2002 VW Golf stealth SQ build (an engineering student's perspective) - Car Audio | DiyMobileAudio.com | Car Stereo Forum

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Old 08-20-2012   #1
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Default 2002 VW Golf stealth SQ build (an engineering student's perspective)

I was first inspired by John Whitledge's sprinter van and his scientific approach. As an engineering student I knew the pursuit of sound quality in Car audio was a matter of breaking down the process into smaller, more simple concepts. My goal was to learn as much as possible, re-use my existing gear, and change my installation a little at a time to hear the differences. I bought some books, found some excellent resources online, and signed up to be a judge for MECA events here on the west coast (best thing ever!).

This is the 2nd Car I've built, the first was a 1995 Jeep Grand Cherokee and I built a transmission line subwoofer to experiment with low frequency extension (the in-car resonant frequency of that sub was 28 hertz and was an absolute joy, I miss it!). The same equipment is being used in both cars except for the subwoofer. Here is the list for the Golf:

Morel Hybrid Ovation 6.5" 2-way component speakers
Alpine PDX-5 5 channel amplifer
Eclipse CD7200 MkII head unit
Focal Polyglass 21 V2 8" subwoofer
sound treatments from Lowe's and sounddeadenershowdown

By experiencing many iterations of the installation in both cars, I am truly astounded by how awesome (and how terrible) the exact same gear can sound. It is so common to see people swapping out speakers and amplifiers because they don't like it, or their midbass driver is muddy, or their tweeter is harsh, or whatever. I am now a strong believer that good installation and good tuning are absolutely essential no matter what price range your speakers and amplifiers fall into.

The subwoofer, sound treatments, and passenger midbass are complete right now. I'll post some more this week, as I want to finish it for the meca competition this saturday in San Jose.


the Jazzy-mobile


my 6 month old austrailian shepherd, she loves to "help" with whatever I'm doing, often borrowing tools and pieces of hardware

Last edited by Jazzi; 08-20-2012 at 02:27 AM..
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Old 08-20-2012   #2
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Subwoofer: Cabin Gain and Getting Good Bass in Small Spaces

This was pretty straight forward and similar to many other subwoofer builds in a small hatchback like my Golf. When I built the transmission line subwoofer for my Jeep, I knew I could change the geometry of the enclosure and drastically alter the frequency response. This was great because I could also measure the cabin gain, or transfer function, of the Jeep and adjust the geometry of the transmission line to match. This photo should help some:


Matching subwoofer frequency response to measured cabin gain


Transmission line built for the Jeep


Installed resonant frequency was 28 hertz, like nothing I've ever experienced

With the Golf however, I needed to keep all of my cargo space and had very limited options for a subwoofer. A sealed enclosure in the rear corner of the cargo area where the factory CD-changer is very common for this car, so I went with it. A sealed enclosure could not be tuned the same way a transmission line could be, so I didn't bother measuring the transfer function of the Golf. I did however pay close attention to the air volume of the enclosure I built.

The newly released Morel Ultimo SC 10" seemed like an excellent match for what I wanted with very low frequency extension and excellent sq reviews from countless sources. I only had about 0.6 cubic feet after the enclosure was built and decided a 10" would not be the best fit. After searching for a while, my local shop recommended the Focal Polyglass 21 V2 (8") and I was skeptical at first but it matched the Power rating of my amplifier, matched the volume requirements of my enclosure, and was about what I wanted to spend on the Ultimo SC. It did have a low resonant frequency for an 8" woofer too.

