Ah, if it were only that easy...
As I have stated many times in the past, speaker design is all about balancing trade-offs. I can't get into all the considerations here as it would be a topic for a book, but here are some thoughts...
The best possible speaker would have a single driver. There are no phase issues to contend with and no comb-filtering. But there are issues. First, a smaller single driver will generally have good off-axis response and fairly good response on top. But they will not play all that deep and will not handle much power. Larger drivers will play deeper, but the off-axis response will not be as good and the top end will suffer as well. Finally, if you want higher efficiency, the drivers will not play very deep and power handling will be limited.
One popular solution is a 2-way design. A tweeter can handle the top end, allowing you to use a larger driver for the woofer. But the trade-off you need to deal with is sensitivity vs. bass extension. If you want high sensitivity, don't expect to get much bass extension. For example, A JBL 2206H 12" woofer is 95 db efficient. But its resonant frequency is 52Hz. So bass extension will be limited. Smaller drivers can play considerably deeper, but these woofers will typically be quite a bit less efficient.
The bottom line is that you have some options... 1) deep bass response; 2) high sensitivity; 3) reasonable cabinet size. Pick any two (you can't have all three).
When it comes to efficiency, it is not normally the tweeter or the midrange that limit the design (unless you want extremely high efficiency). It is the woofer section that will normally determines the sensitivity of the over all design.
If you want a woofer that plays deep in a small cabinet, this will normally be a low-sensitivity driver. Many of these, in fact, are 4-ohm drivers as well. Using a pair in parallel will increase the efficiency by up to 6db. However, the resulting 2-ohm load will not work with most amplifiers. So you have to use four drivers - two series pairs in parallel - to gain that 6db and keep the impedance high enough.
There are two issues here. The first is cost - four drivers means four times the cost if you want to maintain the same sound quality. (You could use less expensive drivers, but the sound quality will suffer as a consequence.) The second issue is comb filtering. The output from each of the drivers will combine in space in front of the speaker. Some frequencies will be in phase and reinforce one another, while others will be out of phase and cancel each other out. The result is a very uneven and jagged frequency response.
The more drivers you have, the more jagged the FR becomes and the more phase anomalies you have to contend with.
In the example you propose, the frequency response would likely be quite a mess. So, in a sense, you traded accuracy for increased sensitivity. To me, that is not a very attractive trade-off.
The speaker you linked to may be quite good. I don't know. But I did notice that they did not publish a FR plot and don't mention what the F3 is for that design (I would suspect that it is not all that low).
You will see that some of our designs are around 84db sensitive. In these cases, we have made a conscious decision to trade efficiency for accuracy. These speaker typically play deep and accurate in a relatively small cabinet. They require a bit more power, but watts are cheap these days. So we feel this is a reasonable trade-off. The alternatives would force us to give up either bass extension or accuracy (or increase cost dramatically).
I have received quite a few emails over the years from people searching for an accurate, high-sensitivity speaker that plays deep in a small cabinet. They can't seem to find it. And the reason is basically that this speaker would defy the laws of physics.
In the world of speaker design, if you want something, you have to be willing to give something else up. Its just the way it is.
I am not in a position to talk about it yet, but we do have an idea we will be working on soon to address the above limitations. It will involve a somewhat unconventional approach, but should allow us to design a relatively high sensitivity speaker that plays deep in a relatively small enclosure. Stay tuned...
- Jim
