Quieting my van

[curtis73]

I have a lot of volume to quiet but I have some ideas/questions to bounce off of you folks.

Van in question is a 99 E350 cargo van. It is stripped bare inside. The goals are to A) reduce the amount of noise that gets into the van from outside and the road, B) acoustically "deaden" the interior so that interior noise won't bounce around, and C) modestly insulate the van since it is used occasionally as a weekend camper.

"C" isn't a big factor since anything I do will insulate enough to make climate control feasible, but if two or more materials provide the same acoustic effect but one produces better R-value, I'd choose the R-value.

Layer 1: I have a few rolls of Raamat (which is a Dynamat knockoff.) Its a butyl adhesive with foil backing. That will go on the inside of the steel body panels.

Layer 2: Some type of insulation - foam, fiberglass, etc.

Layer 3: A hard panel of luan, tempered hardboard, pegboard, etc.

Layer 4: cut-pile carpet.

Layer 2 and 3 are the big question marks. My first thought was that layer 2 should be some open-cell foam (for instance, an eggcrate mattress topper) and layer 3 should be pegboard. The idea there was that the perforated board would allow sound to pass through to the foam where it would get deflected and not come back out. But I also thought that the pegboard would probably allow more exterior sound to come in while driving. Basically it seems like the pegboard/foam idea would help for deadening the interior when parked, but not be as effective on the road.

What are your thoughts on what to use for layers 2 and 3? Any suggestions on the whole project?

[sreten]

Hi,

Transmission of sound through a wall and damping of resonant spaces
are two very different issues often confused under "soundproofing".

I'm not familiar with the construction of your vehicle but generally
speaking affecting the fundamental sound transmission characteristics
in the bass will incur a huge weight penalty and is unlikely to be practical.

My instinct would be to glue (with contact adhesive) rubber backed
carpet tiles to the main central sections of open sheet metal areas.

Damping the metal areas with "dynamat" and building a complete
internal layer separated with fibreglass / rockwool / foam sounds
very complicated, and with foam very expensive.

One thing to note is the "Rt" or reverberation time of the space.

Carpet will kill Rt at higher frequencies, spaced pegboard kills Rt
in the midrange (based on hole percentage and the common
depth behind it). Both can lead to an "oppressive" sounding
space due to the "unnatural" variations of Rt with frequency.

Bitumen roofing sheeting glued between spars with rockwool
behind can be effective lower frequency absorbers if needed.
(Its also a cheap version of "dynamat" for metal panels.)

IMO its the lower frequency damping that is the most critical.

I've never done a vehicle, I've done the acoustics of rooms
with combinations of carpet, pegboard etc to try to get
a good even Rt across the spectrum, noting it had no
real effect on the "soundproofing" of the room.

One tip is if you use pegboard glue on diagonal paper strips
to vary the hole percentage, e.g. have some pure pegboard,
some cover every other row, some cover 3 rows for each
open row, some cover 7 rows for each open row, some
cover 15 rows for each open row. This will tune the centres
of absorption 1/2 an octave apart for each change , but still
the above only covers two octaves (for constant depth).

A variety of techniques is needed for a good sounding space,
and its very difficult to say what would be your best approach.

Some good stuff in the UK's BBC archives about the way they
acoustically treated rooms in their radio stations, very much
mainly on the cheapest approach possible using modules.

I've also used Softboard cut up as cheap acoustic tiles.

Main point I guess is there is no one universal solution.

rgds, sreten.

[kevinahcc20]

Back in the '70s when such vehicles were done up as hippie vans there were conversion shops who would spray foam insulation on the sheetmetal between the interior ribs and screw panels covered with shag carpeting over the foamed interior walls. A raised bed with storage underneath was installed over the rear wheel well area and similarly carpeted. The treatment did a pretty good job of taming the interior resonances and reducing road and wind noise transmission into the interior. Good luck!

[curtis73]

Originally Posted by sreten

Hi,

Transmission of sound through a wall and damping of resonant spaces
are two very different issues often confused under "soundproofing".

I'm not familiar with the construction of your vehicle but generally
speaking affecting the fundamental sound transmission characteristics
in the bass will incur a huge weight penalty and is unlikely to be practical.

