Technical Guides to Soundproofing – Understanding Acoustic Science

Soundproofing vs Sound Absorption: The Critical Difference

One of the most common misconceptions in acoustic treatment is confusing soundproofing with sound absorption. While both play important roles in acoustic design, they serve fundamentally different purposes and use completely different materials and techniques.

What is Soundproofing?

Soundproofing (also called sound insulation or noise reduction) is the process of preventing sound from transmitting through walls, floors, and ceilings between different spaces. The goal is to stop airborne and impact noise from travelling from one room to another.

Soundproofing relies on two key principles:

• Mass: Adding dense, heavy materials that block sound waves. Products like SilentBoard ProMax 30mm panels provide high-mass acoustic insulation that prevents sound transmission through walls.
• Isolation (Decoupling): Creating physical separation between surfaces to prevent vibration transfer. Resilient sound isolation clips mechanically decouple plasterboard from wall studs, breaking the path for structure-borne noise.

Effective soundproofing systems combine both mass and isolation. For example, a high-performance wall assembly might include acoustic mineral wool cavity insulation, resilient clips and channels, and multiple layers of high-density plasterboard.

What is Sound Absorption?

Sound absorption is the process of reducing echo, reverberation, and reflected sound within a room. Absorption materials don't stop sound from leaving a space—they improve the acoustic quality inside the room by preventing sound waves from bouncing off hard surfaces.

Sound absorption uses porous, fibrous materials that trap sound energy and convert it to tiny amounts of heat through friction. Acoustic mineral wool slabs are excellent absorbers when exposed to the room (not hidden in cavities), as are acoustic foam panels, fabric-wrapped absorbers, and specialized acoustic ceiling tiles.

When to Use Each Approach

Key Insight: If you can hear your neighbours or they can hear you, you need soundproofing (mass + isolation), not absorption. Acoustic foam on walls will not stop noise transmission—it only reduces echo within your room.

Understanding the Decibel Scale in Acoustic Design

The decibel (dB) is the standard unit for measuring sound intensity, but it's not a linear scale—it's logarithmic. This means small numerical changes represent significant real-world differences in perceived loudness.

How the Decibel Scale Works

• +3 dB: Barely perceptible change (requires careful listening to notice)
• +10 dB: Perceived as roughly twice as loud
• -10 dB: Perceived as roughly half as loud
• -20 dB: Perceived as approximately one-quarter as loud

This logarithmic nature is crucial when evaluating soundproofing performance. A wall system that achieves 55 dB reduction is not "slightly better" than one achieving 45 dB—it's dramatically more effective, blocking approximately 10 times more sound energy.

Common Sound Levels (Reference)

• 30 dB: Whisper, quiet library
• 50 dB: Normal conversation, moderate rainfall
• 70 dB: Vacuum cleaner, busy traffic
• 85 dB: Heavy traffic, power tools (hearing protection recommended)
• 100 dB: Nightclub, motorcycle
• 120 dB: Rock concert, chainsaw (pain threshold)

Acoustic Performance Ratings

In the UK, soundproofing performance is measured using two key metrics:

• Rw (Weighted Sound Reduction Index): Measures airborne sound insulation through walls and floors. Higher numbers = better performance. Building Regulations Part E typically requires minimum Rw 45 dB for separating walls between dwellings.
• Ln,w (Weighted Impact Sound Pressure Level): Measures impact noise (footsteps, dropped objects) through floors. Lower numbers = better performance. Part E typically requires maximum Ln,w 62 dB for separating floors.

Practical Example: If your neighbour's TV measures 75 dB in their flat, and your separating wall achieves Rw 50 dB, you would hear approximately 25 dB in your flat—roughly equivalent to a whisper or rustling leaves. This demonstrates why high-performance soundproofing systems are essential for acoustic comfort in multi-occupancy buildings.

What is Flanking Transmission & Why It Matters

Flanking transmission (also called flanking noise or indirect sound transmission) occurs when sound bypasses your primary soundproofing barrier by travelling through connected building elements—walls, floors, ceilings, pipes, ducts, or structural connections.

