January 21, 2002

Insulation Tests - the Earmuff Test

Earmuff experiment to test acoustic and thermal insulation.

The purpose of this experiment was to compare various materials and methods of construction for acoustic and thermal insulation.

The following materials were employed in the most successful earmuffs:

  • Dacron (polyester fiberfill) 1" thick
  • Aluminum-coated Mylar (polyester) film
  • High-density urethane rubber foam (grey)
  • Low-density urethane rubber foam (yellow)
  • Perlite (loose-fill mineral)
  • Polyethylene 3/16" corrugated double-wall
  • 3/16" Foamcore (cardboard and rigid foam filler)
  • Polystyrene foam board (beadboard) 1.25"
  • Cellulose sound-dampening board 0.5"

These materials are shown below:

The earmuffs were constructed first as boxes, roughly 4" square and 3" deep, with an open top, from 3/16" Foamcore. The edges were glued together with hot melt glue. First, six boxes were constructed and the three worst performing boxes were selected for replacement. Then these three worst performers were modified to improve upon the three best designs.

The boxes are shown below:


(The boxes that appear white-grey above have been coated with 1/16" milled glass fibers over grey primer. The raw foamcore appears black.)

In the end the three best performers were the following:

Box 4
This box is about 2/3 the effectiveness of standard hearing protector earmuffs.

  • Glass fiber coating (thin film) over foamcore shell
  • 0.5" air gap
  • Polyethylene 3/16" corrugated layer
  • 1.5" loose-fill Perlite (boxed in polyethylene)
  • Dacron (1" compacted to 0.5")
  • 0.75" high dnsity urethane foam

Box 5
This box is similar to Box 4 in performance, but not quite as good.

  • Glass fiber coating over foamcore shell
  • Polystyrene beadboard 1.25"
  • Perlite loose-fill 1"
  • Low-density urethane foam 0.5"
  • Dacron 1"

Box 6
Again, not as good as Box 5

  • Mylar film wrapped around foamcore shell
  • 0.5" LD urethane foam
  • Mylar film
  • 2.5" HD urethane foam
  • Dacron 1"

The best performer in the first round was:

Box 2
This box had no treatment to make the exterior foamcore shell more rigid (no glass coating or mylar)-- it just had plain foamcore exposed to air.

  • 1" air gap (1" thin foamcore rod support)
  • 0.5" cellulose soundboard
  • Mylar film
  • 1" LD urethane foam
  • Dacron 1"

It was difficult to compare the thermal performance of these various earmuffs. In general they appeared to have roughly the same warmth. In order to accurately compare these layered materials it might be necessary to construct a blanket or simple bag-like "mitts".

Dissecting off-the-shelf hearing protectors


I took apart some standard earmuff-style hearing protectors made by "Team Silencio". As shown in the photos below, these protectors have both a reflecting layer (rigid orange plastic shell) and an absorbing layer (0.5" high density urethane foam rubber). The ear is also cushioned by 0.5" hd urethane foam wrapped in vinyl. This exterior cushion also serves to form an airtight seal between the protectors and the ear.

In comparing my earmuffs to standard earmuff style hearing protectors it seems clear that these muffs do not perform as well. The best earmuff had only 2/3 the performance of the hearing protector. My guess is that the hearing protector has a better rigid shell (1/16" - 1/8" rigid plastic) that reflects more noise and is rigid enough to avoid being vibrated by fluctuating air pressure. Thus when constructing an acoustic barrier it is desirable to have at least one strongly reflective layer. This layer could be a rigid layer coated with a high-impedance material such as aluminum or glass.

As for sound-absorbing layers, it seems clear that the naturally occurring Perlite material had roughly the same performance as the synthetic high-density urethane rubber foam material. However, if the Perlite is subjected to a lot of flexing, it might be crushed and compacted over time. Posted by todd at January 21, 2002 11:40 AM