Sound Insulation of Shipping Container Houses

shipping-container-sound-insulation

Sound insulation of shipping container houses is one of the most important parameters characterizing habitable space (be it home, office, work area, etc..). We all know how annoying (if not aggravating) can be noise coming from busy streets or (in large buildings) – from neighboring apartments.

What is Sound?

Before we start battling the sound (noise) let’s start with some physical explication of what it really is and how it travels through the medium from its source to the receptor (our ear). With this knowledge, it will be easier to understand how we can reduce the intensity of unwanted sounds inside our container-based house.
Quoting from Wikipedia: Sound is a vibration that propagates as a wave of pressure (called acoustic wave), through a medium – gas, liquid or solid materials.
Once such wave reaches our hearing system, vibrations are converted into “nerve signals” that are processed by the brain to give us the perception of sound. The only limited frequency range of acoustic waves (roughly 20Hz to 20kHz) can be converted into “perception of sound” by our hearing system.
For the purpose of this article, whenever it makes sense, we will use the general term “noise” instead of sound, as it better describes the character of unwanted acoustic signals you want to battle.

Sound-Insulating Materials

In order to reduce the intensity of noise, an insulation barrier must reflect, absorb or scatter the energy of acoustic waves (vibrations). Soft, porous, rubbery materials have the capability to scatter acoustic signals and to attenuate their intensity by absorbing the energy of vibrations and converting it into heat. Higher the density of such materials (in other words the mass), the better their absorbing characteristics.

In contrast, rigid materials of (poly)-crystal structure (like metals, ceramic, rocks but also concrete, bricks, glass, etc) can partially reflect the noise, but unfortunately, they make a good transmission medium for acoustic waves, passing the majority of their energy through. How does it work in practice? An acoustic wave hits the metal wall, making it vibrate so the air on the opposite side of the wall can pick up vibrations and propagate further, eventually reaching our ears. It’s as simple as that. To make it even worse, such materials can “amplify” acoustic energy at frequencies matching their own natural vibrations. Such an effect known as acoustic resonance is probably well known to all city-dwellers experiencing vibrations of windows’ glass when exposed to noise generated by traffic.

Just for the completeness of the subject, let’s mention that sound (noise) cannot propagate in a vacuum (as there is no medium that can transfer “vibrations”).
Armed with this knowledge, it is obvious that shipping container houses need special means to achieve:

  • a) reasonably good noise barrier
    b) resonance (vibrations)-free operation.

Fortunately, popular thermal insulating materials like fiberglass, rock-wool, polyurethane (PU), and (expanded) polystyrene (EPS) due to their consistency are also good sound absorbers. But there is one serious caveat to this assumption:

Required thermal insulation is determined by the climate zone while the sound insulation is by the location (neighborhood) of the house.

For example, some container-based houses due to the friendly climate (let’s say coastal areas in Southern California) may not need any thermal insulation. However, when located in busy districts with exposure to a wide spectrum of city, industrial and neighborhood generated noise regardless of the favorable climate they will need noise insulation. In general, even the appropriate thermal insulation may not guarantee the required level of noise insulation.

Practically, it is difficult to make container houses soundproof, but frankly, such a goal is not the purpose of this article as we focus here on making container-based houses a comfortable place to live, but not a professional acoustic studio.

Typical Sound-Insulation of Container-based Houses

The main effort is to point out how the intensity of noise infiltrating the interior through the existing (or planned) thermal barrier can be further reduced (if needed). In most cases, the layer of thermal insulation barrier will do the job, because similar physical effects apply to the transmission of acoustic waves and heat. In other words, good thermal insulation will also act as good sound insulation (although the opposite is not always true). That means, fewer empty cavities or smaller sizes in the insulation barrier, lowering its acoustic “conductivity” (air gaps serve as a medium to transfer heat and sound). From this perspective, the spray-foam insulation, thanks to its ability to closely fill all walls’ cavities makes also a good noise insulation barrier, leaving behind in order of performance – blanket and then block insulation.

When adapting cargo shipping containers to habitable spaces you have the opportunity to design the layers of thermal insulation for dual-use (including noise insulation). Well planned effort will certainly bring expected effects. In contrast, Customized Container Houses already come with built-in thermal insulation designed for the given climate zone. In such cases, the level of noise insulation is the “bonus” you may have to accept. With the typical thickness of 50mm (about 2”), the sandwich panels used in customized shipping container houses reduce the exterior noise levels by some 30-to-35db (floor and roof) and 20-to-25db for walls. If needed, you can opt for thicker sandwich panels (typically coming also as 75mm (3”) and 100mm (4”) thick, rarely also as 125mm (5”) and 150mm (6”). Please note however that:

  • a) Increased thickness of sandwich panels eats interior space
    b) Sound (and thermal insulation) does not increase proportionally to the increased thickness of insulation. The total effect is limited by existing “heat” and “sound” bridges and weak spots (like windows, doors, vents, etc.). that’s why at some moment increasing the thickness of walls and ceiling insulation will not make any sense.

Extra Sound-Insulation Materials

In both cases (especially for Cargo Containers), you may improve the level of noise reduction by the following means:

  • a) MLV-type sound-dampening materials
    MLV (Mass-Loaded Vinyl is a flexible synthetic material made from Vinyl mixed with higher density particles (usual metals) in order to increase its mass. Higher mass combined with the “elasticity” of the vinyl offers improved noise-dampening property. MLVs are available in spools of thin (1/8”) sheets easy to install over the walls, ceilings, studs, pipes… They are odorless, non-toxic, and do not take up much interior space, (examples: TMS MLV, Soundsulation, Acoustimac SoundLock….).

Acoustimac-soundproofing-material-insulation

Acoustimac Soundproofing Material (Source: Acoustimac)

  • b. Acoustic Paint Coatings

Noise dampening paints (for example Super Soundproofing liquid compound “Noxudol 3101”), can be used on interior or exterior walls. However, in order to obtain any meaningful results, several consecutive coats must be applied to achieve an insulation layer of up to ¼ inch thick.

Naxudol-3101-soundproofing-liquid

                                                  Naxudol 3101 – soundproofing coating material.

Note:

The noise-dampening/attenuating properties of every insulation layer are frequency-dependent. In practice, low-pitched sounds (bass) will be usually less suppressed than high-pitched ones.

Resonance

The problem of potential resonance is a bit more complex and must be addressed by the design. In practice, it means that structure’s characteristic resonance frequency must be shifted by design up, outside of the spectrum of possible audible signals. It is especially important in an urban environment, where man-made noise and vibrations are omnipresent and may get amplified by container structures if falling within its frequency of resonance. In practice, like in traditional houses, the most likely “victims” of resonance will be windows.
Note that noise-dampening layers (especially closely adhering coatings like paints, spray-foam or glued MLVs) minimize the probability of resonance (in this case of the container’s steel box).

Technical Background

The dB (“decibel”) is a unit describing the ratio of two signals on a logarithmic scale. When applied in the domain of acoustics, it describes the intensity of the signal compared to total silence (dB) or the level of attenuation of the signal passing through the sound-insulation barrier. Note that “deci” (d) comes from the Latin word “Decimus” meaning one-tenth of something, while “bel” (B) is derived from the name of Abraham Bell. In this case, 1 dB represents one-tenth of the “bel”. Note that normal human speech represents an acoustic signal with a power of about 60dB and it is about 1,000 000 stronger than the smallest detectable signal by the human ear (faint whisper). By the same token, 20dB of acoustic insulation means that the intensity of the source signal is reduced 100 times.

Learn more about shipping container insulation here!

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