ArticlesSeptember 2011General Effects of Stuffing and Lining on Subwoofer Performance

Overview

Traditionally, damping is used to improve a subwoofer's low frequency performance which is thought to be similar to increasing the box volume. There is some debate as to how much stuffing material to use, what types of systems to use it in, and the overall benefits or drawbacks to using it. So, we decided to measure three different types of common subwoofer systems (sealed, bass-reflex and horn loaded), and compare the differences in performance with and without the use of stuffing material. The stuffing we used was poly fill enclosed within a pillow case. These are cheap, widely available and effective. The damping used was a dense foam and also cheap egg crate foam which many claim provides no useful acoustic change.

We looked at the following measurements:

  • system impedance
  • basic system frequency response
  • response decay in the time domain
  • system sensitivity

We measured the following systems:

  • a sealed system with and without stuffing
  • a ported system with and without the addition of a minor amount of stuffing
  • a tapped horn system with and without the application of foam lining

These tests were very basic and meant to show what sort of effects these materials have in a general sense.

Sealed System

For the sealed system, we measured an 18” driver in a large sealed cabinet. Without any stuffing or other materials in the cabinet, the impedance showed a peak of 36ohms centered at 25Hz and a resonance or response issue in the impedance trace at about 170Hz. After adding about 9 poly fill pillows to the enclosure, the impedance measurement showed that the peak had dropped to about 21.5ohms in magnitude and about 23Hz in frequency. The issues which showed up as wiggles in the impedance are smoothed out. Resistance goes up just a hair outside of the impedance peak area. 

Looking at the before and after frequency response, it is easy to see that there was a large resonance issue related to the enclosure dimensions near 170Hz. This was completely absorbed and smoothed out with the addition of the stuffing. We can also see that the overall response shape has changed a little with the very lowest and highest bass frequencies boosted just slightly (red curve). However between 16-55hz, the response is flatter but lower in volume. This in addition to the greatly diminished magnitude of the impedance peak, indicates that efficiency has been decreased somewhat over this range.

Vented System

Moving on to a vented system, we measured a large 9 cu ft internal system utilizing a 21” driver and a 10” port. Instead of using damping or lining on the walls as is typical with vented enclosures, we decided to use poly fill pillows as lining. Three pillows were added to the enclosure: One in front of the bracing behind the driver on the top panel of the enclosure, and the other two behind the bracing on the two enclosure side walls. Care was taken to provide a large unobstructed path between the port and driver.

Looking at the before and after frequency response of the system, we noticed that some of the larger response issues related to the enclosure resonances and port resonances were effectively damped out. However, the system tuning was dropped and the overall port contribution was lowered quite a bit as was total system efficiency.

Tapped Horn

Looking at the application of foam to the walls near the mouth and throat of a DTS-10, we can see that it did sap some efficiency from the overall system. The main concern with the TH is the very rough response as well as the multiple areas of severe ringing within its response. The addition of all of the foam did greatly reduce the out of band output above 100Hz and did make some improvement to the ringing at the major resonance points; however it required a good deal of material and overall, the affect near the major points of interest was not significant.

Wrap Up

Overall, stuffing or damping the inside of a subwoofer’s enclosure is generally a good idea to prevent the internal reflections inside of the enclosure from bouncing around and coloring the response and sound. If you are looking for the utmost in uncolored and well-damped sound or if you intend to use the system extra high in frequency up into the lower midrange, it is recommended. If you are trying to be as absolutely loud and efficient as possible, then leave it out.

In general, there is probably a sweet spot for most systems where the system's major response issues are cleaned up and damped, but further addition of material will only serve to further decrease efficiency.  Unfortunately, finding this sweet spot probably involves trial and error or other testing.