Estimate the wall-isolation performance of partitions, party walls and studio shells using the US Sound Transmission Class (STC) rating. Adjust surface mass, single / double-leaf construction, cavity depth and coincidence frequency to see the 125–4000 Hz transmission-loss curve and the STC value update in real time.
Parameters
Wall type
Choose between a single panel and a double-leaf wall (with air cavity)
Surface mass m
kg/m²
Mass per m². 12 mm gypsum ≈ 10; 150 mm concrete ≈ 360
Cavity depth d
mm
Used only in double-wall mode. Sets the mass-spring-mass resonance.
Incidence angle θ
°
Angle at which sound hits the wall. Field incidence is roughly 45° equivalent.
Coincidence freq f_c
Hz
12 mm gypsum ≈ 3000; 12 mm plywood ≈ 1500; 150 mm concrete ≈ 100 Hz.
Results
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Surface mass m (kg/m²)
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500 Hz transmission loss (dB)
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STC rating
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125 Hz TL (dB)
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4000 Hz TL (dB)
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Isolation verdict
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Wall isolation — incident and transmitted sound
Red incident wave on the left passes through the wall and emerges weaker as a blue transmitted wave on the right. In double-wall mode an air cavity appears between the two leaves; wave amplitude attenuates according to the mass law.
Berger's mass law R (normal-incidence transmission loss, dB) and the mass-spring-mass resonance of a double leaf. m is surface mass (kg/m²), f is frequency (Hz), d is cavity depth (m).
$$R_{\text{field}} = R - 5 \,[\text{dB}],\qquad \text{STC} \approx R_{\text{field}}(500\,\text{Hz})$$
Measured field (random) incidence is about 5 dB below normal incidence. STC is set by the ASTM E413 reference contour, but in practice it is close to the field TL at 500 Hz.
The mass law is the simplest single-panel approximation; bending waves, coincidence and leakage make real walls fall short.
Sound Transmission Class (STC) and the Mass Law
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I can hear my neighbour's voice through the apartment wall. What property of the wall actually controls that — the thickness, the material?
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Good question. In building acoustics we rate a wall with a single US number called STC, the Sound Transmission Class — basically "how much sound does this wall block?". The biggest driver is not thickness or material per se, it is the surface mass, kilograms per square metre. 150 mm of concrete is about 360 kg/m², 12 mm of gypsum board is around 10. Heavier walls block more sound — that is the basic principle.
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OK, so heavier is better. If I just keep raising the surface mass on the left, the STC keeps climbing — how far does that go?
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Here is the catch. Berger's mass law R = 20·log10(m·f) − 47 says that doubling the mass only buys you about 6 dB of TL. Going from 12 mm of gypsum to 24 mm bumps the STC from roughly 30 to 36, not from 30 to 60. Returns get worse the heavier you go, which is why standard practice is to add a modest amount of mass and then switch to a double-leaf wall instead.
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A "double wall" — there is an option for that on the left. If doubling the mass barely helps, what does splitting it into two leaves change?
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Sharp question. A double wall behaves like a mass-spring-mass system: two leaves coupled through the air cavity acting like a spring. Well above the resonance f_msm = 60·√(2/(m·d)) you get an extra 10+ dB over a single panel. As an example, a single 25 kg/m² panel versus two 12.5 kg/m² leaves with a 100 mm cavity: the second is lighter overall but is 5 to 10 STC points higher. That is why almost every studio and hotel party wall is built as a double leaf.
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Looking at the chart, TL drops sharply around 1 kHz — what is going on there?
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That dip is the coincidence effect. At the critical frequency f_c the bending-wave wavelength inside the panel matches the trace wavelength of the sound, and the panel resonates with the incoming sound. For 12 mm gypsum f_c is around 3 kHz, for 12 mm plywood about 1.5 kHz. When it falls in the 1–4 kHz band where the ear is most sensitive, STC drops by 3 to 5 points. Fixes are to layer different panels so their f_c shift apart, sandwich a viscoelastic damping layer, or use damped (acoustic) gypsum. Real partitions often use a gypsum + mass-loaded-vinyl sandwich for exactly this reason.
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So what STC do real party walls actually need?
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In the US IBC the party wall in condominiums must be at least STC 50; hotel guest-room walls aim for STC 55, and music studios for STC 65+. A common Japanese steel-stud wall with double gypsum on each side comes in at STC 52–56; 180 mm bare concrete is around STC 55. To break STC 60 you stack tricks: double leaves, fiberglass-filled cavity, mass-loaded vinyl, and structural break (resilient channel) to cut the flanking path.
Frequently Asked Questions
STC is the US single-number wall-isolation rating defined in ASTM E413. The transmission loss TL is measured in 16 one-third-octave bands from 125 to 4000 Hz, and the standard contour curve is fit to the data to give one number. As a practical rule of thumb STC is close to the field-incidence TL at 500 Hz, which is what this tool uses for its instant readout. Higher STC means better isolation: aim for STC 50+ for multifamily party walls and STC 60+ for music studios.
The mass law states that the transmission loss of a single panel grows logarithmically with the product of surface mass and frequency: Berger's equation R = 20·log10(m·f) − 47 dB. Doubling either the mass m or the frequency f therefore raises TL by about 6 dB. For example, going from 25 to 50 kg/m² lifts the 500 Hz TL from roughly 35 to 41 dB. In real buildings, however, bending waves, the coincidence dip and air leaks usually take the wall 3 to 10 dB below the mass-law ideal.
