Just a thought

There's another thread today discussing using perlon for the back pad and if it was quieter or not. Something I've notice is Some GM hood have their inner surface looking like a golf ball. There are 3" round divots and when asked about it I was told it reflects the sound back to the motor and that kills the sound.Anyone know if there is truth to this?

This would of course be in the engine compartment.

Kevin

Perlon is a very lightweight material. You will hardly get any sound reduction from only using it. What you want to do is use an automotive sound deadening material (spray or sheet good) and then cover it with Perlon.

Last week I was at a dealer looking at a Mercedes GLB since I need a new SUV. Popped the hood to look at the engine. The sales guy pointed out the divots in the underhood bracing without my asking (I was interested in the engine - didn't even notice the hood). He said they are designed to reduce head impact forces in a collision with a pedestrian. That is a bit more believable than the reflecting sound theory... but I don't know the real answer either.

Like I said, I had heard this in passing and was curious if it was true.

Kevin

Yes there is truth to it.

Sound is energy and it propagated through the air as "waves" to simplify.


When the "waves" hit an irregualr surfuace and particualry a conical surface they are refocused.


There are other ways as well to do this..series of irregular holes in perforated metal ect.

The size of the dimple depends on the frequency and amplitude of the "wave".

This

All of automotive design has turned into an abomination of government mandates.

Pedestrian impacts are increasing due to wide A-pillar and high belt lines that obscure visibility. Not to mention due to dolts walking around and stepping into traffic while looking at their phones. So now cars are designed for hitting pedestrians. I think it was one of the German OEMs was even playing with pedestrian air bags .

If you pay attention, you’ll notice hood lines moving up. This is to provide more space between the hood and top of the engine to cushion pedestrian impacts.

A-pillars have grown very thick to meet roll over requirement. These wide A-pillars obscure pedestrian visibility.

Belt lines are rising due to side impact requirements. Ever driven a Camaro? It’s like looking out of a port hole. Outward visibility in that car is among the worst out there.

Rear deck lids are rising and rearward visibility is reduced due to rear impact requirements. Result, more children and pedestrians being run over while vehicle is backing up. Government mandated solution: add a useless rear camera that is easily obscured by water, ice/snow, mud, dust. There were other more elegant solutions but that is what was mandated with full compliance required by 2018.

Meanwhile NHTSA basically just proposed mandating light duty trucks out of existence with a 58 mpg CAFE average. Of course that sort of fuel economy is completely at odds with all the weight that keeps being added to vehicles to meet the impact requirements.

https://www.nhtsa.gov/laws-regulations/corp...ge-fuel-economy

Vehicles no longer really designed with the consumer in mind. OEMs just trying to keep up with and meet the endless mandates that are often at odds with other mandates as noted above.

This got me curious... Head impact protection on the hood is what it is.

Found a 2008 GM research paper at the NHTSA...

ABSTRACT
Besides functioning as an engine compartment cover, the hood of modern vehicles can also help manage the impact energy of a pedestrian’s head in a vehicle-pedestrian impact. However, a hood’s ability to absorb impact energy may be impeded by the proximity of the hood to components packaged inside the engine compartment, i.e., by its underhood clearance. For example, for a given hood design, the hood’s ability to absorb impact energy through deformation can be significantly reduced when the hood and engine block are in close proximity. Therefore, a large underhood clearance would be preferred for pedestrian protection. However, it could negatively affect driver visibility, as well as a vehicle’s aerodynamics and aesthetic appeal. This paper presents a sandwich hood design that has a potential to improve the hood’s ability to absorb the impact energy of a pedestrian’s head with a relatively small underhood clearance.


Final conclusion was a corrugated under hood works, but is 27% heavier than the standard hood and was not evaluated for manufacturability. Guess they figured it out and put it into production. But like Superhawk said, designed for regulations to the detriment of the vehicle and does nothing for the driver or driving experience.

Found references to a Mercedes patent for putting in pedestrian hood airbags. Today our 914s would weigh 3000+lbs with this thinking. Isn't that what the 718 weighs?

Agree 100 percent. "A pillars" built like freekin tree trunks is one of the many reasons why I have ZERO interest in owning any modern disposable crap boxes (cars).......
modern cars absolutely suck when it comes to being able to see out of them.

excellent summary of how one regulatory focus on passive safety has created another problem in its wake -- suppression of external perception of surroundings.

also given rise to vehicles rolling over at the drop of a hat.
somewhat harmlessly in low speed intersection collisions, but they just tip over rather than crush and spin. and of course at higher speeds on rural roads its a lot worse.
roll over stats in australia are right up there. and they are wondering why?


when the electronic stability control gives up and hands back control to the retard on auto pilot it all instantly goes to sh$t. yes it really is a brick on wheels that has absolutely zero handling built inherently into its design. a bit like an air france jet liner crossing the equator at the atlantic deciding its going to kick out of auto p and hand back the stick to a couple of seat jockeys who can' figure out they are in a stall even with 35,000 ft to fall through and do something.

Not sure how much value this will add, but here goes.

I spent 3 weekends ripping out the interior and soundproofing my brand new F150. I researched beforehand and learned a lot during the job.

Noise comes from a variety of sources... and the materials you choose for soundproofing should be selected for what noise you're trying to defeat. Stuff I used included:

Constrained layer dampening (CLD). Think dynamat or the factory soundproofing on the floors. It works by reducing resonance in a panel.