My instinct would be to glue (with contact adhesive) rubber backed
carpet tiles to the main central sections of open sheet metal areas.

Damping the metal areas with "dynamat" and building a complete
internal layer separated with fibreglass / rockwool / foam sounds
very complicated, and with foam very expensive.

One thing to note is the "Rt" or reverberation time of the space.

Carpet will kill Rt at higher frequencies, spaced pegboard kills Rt
in the midrange (based on hole percentage and the common
depth behind it). Both can lead to an "oppressive" sounding
space due to the "unnatural" variations of Rt with frequency.

Bitumen roofing sheeting glued between spars with rockwool
behind can be effective lower frequency absorbers if needed.
(Its also a cheap version of "dynamat" for metal panels.)

IMO its the lower frequency damping that is the most critical.

I've never done a vehicle, I've done the acoustics of rooms
with combinations of carpet, pegboard etc to try to get
a good even Rt across the spectrum, noting it had no
real effect on the "soundproofing" of the room.

One tip is if you use pegboard glue on diagonal paper strips
to vary the hole percentage, e.g. have some pure pegboard,
some cover every other row, some cover 3 rows for each
open row, some cover 7 rows for each open row, some
cover 15 rows for each open row. This will tune the centres
of absorption 1/2 an octave apart for each change , but still
the above only covers two octaves (for constant depth).

A variety of techniques is needed for a good sounding space,
and its very difficult to say what would be your best approach.

Some good stuff in the UK's BBC archives about the way they
acoustically treated rooms in their radio stations, very much
mainly on the cheapest approach possible using modules.

I've also used Softboard cut up as cheap acoustic tiles.

Main point I guess is there is no one universal solution.

rgds, sreten.

Wow. that will take a while to sink in but I think I get it. Thanks

[curtis73]

Originally Posted by sreten

Hi,

Transmission of sound through a wall and damping of resonant spaces
are two very different issues often confused under "soundproofing".

I knew that was the case, but never really knew the science behind why. I just assumed that "what converted energy going one way will convert it the same going the other way," and called it good enough.

I'm not familiar with the construction of your vehicle but generally
speaking affecting the fundamental sound transmission characteristics
in the bass will incur a huge weight penalty and is unlikely to be practical.

Agreed. The construction of my vehicle looks like this below. Way too much real estate to dampen low frequencies.

Damping the metal areas with "dynamat" and building a complete
internal layer separated with fibreglass / rockwool / foam sounds
very complicated, and with foam very expensive.

Not as complicated or expensive as it sounds. The dynamat I already have. Foam I already have and it was $15 (sold as an eggcrate mattress topper). As you may be able to see in the photo, the sheet metal is interrupted by periodic braces. The whole interior would receive dynamat, then foam cut to match the large flat "windows" and then the hard backing can simply be a 4x8' sheet of whatever-board screwed to the braces and covered with carpet.

Carpet will kill Rt at higher frequencies, spaced pegboard kills Rt
in the midrange (based on hole percentage and the common
depth behind it). Both can lead to an "oppressive" sounding
space due to the "unnatural" variations of Rt with frequency.

When you say "kill Rt," does that mean reduce Rt? I don't mind having a dead space, unless the "oppressive" nature of the space will be detrimental to proper sound reproduction from the drivers. The cabin will have minimal absorption. I will insulate the doors as well as possible to keep outside noise out, but it won't be carpeted.

Here was my [non-scientific] analogy behind my theory. Let's say you are standing on a busy street trying to listen to a TV in a store window with the traffic at your back. Its nearly impossible since the SPL and frequency range coming from the street is greater than the little speakers on the TV. Now, construct a foam-lined box 6' x 6' x 3' deep behind you. The high frequencies from the TV still make it to your ear unaffected, but the mid and high frequencies that were flooding you from behind are now dampened/absorbed. Make sense? I was hoping to block as much sound from making it INTO the van, but those that DO make it in I wanted to beat them into submission ASAP.

