As any Delica L300 owner knows, these vans are loud, especially on the highway. This is a combination of factors, namely, the cab-over design, lack of insulation and overall geometry. This guide will go through some methods of addressing those issues and attenuating in-cabin noise.
Our Cummins R2.8 engine swap increased the in-cabin noise floor by around 20%. The larger engine with electronic fuel injection produces a lot more acoustic noise. The measures taken to address this noise are still applicable regardless of engine type.
Background
It’s important to have some baseline understanding of how sound works. Slapping Dynamat all over your van is going to do very little if anything. These vans already come with OEM sound deadening material in the appropriate locations, so adding more will do very little for acoustics. Stacking materials of varying densities is the best way to eliminate sound entering the space.
Generally, any hard flat surface will act as a reflector. Soft irregular surfaces help to break up and absorb sound waves. There are some things like windows that will always be hard flat reflectors. There are other places where this can be addressed. Some examples are:
-Having carpeted interior floor/surfaces absorbs sound waves and prevents the bare metal from acting as a reflector
-Having stuff in the back of your van provides more sound absorption. An empty van is noticeably louder.
-Rear window curtains absorb sound and lessen road/wind noise.
Sound enters the cab from external sources, so replacing things like door seals, window seals and run channels can help to seal the cabin up against wind noise. This alone made a noticeable difference in our van.
Preliminary Measurements
To establish a rough baseline a noise floor was established. This was done with a smartphone app. Around 56dB was measured at idle. That went up to around 70dB with some revs. For reference a concert is around 90-120dB.
Note: Our van already had the window and door seals replaced prior to this effort.
Adding Noise Canceling Material
The first step was to add anechoic acoustic foam over our existing Dynamat foam hood liner. After some searching a miniature version of acoustic foam was found (normal versions are too thick). The Sonex Mini Acoustic Foam was selected for its small 1″ profile and 300F temperature rating. This foam is commonly used for small spaces and equipment enclosures, so pretty ideal. The corresponding Acoustic Adhesive was purchased as well.
The one downside is price, The MOQ is 10 2′ X 4′ sheets at around $30/sheet, so way more than we need, but this is a premium commercial product.
This acoustic foam material is also the same thing that Mr. Clean Magic Erasers are made from, so you can save your scraps to clean your kitchen, lol.
1. The foam was cut and applied to the hood, seat panel and air filter cover. The foam was lightly painted with high temp black paint for aesthetics.
2. The interior of the dash (spaces behind the plastic) was insulated with a combination of acoustic foam and Havelock wool. Special under panels were made by gluing the foam to cardboard. This will help block some of the road/wind noise coming from the front.
3. The center console was also filled with acoustic foam.
4. The air filter cover seal was replaced with a new OEM one. The hood seal was also replaced, but with an after market strip, since the OEM seal has been discontinued. This was a huge pain, since the old adhesive was very stubborn. I recommend only doing this if your seal is in truly rough shape. If you do choose to do this, do it before gluing the acoustic foam in place.
We used McMaster Carr PN: 93085K577 This was found to be about the right dimensions, rated to 140F and PSA backed. Its sold in a convenient 10ft length, which is just enough to replace the seal, which is just over 9ft.
5. The old door and window seals are also a big contributor to noise. Replacement of the original window run channels and door seals is a must. The door strike plate can also be adjusted to get the correct amount of sealing force. The window regulator can similarly be adjusted to ensure the window sits correctly in the run channel for a nice seal.
VanderBosch has a great video for how to do this: Door & Window Weatherstrip Replacement
Results
With the noise canceling measures installed, a 6dB drop down to 50dB was measured at idle. Numerically, that may not seem like munch, only 12%, but the dB scale is logarithmic. So that 6dB reduction translates to an almost 4X reduction in perceived loudness or sound pressure level (SPL).
The graph blow depicts this well. It can be easily seen how a difference of 6dB translates to a large reduction in SPL:
This reduction is immediately noticeable at idle, and especially noticeable on the highway! Its no S-Calss Mercedes, but it no longer feels like driving an old ranch truck or piece of machinery.