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Array, or not to array? That is the question

Mar 14, 2012 2:49 PM, By Bob McCarthy

Speaker considerations for best deployment.

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Figure 2: Response comparisons of a single speaker and array pairs in the 500Hz range. (a) Single speaker with 160° coverage (b) 2x160° speaker splayed at 40° combine to 65° (c) 2x160° splayed at 20° (88% overlap) combine to 65° (d) 2x160° speakers at 0° combine to 70°. Overlap is so high in all cases that the differences in splay angle don’t matter. See larger image.

Let’s dig a little deeper. The first reveal is that SPL numbers are as easily manipulated as political polling data. Typical SPL numbers (weighted or not) reduce the entire frequency range to a single value. If we have a 136dB SPL peak spec on our cabinet that does not mean we get 136dB SPL at all frequencies (or for that matter, any single frequency). It is an aggregate total over the entire range. Individual frequencies are unlikely to make it within 20dB of that number before breaking into hard distortion, or worse. The SPL capability distribution over frequency will vary tremendously, with the LF range of any given driver in the system typically falling furthest behind the pack. When we put multiple drivers together in an array, the SPL capability over frequency is substantively revised due to the shared effort of the speakers. The full range SPL number will likely miss the difference in LF power capability of eight 10in. drivers over a single 15in. driver. Maximum SPL is easily the most overrated statistic in audio.

Now, let’s dig a little deeper into coverage pattern. So we have an 80-degree speaker, eh? Is it 80-degree over the speaker’s full operating range of 50Hz to 18kHz? OK, so we know to give 50Hz a pass, but exactly (or even approximately) how wide is the range that we can expect our 80-degree spec to hold true? If you were thinking 1kHz, you would be wrong because there is no standard. The specified value is whatever folks want to print, and it can vary from covering multiple octaves to as small as one-third octave.

Let’s back up a moment and consider what we know about coverage patterns. First, we know that the pattern will widen as frequency falls. It would be fair game to specify nearly every full-range speaker in the world as a 360-degree device (if we used 50Hz as the standard). We also know that a front-loaded cone driver will become more directional as frequency rises—a property known as “proportional directivity.” This makes specifying these drivers a slippery prospect. Third, we know that horns can create a coverage pattern that is a stable frequency even for multiple octaves. Bigger horns can handle lower frequencies and, therefore, extend the controlled range downward. Horn size and depth also play a role in the pattern width. As a general trend, a bigger, deeper horn can create a tighter pattern.

Figure 3: The coverage angle over frequency for eight different versions of 80° coverage. All configurations have 80° coverage in the HF range. As quantity rises the midrange and low-frequency range move downward toward 80° (and even beyond). Note that the three-box configuration uses a large format horn-loaded woofer. Therefore it has a much more directional LF section to begin with. See larger image.

To put this all in perspective: Our little 80-degree frontfill with a 1in. tweeter and tiny horn might actually hold that pattern from 4kHz to 8kHz. By contrast, our example speaker (with a 4in. compression driver and horn) should be able to hold that spec from 1kHz on up. Below 1kHz, it will transition into the LF driver, which is likely to be fairly close to 80 degrees there and then get gradually wider as frequency falls.

Now let’s consider the 40-degree side of our example box. The horn is the same size and depth, but it has a different wall shape to get the narrower pattern. This will work fine enough, but something has to give. The pattern will probably not hold all the way down to the 1kHz frequency range like it did in the 80-degree plane. Therefore, we can think of the following trend lines in coverage pattern specifications: Wide coverage over a wide range can come in any size, but will hold its spec over a wider range as box size increases.

Large boxes with narrow coverage can hold their spec for a large range. Small boxes with narrow coverage only hold their spec for a small range. The smaller the box or the smaller the angle, the smaller the controlled range. That little 5-degree box you are about to fly is unlikely to hold its spec for more than a few notes around 8kHz.

Now we know a few things about a single box. From this we will build the case for how arrays work. But first we will take a brief moment to consider the room and the shape of our target: the audience seating line. Where do I want the sound to land so that it falls on the people? This is the most important consideration in choosing a solo speaker or an array. In the horizontal plane, we need to spread the sound from side to side like butter on bread. In the vertical plane, we need to drop a single line of sound from the front to the back. In either case, these target shapes are the same for all frequencies. We won’t find rooms that are narrow in the HF range and wide in the lows. Therefore, what we need is a speaker system that can maintain its shape over frequency. For solo speakers, this leaves us with two main options: a wide speaker (various sizes) or a large-sized narrow speaker. If we combine speakers into arrays, we can get any shape we want. It just costs money. I could use arrays. We all need arrays, right?

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