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Part 2

Posted by Jon Risch (B) on October 19, 2009 at 19:28:11

In Reply to: Then please define a bass trap… posted by David Aiken on October 17, 2009 at 01:13:40:

What is a bass trap?

Is it a wall panel, consisting of a single 2X4 foot sheet of 2" thick fiberglass?

Is it a Helmholtz Resonator, tuned precisely to one frequency?

Or is it something else?

In response to Dave's post, I replied that of the three aspects he cited:

1. Mechanism
2. Placement
3. Effectiveness

I felt that a device that deserved to be called a bass trap would include some of all three.

It would have an effective and efficient mechanism to absorb bass energy, it would be placed where it could absorb and dampen the maximum amount of bass energy, and it would have to have a certain basic level of performance in order to be called a bass trap.

Let's look at the last aspect first. How much bass does a device need to absorb to be called a bass trap.

Let's look at the absorption in Sabin's, standard ASTM conditions, 3rd party data.

Freq in Hz 63 125 250 500

16"dia 3 ft tall
Cylinder Trap 14.9 17.0 15.0 17.8

11"dia 3 ft tall
Cylinder Trap 10.2 10.9 16.7 16.8

2X4 ft 2"
Thick Panel 1.3 1.4 6.9 9.12
(705)

2X4 ft
Specialty 0.7 4.2 12.3 14.2
Panel 1

2X4 ft
Specialty 4.2 9.8 12.8 16.4
Panel 2

Personally, I consider an 11" diameter cylinder trap to be a sort of 'minimum' amount of performance for a bass trap, but that is a bit arbitrary.

However, I think that most people can see that the simple 2 foot by 4 foot by 2" thick fiberglass panel is just NOT a bass trap, by anyone's criteria.

Keep in mind that I recommend the use of a 4 foot tall cylinder trap, this increases the Sabin numbers in the chart by approx. 133% (multiply by 1.33)

In one post about panel traps vs. cylinder traps, it was stated that one 24" (by 48") by 4" thick fiberglass panel (705) was equivalent to a single 20" cylinder trap.

Balderdash.

Freq in Hz 63 125 250 500

20"dia 3 ft tall
Cylinder Trap 14.9 17.0 15.0 17.8

2X4 ft 4"
Thick Panel 3.3 7.2 14.7 13.1
(705)

Also keep in mind that at frequencies below 63 Hz, the cylinder traps are going to be doing something to the bass room modes all the way down to the lowest mode in the room, while the simple panel traps, EVEN IN THE CORNER, will essentially 'go away' for all intents and purposes.

Before we go any further, I know that the panel company folks are going to crawl out of the woodwork and start talking about all kinds of extenuating conditions, and so on.
So, before they cry foul:
Yes, these are ASTM conditions. Not a corner. In some cases, the wall panel data will improve by a factor of 2 at low frequencies when placed in a corner, and at some resonant frequency for the specialty panels, it might even go up to almost three times as much absorption. There, that is out in the open, not to be denied.
But then, let's be fair to the cylinder traps, they will ALSO increase if measured in a corner, perhaps by a factor of 1.5 to 2 times the above data.
This is the point that never get's made in the comparisons of cylinders to wall panels.

This could be because due to ASC patents, they can not offer a cylinder trap for sale, only wall panels. Or it could just be an oversight, a complete coincidence. You decide.

There will also be the question of where the data came from.
The cylinder trap data came from "The Master Handbook of Acoustics", 3rd Ed. by F. Alton Everest. The data is not quite the same as the data on the ASC site, and appears to be a true 3rd party measurement on the entire trap, not just on a "per linear feet" basis, as is the data at the ASC site (in the flyer PDF)
(ASC data: http://tubetrap.com/tubetrap-flyer.pdf
http://www.tubetrap.com/technical.htm )

The data for the 705 panels is widely available, and it has been measured independently from OC, and I did peruse some of these measurements on various acoustics forums, both years ago, and just recently, to make sure they were still out there.

The data I used for the specialty panels comes from the manufacturer's web site, and some of the various brands of specialty panels have been measured by third parties as well. At least the data given at the manufacturer's web site is close to that taken by the third parties.

I still need to explain about aspects #1 and #2, and why these aspects come into play.
And I will explain them, soon, because they explain why even with equal Sabin numbers, the cylinder trap still outperforms a wall panel type. I just thought a few raw numbers first might be fun.

Look for Part 3 soon.

Jon Risch

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Kimber Kable

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