Experimenting showed that having a subwoofer in the corner of a vehicle is better than placing it midway between the front and rear, or anywhere more than a foot or two away from each boundary of the interior (floor, side, and rear hatch). Reading both books I bought (linked in first post) confirmed placement has a huge effect on which frequencies are reinforced and which are cancelled. Ingvar ÷hman has summarized it better than I can:

Quote:
A good starting point is to try to get as many reflections as possible in phase (preferably all three; floor, rear wall and side wall), to arrive in phase with the direct radiation for the entire working range of the subwoofer (typically 20- 80 Hz). This means that no path via a reflecting surface should be more than a quarter of a wavelength longer than the direct path.
I practice, this means that no adjacent surface should be at a longer distance than 1/8 of a wavelength from the driver in the subwoofer. (Since the sound then will travel a quarter of a wavelength to go there and back). The wavelength at 80 Hz is 340/80 = 4,25 meters, and 1/8 of a wavelength becomes 53 cm. The subwoofer driver should be placed within this 1/8 wavelength sphere.


text and image borrowed from The placement of a subwoofer
Now I knew what driver to use and where to put it and I just had to Build it. This was the first time I've tried to fiberglass into a tight space and my first attempt at using a mold (the factory carpet lining of that cargo pocket). I started by removing that section of carpet, expanding the shape a little using carboard to get maximum air volume, then applied a layer of tape over everything, then a layer of aluminum foil so the resin would hopefully release after curing. The fiberglassing went well and conformed to the shape very well, but unfortunately it did not fit well back into the Car because I made mistakes preparing the mold and didn't allow enough clearance on all sides (also the carpet mold was floppy and not the best idea). trim trim trim trim sand sand sand sand ....


the cardboard expansions, tape, and foil all laid out on top of the carpet mold


first layer of fiberglass applied using lightweight chopped strand mat in case I make mistakes so it will be easier to modify


the factory cargo pocket where I want to install the sub, and the fiberglass shell I made after being sanded and trimmed in many places to clear sheet metal

The materials used were from US Composites and I am very pleased (inexpensive, knowledgeable staff, good quality). I had some other fiberglass projects in mind and bought WAY too much stuff in hindsight haha. I picked up two gallons of their 435 resin, one yard of 1.5oz chopped mat for quick layout work and for complex compound curves, and five yards of their #1708 Biaxial because it was very heavy and would require just a few layers to be super strong. If you decide to fiberglass stuff, be sure to get lots of disposable mixing bowls and cheap paint brushes.

The baffle and carpet were very easy compared to wrestling with the fiberglass shell. The whole thing is mounted to the Car using the same screws used to mount the factory CD changer and is very solid.


the baffle is two layers of 1/2" mdf which sounds thin by some standards, but the small dimensions of this enclosure and modest Power requirements justify it


mating the fiberglass shell to the baffle was a nightmare due to poor planning. I tried to fiberglass the gap seen here but wasn't able to create an air-tight seal. Eventually I went with a resin and bondo milkshake to fill in the holes and it worked great


covered in matching carpet and installed in the rear corner of the Golf. There is a factory cover that rests over the cargo area so when the car is parked, this area is completely blocked from view

The subwoofer would start to look simple in both fabrication and research involved as I started other portions of the car like sound treatments and especially midbass installation in the doors. I'll post some more in the next day or two, thanks for reading.
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Old 08-20-2012   #3
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Cool. I always like to see OEM spaces modified for better aftermarket gear, while maintaining the "stock" look.
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Old 08-20-2012   #4
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Great looking work thus far. Cute little helper you've got there.

------------------------------------------------------
2016 Pennsylvania State Champion- MECA Modex
2016 National Points Champion- MECA Modex
2016 National Points Champion- IASCA Amateur
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Old 08-21-2012   #5
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Be interested to take a listen Saturday.

As an FYI, agree with your food recommendation personally as you probably saw what I was suggesting too, but doesn't look like we'll get consensus.

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Old 08-21-2012   #6
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Quote:
Originally Posted by captainobvious
Great looking work thus far. Cute little helper you've got there.
Thanks for the compliment. She went to get spay'ed today and wow she is unhappy, but better than suddenly! puppies. At the very least, my tools will stay where I leave them for the next few days.


Quote:
Originally Posted by papasin View Post
Be interested to take a listen Saturday.

