Understanding sound flanking: Fire alarm speaker cable conduits in multi-family buildings

In today’s multi-family developments, modern fire safety systems are required for the safety and well-being of occupants.

But how does acoustics come into this?

Smoke detectors and fire alarm speakers can degrade a building’s acoustic privacy. Cabling in these developments is typically independent to each residential suite. But fire alarm and safety announcement speakers may be connected in a loop around the floorplate to a central emergency system in the building.

What’s the issue?

These looped cable conduits provide a more efficient sound path between adjacent suites in multi-story buildings. The cable conduits transfer the sound more easily than the partition and degrade its performance. This article explains why this happens and the solution SLR’s Acoustics & Vibration team has successfully used on various projects.

How does sound go around a wall?

Before we get into the solution, let’s understand how sound travels.

Sound in a room is like water; it will flow easiest through the path of least resistance.

Airborne sound travels between spaces through walls, floors, ceilings, and connections. In fire alarm speaker systems, the conduits used for cabling can create flanking paths, allowing sound to bypass otherwise well-performing partitions and travel between rooms.

Why is it an issue?

During design, wall/floor partitions are selected with minimum Sound Transmission Class (STC) to enhance sound isolation, improve residential comfort, and meet Building Code, LEED, or sound isolation performance standards. It is important to minimize noise flanking so that the acoustic performance of walls/floors is maximized and complies with project requirements. The STC performance can be measured with an in-situ test called Apparent Sound Transmission Class (ASTC).

Learn more about STC, ASTC, and Canada’s National Building Code (NBC) sound isolation performance requirements in our article - Noise separation in multi-family buildings: Is Canada’s building code enough?

Sound through fire alarm speaker system cable conduits

In newer residential buildings, the conduits for the speaker system cables are often installed in a loop around the floorplate, connecting all the suites on each level.

A typical suite and speaker cable conduit layout is shown below.

This layout allows sound to travel through the speaker conduits from one room to another,due to the short length of the conduit connecting any two suites. This compromises privacy and reduces the ASTC performance of the wall partition,even without deficiencies in the partition.

How is this issue identified?

Flanking through speaker cable conduits often produces a distinct sound due to the conduit’s dimensions - something acoustics consultants can hear during on-site ASTC testing.

Let’s investigate some acoustic test data to spot where the issue is. 

The chart below shows the ATL frequency response of partitions with speaker conduit flanking issues. A classic case is shown where there is a drop in ATL around 400 Hz.

Depending on the length of the conduit, this drop can sometimes occur in other bands from 315 Hz – 630 Hz. It’s not one case fits all.

Solving flanking sound through conduits

Speaker system conduits can easily be overlooked as a potential sound flanking path between rooms during the construction of multi-family buildings. This is because they are usually installed during concrete slab pours and are not shown on design drawings, and the issue may not be identified until much later in the build.

When this happens, it is not too late. For example, the flanking issue can be resolved by installing fibrous insulation within the conduit in one of the rooms. Installing insulation in the conduit should be done after the speaker cables have been pulled through but before the speakers are installed.

SLR has conducted ASTC tests with and without conduit insulation. The chart below shows the ATL of a typical wall tested with and without insulation in the speaker conduit.

Using insulation can significantly improve sound isolation, increasing the ASTC rating by 6 – 12 points – that’s a huge difference!

This process is relatively new for trades and contractors, therefore the responsibility for installing the insulation should be clear and discussed with the onsite teams prior to building completion.

Key Takeaways

Fire alarm speaker systems in multi-family buildings, though crucial for safety, can create unintended sound issues between suites and undermine acoustic performance and privacy.

The acoustic implications of conduits used to connect the speakers are commonly overlooked during design.

Speaker cable conduits allow sound to travel between adjoining units, resulting in lower-than-expected ASTC ratings and potential non-compliance with NBC sound isolation requirements.

By incorporating insulation within the conduits during construction, developers can address this issue - enhancing sound isolation, improving residential comfort, and meeting Building Code, LEED, or sound isolation performance standards.