One tip is if you use pegboard glue on diagonal paper strips
to vary the hole percentage, e.g. have some pure pegboard,
some cover every other row, some cover 3 rows for each
open row, some cover 7 rows for each open row, some
cover 15 rows for each open row. This will tune the centres
of absorption 1/2 an octave apart for each change , but still
the above only covers two octaves (for constant depth).

That is an excellent idea, but do you think that its necessary/effective in a vehicle? I know you said you haven't done vehicles, but I think with the floods of sounds all across the range that bombard the cabin of a car on the road, doing small things like altering the spacing of holes to achieve flat response might be like trying to get a date with Kate Beckinsdale. You can do all the right things but still never get exactly what you want.

78% of my audio listening will be while driving, so getting the perfect flat response of a room isn't really in the cards.... at least I don't think it is. I could be wrong.

[curtis73]

sreten... thank you for the well-thought out post.

[curtis73]

Originally Posted by kevinahcc20

Back in the '70s when such vehicles were done up as hippie vans there were conversion shops who would spray foam insulation on the sheetmetal between the interior ribs and screw panels covered with shag carpeting over the foamed interior walls. A raised bed with storage underneath was installed over the rear wheel well area and similarly carpeted. The treatment did a pretty good job of taming the interior resonances and reducing road and wind noise transmission into the interior. Good luck!

I'm kinda shooting for that. I just wanted to run this by you guys first to make sure I wasn't making a rookie mistake. I could go very scientific and use a sound meter and frequency generator, but I'm more interested in putting some stuff in there and saying, "ooh, this makes my drivers sound great."

Kevin, what is your take on foam/pegboard versus something like fiberglass/hardboard?

[Perry Babin]

Fiberglass insulation behind luan or pegboard should work well. The insulation that's fully encapsulated in plastic is easier to work with.

If the sides aren't going to be scraped by whatever you're hauling, headliner material may be a suitable cover. It's designed to damp/absorb sound.

[kevinahcc20]

Originally Posted by curtis73

I'm kinda shooting for that. I just wanted to run this by you guys first to make sure I wasn't making a rookie mistake. I could go very scientific and use a sound meter and frequency generator, but I'm more interested in putting some stuff in there and saying, "ooh, this makes my drivers sound great."

Kevin, what is your take on foam/pegboard versus something like fiberglass/hardboard?

I don't have direct experience that could help determine which would be most effective for sound treatment but there are other factors you'll want to consider. I was an automotive engineer for 39 years and the in-vehicle environment is pretty demanding. Fiberglass is effective in a static application when it is protected from vapor condensation. In vehicle I would expect it to be at risk for compressing due to motion & vibration and at some risk of vapor condensation due to the varying thermal and humidity gradients. A sprayed-on close cell foam that is covered with interior panels has a well established track record of durability and effectiveness in the your application. I can see how pegboard might be more effective at diffusing higher frequencies (wind and tire noise), but at low frequencies (driveline and suspension inputs to the structure) they are probably similar. In the end I think having an interior surface like a fairly deep pile carpeting will do a good job across the frequency spectrum.

[gfiandy]

Hi another consideration is how the panels resonate. As when driving there is a high energy input accros a fairly broad spectrum. The panels will most likely be excited t their resonant frequency.

Materials like bitumen absorb some of this energy and reduce the resonant frequency due to their mass. This tends to make it less audible if you can drive it low enough as your ears sensitivity is lower.

Another approach is constrained layer damping in which you use a material with a different resonant frequency either due to its thickness or different properties coupled by a lossy compliant medium. Commercial materials that do this are sound dead steel and aludamp. However I have heard that you can achieve a similar effect by using stiff floor tiles and carpet tape. The foor tile provides the alternate resonant frequency and the carpet tape the lossy medium.

I have used both aludamp and sound dead steel in hifi applications wih great effect however I have not tried the floor tile approach so cannot guarantee it will be effective.

Almost any half rational approach is likely to make it much better. However I wouldn't want rock wool or fiberglass insulation in a vehicle as the dust s likely to be released in to a fairly small volume by the vibration and it's horrible if you breath it in.

Foam as suggested or sound deadening felt would be my approach.


Regards,
Andrew