A lot of misinformation has been spread in the industry with regard to the issues that affect the SPL capability of a speaker system. The fact is that the factors which control SPL capability are very defined and simple:

It is commonly understood that larger diameter woofers are louder than smaller diameter woofers (assuming equal excursion). In car audio, however, it is not often possible to fit large drivers into vehicles without a substantial sacrifice in usable space. For this reason, car audio subwoofer performance benefits greatly from maximizing displacement through increased excursion capability within a given frame size.

The specification which indicates linear excursion capability is "X_max". This spec designates the amount of cone travel in one direction while maintaining linear motor behavior and is usually listed in inches or millimeters.

Linear motor behavior means that there is always a constant length of voice coil winding in the magnetic gap of the motor structure. If the voice coil is pushed beyond the linear limit, the output becomes more distorted and, if pushed too far, the speaker can suffer a failure of its suspension components or voice coil windings. Well-designed woofers can be played beyond their X_max to some extent without audible low-frequency distortion or damage. The design of the suspension plays a large role in determining how acceptable the non-linear behavior will be.

X_max does not indicate how far the cone can be physically moved. Just because a woofer cone can be moved by hand a great deal does not mean that its voice coil is capable of moving it that far. Just because you can go 100 mph on a bicycle being towed by a Porsche doesn't mean that you can achieve that speed using leg power! You should also be conscious of "peak to peak" X_max specs which need to be divided by two to compare to one-way specs.

Long-excursion woofers require very rugged and precise suspension and motor design as well as sufficient thermal power handling to take advantage of their excursion potential.

Let's compare two 10" speakers and determine their ultimate linear output capability. The first speaker is a JL Audio 10W6 with an X_max of .468" (12 mm), the second is a real 10" woofer from a prominent car audio manufacturer with an X_max of 0.25" (6.5 mm), which at the time of this writing is pretty average in the industry. Let's call the second speaker "Speaker A".

Below you will see the maximum SPL that each speaker can produce at each frequency in a sealed enclosure with a Q_tc of 0.7 (for maximally flat response). Next to the SPL figure in parentheses you will see the amount of power being handled to produce this maximum excursion. This figure is the effective mechanical power handling of each driver at each frequency. The numbers below do not indicate frequency response.

Maximum (Displacement Limited) Output and Powerhandling

	10W6 (Xmax = 12 mm)		Speaker A (Xmax = 6.5 mm)
20 Hz	95.7 dB	189.2 W	90.2 dB	78.2 W
30 Hz	102.7 dB	244.3 W	97.3 dB	81.5 W
40 Hz	107.7 dB	392.6 W	102.3 dB	90.6 W
50 Hz	111.6 dB	705.5 W	106.2 dB	109.7 W
60 Hz	114.8 dB	1275 W	109.3 dB	144.6 W
80 Hz	119.8 dB	3649 W	114.3 dB	290.2 W
100 Hz	123.7 dB	8655 W	118.2 dB	597.5 W

The data show how direct the link is between X_max and ultimate output capability when comparing speakers of equal size. As you can see, the 10W6 outperforms Speaker A by 5.5 dB consistently up the scale. The difference in low-frequency output capability between these two drivers is staggering. You would need two Speaker A's to equal the output capability of one 10W6. That makes sense when you consider that the 10W6 is moving virtually twice as much air as one Speaker A.

If you refer to the plot to the right (clicking on this image will download a full-size version) you will see a comparison to ultimate output with each speaker being driven by the amount of nominal broad-band power necessary to reach its linear excursion limits in that particular sealed box (again with Q_tc = 0.7). You will see that the 10W6 handles twice the power and is easily capable of outperforming Speaker A in this real-world situation. You will also notice that the 10W6 does not begin to approach its excursion limits until the frequency drops below 25 Hz, whereas Speaker A approaches its limits starting at 45 Hz.

For every doubling of excursion capability (X_max) you gain 6 dB of ultimate output capability. This may seem a bit counter-intuitive because we have all been taught that a doubling of acoustic power only produces a 3 dB increase. What we must keep in mind is that the acoustic power is proportional to the square of the pressure, just as electrical power is proportional to the square of voltage. A doubling of excursion requires 4x the input power and produces 4x the acoustic power, all other factors being equal. Here are the relationships in summary form:

 
1.26 x power (watts) = 1.12 x excursion = + 1 dB 1.59 x power (watts) = 1.26 x 
excursion = + 2 dB 2.00 x power (watts) = 1.41 x excursion = + 3 dB 2.52 x power 
(watts) = 1.59 x excursion = + 4 dB 3.18 x power (watts) = 1.78 x excursion = 
+ 5 dB 4.00 x power (watts) = 2.00 x excursion = + 6 dB 5.04 x power (watts) = 
2.24 x excursion = + 7 dB 6.35 x power (watts) = 2.52 x excursion = + 8 dB 8.0 
x power (watts) = 2.83 x excursion = + 9 dB 10.0 x power (watts) = 3.16 x excursion 
= +10 dB

From these numbers you can quickly see that the change in power is always the square of the change in excursion. This is true both for input power and acoustic power as excursion is directly proportional to voltage, not power.

Going back to the comparison between he 10W6 and Speaker A, you can also see that low-frequency power handling is directly linked to X_max. The 10W6 is capable of handling very high power levels in the heart of the sub-bass region range without it coils jumping like suicidal lemmings out of the gap. This means that it is in control and reproducing the signal faithfully. If you pump more than 90 watts into Speaker A at 40 Hz it will begin to distort and could potentially be damaged. The 10W6 handles almost 400 watts mechanically at 40 Hz.

The importance of mechanical power handling is undeniable when it comes to subwoofers. Especially when one considers the output capability of today's high performance car amplifiers. A speaker may be able to handle 1000 watts thermally but if it has a short voice coil and short excursion capability it will not handle power well, mechanically speaking.