Law Firm Soundproofing: ASTC Test Results for Confidential Office Speech Privacy

• Project: Confidential Speech Privacy Assessment for Adjacent Law Firm Suites
• Location: Class A multi-tenant office building, Sarasota, FL
• Tenants Tested: Two adjacent law firms across Suites 800, 840, and 860
• Standard: ASTM E336 airborne sound transmission field testing
• Wall Assembly: 25-gauge studs, 5/8 inch Type X gypsum each side, R-13 batt
• Performance Target: ASTC 38 for Confidential Privacy at Normal Voice Levels

Two adjacent law firms in a Class A Sarasota office building shared a demising wall and a problem. Voices from one suite were intermittently audible in the other. Conversations were not loud. The walls were not thin. The construction documents looked correct on paper.

Commercial Acoustics was retained by the property manager to find the actual sound path and recommend a fix that would restore confidential speech privacy without a full demolition.

Most office tenants can live with an ASTC of 35. Voices are audible as a low murmur but words cannot be made out. For a law firm, that floor is not enough. Attorney-client privilege, opposing-counsel calls, and HR conversations all require what acousticians call confidential privacy.

The accepted ASTC threshold for confidential privacy at normal voice levels is 38. For raised voices and depositions, the target rises to 45 or higher. Standard speculative office construction rarely targets either number. That gap is the entire reason this project existed.

We performed three airborne sound tests per ASTM E336, the field standard for measuring sound transmission between rooms. The same wall assembly was tested in three locations within the same building. The results were not consistent.

Six points of variance from the highest to the lowest result. The wall is identical in all three locations. Something else is moving the number.

To model how a target ASTC translates to real wall construction, our STC calculator walks through the assembly trade-offs side by side.

Walking the perimeter with a sound level meter, the dominant flanking path became obvious. The exterior wall used a continuous extruded aluminum window mullion system, only one-eighth of an inch thick. That mullion runs uninterrupted past every demising wall in the building.

Sound enters the mullion in one suite, travels along the aluminum, and re-radiates into the next suite. The demising wall is bypassed entirely. Mass alone determines how much sound a thin panel transmits, and one-eighth-inch aluminum has almost none.

Secondary flanking was identified at the baseboards, where small air gaps between the gypsum and the slab allowed a low level of leakage. The plenum above the ceiling tile was tested and ruled out. No flanking path through the ceiling.

• Mullions: Apply a mass-loaded membrane to both interior faces at the demising wall intersection
• Baseboards: Remove existing base, run a continuous bead of acoustical sealant along the wall-to-slab joint, reinstall
• Walls (Optional): Add one layer of 5/8 inch Type X gypsum to the office side of the worst wall for 3 to 5 ASTC point gain
• Sound Masking (Optional): Pink-noise system tuned to NC-40 in private offices to mask residual transmission once flanking is closed

The fix order matters. Adding another layer of drywall to a wall that is being bypassed by a mullion will not move the ASTC number. The flanking path has to close first.

For mullion mass, we specify Wall Blokker mass-loaded vinyl membrane as the standard treatment. Closing the mullion path alone is projected to bring the worst wall (ASTC 37) up to 41 to 43 — into the confidential privacy range without touching the wall itself.

For the masking layer, a commercial sound masking system covers any residual speech that survives the flanking fix.

Not every office needs the same number. The target should match what is being said in the room. The table below maps the working windows across four common office tenant types.

For more on how STC numbers translate into the real-world experience of speech privacy, see the STC rating chart.

Lab-rated STC numbers come from idealized assemblies built in isolation. Field-tested ASTC numbers come from the actual wall in the actual building, with all of its real-world flanking paths included. The two numbers are almost never the same.

For this Sarasota project, the lab STC of the tested assembly is rated near 45 per the ASTM E336 standard. The field ASTC measured between 37 and 43. The delta is the flanking.

Without the field test, the property manager would have specified another layer of drywall, paid for the work, and watched the number barely move. An ASTM E336 test is the cheapest line item on a soundproofing project, and the only one that tells you what to actually fix.

Law firms cannot run on hope when it comes to speech privacy. The wall on the drawing is rarely the wall in the field, and the flanking paths around it almost always carry more sound than expected. A short ASTM E336 test, an honest read of the perimeter mullion and baseboard conditions, and a targeted fix — in that order — restore confidential privacy without rebuilding the suite. Talk to an acoustical engineer about a field assessment before the next tenant move-in.