Reducing Diesel Engine Noise in Bus Cabs: Practical Solutions and Limitations

[FawnaFox]

My bus, along with many busses is loud in the cab. I know for sure that it's just the diesel engine being loud, not exhaust but, it's a big mechanical machine, it's loud! I don't want it to be rolls royce quiet, but currently driving it is exhausting because of the loud drone the engine makes.

Right now I plan to redo the hood insulation, as it's falling apart and half eaten by wasps (I guess they take that to make nests) I really don't want to tear apart the dashboard to do insulation, not with the 10,000 wires behind it. (90s bus problems). There is a bit of room on the firewall for insulation, but i don't feel like it's enough. My bus has a small doghouse, not a removable one, it's meant for the larger engines that could have been optioned, right now it's lined with insulation just like the hood.

Any help is appreciated!

 

[Mr. Beefy]

It's a tough nut to crack.
The nature of the firewall makes it very challenging to insulate effectively against sound.
I think the best bet is to:

A) seal every penetration that allows any air to pass, if present. You'd think there wouldn't be any for all kinds of reasons, but there were with ours.
B.) If there are any sheet metal panels that aren't serving an irreplaceable purpose, nix 'em. With the bus vibration they're like cymbals lol.
C) Use CLD tiles on the interior-facing metal of the firewall where you can. You don't have to cover everything. It's my understanding that past 50% coverage the law of diminishing returns begins to come into play. The firewall is pretty thick so you won't get the same impact as you would on thinner sheet, but they still help so a small degree, particularly in the higher frequencies).
D) Since it's all but impossible to insulate effectively around everything behind the dash and penetrating the firewall, build out over it, using the densest / heaviest / most sound-absorbent material you can. Mass = sound absorption. In the spots where you can't build out (like where you sit), thick carpet and padding would probably help (something I have yet to do but have planned on). We used this area for the bulk of our electric components, but if you need storage for something 'non-rattly' and dense (like maybe tightly-packed clothes?), this might be a good place for something like that.

Research the topic of sound insulation to guide your decisions.

 

[Mr P]

I have some home HVAC "duct board" that I hope to repurpose for sound deadening under the hood and on the cowl.

 

[ewo1]

I helped a friend build her bus, FE amtran and boy was it LOUD (DT466e)!!!

We put in some sound deadener on the dog house cover. That helped a tiny bit.
we then built a second doghouse cover that fit on top of the OEM cover, sound deadener and insulation board.

It is larger and with some creativity you can make it functional too. To get to the engine you need to get it out of the way, yes, but other than pre-trip inspection (oil/water/belts) it really is not inconvenient for them. The results outweighed the extra work to remove it when needed.

Now the driver can calmly talk to the co-driver while going down the road. The new cover was designed so that not it acts as a drivers tables, drink cups, laptop....

Your gonna have to get creative but yes, you can reduce the noise for sure!

Here are some pics of what we did...

 

[FawnaFox]

It's a tough nut to crack.
The nature of the firewall makes it very challenging to insulate effectively against sound.
I think the best bet is to:

A) seal every penetration that allows any air to pass, if present. You'd think there wouldn't be any for all kinds of reasons, but there were with ours.
B.) If there are any sheet metal panels that aren't serving an irreplaceable purpose, nix 'em. With the bus vibration they're like cymbals lol.
C) Use CLD tiles on the interior-facing metal of the firewall where you can. You don't have to cover everything. It's my understanding that past 50% coverage the law of diminishing returns begins to come into play. The firewall is pretty thick so you won't get the same impact as you would on thinner sheet, but they still help so a small degree, particularly in the higher frequencies).
D) Since it's all but impossible to insulate effectively around everything behind the dash and penetrating the firewall, build out over it, using the densest / heaviest / most sound-absorbent material you can. Mass = sound absorption. In the spots where you can't build out (like where you sit), thick carpet and padding would probably help (something I have yet to do but have planned on). We used this area for the bulk of our electric components, but if you need storage for something 'non-rattly' and dense (like maybe tightly-packed clothes?), this might be a good place for something like that.

Research the topic of sound insulation to guide your decisions.

The front of my bus is an awkward space to try and build anything out on. I have a manual door and I don't plan to remove it. I'd have a hard time building anything in front of the wall because there is a fuse panel and a panel that needs to come off for the A/C relays.. which I need access to. What should I use to seal all the holes? Someone in the past tried this with that reflective tape, but I don't think it's doing much. I also need to have at least some airspace because I have the Hurri-Hot step well heaters that draw air from the side, and they work very well, so I have no intention to remove them.

