DIY Tube Traps
When I built my first dedicated listening room the added space allowed me to
install a number of DIY pressure zone tube traps. The traps significantly
reduce room effects that smear and obscure the musical information. They do
much more than just create the right amount of sound absorption. Adding the
traps resulted in improved clarity, detail and transparency. The most profound
effect is in the bass but there are improvements across the frequency
spectrum. The cost of the traps is very modest ($300) and they are
not difficult to construct.
Given the level of interest in these DIY tube traps I put together a
brief tutorial for the relatively simple process for constructing these
excellent tube traps. While I obtained good results I am in no means an
expert on this subject so don't hesitate to experiment or make changes.
The original concept for building these traps was
published by Jon Gale.
Materials & Tools
The compressed fiberglass is the only item that may be difficult to
locate. The fiberglass I used was manufactured by Knauf and they
apparently have distributors in most major cities. I suggest going to
the Knauf website to contact Knauf
about the location of local distributer. International Technifab is the
distributor in the Denver area. The folks at both Knauf and International
Technifab were accommodating and helpful.
- Compressed fiberglass pipe wrap
- Particle board, MDF or hardboard 1/4 to 1/2" thick.
- Construction adhesive (Liquid nails or equiv.)
- Caulking gun
- Router or saber saw
- Staple gun and lots of staples
- 2-3 mil plastic film
- Fabric of your choice
- Replacement vinyl vertical blind slats - Optional
The pipe wrap from Knauf is only available in 36 inch lengths but a
wide variety of diameters. The diameter is specified by the inside diameter
of the pipe is is normally used on. This makes it a bit confusing to
figure out what size you need for a given outside diameter. The pipe wrap
is available in a variety of wall thicknesses and the 1 inch thickness is
The top and bottom of the fiberglass tubes need to be covered but it's
not clear how rigid the cover needs to be. For smaller traps I selected
3/8" particle board and 1/2" particle board for large (16+ inches) traps.
In retrospect thinner material probably would have been sufficient, but
there is no harm in making them more rigid than necessary.
I also had some question about what thickness of plastic film to use.
The plastic film acts as a low pass filter, allowing low frequencies pass
into and be absorbed by the trap while reflecting the higher frequency
energy. The thickness of the film affects the frequency of the filter.
I used 2mil film and was happy enough with the results that I never
experimented with any other thicknesses. If you are thrown by the
name "plastic film", its just the cheap vinyl used as a drop cloth for
painting and available at hardware stores.
Since the tube traps are designed to reflect higher frequency energy
the choice of fabric is relatively unimportant from a sonic perspective.
Heavy fabrics, like burlap, will be far more difficult to wrap around the
traps. I selected an inexpensive muslin that had the appearance and price
that I liked.
To finish the trap edges and joints vinyl slats from vertical blinds
may be used. I found packages of 2-1/2 inch wide replacement slats for
$10 at a Home Depot store.
- The first step is to cut round circles to go over the top and bottom of the
fiberglass tubes. This may be done with a saber saw or better yet a
circle cutter fitted to a router. Where
a number of traps are being constructed a router circle cutter makes this
process quick and easy. Make the circles the same diameter as the measured
outside diameter of the pipe wrap.
- Place a bead of construction adhesive along the entire length of the
open seam on the pipe wrap. Be sure that the adhesive extends to each end
and does not have any gaps. Press the seam together to get a good seal.
- Place a bead of construction adhesive around the end of the pipe
wrap. As with the seam make sure that there are no gaps. Place the end
cap in place and insure that a good seal is obtained.
- Carefully remove the paper pipe wrap covering. Some fiberglass will
come off with the covering but try to keep this to a minimum.
- Using a staple gun attach plastic film to the trap so that it covers
half of the circumference of the trap.
- The next step is to wrap the trap with fabric. There are lots
of ways to do this. With a little practice and experimentation you will
be able to refine a technique that result in a smooth nice looking covering.
What worked well for me was to stretch the fabric along one
edge and staple on the top and bottom. Then alternating between the top
and bottom, pull the fabric tight in both directions (along the length of
the trap and also around the trap) and staple.
- To create a nice looking finished edge I used 2-1/2" wide vinyl slats
from vertical blinds. Just cut the slats to length and make a loop around
the trap and join the ends with clear packaging tape. Make the vinyl loops
so that they fit snugly around the end of the traps. Using these vinyl
loops at the joint of two stacked traps makes them a bit more stable.
For trap ends that will be visible cut an extra end cap and wrap it with
fabric. The fabric wrapped end cap may then be glued to the end of a trap with
construction adhesive and the joint covered with a vinyl loop.
An excellent discussion of tube
trap theory and placement is available
from Ariel Audio.
My experience with optimizing the traps follows very closely with what is
laid out in this article. At first I thought that the
recommendation of placing traps every 3 to 4 feet was excessive. However,
this produces excellent results and it is clearly audible to remove a single
trap. I currently am using a total of 32, 3 foot tall traps (4-22", 2-16",
20-9" and 6-16" half rounds). Not pictured is a single row of three traps
across the ceiling at the first reflection point. On the back wall I use
a pair of 16" full rounds in the center flanked on both sides with 2 high
9" traps. The rear corners are occupied with a door and a desk otherwise
I would also place traps there.
Accurately placing and rotating the traps makes a significant difference.
I was surprised at how sensitive both placement and rotation are. Also it
is quite important to place the traps so that there is accurate left to right
symmetry. Use lots of traps and get real anal about how they are placed
and you will love what these things will do for your system.