I’ve seen a number of posts and comments in various mixing and mastering groups relating to speaker excursion which perpetuate misinformation so hopefully this post helps to educate engineers on this matter. This is not at all an in-depth look at excursion, speaker design, or the various pro and cons associated with it. This is an area I’m actively researching and am certainly no expert on.
Excursion is all about how much a woofer (or any speaker driver really) is moving. A woofer produces sound by moving the cone back and forth. The range of that motion is the excursion (usually measured in mm). Peak-to-peak is the convention used as opposed to one-way travel. The Xmax of a woofer is the maximum linear excursion which represents how far the cone can move without leaving the magnetic gap. I won’t get into details about this but the important thing to note is that beyond Xmax the distortion rises very quickly. Drivers should ideally always stay within Xmax. Xlim (sometimes referred to as Xmech) defines the mechanical limit which the cone can move. If you try going beyond this point you will likely damage the driver.
The point that I’d like to get across in this post is that excursion is not an absolute value to be used when comparing systems. Excursion is very closely tied to a driver’s sensitivity. The sensitivity defines how much power you get when you put in a given amount of power. A higher sensitivity driver will produce more sound with the same amount of power from an amplifier. It may seem strange that drivers can have different sensitivities but a woofer is a complex electrical and mechanical (and acoustic) system with a number of different parameters which are all tied. As with speaker design in general, woofers are all about tradeoffs. Some of the parameters which affect sensitivity are the weight of the cone, compliance of the system, shape of the cone (how much air is “held” in front of the woofer by the conical shape), the way the voice coil is wound, and the strength of the motor.
Some of the tradeoffs which the sensitivity of a woofer can affect are the frequency range that the woofer extends through, the size of the woofer, the size of the cabinet for the woofer, and lastly, the excursion. A higher sensitivity driver will require less excursion to produce the same SPL as a lower sensitivity driver. In general a higher sensitivity driver will be capable of higher max SPL levels than a lower sensitivity driver (of course there are many other factors to consider such as the Xmax and power handling).
Why does this matter? I’d like to mention two situations which I feel should have been handled differently. The first was a comment made on a clip of D&D 8C's showing their subwoofers moving with pretty high excursion. The commenter mentioned that one should never be working at those levels and that the levels they work at produce next to no excursion on their speakers. What does a clip of a sub with high excursion tell you about the playback levels? Next to nothing. Were they listening at levels which would cause hearing damage? I have absolutely no idea. I wasn’t in the room. The speakers could’ve been producing next to no sound.
The woofers on the 8C are fairly low sensitivity and reasonably high Xmax woofers. They’re made to move. They’re small woofers in an extremely small cabinet and the system is EQed to deliver deep bass (could easily be +10dB at 20Hz). What does that mean about the sensitivity of those subs (factoring in the corrective EQ)? They’re very low sensitivity and as a result will take a lot of excursion to produce high SPL levels in the low end. If I put them next to my 15” subs and played them at the same levels, the 8C’s could easily hit Xmax and you wouldn’t even be able to see my subs move. There are certain conclusions that can be drawn about the resulting sound quality but I won’t get into that here.
The second situation I’d like to mention is someone who posted a clip of their brand new speakers playing something with high levels of excursion and implying that this made the speakers awesome. These were a small Scan-Speak midrange woofer which I’m familiar with. They’re designed to have fairly high sensitivity and low Xmax. While many speakers can produce high excursion (like the subs on the 8C’s), these woofers are not designed to move much. I’m sure they were close to pushing the woofers to the point of damage. I don’t believe that was intentional but just a result of not knowing that different woofers have different Xmax values. Just because you can make one woofer move with 90mm of excursion doesn’t mean any woofer can handle that. The woofer in question has an Xmech of 9mm.
To add to the problem, these particular speakers were ported without a subsonic filter (which I know as they’re passive). A ported speaker will stop loading (supporting) the woofer below a certain frequency at which point the woofer is essentially moving uncontrollably (as if the woofer were in free air). This produces high excursion without making any sound. At the same time it distorts higher frequencies thanks for doppler distortion. I’m sure a lot of the excursion in this clip mentioned was producing no sound. It was not at all a reflection of the high quality nature of this particular speaker. Just reckless woofer abuse.
To summarize, if you see a clip of a speaker producing high levels of excursion, that tells you very little about the system. You can’t draw any conclusions about how loud it was playing in the clip, the quality of the speaker, or how low it goes without more information about the system. If anything, it’s a poor indication since the higher the excursion, the more potential for distortion that will be introduced. If you don’t know how much Xmax your woofers can handle, you should be very careful and listen closely for any signs of distortion. If you hear any distortion from your speakers, back the level off. Ideally (at normal playback levels) there should be as little excursion as possible but that’s a topic for another post once I’ve had a chance to further educate myself on the matter.