Design Acoustic Separation

A wall is specified at a laboratory rating and the built separation comes out lower, every time. The reason is that the rating describes sound going straight through the middle of a wall in a test chamber, and in a building the sound goes around it: through the floor, along the facade, over the ceiling, through a shared duct. The assembly is the easy half.

Workflow

Step 1. Establish the source, the receiver and the target

Ask, in one message:

1. What are the two spaces, and what happens in each? A source and a receiver, not just two rooms.
2. What kind of sound: speech, music, machinery, footfall, plumbing? Each behaves differently and airborne and impact are separate problems with separate ratings.
3. What is the target, quoted, and from where: the code, the brief, a certification, the acoustician?
4. What is the structure between them, and what is the structure around them?
5. What is above and below each space, and what is on the other side of each of their other walls?
6. What services run between them or serve both?
7. Is an acoustician engaged?

Airborne and impact are separate. A floor separating a corridor from a unit below is two problems with two ratings, and the assembly that solves one may not touch the other.

Step 2. Trace every path, not just the wall

The separating assembly is one path. Set out all of them:

#	Path	Through what	Addressed by	Status

Work through at least:

• Direct. Through the separating wall or floor.
• Flanking along the structure. A continuous slab, a continuous facade, a continuous corridor wall, a structural frame carrying sound around the separation. This is usually the path that decides the result.
• Over the ceiling. A partition stopping at a suspended ceiling with a continuous plenum above. Point at design-partition.
• Under the floor. A raised floor, a continuous screed, a service void.
• Around the facade. Along the exterior wall, through the mullion, through a continuous spandrel or a shared window.
• Through services. A shared duct, a shared riser, back-to-back outlets, plumbing in a shared wall, a shared plenum return.
• Through openings. Doors, glazing, transfer grilles, undercuts.
• Structure-borne. Anything rigidly connecting the two: a pipe, a hanger, a stud, a fixing.

Report the count of paths that nothing addresses. That count, not the assembly's rating, is the honest description of the separation.

Step 3. Name the decisions that have to be made early

Some of this is settled in the structure and the layout, and cannot be improved afterwards at any price:

• Whether the structure is continuous through the separation, and whether a break is possible.
• Whether the floor is a bare slab, a floating floor, or a slab with a resilient layer. This is a build-up thickness and a floor-to-floor decision.
• Whether the layout puts a quiet room against a noisy one at all. Moving a room is cheaper than any assembly.
• Whether risers and shafts are shared. Point at /size-shaft.
• Whether the ceiling is continuous, and whether the partition can reach the structure. Point at design-partition.
• Whether the facade is continuous past the separation, and what breaks it.

State which of these are still open and which are already fixed. Anything already fixed constrains the rest, and saying so early is the point.

Step 4. Work the openings and the services

Openings and services usually dominate the result:

• Every door in the path: its own rating, its seals, its threshold, and whether two doors in series would do what one cannot.
• Every glazed element, including internal glazing and borrowed light.
• Every duct crossing the separation, and whether it is lined, offset, or has an attenuator. Named as a question for the mechanical engineer.
• Every penetration, and whether the seal is acoustic as well as rated. Point at penetration-review, which logs both.
• Back-to-back and near-adjacent boxes, which defeat a wall on their own.
• Plumbing in or against the separation, which is structure-borne and is not fixed by mass.

Step 5. Report

• The paths table, with the unaddressed count first.
• The early decisions, split into open and already fixed.
• Openings and services, with what has to be established for each.
• Every value that was quoted and every value still open.
• What has to reach the acoustician, the structural engineer and the mechanical engineer.

Close with the standing limits, every run: no result was predicted, a laboratory rating is not a built result, and flanking has been found rather than assessed.

Step 6. Save, if asked

Ask whether to write the assessment to a file and where.

Rules

• Never supply a rating, a laboratory value, or a criterion.
• Separate airborne from impact, every run.
• Trace all the paths, and report the count nothing addresses before anything else.
• Never present an assembly's laboratory rating as the separation achieved.
• Name the early decisions and say which are already fixed.
• Say on every run that flanking was found here, not assessed.

Anti-patterns

• Specifying a wall rating and calling the separation designed.
• Treating airborne and impact as one problem.
• Ignoring the corridor wall and the slab because they are not the separating assembly.
• Assuming a plenum above a partition is not a path.
• Back-to-back outlets in a separating wall.
• Recalling a rating for a common assembly.
• Leaving plumbing in the separating wall and expecting mass to fix it.
• Proposing an assembly upgrade when moving the room was available.

Resources

None. This skill is one file. Output is written directly at the path you choose.