My idea is to try and insulate the wall against the footwell with insulating tile, and possibly some in the engine bay, I plan to put velcro astroturf on the steps and landing area. I will also replace the ac relay panel cover with something that isn't metal.

 

[skydawg]

Probably not the answer you're looking for, but I wear foam ear plugs and noise cancelling bluetooth headphones to listen to music on any lengthy trips at the moment.

It's simple and it works for me, as I enjoy listening to music while I drive and no amount of sound suppression will probably be enough to do so without headphones, anyway. You do have to be careful with the noise cancelling settings as far as hearing sirens, etc.

Even if you continue with trying to dampen the noise source itself, it's a viable interim solution

 

[FawnaFox]

Probably not the answer you're looking for, but I wear foam ear plugs and noise cancelling bluetooth headphones to listen to music on any lengthy trips at the moment.

It's simple and it works for me, as I enjoy listening to music while I drive and no amount of sound suppression will probably be enough to do so without headphones, anyway. You do have to be careful with the noise cancelling settings as far as hearing sirens, etc.

Even if you continue with trying to dampen the noise source itself, it's a viable interim solution

I would just use headphones but I have multiple peoples, one of which is going full time with me. If they want to sleep while we're driving it would be hard. It doesn't have to be everything, just the harshness.

 

[nikitis]

I've been thinking on this for myself. I don't have any sort of dog house in the interrior cab, my dash area is completely open and roomy for legs etc. I only have the interrior of the firewall to use. It does have an insulation in my engine compartment already, but I haven't tried adding any to the side panels inside the firewall. I also have a new hole in the firewall I cut out so I could access the VPM back when my VPM was dead. They made it so you have to basically rip apart the whole inside dash in such a tight spot. They REALLY didn't want people accessing that computer. You had to take out the ECM to even get to it so it made sense to add a small access hole there. I planned to add a door that opens and closes there with some sound deadening on top of the door as well as a modification.

I guess what I'm saying is, other than a few kill-a-mat's on some inside panels I'm not sure what else I could do for my bus in this regards.

 

[FawnaFox]

I was gonna buy some 80 mil Killmats, how many square feet should I get? I was going to do the engine bay firewall, and as much as I can do inside, most likely the footwell and the little i can reach behind the dash.

 

[Mr. Beefy]

I was gonna buy some 80 mil Killmats, how many square feet should I get? I was going to do the engine bay firewall, and as much as I can do inside, most likely the footwell and the little i can reach behind the dash.

I think you'd probably need to figure that out for yourself. Measure everything you can cover, sum it up, and that will give you a close approximation of the max amount you'll need. From there you can decide what percentage of coverage you desire. Again, you can go with 100%, but it's generally accepted that past 50% you're getting into 'diminishing returns' territory. So at least 50%, and up from there depending on what your goals / budget allows.

Just FYI, I doubt this alone will make much difference. As part of a multi-point strategy, I think it's worthwhile, but if it's all you do, I fear you might be underwhelmed. As I said above, the thickness of the firewall makes CLD tiles far less world-changing than they are on thinner sheet metal. We did many things at once - including CLD tiles (~75% coverage excepting penetrations), so while we defiintely decreased noise transmitting through the firewall, exactly what contributed how much to that result I couldn't say.

If you do go this route, get a good roller, and put some real effort into adhering those tiles well (generally, working from the centers out to the edges). Their performance depends on it.

 

[FawnaFox]

I think you'd probably need to figure that out for yourself. Measure everything you can cover, sum it up, and that will give you a close approximation of the max amount you'll need. From there you can decide what percentage of coverage you desire. Again, you can go with 100%, but it's generally accepted that past 50% you're getting into 'diminishing returns' territory. So at least 50%, and up from there depending on what your goals / budget allows.

Just FYI, I doubt this alone will make much difference. As part of a multi-point strategy, I think it's worthwhile, but if it's all you do, I fear you might be underwhelmed. As I said above, the thickness of the firewall makes CLD tiles far less world-changing than they are on thinner sheet metal. We did many things at once - including CLD tiles (~75% coverage excepting penetrations), so while we defiintely decreased noise transmitting through the firewall, exactly what contributed how much to that result I couldn't say.

If you do go this route, get a good roller, and put some real effort into adhering those tiles well (generally, working from the centers out to the edges). Their performance depends on it.

My other problem I'm guessing is coming from all of the cheap plastic dash material flopping around, and the metal components. I'm going to try the aforementioned clothes box, although I have a place for my clothes planned already.. but a sleeping bag will do! I'll just have to make the box removable so I can access the AC relay box when the time comes.