As an FYI, agree with your food recommendation personally as you probably saw what I was suggesting too, but doesn't look like we'll get consensus.
I look forward to hearing lots of cars Saturday! Looks like there will be many in the Modified class, should be fun.

As for food, I'm happy with most anything.
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Old 08-21-2012   #7
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

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Originally Posted by Jazzi View Post
I look forward to hearing lots of cars Saturday! Looks like there will be many in the Modified class, should be fun.

As for food, I'm happy with most anything.
I was at the meet couple months ago so heard some of the cars, but I think more folks heard mine. I'll be competing mainly because I'm curious and due to some of the urgings of the folks from the meet...never really thought of competing until then.

Being a native of the South Bay Area for nearly 30 years, have experienced lots of good places...problem is making everyone happy. You'd be surprised at some of the food discussions we had at the meet .

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Old 08-21-2012   #8
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

I wanted to post some resources I found useful, and I will be referring to these later when this thread catches up to my installation.

Webpages:
The Car audio System Nobody Would Build 6 part series

Roxul Safe'n'Sound is a mineral wool insulation similar to fiberglass in acoustic absorbtion, but does not absorb water and will not grow and fungus or mold. I installed a bunch in my Car doors with great success. Lowe's was the only hardware store near me that carried it.

SoundIsolationStore.com has some detailed and easy to understand articles about sound isolation that is very relevant to cars even though their products are intended for residential homes.

Help my soundstage ate my windshield a thread here on how to tune using independent L/R eq, and my inspiration to aim my midrange drivers more on-axis but still symmetrical (I'll write about this later)

Viscoelastic Damping 101 by Palu Macioce, Roush Industries. A technical description of damping and a good read, though not very sensational

Cross Spectrum Labs sells inexpensive calibrated measurement microphones, and is run by a very knowledgeable guy who has answered many of my questions

Audioholics Online A/V Magazine has an outstanding free educational article section

Sound Deadener Showdown has excellent sound treatment products for a great price

Tricks to getting 500hz response a great thread on this forum regarding midbass holes in car doors


Books:
Master Handbook of Acoustics 5th Edition by Everest & Pohlmann
This book covers all the basics very well and goes into more depth than most people would care to read. I found it an excellent resource for understanding how sound waves move and what they are affected by, but there is little emphasis on how to use this technical information to make a system sound "good".

Sound Reproduction - The Acoustics and Psychoacoustics of Loudspeakers and Rooms by Floyd Toole
Toole goes into great depth citing study after study about what sound artifacts are audible and how much they effect our perception of sound. This book clearly helped me decide which artifacts I wanted to put the most effort into controlling (resonances, reflections, standing waves) and which ones I could reasonably ignore (high frequency comb filtering, diffraction). It is not an easy read nor would I call it a cookbook, and I re-read many chapters to understand some concepts. Overall though, exceptionally useful.
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Old 08-23-2012   #9
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Just returned from a trip to Tahoe to move some furniture around. The Golf towed a 5x8' trailer up and back fairly well, but after having to downshift into second(!) gear a couple times, I really started to miss the 5.2L engine of my old Jeep.

The midbass in both doors is complete, I have a ton of writing to catch up on. Tonight I was looking forward to sharing experiments with damping tiles vs asphalt-based roofing material (a commonly suggested alternative), but that will have to wait. Good night, and I look forward to meeting everyone this Saturday in San Jose at the competition.
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Old 08-23-2012   #10
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Quote:
Originally Posted by Jazzi View Post
Just returned from a trip to Tahoe to move some furniture around. The Golf towed a 5x8' trailer up and back fairly well, but after having to downshift into second(!) gear a couple times, I really started to miss the 5.2L engine of my old Jeep.

The midbass in both doors is complete, I have a ton of writing to catch up on. Tonight I was looking forward to sharing experiments with damping tiles vs asphalt-based roofing material (a commonly suggested alternative), but that will have to wait. Good night, and I look forward to meeting everyone this Saturday in San Jose at the competition.
Nice job on the sub enclosure, looks really clean.

