Sound isolation

The ninth week begins with a complaint letter. Claire Beaulieu reads it to the students: it comes from the head librarian of a branch library that shares a building with a community hall, opened three years earlier. 'Every Tuesday and Thursday evening the seniors' Zumba class makes the shelves hum,' it says. 'On Saturdays, wedding receptions shake the reading room until midnight. We have stopped holding children's story time on Saturdays.' The wall between the hall and the library, Beaulieu explains, was a single steel stud wall with one layer of gypsum board on each side, rising to the underside of a suspended ceiling, with a door in it for staff and a shared duct running over the top. 'Every one of those decisions was cheaper than the alternative,' she says. 'Together they cost the library its Saturdays. The hub you are designing has the same neighbors, a hall beside a library and a daycare, and the same budget pressure. Today's lesson is about keeping sound where it belongs.'

Room acoustics and sound isolation

Room acoustics, the subject of Lesson 8, concerns how sound behaves inside a room: its reverberation, reflections and clarity. Sound isolation concerns how much sound passes from one space to another, through walls, floors, ceilings, doors, windows, ducts and structure. The two are often confused, because both involve the word absorption, but they need different materials. Absorbent materials, such as acoustic tiles or panels, make a room quieter inside but do almost nothing to stop sound passing through a wall. Isolation needs mass, airtightness and the separation of layers.

Sound travels between spaces in two forms. Airborne sound, such as voices, music and traffic, sets the air in motion, which sets a wall or floor vibrating, which sets the air on the other side in motion. Impact sound, such as footsteps, dropped objects and jumping, sets the structure itself vibrating directly; it is the main problem in floors between homes and above quiet rooms. Both can also travel by flanking paths that bypass the separating element: through a continuous floor slab, along a ceiling plenum, through a duct, or around the edge of a wall.

Transmission loss and the mass law

The ability of a wall or floor to stop airborne sound is measured by its transmission loss, the difference in decibels between the sound striking one side and the sound passing through. For a single, solid panel, transmission loss depends mainly on its mass per unit area and on the frequency of the sound. This relation is called the mass law: each doubling of the mass, or of the frequency, adds about 6 decibels of transmission loss in theory, and about 5 in practice. Heavy walls of concrete or masonry are therefore good isolators, and low-frequency sound, such as the bass of amplified music, is hardest to stop.

Single panels also have a weakness called the coincidence effect: at a certain frequency, which depends on the stiffness and thickness of the panel, the bending waves in the panel match the sound waves in the air, and the panel transmits sound far more easily than the mass law predicts. For gypsum board this dip falls in the upper speech range, which is one reason why single-layer partitions perform poorly.

Double walls and decoupling

Light buildings cannot rely on mass alone. A wall or floor made of two separate layers, with an air space between them filled with sound-absorbing insulation, can do much better than a single layer of the same total mass, provided the two layers are structurally separated. If they are joined rigidly, by shared studs or screws, vibration passes from one to the other and much of the benefit is lost. Decoupling methods include staggered studs, in which each face is fixed to its own row of studs on a wider plate; double stud walls with two completely separate rows of studs; resilient channels or sound isolation clips that hold the gypsum board away from the studs on springy metal; and floating floors on resilient layers. Extra layers of gypsum board, a deeper cavity and insulation in the cavity each add a few decibels more.

Single-number ratings

Transmission loss varies with frequency, so laboratories measure it in many frequency bands. The sound transmission class, STC, defined in an ASTM standard, condenses these measurements into one number by fitting a standard curve, weighted toward speech frequencies. A wall of STC 35 lets loud speech be heard and understood; at STC 45, loud speech is heard but not understood; at STC 50 to 55, loud speech is barely audible; and at STC 60 or more, most sounds are inaudible, although amplified music may still be heard. Because STC gives little weight to low frequencies, it underrates the problem of music and traffic; the outdoor-indoor transmission class, OITC, gives more weight to low frequencies and is used for windows and walls exposed to traffic and aircraft.

STC is measured in a laboratory. In a real building, flanking and workmanship lower the result, so field tests give the apparent sound transmission class, ASTC, which is typically several points lower. Impact sound is rated by the impact insulation class, IIC, measured with a standard tapping machine on the floor above; higher is better. Codes set minimum ratings between dwellings: the International Building Code requires an STC and IIC of at least 50 in the laboratory, or 45 in field tests, between dwelling units. Codes say nothing about public buildings like the hub, so the design team must set its own targets.

Weak links: doors, gaps and windows

A wall is only as good as its weakest part. Sound leaks through any gap that air can pass through, and small openings have a disproportionate effect: a crack around a door or an unsealed electrical box can lower the performance of an excellent wall by ten decibels or more. Doors are the commonest weak links. An ordinary hollow door with gaps around it is worth about STC 20; a solid-core door with good seals and a threshold seal about 30 to 35; and a specially built acoustic door, or a vestibule with two doors, 45 or more. Windows depend on the thickness and spacing of the panes: laminated glass and wide, unequal gaps between panes perform best, and triple glazing chosen for energy is not automatically good for sound.