Edit: I also am going to see if I can either secure or remove the heater intake, It rattles around a lot.. What I mean is that the step well heaters have an inlet vent on the side, I doubt the bus is airtight enough where If i remove that it's going to cause damage, that or I'll redirect the intake to intake from the top, as I sealed the end of the defroster chain. (for context, my bus has a vent at the end of the defroster line that acts as a small step well heater)

 

[Mr. Beefy]

I was just thinking of a way to kill 2 birds w/ one stone w/ the clothes suggestion. If that bird is already dead, I'm sure you could find another nesting somewhere

Could you post pics of your dash area (and/or you, @nikitis)? Maybe a few more sets of eyes on the problem might come up with a unique solution?

 

[FawnaFox]

I was just thinking of a way to kill 2 birds w/ one stone w/ the clothes suggestion. If that bird is already dead, I'm sure you could find another nesting somewhere

Could you post pics of your dash area (and/or you, @nikitis)? Maybe a few more sets of eyes on the problem might come up with a unique solution?

I will when I get home,

I did manage to get the heater intake re-routed and unnecessary metal removed and what needed to stay I made sure it was all tight.

 

[s2mikon]

Back in the early 90's my dad bought a p-30 step van with a BT3.9 to do a conversion on, and that thing was all aluminium and as noisy as a jackass in a tin barn. This is what we put down on the floor (2 layers) and 4 layers over the dog house. My dad bought my wife an upholstery sewing machine and she custom cut carpet and sewed it to the first layer for looks and we spray glued the other layer to it. It worked very nice at both heat and noise reduction. She left enough margin around the dog house cover we installed snaps to secure it. The floor carpet and padding was held down with trim screws around the edges. Automotive Carpet jute padding 40 OZ 36" wide by the yard (free shipping) | eBay

 

[FawnaFox]

I was just thinking of a way to kill 2 birds w/ one stone w/ the clothes suggestion. If that bird is already dead, I'm sure you could find another nesting somewhere

Could you post pics of your dash area (and/or you, @nikitis)? Maybe a few more sets of eyes on the problem might come up with a unique solution?

Here it is, it's 19 inches wide, 17 1/2 deep and 18 tall

 

[ewo1]

View attachment 2412915Here it is, it's 19 inches wide, 17 1/2 deep and 18 tall

covering that hole is not a big project.
custom cut plywood OR custom bent sheet metal. With either one you can line the inside with insulation board.

Your pic doesn't show well the top of that space. maybe squatting a bit and stepping back 2-3 feet we can get a better view.

That center console in front of the shifter, does it come out?

If so, I would start from that space, seal it up so air can't make it way in and then make the cover for the right side.

my .02c !

 

[FawnaFox]

covering that hole is not a big project.
custom cut plywood OR custom bent sheet metal. With either one you can line the inside with insulation board.

Your pic doesn't show well the top of that space. maybe squatting a bit and stepping back 2-3 feet we can get a better view.

That center console in front of the shifter, does it come out?

If so, I would start from that space, seal it up so air can't make it way in and then make the cover for the right side.

my .02c !

The console does not come out, It's this giant hunk of plastic that was meant for the busses with the automatic shifter... So taking it off would just expose the engine. The manual was a total afterthought.. Here is what the entire dash looks like. (I'm not at the bus right now so this is the picture I have) As you can see there is that plastic dash material.. and also the door rod, which I don't want to remove.

 

[Mr. Beefy]

I appreciate the pics. It's still hard for me to see exactly what could potentially be done there. You can't build out over the area behind the door actuator rod? Not enough clearance?

 

[FawnaFox]

I appreciate the pics. It's still hard for me to see exactly what could potentially be done there. You can't build out over the area behind the door actuator rod? Not enough clearance?

the clearance between the rod and the dash when the door is open is probably 5 inches, that tanish plastic I need access to because it is where the fuse panel is.

 

[Mr. Beefy]

So 5" is a lot. You could certainly build out over that if you felt so inclined. I'm assuming there's some way to maintain access to the fuse panel and still create some measure of sound barrier.

Don't know if it helps, but here's (link below) is what we did with your area (which admittedly is completely different from yours. Steel framework comprises a skeleton upon which wood panels are installed (via rivet nuts). Pretty much everything comes off with 4 bolts... 4 bolts to remove the battery box cover, 4 bolts to remove the inverter / electric area cover, 4 bolts to remove the upper dash, etc. So everything remains accessible.

Hit this link. Pretty much everything I'm talking about is on that page of our build thread (scroll up & down)...

Our dash build