the phase responses of the crossover filters, the shape of the cabinet, and other things. However, if we were to simplify, we could say that a two-way loudspeaker (say, with a 4th order Linkwitz-Riley crossover, just to pick one type…) that is flat on-axis, will have a dip in its power response at the crossover frequency. This is because, although the two drivers (the tweeter and the woofer) add together nicely in one location (on-axis, directly in front of the loudspeaker) they do not add together nicely anywhere else (because the distances to the two drivers from the listening position are probably not matched, particularly when you go vertical…).<\/p>\n

So, one basic problem with building a “simple” two-way loudspeaker is that you have to\u00a0choose to either have a flat magnitude response on-axis, or a smooth power response (without a dip) – but you can’t have both. (If you want to really dig into this, I’d recommend starting with\u00a0J. Vanderkooy & S.P. Lipshitz, “Power Response of Loudspeakers with Noncoincident Drivers — The Influence of Crossover Design,”\u00a0J. Audio Eng. Soc.<\/i>, Vol. 34, No. 4, pp. 236-244 (Apr. 1986).)<\/p>\n

This basic problem raised a question in the head of one of our acoustical engineers, Gert Munch. He started to wonder how we could build a loudspeaker that could have a flat magnitude response on-axis and still have a smooth power response that didn’t suffer from a dip at the crossover. One possible solution is to build a loudspeaker with the desired on-axis response, and then somehow create an additional portion of the loudspeaker that could “fill up” the dip in the power response by sending energy into the room without it affecting the on-axis response.<\/p>\n

One possible way to do this is to use an extra loudspeaker with a “dipole” characteristic – a two-sided loudspeaker where the same signal is sent to both sides, but in opposite polarity. This means that, when one side of the loudspeaker produces a high pressure, the other side produces a low. When you sit on one side of the loudspeaker or the other, then you hear a signal. However, if you sit “on edge” to the loudspeaker, you hear nothing, since the two sides of the loudspeaker cancel each other out (in theory).<\/p>\n

So, Gert’s idea was to add a two-way dipole loudspeaker on the top of a normal two-way loudspeaker and to just use the dipole (which became known as a “shark fin” – shown in Figure 2) \u00a0to add energy around the crossover frequency of the “normal” loudspeaker. Since the edge of the dipole was facing forwards, the theory was that none of its sound would have an effect on the on-axis response of the two-way loudspeaker below it.<\/p>\n

\"Figure<\/a>
Figure 2. A two-way loudspeaker with a side-facing dipole loudspeaker on top. Note that the other side of the dipole looks identical to the one you see.<\/figcaption><\/figure>\n

 <\/p>\n

So, the question to answer was:<\/p>\n

“Which sounds better:<\/p>\n