Here's a good thread on the asphalt tile topic.

Why Not Asphalt?

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Old 08-23-2012   #11
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

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Nice job on the sub enclosure, looks really clean.

Here's a good thread on the asphalt tile topic.

Why Not Asphalt?
That is an excellent introduction to the topic, thank you very much for the link. I echo much of what was expressed there. I have also taken it a step further and done some experiments, gathered data, and made some cool charts to really show the difference between the materials I gathered. After reading that one though, I'm tempted to expand my collection of materials and tests
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Old 08-25-2012   #12
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Damping Materials: name-brand product vs asphalt-based alternative

When I started learning about damping resonances and sheet metal, I was (like many people) surprised by how expensive materials like Dynamat, Hushmat and other name-brand CLD tiles were. Many forum warriors were convinced it was a grand scam and we could easily substitute products from other industries with similar materials for much less money. But there was much controversy about asphalt vs butyl, and weather or not the presence of foil backing made a difference, or how thick it should be, and so on.

At the time, I was in the middle of a physics class dealing with light, optics, waves and whatnot. As an extra credit project I ran an experiment to see how effective a single layer of asphalt-based roofing sealer (Grace Ice and Water Seal as commonly suggested during my reading) would be compared to a single layer of Stinger Roadkill (the first name-brand product I found at a local shop). The results were astounding.

If you are interested in the specifics, this report from E-A-R Specialty Composites is a great primer on the difference between a free-layer damping system (FLD) and a constrained-layer damping system (CLD). One of their illustrations is below. From what I understand, it seems a single very thin constrained layer sandwiched between two metals that are similar to eachother will give the best results. So when shopping for a damping tile, find one with excellent adhesion, high temperature rating, a thin viscoelastic (usually butyl) layer, and a thick aluminum backing (ie. almost every name-brand product meets this).


Image borrowed from E-A-R Specialty Composites
Understanding Damping, E-A-R Specialty Composites


In this example, a single layer of asphalt-based roofing material material with adhesive on one side attached to sheet metal is a free-layer damping system. A layer of Stinger Roadkill is a constrained-layer damping system because it sandwiches the butyl between the aluminum backing and the sheet metal of the car. Hopefully the above diagram helps.

The experiment was simple: measure the damping factor of resonances of my Car door as it came from the factory, after applying asphalt-based product, and again after removing the asphalt and treating it with Roadkill. In addition, I would test a single piece of sheet metal on the bench to see if there was a significant difference between trying to damp the resonances of my Car door (a complicated system) vs a single sheet of steel (a simple system).

Damping factor is easy to visualize using an RTA and some division. No complicated maths here. Viscoelastic Damping 101, Roush Industries is an article describing how to calculate it, and here is an example from TrueRTA in my measurements.


A sample calculation: damping factor of 95hz resonant peak: η = (106hz - 82hz) / 95hz = 0.25
(a higher number is better)




Above is test 1: this is what the outer door skin looked like from the factory. Notice the single free-layer damping material that has been applied in the top-left corner.



Above is test 2: this is two layers of asphalt-based roofing applied over the top of the factory damping material.



Above is test 3: all asphalt and factory damping materials have been removed, and about half of all accessible surface area has been treated with Stinger Roadkill, a constrained-layer damping material.

After each treatment the doors were fully assembled before taking measurements. Essentially I hammered the outside of the door using a soft mallet and recorded the sound from the inside, used TrueRTA to help calculate the damping factor, then made some graphs. First is the test using the car, the second graph is from a single piece of sheet metal suspended in air, and tested the same way.


