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Soundproofing Solutions

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Soundproofing Solutions
Wed May 20, 2009 1:36 am


I am initiating this topic in response to Dave’s (NormaJean) interest in effective sound isolation (see discussion thread about slide-out rooms). And since Dave is ready to install a floor into his Eagle 15, it may be best to touch on the pitfalls and folly when one discards soundproofing, including the prevention of resonance and critical vibration.

Proper sound isolation is essential. For example, you may find a converted bus or RV parked next to you that has an impressively quiet generator set. You can even have a normal conversation with the proud owner only a few feet away from it. Yet, when you are invited to go inside the coach, you will more often than not feel vibration and hear an annoying rumble, droning, or other subliminal noises. This is because the coach builder has noticeably designed and assembled a proper enclosure for the generator set, but failed to pay attention to thoroughly sound isolating the coach and especially the floor.

There are many materials one can use for sound isolation, namely Plywood, Acoustical Deck, Acoustical Foam, Acoustical Plaster, Acoustical Tile, Fibrous Board, Fibrous Plank, Fibrous Spay, Insulation (Loose), Laminated Glass, Lead Sheet, Petal Pans, Mineral Fiber, Sealants, Slats and Grills, etc.

For the floor in our coach, we decided to use a three layer arrangement, a sandwich of two layers of ¾” marine plywood with a layer of lead in-between.

Corollary:
When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation.

Plywood by itself is a laminate of several layers of wood veneer. It is commonly used as an underlayment of floors, as sheathing on studs or rafters, or as finished paneling on walls. Mainly because of its modest mass [approximately 0.6 kg/dm^3 (36 lb/ft^3)], specifically its mass per unit area, plywood is relatively ineffective as a sound attenuator. However, it is often adequate in combination with other materials or where high performance is not required. Thin plywood, if furred out from a solid wall, is a potent low frequency absorber. Specially detailed resonant absorbers, made of plywood, are sometimes used to “tune” special purpose rooms. At the higher frequencies, plywood is quite reflective.

Lead sheet, on the other hand, provides excellent attenuation per unit thickness because it is heavy [11.34 g/cm^3 (grams per cubic centimeter) or 0.40968 lb/in^3 (pounds per cubic inch)] or roughly 11kg/dm^3 or 700 lb/ft^3] and limp. Furthermore, lead is easily shaped to conform to irregularities, which avoid holes in barriers that must be tightly sealed.

In conclusion, the combination of plywood and lead sheet is widely used in high density housing, cruise ships, luxury yachts, sound studios and other noise-sensitive environments. Because of its high physical density, lead is the ideal material for reliable soundproofing solutions and that includes the floor of bus conversions!

Christian
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Re: Soundproofing Solutions
Wed May 20, 2009 6:58 am


Hey Christian , Welcome to the Board. Nice post on the insulation. I have a ? Our Bus was a finished product when we bought it and the pervious owner did a nice job , However like you mentioned when the gen is running you can notice it in the kitchen , not real loud but noticable. We have 3/4 inch decking already and tile in the kitchen . Any thought's on redoing the Generator compartment. Thanks Andy
Andy Wright 68 Silver Eagle Model 5 8V92 #7481
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Re: Soundproofing Solutions
Wed May 20, 2009 10:55 am


Thanks Christian! We were considering using lead in combination with the plywood. We do have a steel floor over the entire engine compartment for fire retardation. We will use t&g plywood and cut foam insulation wedged into the tubing forms that are above the steel over the engine compartment. Still thinking about the lead.

Dave
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Re: Soundproofing Solutions
Wed May 20, 2009 11:52 am


I used a somewhat different approach than Chris. I used DB plywood for the complete floor. This product is a 3/4" material with a sound decoupler (film) mounted in the center. It is a full 3/4" thick and is heaver than standard 3/4 plywood. Sound Down used to sell the film but installing it between two 3/8" sheats of plywood could be quite a challange. The DB is sold by Dick Wright at WRICO International for generator boxes.
I'm sure lead may do a better job on the sound but the DB ply seems to work very well with considerably less weight-something I had to be very careful of with a 102" wide 45' slideout bus with a S60.
I also used block foam insulation under the plywood to insulate and slightly help in the sound and heat transmission. The insulation is also important when using lead as the lead tends to hold the heat and re-radiate it into the bus when stopped.
My generator is mounted in the right rear corner of the bus with the same material used for its enclosure. With the generator running it is sometimes impossible for me to hear it running from the front living room while parked(of course I don't hear as well as I once did :lol: )
That being said, I'm currently building a small diesel powered dc generator that will use a combination plywood , felt and lead sandwiched together with foam sound material for hopefully the ultimate sound control. It must be quiet enough that the sound is undetectable from outside of the bus while running. Bob Belter uses this method on his VW powered generator and it is quite remarkable in the noise pressure reduction he achieves.
Lots of different ways to achieve the same end-isn't it fun!
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Re: Soundproofing Solutions
Wed May 20, 2009 1:59 pm


It sure is wonderful to have so many ideas! I'm learning a lot from reading what has been tried and tested.

