Interior noise reduction ... a different take... Frame to tub decoupling

[nkemp]

I just read through: http://dmctalk.org/showthread.php?439-New-carpet-soundproofing-and-dynamat-any-tips/page2 which is a good read. I was going to post to that thread but this really deserves a thread of its own.

Noise reduction as indicated in the above link has its place. But adding sound reduction materials is kinda like mopping up the mess after the water leaked through the roof. There may be advantages to "patching the roof". What about reducing the mechanical energy that gets transmitted to the tub in the first place? The tub is not rigid enough to prevent mechanical energy conversion to acoustical energy. So lets reduce the energy going to our tubs.

For example,

Are there better alternatives to the engine mount? The Lotus folks reduced noise via a different engine mount with the Turbo introduction. Pre-Turbo versions have retrofitted the "Turbo mount" to reduce their noise level. Some modern cars go so far as to have active engine mounts that are computer controlled. In essence, our engine sits on the frame decoupled (or coupled as the case may be) with a rigid rubber like pad between the two engine mount plates.
Are there better alternatives to the tranny mounts? At least the current mounts are not squished between the tranny and the frame like the engine mounts.
How about where the tub mounts to the frame. It may be possible to reduce the energy transferred from the frame (which gets its energy from the road and motor) to the tub.
Have you ever noticed how much shifter noise can be heard through the shifter opening? One could guess that some amount of engine noise enters as well.


Also, this might be a good application of some audio spectral analysis. It appears there is some free and for fee software products for use on the PC. These are not simply sound levels but look at the frequency distribution including harmonics. This would add a bit of "science" to the results.

I don't have any specific ideas at this point. I wanted to start the discussion to see where it goes.

Nick

[Rich]

Have you ever noticed how much shifter noise can be heard through the shifter opening?


Yes, I had noticed it and I reduced it a lot on 0934. Based on the nearby VIN of your car it may not have one of these over the hole in the frame under the shifter boot - mine didn't:

http://store.delorean.com/p-8649-noise-isolator-mt.aspx

There is an A/T version as well. The 5-speed version pops in without tools and with with some trimming in less than 10min. Big improvement.

As for the rest of your list, no answer other than to add that Dynamat or similar sheet-type acoustic products are mentioned in this forum from time to time.

[nkemp]

Thanks, I didn't know that part existed.

[jawn101]

Rich knows I always agree with him but I'll chime in on the shifter noise isolator as well - I didn't have one in my car either. My shifter was completely full of crap - grease, degraded foam, toothpicks, candy wrappers, you name it. I cleaned all that out and installed a new isolator and the transmission noise was reduced dramatically instantly. And the isolator will help ensure that trash doesn't find it's way into the gearchange mechanism... :)

[Rich]

Thanks, I didn't know that part existed.

Right. Neither did I til about 5 years ago when another NCDMC member mentioned it to me. Another example of a benefit of getting into a local club chapter if one can manage it.

The part is listed in the correct category but is not shown in the parts schematic for that category.

It leads me to wonder the noise isolator was added to the bill of materials as a running change when production ramped up in the first months. The schematic might never have been re-drawn to show it.

And thanks for the shout-out, Jon. Glad to hear you enjoyed the same payback I did for that small investment in the "secret" noise isolator.

[nkemp]

I'm adding "NOISE ISOLATOR, M/T 108903" to the list of things to order (that list keeps getting longer and pricier). I'm also forwarding the info to our local collection of D owners.

With regard to the engine mounts, fit issues aside for a while, I've often wondered if the fluid filled engine mounts used on some higher end cars would reduce vibration transmitted to the frame. Seems to me that that solid ribber block does little to reduce vibration transmission to the frame.

[nkemp]

One of the local Lotus owners that participates in the MN DeLorean group added the following:

That part looks like the one Lotus used at the time. In
Lotus-speak, it's a rubber gaitor (or boot). It's interesting to me
that DeLorean refers to it as a Noise Isolator. While it may reduce
noise levels, it's real purpose is to seal off the shifter hole in
the chassis in order to isolate the cabin from the engine bay fumes.
The leather boot is largely cosmetic, and is not effective at sealing
off the hole by itself.

If your cabin smells of oil, gas, exhaust, and general engine bay
stink, then it's possible the rubber gaitor is missing, or has dried
out from age and cracked/ split. While the noise level may go up
without the gaitor, I'd be more concerned about the fumes and stink.

FYI...

[jawn101]

One of the local Lotus owners that participates in the MN DeLorean group added the following:

That part looks like the one Lotus used at the time. In
Lotus-speak, it's a rubber gaitor (or boot). It's interesting to me
that DeLorean refers to it as a Noise Isolator. While it may reduce
noise levels, it's real purpose is to seal off the shifter hole in
the chassis in order to isolate the cabin from the engine bay fumes.
The leather boot is largely cosmetic, and is not effective at sealing
off the hole by itself.

