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I need the box to block at least 40 dB of sound. The interior size can be as large as a shoebox. I'm thinking about using a plastic box as a frame, and then pad 2 layers of Dynamat or (one layer on the inside, another on outside). And then I want to add 2 layers of styrofoam padding, on top of the Dynamat. And then finally, I plan to add 1 inch thick of acoustic foam to the interior.

Similar thing would be done on the flat surface that the box would be place on, so all 6 sides would have 2 layers of Dynamat, and 2 layers of styrofoam, and 1 layer of acoustic foam. Do you think 40 db is possible?

I'm not sure about using plastic box as the frame. A webpage I looked at recommended particle boards but those are too heavy for the application.

Thanks

Edit: Thanks. It's valuable knowing that mass is important for insulation. Due to various reasons, I have decided to use 2 layers of acrylic (each being 12mm thick), separated by a layer of acoustic foam for absorption. Is it critical that the absorption foam cover all the areas between the 2 insulation layers?

The box's purpose is to cover up a device from outside noise. It will need to be frequently removed so weight is an issue. For that reason, I have decided to put the device inside a 5 sided box (with the opening facing the top), and just have a flat cover that user can remove easily, rather than removing the whole box. Air tightness has been brought up as a key, so I wonder if it's acceptable to just have a layer of the foam under the cover which would create a seal with the walls of the box due to the weight of the cover itself. Or should I use some sort of rubber seal like they use for doors (that will create a more "airtight" seal).

Thanks

  • What application? Weight is an essential component of damping, that's why MDF is often recommended. – Hobbes Aug 5 at 15:48
  • So a heavy steel box 1/2 inch thick would work great? The application is to to isolate factory noise from the inside of the chamber. Inside the chamber there would be a noise measurement application for the product. – user173729 Aug 5 at 21:15
  • For your measurement, what is the lowest frequency of interest? – Chris K8NVH Aug 6 at 11:14
  • I can ignore frequencies under 1k and above 8k – user173729 Aug 6 at 13:15
  • Lots of foam earplugs can block 30db of sound and they are not heavy and they are only "1 layer" of absorption. How do you explain that? – user173729 Aug 6 at 13:23
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Understand that sound-proofing is a frequency-dependent exercise. You can't simply quote an arbitrary decibel number without understanding what properties different materials have at different frequencies.

Frequency absorption is usually measured at 125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz and 4000Hz.

Different materials will have different absorption coefficients at different frequencies.

Before attempting to pose this question, understand what frequencies you want to work at, and then find materials that have the appropriate absorption coefficients at those frequencies.

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Soundproofing consists of several elements:

  1. absorption: this stops sound from bouncing around inside the space you're working on. Acoustic foam is an absorption material.

  2. insulation (soundproofing): this blocks sound from entering the space.

Styrofoam is not a good material for soundproofing, because it doesn't work for either of these.

Dynamat is an insulation material: it reduces wall vibration, so sounds don't transmit through the wall. It is often used on thin sheet walls (steel, plastic). MDF is another insulation material: a thick sheet of MDF is very vibration-resistant.

I would try this:

  • build a box from 18-24 mm MDF. Make sure the lid closes tightly and create a labyrinth seal.
  • Cover the box on the inside and outside with absorption material
  • You need to make it difficult for vibrations from the floor or wall to get into the box. Don't attach the box to the wall or floor with hard mountings. If you want to place the box on the floor: place the box on spikes (as used for high-end loudspeakers), not directly on the floor. Rubber mountings or bungee cords are also possible.
  • use 2 insulation layers, separated by an absorption layer (this is often overkill but 40 dB is Not Easy);

Even then, 40 dB is a difficult target.

more resources:

soundproofing basics

  • Thanks for the additions. I've used acoustic foam in the past: silentcoat.co.uk/collections/sound-absorber This is a lot heavier than the foam sheet you can get in the DIY store. It works well in my application (speakers installed in a car door), don't know how well it would do for soundproofing. – Hobbes Aug 6 at 12:14
  • Is polyester foam a good material for absorption? – user173729 Aug 6 at 13:20
  • It depends. You need to find the sound absorption coefficient specs of your foam to judge how efficient it will be. – Hobbes Aug 6 at 13:27
  • Also, can I use solid wood rather than MDF? – user173729 Aug 6 at 13:27
  • MDF is often used (e.g. for loudspeaker enclosures) because it is more resistant to vibration than most woods. – Hobbes Aug 6 at 13:41
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In addition to the valuable and relevant information provided by @Hobbes and @Mark, a 35mm thick plastic box should provide about 40dB attenuation at 1kHz and much more than 40dB at 8kHz provided there is an air-tight seal and there are no odd vibration phenomena present. There is software available, such as Insul, which can verify this.

You mentioned Particle board is too heavy for this application

Be advised, you need mass here, regardless of the material. If you choose a plastic that is twice as heavy as a "traditional" plastic you only need 17.5mm thickness. Particle board, being heavier, can be thinner. To estimate your required thickness, consider that plastic generally has Specific Gravity =1 and scale thickness as appropriate for the density of whatever material you prefer.

If you use 2 insulation layers separated by an absorbing layer, each insulation layer can be thinner than half the "required" thickness, but the amount depends on how good the absorber is. Again, there is software which can quantify this. The absorber does not have to completely cover all of the area, leaving a few --or many-- spots uncovered may be barely noticeable. An extreme example is acoustic double pane windows; an absorber is placed only around the edges and it still does a reasonable job of absorbing.

It is also worth remembering that these sorts of enclosures never quite live up to their theoretical expectations. For example, there may be penetrations for sensor wires; those will allow noise to sneak in (caulk all of those with acoustic caulk, don't just stuff foam into the gap and call it good). The door and hinges will be less-than-perfect. The enclosure might vibrate. The foam-lined cover gasket might be imperfect or deteriorate. The defense against this is to overdesign a little or perhaps just be prepared to modify later; you will learn a lot from this exercise!

One modification to keep in your "back pocket" (so to speak) is the possibility of a thin steel shield with 10mm acoustic foam glued to it (spray glue on the steel, not the foam so it does not seep into the foam); if one side seems to be allowing too much sound in, just adhere the foam side to your box and that side will now have a 2-layer barrier. When using a middle absorption layer like this, it is best for the absorber to be loose --ideally a small air gap between the absorber and each wall-- but that is often difficult to realize in practice.

Or perhaps be ready to duct-tape the door seams.

You mentioned using a (crushed) acoustic foam as a gasket for the lid. @Hobbes already mentioned a labyrinth seal; if you are careful with that, the gasket will be less important. And if it turns out to be "not quite good enough" you can always place the aforementioned "steel-foam second barrier" on top.

If you need the enclosure to properly work immediately and without further mods then some serious theoretical work would be appropriate. But if you can measure and modify and interact with it, this can be a fun and straightforward project. Please consider posting back here with your finished product, perhaps as an answer, as many of us would find that both interesting and informative.

Good luck on your project!

  • thanks for the helpful response. I edited my post with some follow up questions. Hopefully you can answer. Thanks – user173729 Aug 7 at 14:39

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