Common Flanking Paths

Even with a perfectly soundproofed separating wall, noise can still reach the adjacent room through:

• Solid floor/ceiling connections: Sound travels through the floor slab, into the separating wall, and radiates into the next room
• Cavity wall ties: Rigid ties connecting inner and outer wall leaves create vibration bridges
• Structural columns and beams: Sound travels through the building's structural frame
• Service penetrations: Pipes, electrical conduits, and HVAC ducts that pass through walls
• Back-to-back electrical boxes: Outlets or switches on opposite sides of a wall create acoustic weak points
• Perimeter gaps: Unsealed junctions between walls, floors, and ceilings

Controlling Flanking Transmission

Effective flanking control requires a systematic approach:

1. Structural Isolation: Use SilentCloud isolation strips under wall tracks and at floor/wall junctions to break vibration paths
2. Resilient Mounting: Install resilient clips and channels to mechanically decouple plasterboard from structure
3. Acoustic Sealing: Seal all perimeter gaps with acoustic sealant (not standard decorator's caulk)
4. Service Isolation: Wrap pipes in acoustic lagging, use flexible connections, avoid back-to-back penetrations
5. Cavity Barriers: Install mineral wool in wall cavities to absorb airborne sound within the cavity space

Critical Point: Flanking transmission is why DIY soundproofing often fails. You might invest in expensive acoustic plasterboard and high-density insulation, but if flanking paths aren't addressed, sound will simply travel around your barrier. This is particularly important for Part E acoustic testing, where flanking failures are a common cause of test failures.

Design Considerations

When specifying soundproofing systems, always consider the complete acoustic envelope:

• Extend soundproofing treatments to structural soffits and floor slabs
• Maintain continuous isolation layers at all junctions
• Specify acoustic-grade sealants and gaskets at penetrations
• Coordinate with MEP (mechanical, electrical, plumbing) trades to minimize penetrations
• Consider pre-construction acoustic modelling for complex projects

Resilient Clips vs Resilient Bars: Performance Comparison

Both resilient clips and resilient bars aim to achieve the same goal—acoustic decoupling—but they use fundamentally different mechanical approaches with significantly different performance outcomes.

How Resilient Bars Work

Resilient bars (also called furring channels or hat channels) are long metal channels that run horizontally across wall studs or ceiling joists. The plasterboard is screwed directly to these bars, which are designed to flex slightly and absorb vibration.

Limitations:

• The bar maintains continuous contact with the structure along its entire length
• Vibrations can still transmit through the bar, especially at screw fixing points
• Performance degrades if screws are over-tightened or penetrate through to studs
• Limited low-frequency isolation (bass sounds still transmit effectively)
• Typical performance improvement: 5-10 dB over direct-fixed plasterboard

How Resilient Clips Work

Resilient sound isolation clips use a completely different approach. Each clip contains a rubber or neoprene isolator that creates a true mechanical break between the structure and the plasterboard. The clips attach to studs/joists, then metal furring channels slot into the clips without touching the structure.

Advantages:

• Complete mechanical decoupling—no rigid connection between plasterboard and structure
• Rubber isolators absorb vibration across a wide frequency range
• Excellent low-frequency performance (critical for bass, footsteps, machinery)
• More forgiving installation—less risk of acoustic short-circuits
• Typical performance improvement: 15-25 dB over direct-fixed plasterboard
• Can achieve Rw 60+ dB when combined with proper mass and cavity absorption

Performance Comparison

When to Specify Each System

Resilient Bars: Suitable for light-duty acoustic improvement in single-family homes where Part E compliance isn't required, or as a budget-conscious upgrade over direct-fixed plasterboard.

Resilient Clips: Essential for Part E compliance projects, multi-occupancy buildings, home cinemas, recording studios, or any application where serious noise control is required. The higher upfront cost is offset by superior performance and reduced risk of acoustic test failures.

Professional Recommendation: For new-build flats, conversions, or HMOs requiring Part E acoustic testing, always specify resilient clip systems. The performance difference is substantial, and the cost of failing an acoustic test (remedial work, project delays, re-testing fees) far exceeds the incremental cost of clips over bars.

Key Takeaways & Next Steps

Recommended Next Steps

Now that you understand the fundamental principles of acoustic science, you can make informed decisions about your soundproofing project:

1. Assess Your Requirements: Determine whether you need Part E compliance, what noise sources you're controlling, and what performance level is required
2. Specify Complete Systems: Don't just buy acoustic plasterboard—design a complete assembly with mass, isolation, cavity treatment, and sealing
3. Choose Quality Products: Explore our range of high-performance soundproofing materials designed for UK building standards
4. Plan for Flanking Control: Budget for isolation strips, acoustic sealant, and proper detailing at junctions
5. Consider Professional Testing: For Part E projects, factor in pre-completion acoustic testing to verify performance before final sign-off

Related Technical Resources

• Building Regulations Part E Explained: Complete Compliance Guide
• Part E Acoustic Testing Requirements: What You Need to Know
• Floating Floor Installation Guide: Acoustic Underlayment
• Acoustic Performance Standards in UK Construction