Airborne vs Structure-Borne Sound: Understanding Two Noise Paths

Not all noise reaches a room the same way. Voices from the next office, music from a neighbouring apartment and traffic outside travel through the air before they strike a wall. Footsteps from the floor above, a pump in a plant room or a lift motor send vibration straight into the building structure, which carries it to rooms far from the source.

These are the two fundamental noise paths in buildings: airborne sound and structure-borne sound. They behave differently, are measured differently and are controlled with different solutions. Treating one as if it were the other is one of the most common reasons acoustic upgrades fail.

For residential, hospitality, commercial, healthcare, data centre and industrial projects in Saudi Arabia, AcouStick addresses both paths with KSA-manufactured barriers, isolated wall and ceiling systems, acoustic doors, floating floors, structural isolation mounts, enclosures and attenuators.

This guide explains the difference between airborne and structure-borne sound, impact sound and flanking, how each is measured, how to tell which one you have, and how each is controlled.

What Is Airborne Sound?

Airborne sound starts as sound waves in the air. When those waves strike a wall, floor, ceiling, door or window, they set the element vibrating, and the vibrating element radiates sound into the space on the other side.

Typical airborne sources include:

• Speech and conversation

• Television, music and home cinema

• Traffic, aircraft and outdoor activity

• Machinery noise radiated into the air

• Noise travelling through ducts and openings

ISO 12354-1, the international standard for estimating airborne sound insulation between rooms, describes direct transmission as sound incident on a separating element that is then radiated directly by that element — or that passes through openings in it.

What Is Structure-Borne Sound?

Structure-borne sound starts as vibration in the building itself. A University of Liverpool review by Gibbs and Villot describes structure-borne sound as generated by vibrating or impacting sources that are in contact with building elements; the vibration travels through the structure and radiates sound into adjacent and even non-adjacent spaces.

Typical structure-borne sources include:

• Pumps, fans, compressors and chillers

• Lifts and escalators

• Pipework and water services

• Washing machines and other appliances

• Gym equipment and dropped weights

• Footsteps and moving furniture (impact sound)

Gibbs and Villot note that a single installation can have multiple contacts with the structure, feeding several building elements at once — one reason structure-borne problems can be hard to trace and control.

Impact Sound: A Special Case of Structure-Borne Sound

Impact sound is structure-borne sound caused by objects striking a floor or other element — footsteps, dropped objects, chairs scraping. It is measured separately from airborne sound:

• ASTM E492 — laboratory impact test using a standard tapping machine with five hammers; result classified as Impact Insulation Class (IIC) under ASTM E989 — higher is better.

• ASTM E1007 — the field version of the same test.

• Code example — the 2021 International Building Code, Section 1206.3, requires IIC 50 (laboratory) or a field rating of 45 for floor-ceiling assemblies between dwelling units.

• International ratings — impact is reported as a level, where lower is better; England's Approved Document E sets a maximum L'nT,w of 62 dB for floors in purpose-built dwellings.

Flanking: When Sound Takes the Long Way Round

Flanking transmission is sound that bypasses the main separating element. ISO 12354-1 defines it as transmission of sound energy from an excited element in the source room to a receiving room via the building structure, and labels the paths:

• Dd (direct) — sound enters via: Separating element; sound radiates from: Separating element.

• Df — sound enters via: Separating element; sound radiates from: Flanking element in receiving room.

• Fd — sound enters via: Flanking element in source room; sound radiates from: Separating element.

• Ff — sound enters via: Flanking element in source room; sound radiates from: Flanking element in receiving room.

The standard also identifies indirect airborne transmission through separate air paths such as ventilation systems and corridors. NRC Canada research warns that flanking along ties, floors, ceilings and abutting walls can seriously impair the effectiveness of a wall's insulation.

Key point: airborne sound in one room can become structure-borne sound in the building, and then airborne sound again in another room. The source type and the path are not always the same.

Airborne vs Structure-Borne Sound: Comparison

Airborne Sound

• Starts as — Sound waves in air

• Typical sources — Speech, music, TV, traffic

• How it travels — Through air, then through elements and openings

• Reach — Mainly adjacent spaces

• Laboratory ratings — STC (ASTM E90/E413); Rw (ISO 717-1)

• Main controls — Mass, decoupled leaves, damping, sealing, acoustic doors and glazing

• Common mistake — Ignoring doors, gaps and ducts

Structure-Borne Sound

• Starts as — Vibration or impact in the structure

• Typical sources — Pumps, fans, lifts, pipes, footsteps, gym equipment

• How it travels — Through slabs, walls, columns, pipes and ducts

• Reach — Can reach distant, non-adjacent rooms

• Laboratory ratings — IIC (ASTM E492/E989); impact levels such as Ln,w

• Main controls — Vibration isolation at source, floating floors, resilient mounts and connections

• Common mistake — Adding mass to walls instead of isolating the source

How Airborne Sound Is Measured and Rated

Airborne sound insulation is measured as transmission loss in the laboratory to ASTM E90 and rated as STC under ASTM E413, or measured under ISO 10140 and rated as Rw under ISO 717-1. Riverbank Acoustical Laboratories describes STC 50–55 as typical minimum code performance for shared walls and STC 65+ as the range where most sounds are virtually inaudible. In completed buildings, field ratings such as DnT,w apply; Approved Document E, for example, requires at least 45 dB DnT,w + Ctr for walls between purpose-built dwellings.