Above is damping factor of the car door untreated from the factory, after applying a single layer of asphalt-based roofing material (Grace), after a single layer of name-brand product (Stinger), and finally with additional Stinger Roadkill plus a layer of MLV for sound isolation.
(higher is better)




Above is the bench test of a single piece of sheet metal. (higher is better)

In the bench test, the result is obvious. A single layer of asphalt-based roofing material provides some damping to the sheet metal, an increase of about 29% average damping factor. But a single layer of CLD is phenomenally more effective with an average increase of 770%! So, myth busted, Grace Ice & Water Shield may be a fine roofing material, but it is nearly worthless as a damping material.

In the car door test, the results are much less clear. Two layers of Grace yields an average decrease (!) in damping factor by 1%, but a single layer of Roadkill only has an increase of 13% on average. Confused by this, I tried to go crazy and added more Roadkill to the outer door skin, applied some to the inner door skin, and then added a mass-loaded-vinyl barrier to the inner door skin to further isolate noise and prevent it from entering the cabin. Even with these stronger measures I only saw an increase of 26% damping over the factory car door.

So what is going on? Clearly from the bench test a single layer of Roadkill (on 25% surface area) should have a drastic effect on damping factor for sheet metal. However the car door behaves differently. I believe the car door having (1) an outer wall of sheet metal and (2) an inner wall of sheet metal and (3) another inner wall of plastic and vinyl door card and (4) air chambers between all these layers are responsible. I believe the air chambers themselves have resonances that cannot be affected by damping the sheet metal and must be addressed other ways.

Conclusion? Buy a CLD tile from a common brand (or sounddeadenershowdown) and apply it in a single layer over about 25% to 50% surface area for excellent results (adding more doesn't increase benefits for the high additional cost). Also, further study needed to find sources of resonance in car doors. I'll have measurements for a new idea to address this soon.
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Old 08-25-2012   #13
 
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

An additional problem with asphalt based products is that they will not remain bonded to vertical panels or when installed upside down (like on the roof). People should have been done using asphalt based deadeners 20 years ago but cheap people are cheap.
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Old 08-26-2012   #14
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Great sounding car, and glad to meet you. Seems like we share common interests besides Car audio . I think your Car really does belong in street, and depending on number of eq bands on your HU, maybe can pass for stock if you have your spare??

It really doesn't pay to be in modified as you probably noticed. If you took a look at the full scores across classes, the top 3 in modified scored higher than top person in modex and this is with the same judge...so I think that's saying something. So flush your mids, but I guess it might be battle of the VWs .

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Old 08-28-2012   #15
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

I may try flush mounting the woofers into the doors again for the purpose of competition in the future, but this Car is more for me and for learning than for winning. I want to further explore the symmetrical on-axis mounting I built before going back to flush, if I do.
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Old 08-28-2012   #16
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

You're the judge . Not sure if it was me, but I was wondering if you noticed that your center image seems to wander at times. What was the other feedback you received from Fred?

EDIT: Also wondering...do you have room for kicks? That maybe another solution.

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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

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You're the judge . Not sure if it was me, but I was wondering if you noticed that your center image seems to wander at times. What was the other feedback you received from Fred?

EDIT: Also wondering...do you have room for kicks? That maybe another solution.
Yeah the center moves around a bunch, I know it, you know it, Fred knows it, but surprisingly many of the people that listened to the Golf said it was "rock solid" and the center was "right there" as they emphatically defined a tiny spot in the air with their hands.

I don't like kicks. The ones I have heard so far translate too much tactile feedback to my feet and are super distracting, plus I have long legs and enjoy having room to move around. I'm sure there are kicks done "right" that don't have either of these problems, but I haven't experienced one yet.

One day soon I'll continue the this Build log, there is lots to catch up on.
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Old 08-28-2012   #18
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Sound Barrier Installation

Deadening vibrations is a great thing, but it will not prevent airborne noise from entering the cabin from the outside. The sound of my tires against the pavement and the sound of other cars on the road next to me were two of the loudest intrusions on an otherwise quiet interior. Engine noise is also up there, but not easy to hear over how loud the Car rolling down the highway was.

