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What’s the Difference Between Airborne and Structure-Borne Sound?

Property Noise Investigations 101: Diagnosing Airborne vs. Structure-Borne Sounds

Property Noise Investigations 101: Diagnosing Airborne vs. Structure-Borne Sounds

In noise‑sensitive buildings—multi‑residential towers, hotels, offices, and schools—the most common complaints are about noise from the neighboring space or the suite above. A resident hears voices through a party wall. An office tenant picks up their neighbor’s calls. A teacher hears chairs scraping in the classroom overhead.

Before an acoustical consultant like HGC can fix a noise problem, we first need to identify what kind of sound you’re dealing with: airborne or structure‑borne.

Airborne Noise and STC (Sound Transmission Class)

Airborne sound—such as speech, music, or HVAC noise—starts in the air and passes through or around separating assemblies like walls, ceilings, or doors. That’s why tenants might hear mid‑to‑high‑frequency sounds from next door: conversations, TV shows, or the hiss of an air diffuser fan.

In North America, airborne sound isolation is measured using the STC (Sound Transmission Class) rating. A higher STC value means a wall or floor‑ceiling assembly is better at reducing airborne sound transfer.

STC Rating Guide
STC Rating Perceived Sound Isolation Typical Building Performance
30–34 Very poor – Normal speech easily understood Lightweight or minimal partitions with very little mass or insulation. Often thin drywall and uninsulated cavities. Neighbour activity is clearly audible and privacy is very limited.
35–39 Poor – Normal speech heard and understood Basic wall construction with limited insulation or single stud framing. Everyday conversation, TV, and general activity remain clearly audible through the partition.
40–44 Fair – Loud speech can be understood Standard insulated stud walls with modest upgrades such as improved drywall or tighter construction, but without decoupling. Provides some noise reduction, but raised voices and televisions remain noticeable.
45–49 Fair – Loud speech heard but not understood Walls with insulation and some acoustic improvements such as heavier drywall. Normal speech becomes muffled but is still perceptible as background sound.
50–54 Good – Loud speech barely audible More robust assemblies using added mass or decoupling elements (e.g., resilient channels, staggered studs, or double drywall). Provides solid everyday privacy with only faint traces of louder activity.
55–59 Very good – Very loud sounds faint High performance partitions such as double stud walls or assemblies with multiple gypsum layers and dense insulation. Strong sense of acoustic separation; only very loud sounds transmit faintly.
60+ Excellent – Shouting is generally inaudible Premium acoustic assemblies with substantial mass and full decoupling. Neighbour noise is rarely heard, offering near complete privacy even under loud conditions.

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Building Code Comments Between Adjacent Dwellings

US (IBC) Requirement: Minimum STC‑50 (lab) or NNIC‑45 (similar to ASTC‑45, if tested in the field).

Canada (NBC) Requirement: Minimum ASTC‑47 (calculated or tested), or STC‑50 if meeting certain junction conditions outlined in the code.

Structure‑Borne Sound and IIC (Impact Insulation Class)

Structure‑borne sound, also known as impact noise, travels as vibration through the building’s structure—slabs, framing, or steel members—and re‑radiates as sound in the receiving space. It has a deeper, low‑frequency “thud” or “boom,” typical of footsteps, dropped objects, or mechanical vibration.

This performance is described by the IIC (Impact Insulation Class) rating. A higher number indicates better isolation against impact sounds.

IIC Rating Guide
IIC Rating Perceived Impact Noise Typical Building Performance
30–34 Very poor – Footsteps and impacts clearly heard; strong thumping Lightweight floors with rigid connections, hard surface finishes, and no acoustic underlay. Structure easily transmits activity noise.
35–39 Poor – Heavy footsteps and movement easily heard Basic floor systems with little isolation. Wood joist floors with minimal insulation and direct fastening between layers.
40–44 Fair – Footsteps noticeable; sharper impacts still heard Floors with thin integrated backing or factory applied acoustic layers on hard surface flooring. Provides modest improvement but walking noise remains evident.
45–49 Fair – Footsteps muted but still perceptible Floors with basic underlayments installed below (foam, thin rubber, cork composite). Reduces some impact noise but heavier steps and movement continue to transmit.
50–54 Good – Normal walking barely noticeable; heavier impacts faint Floors using reasonable floating underlayments that create partial decoupling. Everyday activity becomes much quieter, with only occasional thuds.
55–60 Very good – Heavy footsteps faint; strong sense of separation Floors incorporating a proper acoustic underlayment below the finished surface. Provides high impact isolation across a wide range of frequencies.
60+ Excellent – Even forceful impacts barely audible Carpeted floors or hard floors installed over floating concrete or floating plywood toppings, offering very high impact insulation and near complete isolation.

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Building Code Comments

US (IBC) Requirement: Minimum IIC‑50 (lab) or NISR‑45 (similar to AIIC‑45, if tested in the field).

Canada (NBC) Recommendation: IIC‑55 for bare floors (tested without a carpet).

STC vs. IIC: Why Both Matter

STC and IIC ratings are independent. A floor or wall assembly might block speech effectively (high STC) yet still transmit footfall vibration (low IIC), or vice versa.

Successful residential, hospitality, and mixed‑use projects in the US and Canada establish explicit design targets for both STC and IIC and ensure architectural detailing supports each.

At HGC, we often illustrate this distinction using a simple music box demonstration (see video below). When held in the air, the sound you hear is airborne. When it rests on a surface, the vibration transfers—creating structure‑borne sound. The tonal shift from “pitchy” to “boomy” mirrors how occupants perceive voices through walls versus footsteps through floors.

 

Author

Simon Edwards
Simon Edwards

Senior Acoustical Consultant

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