Should you add stuffing in a Subwoofer
I just built a pair of woofer enclosures and when they were finished the question was whether or not to add some wall treatment or stuffing. Commonly the answer is don’t bother.
I usually use stuffing but this was a great opportunity to test a few different treatments.
The Enclosure
This was built to add additional low-end heft to a smaller 2-way speaker. I usually don’t put the port in front because of midrange leakage but here it’s just a (sub)woofer crossed over around 200Hz so it has little midrange to speak of.
First Test
In order to test the stuffing we start by measuring the impedance of the bare enclosure with driver installed. The enclosure internal size is approximately 11x13.5x19.5 inches.
This looks ‘fine’ but notice the tiny bumps at about 340Hz and 500Hz. If the driver were in free air (no box) these would indicate resonances. In the box it’s the same…
The worst box effects are when sound is bouncing off something a quarter or half-wavelength away. This produces a 180 or 0 degree phase shift and abrupt sound changes. So,…
350Hz half wavelength is 19.5 inches and 500 Hz half wavelength is 13.5 inches. So — bounce off the inner walls is what causes the two little bumps and makes a great way to objectively test the effect of stuffing.
Here’s a graph of the nearfield port test acoustic (SPL) test.
Notice that we have very large spikes in acoustic output at 340 and 500Hz — at roughly the same level as the max bass output of the port! So, if the ‘subwoofer’ has any 340 or 500Hz energy in the signal it’s going to interfere with the midrange.
Sound Treatment
The issue with this enclosure is wall bounce. At these frequencies the usual treaments are lining walls with eggcrate closed-cell foam or adding ‘stuffing’ (cotton or fiberglass or long wool or acousta-stuff). The stuffing is most effective away from the walls but I glued balls of ‘stuff’ to the wall to avoid having it shift when the speakers are moved.
Other treatments (such as painting stuff on the walls or adding insulation to the walls) won’t have any real effect on wall bounce.
I tried eggcrate foam and acousta-stuff (synthetic long wool). Here’s the comparison plot zoomed-in.
The three impedance lines clearly show the impact of stuffing. The eggshell foam works moderately effectively and the acousta-stuf almost completely removes the peaks — acoustically and impedance. Wow!
I ended up using a bit less acousta-stuf but enough to lower the peaks to inaudible.
Acoustic Testing
So, does this really matter? Let’s take a look at the nearfield woofer response with and without stuffing.
The response curves above are kind of jaw-dropping if you think woofer enclosure stuffing is irrelevant. I have a very sharp crossover around 220Hz so the woofer output at 350Hz is down about 9dB but it may be audible.
Summary
This example is kind of a worst case because of the front port. It is a great example of why you should use speaker stuffing.
If this had a rear port and a low crossover point the stuffing would be unnecessary — but it has a front port and a higher-than-100Hz crossover.
