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Do bass waves go through plastic?

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Posted on October 17, 2009 at 03:08:50
amdan
Audiophile

Posts: 416
Location: Sydney
Joined: August 4, 2004
Contributor
  Since: July 28, 2009
 I have semi rigid fibreglass panels in my listening room acting as bass traps. However, they are wrapped in plastic to avoid the fibres escaping into my breathing space. I wonder if this reduces the effectiveness of the bass traping.

I know that this question has been asked before and the answer has been that bass travels through plastic but I think this should be revisited. I accept that some bass will travel through but I don't think that all will. In fact I wonder if most of the bass waves do not. What are your thoughts?

Why shouldn't it?, posted on October 17, 2009 at 13:57:07
David Aiken
Audiophile

Posts: 5108
Location: Brisbane
Joined: September 25, 1999
 After all, it can go through steel and glass in a sealed car driving along the street with its subwoofer thumping like mad. You don't hear most of the musical content if you're standing in the street or sitting in your home when the car drives by but you most certainly can hear the bass content.

If the car body and windows don't stop the bass, why would a thin sheet of plastic stop bass reaching the fibreglass in your panels? It will certainly let a lot more through than the car body and windows do, and they obviously let a hell of a lot through anyway.

I think you can be quite assured that most of the bass will get through your thin plastic sheet.



David Aiken
I believe your example supports my 'theory', posted on October 17, 2009 at 14:10:28
amdan
Audiophile

Posts: 416
Location: Sydney
Joined: August 4, 2004
Contributor
  Since: July 28, 2009
 My panels are sealed in plastic. In the case of the car you mention - they are not air tight. Even so you could be standing right outside the car and not hear all the bass coming through.

Also, the panels that I have put in my room seem to make no difference. I have not measured the results. This statement is based on listening tests. I did hear a difference when I installed acoustic foam bass traps in the room. I heard an even greater difference when I put in a sofa bed in the room. Both these articles have exposure to the air. Hence my question.

RE: I believe your example supports my 'theory', posted on October 17, 2009 at 16:12:02
David Aiken
Audiophile

Posts: 5108
Location: Brisbane
Joined: September 25, 1999
 If foam traps make a difference but fibreglass wrapped in plastic does not, then I'd look to reasons other than a thin plastic sheet. If you search here, you will find many people who report great success with fibreglass covered with a thin sheet in order to prevent fibre release and, as Ethan says, actually bonding the sheet to the fibreglass as a limp membrane can actually improve bass absorption.

So, what other reasons could there be? The following come to mind but I wouldn't suggest this is a full list:

- you've got the trap placed behind something solid that's blocking the sound path, or you've placed the fibreglass trap behind another trap which is doing the bulk of the work so the fibreglass trap adds little/nothing to what you were getting previously;

- the fibrglass isn't dense enough to absorb bass or it's so dense it's reflecting bass frequencies;

- where you have them placed isn't an ideal location for bass trapping;

- you've already got so much bass trapping in the room that you're getting significant bass smoothing already and the addition of the fibreglass traps isn't sufficient to make a significantly audible improvement to the results you're already getting



David Aiken
RE: I believe your example supports my 'theory', posted on October 18, 2009 at 02:00:22
amdan
Audiophile

Posts: 416
Location: Sydney
Joined: August 4, 2004
Contributor
  Since: July 28, 2009
 Hi David. Please have a look at my response to Ethan for comments regarding his experimental results. They don't apply to my situation. Unfortunately non of the possibilities you suggest apply either. I packed one corner of the room from floor to ceiling with fibreglass sealed in plastic and it did nothing for the sound. It measures 1.2 metres across and 2.4 m high on the outside of this fibreglass pack. The other cross section is triangular. Yet when I removed a sofa bed from the room the difference is significant. I noticed this significant difference when the sofa bed was moved in as well. It also did not occupy the best position for bass trapping.
I suppose I should try the fibreglass without the plastic and see what happens though I need to find an alternative covering to avoid any health hazards.

RE: I believe your example supports my 'theory', posted on October 18, 2009 at 11:07:25
Ethan Winer
Manufacturer

Posts: 1554
Location: New Milford, CT USA
Joined: December 3, 2003
 > I packed one corner of the room from floor to ceiling with fibreglass sealed in plastic and it did nothing for the sound. <

This is why using measuring software like Room EQ Wizard is much better than listening. One problem with listening is the key of the music you play may or may not interact with room resonance. As an oversimplified example, if your room has a major peak at 55 Hz, which aligns with the key of A, but you play music in the key of G, bass traps may not have much audible affect. But if you play a different tune that's in the key of A, the difference will be much more obvious.

--Ethan

What he said…, posted on October 18, 2009 at 13:37:23
David Aiken
Audiophile

Posts: 5108
Location: Brisbane
Joined: September 25, 1999
 Bass traps smooth response, lowering peaks and filling in dips. If there's no musical content at the frequencies where your peaks and dips are occurring, you won't hear a change.

But if you've played a fair bit of different music then one would expect a variety of keys to be used and for the effect of the panels to have shown up.

I can't explain what you're reporting but I do know that a thin sheet of plastic isn't stopping bass absorption.



David Aiken
My situation is different, posted on October 17, 2009 at 14:04:19
amdan
Audiophile

Posts: 416
Location: Sydney
Joined: August 4, 2004
Contributor
  Since: July 28, 2009
 ff

RE: Do bass waves go through plastic?, posted on October 17, 2009 at 08:21:23
Ethan Winer
Manufacturer

Posts: 1554
Location: New Milford, CT USA
Joined: December 3, 2003
 Not only does thin plastic wrap not block bass, it actually makes the fiberglass inside absorb more. This article shows test results of plain and FRK type rigid fiberglass:

Density Report

The FRK is a paper barrier, not plastic, but the principle is the same.

--Ethan

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