If your cabin smells of oil, gas, exhaust, and general engine bay
stink, then it's possible the rubber gaitor is missing, or has dried
out from age and cracked/ split. While the noise level may go up
without the gaitor, I'd be more concerned about the fumes and stink.

FYI...

I wonder if that might be more true because of the mid-engine design in many Lotuses? I didn't notice any difference in smell at all (though if it was something like CO, you would not smell that). But like I said, my shift boot was torn and the mechanism was all full of trash when I got down there - so there's no question this part functions to keep things tidy. But the noise reduction was palpable as well. It also gives the shifter a 'stiffer' feel as there is some resistance on-center with the isolator installed.

[nkemp]

Other than gas filling problems, I've not noticed any engine smell. To that point I've wondered if the pressure differentials across the car result in air being pulled up & out of the louvers when driving (or maybe the other way). Lately, being that it is fall, I've noticed an occasional dried leaf getting under the louvers and on the engine cover. If they are up front, they tend yo not get pulled out very quickly. So I don't think there is a huge air current.

[David T]

In the area of noise control it is always more effective to treat the problem as close to the source as you can. Stopping or reducing the noise is a lot better than trying to shield from it. There are many sources of noise. Depending on your speed, wind noise and tire noise can be a bigger problem than mechanical noise. Make sure the door seals (inner AND outer) are in good condition. Check that the rear window is sealed all around. Press on the fiberglass around and underneath the glass to see if it has separated. Make sure the air cleaner assembly is is all there and properly assembled. Check that the windows go all the way up and the seals are in good shape. Adding Dynamat is not the only or best solution, it adds a LOT of weight. For the era the Delorean was built in cars were not as quiet as they are today.
David Teitelbaum

[nkemp]

The noise I'd like to suppress is the "DeLorean Drone". That constant low frequency sound that seems to resonate in the cabin. Tires would help and there are any sorts of other noises. But that droning sound gets tiresome. I agree that going for the source is the best bet and Dynamat, although it has its place, maybe isn't the first solution.

If the constant hum is being generated from the road, tires and engine, the question is if we can better deter its transmission to the tub which seems to act as a speaker. The potential solutions would then include different motor mounts, tranny mounts and isolation between the tub & frame (which is a lot harder than the first two which are not necessarily easy).

If anyone has more ambition than I have, or already has a car near this state, it would be interesting to:
- Measure the sound frequency profile with the engine running at some RPM (in idle) as is
- Separate the engine from the motor mounts and support with proper blocking to the ground and redo the measurements
- Separate the tranny mount and support with proper blocking to the ground and redo the measurement

[Spittybug]

I'm not an acoustical engineer, although I have stayed in a Holiday Inn Express.

I don't believe that it is reasonable to believe that the flex associated with the fiberglass tub can be fully eliminated. It is this large volume "container" that creates the low frequency interior cabin noise that plagues us in my opinion. As a student of sub woofer enclosure design, I believe that the Delorean body tub is actually a very good enclosure for creating low frequency harmonics, albeit unintentionally! To mitigate such, the tub needs to be made less resonant by stiffening walls, dampening with foam, clamping down...etc. The best approach may be an active cancellation system. Maybe Dave M. or other enterprising inventor can design a microphone/amplifier/sub woofer setup that sends equal and opposite sound waves to silence the interior of our cars? Wouldn't that be worth buying?

[nkemp]

I've done some research on active systems and there are a fair number of problems trying to actively manage noise in large areas. Active noise reduction works well for headphones due to the confined space. You MIGHT be able to create a quieter space around your head but that seemed to be VERY problematic. But you could actually increase the noise level if the phasing and timing gets out of alignment.

That's why reducing the energy going to the tub seems worth pursuing. We won't be able to get it all but less would be better.

[David T]

I have noticed some tires make a LOT more noise than others. That droning could just be the type of tire you have. At least make sure you have the proper air pressures in them.
David Teitelbaum

[Bitsyncmaster]

I've never been a fan of active noise canceling headsets. These are a big thing for private pilots. You really want to hear if that engine starts making a different sound.

I think you need to contain the feedback of the active circuit so the microphone does not pick up the feedback. Hence, the mic would go into the engine compartment. Maybe road noise could also be reduced.

All you need is an amp with inverted output phase. Just swap the wires on the speaker terminals or mic terminals.

[nkemp]

I've done some research on active systems and there are a fair number of problems trying to actively manage noise in large areas.

Here is where I quote and contradict myself to some extent.

GM added active noise cancellation to the Cruze in order to lower the RPM and increase gas mileage. So it could be done but I suspect that GM has a lot more resources than we have here. It would be interesting to see if you can move their system to our car or is it specifically tuned to the car(Cruze).

On a side note, BMW now does the exact opposite by adding engine noise to the cabin to add to the sense of performance.

Also, our local Lotus expert indicated that Lotus did cabin active noise cancellation with Jensen as well as sound adding MANY years ago before noise cancelling headphones. I believe he said there were 3 cars made with this device and that it added $100 to the cost of the car back then (early 80's?)