Sound Insulation vs Absorption Explained | Essential Knowledge for Choosing Soundproof Rooms in Japan

“If I put absorption foam on the walls, will that stop sound from leaking?”

Many people ask this question, but the answer is “No.”

Soundproofing involves two distinct concepts: “sound insulation” and “sound absorption,” each serving completely different roles. Insulation prevents sound from escaping, while absorption improves indoor acoustics.

Choosing a soundproof room without understanding this difference leads to failures like “more sound leakage than expected” or “poor indoor acoustics.”

This article thoroughly explains the difference between sound insulation and absorption from a professional perspective.

Sound Insulation | Technology to “Reflect” Sound

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Definition of Sound Insulation

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Sound insulation is the technology of blocking sound and preventing it from escaping to the outside.

Sound travels as air vibrations. Insulation physically “reflects” or “blocks” these vibrations, preventing sound transmission to external spaces.

Principle of Sound Insulation:

• Uses heavy, high-density materials
• Reflects air vibrations
• Reduces sound transmission to outside

Sound Insulation Performance Indicator: “D-Value”

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Insulation performance is expressed by the “D-value” mentioned earlier.

Types of Sound Insulation Materials

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Materials used for insulation are fundamentally heavy and high-density.

Major Insulation Materials:

[!IMPORTANT] For insulation materials, “weight” is everything. Light materials cannot achieve high insulation performance.

Sound Absorption | Technology to “Absorb” Sound

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Definition of Sound Absorption

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Sound absorption is the technology of absorbing sound energy and reducing sound reflection.

Sound reflects within spaces, creating echoes (reverberation). Absorption reduces these reflections, creating a clear acoustic environment.

Principle of Sound Absorption:

• Uses porous (hole-filled) materials
• Converts sound energy to heat energy
• Suppresses sound reflection

Important Point: Absorption has almost no effect on preventing sound from escaping outside.

Absorption Coefficient Indicator

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Absorption performance is expressed by the “absorption coefficient (α),” ranging from 0 to 1.

Types of Sound Absorption Materials

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Absorption materials are fundamentally light and porous.

Major Absorption Materials:

| Urethane Foam | Sponge-like material | Recording studios, streaming booths | | Absorption Panels | Panels with textured surfaces | Wall decoration + function |

[!TIP] Absorption materials are fundamentally “light and porous.” Heavy materials are not suitable for absorption.

Insulation vs Absorption | Comparison Table

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Key Point: Insulation and absorption are different. Combining both creates the ideal soundproof environment.

Common Misconceptions and Correct Knowledge

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Misconception 1: “Adding Foam Stops Sound Leakage”

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× Misconception: Applying absorption foam (urethane foam, etc.) to walls prevents sound from escaping
○ Truth: Absorption materials only absorb sound; they don’t prevent leakage

Reason:

• Absorption materials are light and porous, allowing sound to pass through
• Insulation performance (D-value) is nearly zero
• Insulation materials are essential to prevent external leakage

Real Example:

• You may have seen YouTube streaming booths with walls covered in urethane foam. This is “to improve indoor sound quality,” not “to prevent sound leakage.”

Misconception 2: “Insulation Materials Alone Are Perfect”

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× Misconception: Just thickening walls with gypsum board or concrete is sufficient
○ Truth: Insulation materials alone create poor indoor acoustics

Reason:

• Insulation materials reflect sound, causing excessive indoor reflection
• For musical instruments or recording, echoes worsen sound quality
• Combining with absorption materials is essential for comfortable acoustics

Misconception 3: “Soundproof Curtains Are Enough”

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× Misconception: Installing soundproof curtains prevents sound leakage
○ Truth: Soundproof curtains primarily provide “absorption,” with limited insulation

Reason:

• Soundproof curtains are fabric-based and light, offering very low insulation
• They suppress high-frequency reflections (absorption) but low frequencies pass through
• Effect is limited to “slightly reducing neighbor noise”

Combining Insulation and Absorption in Soundproof Rooms

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Soundproof rooms are designed by combining insulation structures with absorption materials.

Standard Soundproof Room Structure

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Exterior (Insulation Layer):

1. Outer wall: Gypsum board + insulation sheet
2. Air gap: Blocks vibrations
3. Inner wall: Gypsum board

Interior (Absorption Layer): 4. Absorption material: Glass wool, rock wool 5. Interior finish: Absorption panels, wallpaper

This structure ensures:

• Insulation layer prevents external sound leakage
• Absorption layer optimizes indoor acoustics

Optimal Balance by Use Case

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Summary | Using Insulation and Absorption Appropriately

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Sound Insulation:

• Prevents sound from escaping outside
• Uses heavy, high-density materials
• Performance measured by D-value

Sound Absorption:

• Improves indoor acoustics
• Uses light, porous materials
• Performance measured by absorption coefficient

Ideal Soundproof Room Configuration:

• Exterior insulation layer (gypsum board + insulation sheet)
• Interior absorption layer (glass wool + absorption panels)

Final Advice:

1. Insulation is essential to “prevent sound leakage”
2. Absorption is essential to “improve indoor sound quality”
3. Combining both creates the ideal soundproof environment

Use this knowledge to choose the optimal soundproof room for your needs.

Related Articles

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• [Technical Guide]: The Truth About D-Values
• [Basics]: Soundproof Room Prices in Japan【2025 Latest】
• [Comparison]: Top Soundproof Room Manufacturers Compared