Thanks for the contribution everyone!

~Paul~
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Re: Soundproofing Solutions
Wed May 20, 2009 3:27 pm


I used the same stuff Hal is talking about along with another 3/4 layer of MDF plywood and a sound proof insulation around my generator. Outside you can bearly hear my generator. Inside I did two things wrong and will correct on my 15. One I raised the generator up off the bay floor closer to the living floor and some noise is transfered to the living area. Two I ran the exhaust out through the roof. I made sure that the living aera was protected from heat and it is the outer most layer is the same temp as the surounding area. The problem is I forgot about noise and some is transfer into the living area (A small hum ). The 15 will have a final wrap of lead.

Wayne
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Re: Soundproofing Solutions
Thu May 21, 2009 11:24 am


Ahoy Eagles,

I've mentioned before my genset soundproofing box: 1/2" birch plywood, two layers of 30# roofing felt, one layer of 2lb/sqft roofer's lead, two layers of 30# roofing felt, one layer of 1/2" birch plywood. with similar treatment plus fibreglas pads in the labyrinth in/out ducts. It is quite effective, and low cost. Mufflers are two in-line cheap 1 1/2" oval mufflers, with a long glasspac return. ~~200 hours, and it's not plugged up yet. Cheap and fairly easy to change if they plug from low temps.

The air discharge from the dynamo is a very significant box heating issue. I take in ambient air to the dynamo, and I measured my dynamo temp out at 225f when under high load. Concerned, I called Gillette (A very good dynamo maker). No problem --- Limit is 350f !!!!!! I have since collected and seperately discharged my dynamo air, with some noise which I've not yet baffled.

Enjoy /s/ Bob
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Re: Soundproofing Solutions
Thu May 21, 2009 10:21 pm


Bob is on-the-mark when it comes to successfully building a generator enclosure without dipping too deeply into ones pocketbook.

The use of roofer’s felt and lead sheet amid two sheets of birch plywood can undoubtedly attenuate much of the sonic energy from a diesel-powered generator set -- keeping in mind that what we are discussing here does not relate to the absorption of sound within a material itself, since it is not of intrinsic interest. The natural properties and loading conditions of materials that can be used in bus conversions, however, is related to acoustic noise attenuation.

In your particular case, Andy, I’d like to suggest finding a more practical solution to your problem and tearing up your sub-flooring certainly doesn’t make sense. Unfortunately, this leaves you with annoying generator noises in your galley, which I believe is spawned by vibration and acoustic resonance and, therefore, the root cause of your problem.

Now, there are many ways to Rome…

All the same, I’d like to suggest the following approach (My lack of familiarity with formatting and pasting illustrations/pictures could possibly mess up this post. Yet, I’ll be brave and try it anyway):

Image

If you install your generator set onto a floating base supported by neoprene mounts, then you will significantly decouple resonance and other acoustic vibration from the structure of your coach.

Christian
Last edited by chrisber on Fri May 29, 2009 9:39 pm, edited 3 times in total.
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Re: Soundproofing Solutions
Sat May 23, 2009 6:15 am


Thanks for the info fella's ,Iam going to be in Ennis in july and hopefully I will beable to discuss this with some of you .
Andy Wright 68 Silver Eagle Model 5 8V92 #7481
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Re: Soundproofing Solutions
Sun Sep 06, 2009 1:13 am


Rusty, why was it a mistake to vent out the roof? I had intended to so the same thing.
I intend to properly soundproof the genset, but would like to do some basic sound proofing on one of the rooms. I am going to be about at the end of my budget when it comes to adding soundproofing, so I wont be able to use lead or anything else high cost. I am only going to carpet the floor so I figure that I will be able to go back and do that right later without too much reworking. The room will be set up for home theater so it is going to be to sound proofed and sound dampened for audio quality as well.
My question is what is the least expensive things that I can use, knowing that I am willing to go back and add more later as funds are available, without just throwing away what I put in now.
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