# Combined transmission loss of a wall with a door, and with an unsealed gap
import math
def tl_mass(mass, f):
    return 20 * math.log10(mass * f) - 47                # mass law, mass in kg per m2
def composite(parts):
    area = sum(a for a, _ in parts)
    tau = sum(a * 10 ** (-tl / 10) for a, tl in parts)  # transmitted fraction, weighted by area
    return 10 * math.log10(area / tau)
to_kg = 4.882                                            # mass unit to kg per m2
for name, m in (("one layer of gypsum board", 2.0), ("four layers of gypsum board", 8.2), ("grouted block wall", 61)):
    print(f"Mass law at 500 Hz, {name:27s} ({m} lb/ft2): {tl_mass(m * to_kg, 500):4.0f} dB")
wall = 650                                           # wall area (ft2)
door = 21                                           # door area (ft2)
for label, parts in (("wall alone (STC 60)", [(wall, 60)]),
                     ("with a sealed solid-core door (35)", [(wall - door, 60), (door, 35)]),
                     ("with an acoustic door (45)", [(wall - door, 60), (door, 45)]),
                     ("acoustic door with a gap at the bottom", [(wall - door, 60), (door - 0.2, 45), (0.2, 0)])):
    print(f"{label:40s} about {composite(parts):4.0f} dB")

Mass law at 500 Hz, one layer of gypsum board   (2.0 lb/ft2):   27 dB
Mass law at 500 Hz, four layers of gypsum board (8.2 lb/ft2):   39 dB
Mass law at 500 Hz, grouted block wall          (61 lb/ft2):   56 dB
wall alone (STC 60)                      about   60 dB
with a sealed solid-core door (35)       about   50 dB
with an acoustic door (45)               about   57 dB
acoustic door with a gap at the bottom   about   35 dB

The calculation shows why the details matter more than the wall type. A double stud wall rated about STC 60 drops to about 50 with a well-sealed solid-core door in it and stays in the high fifties only with an acoustic door; a gap of about 3/8 in under that acoustic door brings the whole wall down to about 35, no better than a single stud partition. The cure is to keep doors out of walls between noisy and quiet rooms wherever possible, to use vestibules where a connection is needed, and to seal every gap: perimeter joints with acoustic sealant, electrical boxes with putty pads and offset positions, and pipe and duct penetrations with sealed sleeves.

Flanking and the whole building

Flanking defeated the library wall in Beaulieu's letter. The partition stopped at the suspended ceiling, so sound passed over it through the shared ceiling space; the duct over the top carried sound from one room to the other like a speaking tube; and the continuous concrete slab and steel structure carried the thud of the Zumba class into the library's floor. Good isolation needs the separating wall to run from the floor slab to the underside of the roof deck or the slab above, sealed at the top; ducts that cross it to have silencers or lined bends; and, for impact noise from active floors such as halls, gyms and fitness rooms, floating floors or structural breaks.

The most powerful tool is planning. Rooms that make noise and rooms that need quiet should be separated by buffer spaces, such as storage, corridors, washrooms and stairs, rather than sharing a wall. Mechanical rooms should be far from quiet rooms and sit on slabs on grade where possible. Stacking matters in multi-story buildings: a fitness room above a library or bedrooms is asking for trouble. In the hub, the students moved the hall's stage and storage to the wall it shares with the library, so that a 20 ft band of storage rooms and a corridor separate the two main spaces.

Noise from outside and from equipment

The hub stands at the corner of a busy avenue with a bus stop, where the noise survey in the previous term measured about 70 decibels, and beside a greenway where it is much quieter. Exterior walls and windows on the noisy side must reduce traffic noise to the indoor targets set by the noise criteria in Lesson 8; heavy cladding, laminated glass in deep frames and acoustically lined ventilation openings help. Equipment noise, from fans, pumps, compressors and transformers, is controlled at the source by choosing quiet equipment, mounting it on vibration isolators, lining ducts and keeping air speeds low, and by placing outdoor units such as heat pumps away from windows and neighbors. Lowering the noise reaching a quiet room is always cheaper at the source than at the room.

The noise reduction between two rooms depends on the transmission loss of the wall and on the rooms themselves. The receiving room's absorption lowers the level a little; a large separating wall raises it, because more area radiates sound.

Exercise 9: sound isolation plan

This exercise takes about one week.

• Color the hub's plans by acoustic zone: noisy, normal and quiet, and identify every place where a noisy and a quiet room share a wall, floor or ceiling.
• Set target ratings for each separation and choose assemblies to meet them.
• Draw the key separating details: wall to roof deck, wall to slab, door and duct penetrations.
• Calculate the composite transmission loss of one wall with its doors and windows.
• Locate the mechanical rooms and outdoor equipment, and explain how their noise will be controlled.