Originally Posted by Axxion
Is the rumble filter to actually protect the speaker from damage, because of overexcursion, or is this just protecting the sound quality?
The answer depends on type of subwoofers. For sealed subs, the back pressure actually limits the amount of excursion at the subsonic frequency. So there is no need for a rumble filter to protect the drivers. When the sub is over-driven, it has this amp clipping noise that is sufficient for alert the listeners. But turn on the limiter function will serve the purpose. But if you have some of those recordings that you can see the cone moving in and out slowly when you playing them, it means those recordings do not use rumble filter during mastering and a lot of those subsonic noise is from each people walking on stage, or air conditioning vibration, or even wind noise. For this type of sub-sonic noise, you can turn on the rumble filter to prevent the energy being wasted. However for other cleaner recordings, turn on rumble fitter can filter out vital spatial information. When we talk about a single tone 20hz, people have this impression that the signal will stay at 20hz even when the signal comes in and goes. The reality is the spectral content of dynamically changing signal is always wider than the static signal by itself and therefore a rumble filter may filter out some of the "real" signal. For instance, if a 20hz signal slowly comes in and out at 10 times/sec rate, there will be a spectral content at 20hz -10hz=10hz and 20hz + 10hz = 30hz. This is called frequency modulation. Now if the same signal slowly comes in and out at 18hz rate, there will be a spectral contents at both 20hz-18hz=2hz and 20hz+18hz=38hz. A lot of techno sound actually uses this type of techniques to produce interesting low bass sound effect to a degree that it comes un-natural. Other naturally recorded sound will also have similar sound effect, albeit the subsonic content is much lower. The purpose for our sealed subs to have low extension is to faithfully reproduce this type of sound effect with already very weak subsonic energy. Very often there will be some bass nuts make the comment that your subs have 14hz extension, but how loud they can play at that frequency, sort of implying if we cannot produce the 14hz signal at the SPL high enough to satisfy them, then the extension of 14hz is meaningless. Those remarks completely missed the point. The reason I explain the above is customers can turn on and off rumble filter based on his objective of a sound system. If he wants to play loud, then it is best to turn on the rumble filter. If he wants to hear a complete spectrum of the recorded signal with reasonable subsonic energy contents, then he can turn off the rumble filter.
On the other hand, vented subs have no back pressure to protect the driver from over-excursion. Then it is best to do two things together: 1) turn on limiter, and 2) use the rumble filter and extension filter as we have suggested. For instance, for LV12R, the extension filter can be at any position. Same is FV15HP/FV15 in 2port mode with rumble filter on. However, in FV15HP/FV15 1 port mode and rumble filter off (it has to be in the off position), it is recommended to use 14hz/low damping for better excursion protection. We are considering of adding a third position of the rumble filter that we can add 13hz rumble filter so that we have more protection in 1 port mode. Let me know if you guys have any suggestion.