How to Tell Which Path You Have

1. Identify the source — is it a voice or loudspeaker (airborne), or a machine, pipe, lift or footstep (structure-borne)?

2. Check where it is heard — noise only next to the source room suggests airborne; noise heard in distant rooms suggests structure-borne.

3. Feel for vibration — vibration felt in floors, walls or fixtures points to structure-borne transmission.

4. Listen to the character — thumps and low-frequency rumble are typical of impact and machinery; speech and music content suggests airborne.

5. Switch sources on and off — confirm which equipment is responsible.

6. Look for openings and ducts — indirect airborne paths through ventilation and corridors are easily missed.

7. Measure where needed — sound level, frequency and vibration measurements remove guesswork.

How Airborne Sound Is Controlled

Mass

The mass law (Acoustical Society of America terminology): doubling mass per unit area adds 6 dB of transmission loss. NRC Canada reports a sealed concrete block wall reaches about STC 45 to 55.

Decoupled Leaves

Adding gypsum board on furring or studs set away from a block wall can greatly improve insulation (NRC Canada), with attention to the mass-air-mass resonance around 100 Hz.

Damping

Viscoelastic layers reduce vibration in board assemblies (Davy, Phillips and Pearse, Applied Acoustics, 2015).

Sealing and Openings

Acoustic doors, glazing, sealed penetrations and attenuated ducts close the weak points.

How Structure-Borne Sound Is Controlled

Isolate at the Source

Vibration isolators and mounts reduce the vibration passed into the structure. Gibbs and Villot note that isolator benefit depends on the dynamic properties of the source, the isolator and the supporting structure, and that isolators are most effective on stiff structures.

Design the Natural Frequency

Vibration isolation theory (E-A-R Specialty Composites) shows transmissibility only falls below 1 above about 1.4 times the system's natural frequency; below that, vibration is amplified.

Floating Floors

A heavy topping on a resilient layer isolates impact sound and equipment vibration. A 2013 Journal of Sound and Vibration study found low-frequency performance depends on resonances in the real building, not only laboratory models.

Resilient Connections for Services

Flexible connectors and resilient supports stop pipes and ducts carrying vibration into the structure.

Control Flanking

Isolate floating floors and walls at their perimeters and avoid rigid bridges between isolated and structural elements.

Typical Problems and Solutions

• Voices heard through an office wall — main path: Airborne; typical solution direction: Upgraded partition, sealed head and penetrations, acoustic door.

• TV or music from a neighbouring apartment — main path: Airborne (plus flanking); typical solution direction: Decoupled wall lining, sealing, flanking control.

• Footsteps from the floor above — main path: Structure-borne (impact); typical solution direction: Floating floor or resilient underlay above; isolated ceiling below.

• Pump or chiller hum in rooms far away — main path: Structure-borne; typical solution direction: Vibration isolators, inertia bases, flexible connectors.

• Gym weights heard in hotel rooms — main path: Structure-borne (impact); typical solution direction: Heavy-duty floating floor in the gym.

• Noise through ventilation grilles — main path: Indirect airborne; typical solution direction: Duct attenuators, lined ducts, cross-talk silencers.

• Generator noise at the boundary — main path: Airborne (plus vibration); typical solution direction: Enclosure, louvers, exhaust silencer, isolation mounts.

Airborne and Structure-Borne Noise in Saudi Buildings

Modern Saudi developments often combine residential, hospitality, retail, office and plant spaces in the same building, with large rooftop and basement mechanical installations, gyms and amenities near sensitive rooms, and lightweight internal partitions. Both noise paths are therefore common — often in the same project.

Identifying the path early — and coordinating structural loads for floating floors, plant isolation and ceiling voids — avoids expensive retrofits after occupation.

Common Mistakes

Mistake 1: Adding Mass for a Vibration Problem

Heavier walls do little if the noise enters through the slab from a machine.

Mistake 2: Treating Only the Ceiling Below for Footsteps

Impact noise is usually best controlled at the floor where it is generated.

Mistake 3: Ignoring Flanking

A high-STC wall can be bypassed by floors, ceilings, abutting walls and ducts.

