Principles of Soundproofing and Noise Control Techniques

Understanding Acoustical Soundproofing

Hearing Sensitivity and the Masking Effect

The human ear is not equally sensitive to all frequencies. The sensitivity observed for pure tones differs when dealing with complex sounds and noise, which are composed of several tones. This phenomenon is known as the 'masking effect'.

Defining Vibration and Resonant Sense

Vibration refers to the resonant sense of excitement that occurs by direct contact of the human body with a solid vibrating object. There is often not a clear separation between the sensation of sound and the sensation of vibration.

Reducing Noise Levels

Sound Level Reduction by Controlling Reverberation

Noise abatement strategies often prioritize reducing the sound power emitted at the source. However, in cases where source reduction is not feasible (e.g., gyms, restaurants), the primary possibility for reducing the noise level is by:

• Increasing the total sound absorption area.
• Reducing the reverberation time.

Acoustic Absorbent Materials

Acoustic absorbent materials are those that exhibit high coefficients of sound absorption across all or part of the audible frequency spectrum. The most important types include:

• Porous Materials: Materials with a skeleton structure (rigid or flexible) and internal pores (e.g., mineral wool, foam).
• Resonators: Systems using plates, panels, or sheets that vibrate on a cushion of air.

Noise Isolation Techniques

Airborne Sound Isolation (Air Gaps and Flanking Paths)

When a barrier is placed between two locations to achieve airborne sound insulation, noise transmission can occur through several paths:

• Direct Route: Through holes, gaps, and crevices in the barrier.
• Indirect Means (Flanking Paths): Through conduits and adjacent walls.

Simple Wall Insulation Requirements

Simple wall insulation consists of one or several layers of similar or different materials that are not independent when vibrating under the stimulus of a sound wave. Effective acoustic insulation for these walls requires the material to be:

• Sufficiently heavy (high mass).
• Weakly rigid (low stiffness).
• Airtight.

Multiple Wall Insulation (Mass-Spring Systems)

Multiple wall insulation functions as a mass-spring system, where thermal insulation or an air gap acts as the spring interposed between two sheets (masses). Sound insulation can be significantly improved by:

• Ensuring the two leaves are not equal in thickness, even if they are made of the same material.
• Using sheets of different materials, especially ensuring one of them is soft to bending (flexible).
• The ideal scenario involves providing both leaves with materials soft to flexion.

These multi-wall systems must be complemented by an absorbent element inside the air gap (e.g., glass wool). Crucially, the layers of a multi-wall system should have no rigid connection.

Impact Sound Insulation

Impact sound insulation addresses noise produced directly on the floor surface (e.g., footsteps), which is immediately transmitted to the room below or adjacent areas. To mitigate this:

• Elastic breaks must be introduced between the pavement and the structural floor.
• The best solution is constructing a floating floor built upon resilient layers, such as blankets or panels made of mineral fibers or elastic polymers.

Vibration Isolation (Active and Passive)

Vibration isolation aims to prevent unwanted transmission of mechanical energy. This can be achieved in two ways:

• Active Isolation: Preventing vibrations originating from a source (e.g., machinery, elevators) from being transmitted to the ground or attached structures.
• Passive Isolation: Preventing ground vibrations from being transmitted to a sensitive device.

Effective vibration isolation requires the presence of soft elements that reduce transmission, such as steel springs or specialized elastic materials.