Mass Loaded Vinyl (MLV). Think a sheet of heavy rubber that floats on the floorboard between the car and the carpet. It absorbs the energy of noise from below. Imagine throwing a football at a bedsheet hung on a laundry line. A flimsy, free-floating bedsheet can absorb the energy from the football and not transfer it to anything else - resulting in a much quieter passenger compartment.

Insulation. Think EZ Cool (closed cell foam) or 3M Thinsulate SM600 (batting). These materials will absorb ambient noise as well. They can also function as thermal insulation. Van-dwellers use the EZ Cool stuff to help make their abodes more pleasant.

I think our 914s would benefit from strategic placement of these materials, depending on what noise you're trying to diminish. The factory engine compartment pad was probably an attempt to do CLD & insulation in one package. It was probably the best solution they had 50 years ago, but we have better options today.

Of course, none of these will protect a pedestrian's head during an impact.

best,
Rusty

Perlon will offer little sound deadening effect.

The only way to "kill" noise is to
1- have active noise reduction that creates a sound wave opposite the sound
https://www.youtube.com/watch?v=oVZEtDyTdBI
2- provide a barrier that will absorb the sound ie. dyno Mat or a product similar.




The problem is that when you eliminate noise from the engine compartment, you will get the road noise. There are really too many sources in a 914 to have a "quiet" ride.

So your time and money spent have decreasing returns.

The heavy "tar" mat that is factory works pretty well. If you need to have perlon for an aesthetic you are looking to achieve you could take a back pad and cover it with perlon. Either a stripped-down stock one or one of these-

https://914rubber.com/thermal-formed-3-piece-back-pad-core-1



Matt used this on his carClick to view attachment

Really good summary

The only other thing I’ll add is that treatments are also specific to the frequency.

Low frequency noises (<100 hz) usually respond better to mass damping and constrained layer damping.

High frequency (>100 hz) usually respond better to blockage and absorption. Good old fashioned cotton shoddy pad is still highly effective.

One of the most import aspects of reducing noise is to make sure you don’t have pass-through holes that allow noise to enter the cabin unimpeded. Talking about things like DAPO holes drilled for stereos, wiring, a/c installs, oil cooler lines, etc.

OEMs do a pretty good job of optimizing NVH but they have to do that against cost constraints, manufacturing constraints, and weight constraints that affect fuel economy. You can always do more when those constraints are removed. However as previously stated, the further you go, the more you’re chasing diminishing returns.

This really does work.
I use Apple's gen 2 AirPods - the noise cancelation is absolutely awesome in the 2.4L (with the motorcycle exhaust - glass pack pipe). Gen 3 AirPods don't have active cancellation.

I'm probably breaking another law somewhere by driving with them in my head...

@Mikey914
Mark, do you all offer the 3 piece backpad with Perlon already installed like Matt’s car?
(Edit- Mark I see from another thread that you all are currently short staffed. No big deal on my question here.)
Thanks!
(Sorry for the hyjack)

I haven't yet, but was going to play with this this week. I'll post up some pics when I have done one up.
-Mark

You can attack the firewall problem but in the end the rear window will become the loudest transference of noise.

I think the OEM engine bay rubber pad glued to insulation gave pretty decent results.
I might suggest adding dynamite to the interior of the firewall in addition to the oem engine bay pad, then call it quits.

Rich

I used Perlon on the panel behind the seats and Dynamat on the firewall in the engine compartment. I also have a 3/4 inch thick aluminum foil covered pad covering the firewall, works pretty well with the windows rolled up. But most of the time I am cruising with the windows down listening to the tunes from the Triad West exhaust!

Just drive very cautiously if you add that dynamite…

Good morning Rich, I did a dual glaze rear window on my 914 almost 30 years ago. Look at the picture below, the identification on the glass is mirrored or doubled.

In fact, right after I put in the 3.2 L motor I ran without a back window for almost a year. Rain does not get into the car when you’re going forward. Lol


Click to view attachment

Creative - I didn’t realize you can fit two layers of glass in there. How did you secure the second pane of glass?

Modern luxury cars often use thicker and sometimes laminated glass on side and rear windows as a way to reduce NVH. The lamination layer acts like constrained layer damping impeding sound transfer from one pane of glass to the other.

I like the double glazing. Nicely done.

Laminated glass is a good idea too... Lots of high end Mercedes use this trick.

I have a spare 914 b/w that I will need to look at. I think it is basically a flat piece of tempered glass with little to no cross car curvature. You can buy panes of laminated flat glass - doubtful it is DOT rated, but... digitize a sample b/w and drop a laminated blank into a FlowJet water cutting machine - and an instant laminated back window glass for the 914. Of course your would need to mask-off the cut area to prevent etch from the garnet abrasive. I had water jet cutting done on a small 12"x12" piece of laminated glass that worked out well for an industrial porthole.

I am going to have to look into this.

Yes, you can fit two pieces of glass in the space behind the seatbelt anchor point vinyl interior trim. I used balsa wood as an intermediate gasket piece between the glass, (it allowed minor ventilation, so condensation does not fog up the dual glaze window) I then sandwiched the two pieces of glass together with silicone.

@vitamin914

By the way, I did get the idea after looking at a Mercedes sedan that had dual glazed driver and passenger side windows..!

I had a 914 rear window cut from laminated glass once just so it'd be tinted. Surprised how much noise was reduced. I'll be doing that to my current 914 at some point.