Mistake 4: Rigid Pipe and Duct Connections

Unisolated services carry vibration through the building.

Mistake 5: Bridging Isolation

A single rigid fixing can short-circuit a floating floor or isolated wall.

Mistake 6: Using Acoustic Panels for Transmission

Absorption controls sound inside a room, not transmission through it.

Checklist

• Has the noise source been identified?

• Is the path airborne, structure-borne, impact or indirect through ducts?

• Is the noise heard only next door, or in distant rooms too?

• Is vibration felt in floors or walls?

• Have flanking paths been considered?

• Are services isolated with flexible connections?

• Are floating and isolated elements free of rigid bridges?

• Will performance be measured after installation?

How AcouStick Addresses Both Noise Paths

AcouStick's engineering process reviews drawings and noise or vibration requirements, then models, specifies and supports installation of the right combination of systems from its KSA-manufactured range:

• Airborne — mass loaded vinyl, damping products and barriers; floating walls and ceilings; acoustic doors and windows

• Structure-borne — floating floor systems and mounts, resilient wall ties, isolators, acoustic underlay

• Indirect airborne — attenuators and acoustic louvers

• Source containment — acoustic enclosures and booths

Frequently Asked Questions

1. What is the difference between airborne and structure-borne sound?

Airborne sound starts as sound waves in the air and passes through building elements. Structure-borne sound starts as vibration or impact in the structure and travels through it before radiating as sound.

2. Is footstep noise airborne or structure-borne?

Footsteps are impact sound — a type of structure-borne sound — measured using tapping-machine tests such as ASTM E492.

3. What is flanking transmission?

Sound that bypasses the main separating element through adjoining floors, walls, ceilings or ducts.

4. Can a soundproof wall stop structure-borne noise?

Not on its own. Structure-borne noise usually needs isolation at the source or a floating construction.

5. How is impact sound rated?

By IIC (higher is better) under ASTM E492/E989, or by impact sound levels such as Ln,w or L'nT,w (lower is better) internationally.

6. How do I know which noise path I have?

Identify the source, check where it is heard, feel for vibration, switch equipment on and off, and measure where necessary.

Final Takeaway

Airborne and structure-borne sound are two different noise paths that need two different strategies. Airborne sound is controlled with mass, decoupled leaves, damping and sealing. Structure-borne sound is controlled by isolating vibration at the source and breaking rigid paths through the structure. Identifying the path first is what makes the solution work.

Not sure which noise path is affecting your Saudi project? Share your drawings and noise concerns with AcouStick for an engineering review.

If noise is reaching your rooms through the air, the structure or both, AcouStick can help you identify the path and specify the right isolation.

Email: info@acoustick.co | Call: +966 51 130 809 | Web: www.acoustick.co

REFERENCES

• International Organization for Standardization. ISO 12354-1:2017, "Building acoustics — Estimation of acoustic performance of buildings from the performance of elements — Part 1: Airborne sound insulation between rooms" (preview edition, iTeh Standards).

• Gibbs, B.M., & Villot, M. "Structure-borne sound in buildings: Advances in measurement and prediction methods." University of Liverpool Repository (author version): livrepository.liverpool.ac.uk/3077422

• HM Government. Approved Document E, "Resistance to the passage of sound," 2003 edition incorporating 2004, 2010, 2013 and 2015 amendments (England) — Section 0 performance tables: assets.publishing.service.gov.uk

• Warnock, A.C.C. "Controlling Sound Transmission through Concrete Block Walls." Construction Technology Update No. 13, National Research Council Canada, 1998: publications-cnrc.canada.ca

• Riverbank Acoustical Laboratories. "Understanding Sound Transmission Class (STC) Ratings and Testing" — ASTM E90 / ASTM E413 procedure and STC ranges: riverbankacoustics.com

• International Organization for Standardization. ISO 717-1:2013, "Acoustics — Rating of sound insulation in buildings and of building elements — Part 1: Airborne sound insulation" (preview edition, iTeh Standards).

• Acoustical Society of America Standards. Terminology entry "mass-law sound transmission loss": asastandards.org/terms/mass-law-sound-transmission-loss

• Davy, J.L., Phillips, T.J., & Pearse, J.R. "The damping of gypsum plaster board wooden stud cavity walls." Applied Acoustics, Vol. 88, 2015, pp. 52–56 (ScienceDirect).

• Masterson, P.A. "The Basics of Vibration Isolation Using Elastomeric Materials." E-A-R Specialty Composites — transmissibility and the √2 crossover: vibrationdata.com/tutorials_alt/vib_iso.pdf

• Cho, T. "Experimental and numerical analysis of floating floor resonance and its effect on impact sound transmission." Journal of Sound and Vibration, Vol. 332, Issue 25, 2013 (ScienceDirect).