I learned a lot from the great examples set here (2008 Yaris Sedan Deadening and Sound System by derickveliz) and here (sounddeadenershowdown). Essentially I wanted to create a massive, isolated layer of material between the outside of the Car and the interior. I would do this with the front doors later, so I started with the floor and as much of the lower firewall as I could reach. I also added what I could to the rear wheel wells.

There is nothing special here other than lots of labor and fighting to re-install the factory carpet, seats, and interior trim panels with the extra material sandwiched between them. I purchased mass-loaded-vinyl from a company called Acoustiblok in Florida (mistake! shipping was expensive). It was a recommendation from a friend who builds broadcast trucks and they use it for sound isolation in their builds. I eventually ran out of that and purchased more MLV from SoundDeadenerShowdown, but if I need to get any more in the future I would try hard to find a local place to avoid paying for shipping.

The material is a thick vinyl sheet with a density of 1lb. per square foot. It is isolated from the factory sheet metal on the floor by the factory carpet pad as seen in the photos, so I did not have to use any additional closed cell foam as some installations require (yet another example of why the Golf is a great car to install in).


All of the factory trim panels, seats, and carpet is removed first


Factory carpet removed and then re-installed in one piece, the most difficult step


A "before" shot showing the carpet pad that sits just below the carpet. I laid the MLV on top of this to mechanically isolate it from the floor of the car.


MLV initially laid on top of the carpet pad, lots of detail work to seal it up nice later
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Old 08-28-2012   #19
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Got to LOVE the smell of MLV!

Awesome!

D.

My system may not blow your ears, but it will sweeten your senses.
Mercedes Benz CLA 250, Pioneer P-99, IDQ12d4v3 Subwoofer, ... ID min Horns, 6.5" Midbass!
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Old 08-28-2012   #20
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Been there as well for 3 or 4 cars now . I got my MLV from soundproofing.org

Mass Loade Vinyl Mat

which is in CA, so shipping isn't as bad. I tried calling around locally, and even with shipping, I found soundproofing.org cheaper up to the Bay Area. I was talking to JT during the meet back in June and mentioned this to him, and he told me at the comp he just drove down to pick some up .

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Old 09-02-2012   #21
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Quote:
Originally Posted by derickveliz View Post
Got to LOVE the smell of MLV!

Awesome!

D.
Thanks Derick !


Quote:
Originally Posted by papasin
Been there as well for 3 or 4 cars now . I got my MLV from soundproofing.org

Mass Loade Vinyl Mat

which is in CA, so shipping isn't as bad. I tried calling around locally, and even with shipping, I found soundproofing.org cheaper up to the Bay Area. I was talking to JT during the meet back in June and mentioned this to him, and he told me at the comp he just drove down to pick some up .
Good to know, they have lots of great products to choose from.
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Old 09-02-2012   #22
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

Sound Absorption

With the results from my resonance tests above (damping sheet metal with cld tiles), I was really confused about the huge difference between the bench test and the in-car test. The door still had some huge resonances in it, but if damping the sheet metal wasn't the trick, what else could I do?

In a recording studio or very serious home theater build, sound absorption is a huge factor to consider. Essentially you want the decay time of each frequency to be the same from the lowest bass notes up to the highest treble. If some frequencies "bounce" around a room much longer than others and take a noticable time to decay, this can be heard as ringing, or in the case of low frequencies, "one note bass" because certain frequencies will dominate over time. The measurement is called RT60, the amount of time for a sound to decay to -60db (about one millionth) from when it stops. It requires an impulse (sound, gunfire, hammer, etc) and a 3-d plot of frequency vs amplitude vs time. Below is an example 2d plot, excluding the frequency domain:


Image borrowed from RT60 calculator Sabine calculation reverb time reverberation time Sabin formula - sengpielaudio Sengpiel Berlin

Realtraps.com has an excellent introduction of this concept and how to measure it using REW, a free software tool and rta.
RealTraps - Room Measuring Series

I love this 5 part series on practical sound treatments. It has a light touch of math for anyone interested in numbers, lots of very practical information, and isn't overly complex to be accessible by most anyone.
Practical Acoustic Treatment, Part 1

Frequencies in the kilohertz range are easy to attenuate and reduce their decay times. A simple dash pad 1/2" thick will absorb almost everything above a kilohertz. In a studio or theater, any padded furniture or drapes several inches thick will severely attenuate frequencies down to around 300hz.

