[Paul Spencer]
| Disclaimer: Many diy audio members don't feel that a rumble filter is necessary, so I will mention from the outset that the opinions are those of the original author. Others with different views are welcome to present theirs also. You may wish to put a link here to your view if it is different. |
What is a rumble filter?
A rumble filter is also known as a low cut filter, a subsonic filter, or
a high pass filter. Its function is to filter out low frequency content,
usually below 20 Hz, to reduce excessive cone excursions.
Do I really need one?
In most cases, an emphatic
YES is an appropriate answer.
There are some exceptions. You don't need one if:
- You never listen to music or watch movies with significant sub 20 Hz content
- You never listen at anything more than moderate levels
- Your system can reproduce 120db @ 2 Hz without exceeding maximum excursion (xmax)
- Watching cones rip apart is your idea of fun
- You want to destroy your speakers as an excuse to get some drivers with real power!
- You have a horn subwoofer where the design ensures xmax will never be exceeded
But I only listen to music!
"Most music has little content below 40 Hz." This is a common argument for not
needing one. Some even make similar statements about movies, however, I doubt those
who make those statements have actually looked at a real time analyser (RTA) for
any amount of time. I have one operating all the time, and I'm surprised at how
much low bass content there is ''even when watching TV! It's quite common for music
to have strong bass down to 25 Hz, although it's rare that it will go below this.
Sometimes, however, I find that music has
very low bass, less than 1 Hz!
Even with the rumble filter on, this causes much larger cone excursions than the
intended signal. This is clearly a mistake in the recording, but it's more common
than you would expect.
Recently I was watching the start of Top Gun, and there was some very low bass ~ 1 Hz.
With the rumble filter on (3rd order @ 20 Hz), cone excursions from this bass that
should not be there were around 25mm peak to peak. The rumble filter attenuates this
signal by at least 72db. Can you imaging the excursion required to try to reproduce
this signal if it were not attenuated? The driver would have found the suspension limit
of 30mm one way in this case. I've heard of guys with drivers capable of 30mm one way
suspension limit having dinted cones! The excursion was so extreme that the cone moved
back on the speaker terminals and dented the cone!
But I have an Infinite Baffle subwoofer!
This is in many cases a very good reason to have a rumble filter. An IB sub doesn't
have anything to limit driver excursion, but the suspension system. If you have a large
number of high excursion drivers, you may have so much excursion to spair that a
rumble filter is not a necessity. It then comes down to a question of trade offs.
You must choose a balance between extension and output. A rumble filter will allow
better dynamics and more SPL. If you have a very large amount of displacement, you may
decide that 15 Hz extension is enough and you then set your rumble filter at this point.
One more reason to use a rumble filter
Even if you have determined you don't need one, where a rumble filter reduces cone excursions
which aren't associated with sound you want to reproduce, it also increases distortion.
Consider a 15 Hz tone - inaudible. Most systems won't reproduce it with enough SPL for it
to have any direct impact. Higher order distortion will be in the range where you can hear it.
What makes this worse than you may at first think, is that the ear is more sensitive to
higher frequencies. 5th order distortion of the 15 Hz fundamental will be in the upper bass
range, and clearly audible!
Conclusion
I have written this article as I think rumble filters are often neglected. The result is that
a system is robbed of its potential in terms of dynamics and maximum output. Good engineering
is about allowing for worse case scenario conditions. While they are most needed for vented
subwoofers for home theatre use or for use with a turntable, at least 90% of all good audio systems
could benefit from a rumble filter!
Other references
[Bill (Collo) on rumble filters and their effect on excursion] Shown visually with a chart.
[Bill (Collo) on building your own rumble filter]
