Why you need a rumble filter

In this article I show why high pass filters, also referred to as rumble filters, are important. In the following simulations I'll demonstrate with a high excursion driver, the Exodus Audio Tempest X2. It has a one way xmax of 27mm. In the first example, we'll investigate a sealed box, where rumble filters often aren't considered necessary. 

Sealed box - 200L each with a Linkwitz Transform included, excursion plot:

Yellow - input power limited to avoid over excursion at 5 Hz

Blue - input power limited to avoid over-excursion at 20 Hz

Red - rumble filterinput power limited to avoid over-excursion at 20 Hz 

You can see that at high output (blue), the excursion gets out of control below 20 Hz, even in a sealed box. If we reduce the input power dramatically, we are safe to 5 Hz. This is probably good enough, however, the output as we will see in the next chart is limited. With a rumble filter, we can set up the system to avoid ever exceeding xmax. We can be sure our driver is safe.

SPL for the input levels previously shown

Yellow - input power limited to avoid over excursion at 5 Hz

Blue - input power limited to avoid over-excursion at 20 Hz

Red - rumble filterinput power limited to avoid over-excursion at 20 Hz 

You can see that output is much greater when a rumble filter is used. We've gained around 8 dB of output, which is equivalent to more than doubling up on drivers and box size. 

Now lets consider the same driver in a large low tuned vented box. 

Excursion

Orange - input power low to avoid over-excursion below 20 Hz

Green - no rumble filter

Magenta - rumble filter in place

 

Again we see much higher SPL as a result of using the rumble filter. We also avoid vent chuffing as a result of very high velocity in the vent.