But to attenuate bass frequencies is much more difficult because the wavelengths are so much longer. A common tool is a tuned bass trap placed at the high-pressure node of as many relevant frequencies as possible (a corner is great). Because these traps are enormous relative to the interior of a car, I had to try something different.

While reading The Master Handbook of Acoustics 5th by Everest and Pohlmann, I was intrigued by the section of absorption and this diagram in particular. It is comparing thickness of glass fiber material to the absorption coefficient when mounted flush against a wall.


Image borrowed from The Master Handbook of Acoustics 5th by Everest and Pohlmann, page 191

If I could somehow get 1-2" of glass fiber with a density of 3lbs./sqft into my doors as an acoustic absorber, it should attenuate the resonances I measured earlier when applying cld tiles. The effectiveness of the glass fiber material falls off as it approaches 125-200hz, but my resonances are worst in the 125-300hz range so I expected some improvement.

Glass fiber insulation materials notoriously absorb water and promote all kinds of fungal and mold growth. The inside of the door cavity is prone to moisture and I couldn't use this material. Luckily, there is a basalt rock-based alternative that shares fiberglass's excellent sound absorption properties (and r-value thermal insulation) that does not absorb water and inhibits all kinds of mold and rot.

Mineral Wool or Rockwool is perfect. Roxul Safe'n'Sound home insulation is perfect for this application. It is designed for sound insulation, fire retardant, and moisture resistance in residential environments.


This stuff is only sold in a large package of 12 batts, each 4 feet long. The whole package was about $40 and I only used 2 batts. Maybe it'll be useful for other projects.


Mineral Wool is super easy to work with and cuts very well with a hack saw blade or razor. I used this simple jog to get a consistent 2" thickness.


I used some Wire fencing from a previous project to create a mold to hold the mineral wool into the door.


The first mold and mineral wool is installed, the second mold above is seen without any material yet.


All mineral wool is installed. Some steel beams are used to hold it tight against the outer door skin so it will not bulge inwards and interfere with the window motor track.

This was the easiest part of the entire install on this Car so far.

I'll have some measurements posted soon, good night for now.
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Old 09-02-2012   #23
 
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

I didn't see how you're dealing with water retention in the Rock-wool.
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Old 09-02-2012   #24
 
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

That wool will absorb water like a sponge. And stay wet even after the world ends.
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Old 09-02-2012   #25
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Default Re: 2002 VW Golf stealth SQ build (an engineering student's perspective)

From Roxul's website: Water-Repellent

Quote:
Water-Repellent
Roxul insulation does not absorb water or hold moisture. Its unique stone wool orientation is ideal for repelling and draining water away from the exterior walls, industrial pipe, interior walls, or whatever Roxul insulation is protecting. And, since Roxul insulation does not absorb water, it will not sag or lose its shape, and will retain its R-value. So the R-value installed today, will be the same for years to come. Being water-repellent also means Roxul insulation does not rot, corrode, or promote fungi or bacterial growth.
Perhaps you are thinking of rockwool hydroponics material, which does absorb water. Depending on the manufacturing process stone wool can be hydrophilic (water absorbing) or hydrophobic (water repelling). The former is great for hydroponics, the latter is great for in-home insulation.

I admit I have not tested it like this, and just to be sure I'll do this test tomorrow, but this is what I am expecting:
https://www.youtube.com/watch?v=gLDxhbKfVeo
start at 4:05 mark

Last edited by Jazzi; 09-02-2012 at 05:44 AM..
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