A double wall behaves like a mass-spring-mass system, with the two leaves coupled through the air cavity. Above the mass-spring-mass resonance f_msm = 60·√(2/(m·d)) Hz it greatly exceeds the mass law of a single panel. For two 25 kg/m² panels with a 100 mm cavity, f_msm ≈ 54 Hz, and from there upward you can pick up 6 to 15 dB of extra isolation. Near f_msm itself the TL dips, so for low-frequency control increase the cavity depth, fill it with fiberglass, and avoid rigid connections between the two leaves.
The coincidence effect is the dip in TL that appears when the bending-wave wavelength inside a panel matches the trace wavelength of the incident sound. The critical frequency f_c depends on thickness, density and stiffness — about 3000 Hz for a 12 mm gypsum board, 5000 Hz for a 5 mm lead sheet, and around 1500 Hz for 12 mm plywood. Mitigations include using multiple layers with different f_c, sandwiching a viscoelastic damping layer, or raising the mass to lift the overall TL. This tool drops TL by 5 dB around f_c to visualize the dip.
Real-World Applications
Multifamily party walls and floors: The walls between adjacent apartments and the floor / ceiling assemblies between stacked units are the number-one source of "I can hear my neighbour" complaints. In the US, the IBC requires STC 50 minimum for the party wall in condominiums; for-rent multifamily often targets STC 55. A typical steel-stud party wall with two layers of gypsum on each side plus fiberglass in the cavity lands around STC 52–56. Use this tool to compare a single-panel idea against a double-leaf option before specifying details.
Music studios and recording rooms: Both keeping outside noise out and keeping playback in require very high isolation — STC 65 to 75. Achieving it requires a box-in-box construction (a floating inner shell), double walls with different leaf masses (to spread the coincidence dips), heavy gasketing on doors and windows, and rigorous structural break detailing. Use the tool to see the ceiling of a single panel (the mass-law roof) and quantify the benefit of going to a double leaf.
Office and meeting-room partitions: For private offices, medical exam rooms or call-centre booths the privacy target is STC 40 to 50. Dry-wall partitions (steel stud + gypsum) dominate, and at this STC range you usually win more by sealing leaks and packing the cavity with fiberglass than by piling on mass. Remember that the calculation here is the upper bound for an ideal isolated wall; assume the field result will be 5 to 10 dB lower.
Home theatre acoustics: A 5.1 subwoofer pushes hard at the low frequencies and is the classic source of neighbour complaints. At 63 to 250 Hz the mass-law TL is small and the mass-spring-mass resonance f_msm of a double wall sits inside the band of interest. Use the cavity-depth slider to push f_msm below the lowest band of concern, fill the cavity with fiberglass to damp the resonance, and add a floating floor to break structure-borne flanking.
Common Misconceptions and Pitfalls
The biggest pitfall is "the calculated STC value is not what you measure". This tool returns the upper bound from Berger's mass law for an ideal isolated wall. In actual buildings the on-site STC is typically 5 to 10 dB lower because of (1) the coincidence dip in the mid-high band, (2) flanking transmission through the surrounding structure (columns and slabs), (3) leakage through electrical outlets, pipe penetrations and window-frame perimeters, and (4) rigid connections in double walls (a single fastener that pierces both leaves). On-site STC measures the system, not the wall. Budget calculated value minus 5 to 10 dB when planning.
Second, "STC is not what your ears actually hear". STC weights the 125 to 4000 Hz mid-high range and barely scores the deep bass below 125 Hz — the kick drum in music, the boom of car subwoofers, the low-frequency content of footsteps. As a result, an STC 60 wall can still let a neighbour's 60 Hz subwoofer through clearly. For low-frequency isolation use OITC (Outdoor-Indoor Transmission Class) or look at the full one-third-octave TL curve rather than the STC number. And in multifamily living, the dominant noise complaint is often impact noise on the floor (IIC rating), which STC does not address at all.
Finally, "heavier always means higher STC" is not strictly true. The mass law is an ideal for a single panel. Stiffer panels push the coincidence frequency f_c down into the audible band where it cuts a hole in the TL curve. A 5 mm lead sheet and a 12 mm plywood panel have roughly the same surface mass, but the stiff plywood has its f_c near 1500 Hz with a major dip, and ends up isolating less. Practical practice is to use heavy but lossy materials (mass-loaded vinyl, damping gypsum, lead-vinyl composites), or layer panels with different f_c so their dips do not align.
How to Use
Enter surface mass in kg/m² (typically 100–500 for gypsum board, 400–800 for concrete blocks)
Set cavity depth in mm (50–200mm air gaps increase isolation by 3–6 dB per octave)
Input critical frequency in Hz (lower values improve high-frequency attenuation; steel studs shift critical freq upward)
Read STC rating and transmission loss (TL) across 125 Hz, 500 Hz, and 4000 Hz bands
Worked Example
Double-stud wall: two 5/8" gypsum layers (surface mass 120 kg/m²) + 150 mm fibreglass cavity. Normal incidence (0°), critical frequency 250 Hz. Result: 500 Hz TL = 32 dB, 125 Hz TL = 18 dB, 4000 Hz TL = 44 dB, STC = 42. Adding 25 mm rockwool increases cavity damping; raising surface mass to 180 kg/m² (Type X board) yields STC = 51 at same configuration.
Practical Notes
Mass-air-mass configurations (staggered studs, decoupled plates) reduce coincidence dip near critical frequency, preventing 3–5 dB notch at 250–500 Hz
STC underrates low-frequency isolation: a partition scoring STC 50 may transmit footfall/bass at 63 Hz (typically 20–25 dB TL). Specify weighted sound reduction index (Rw) for music studios
Grazing incidence above 60° degrades STC by 2–4 points; orient partition perpendicular to dominant noise path
Polyurethane foam backing (25 mm) improves STC by 1–2 points; mineral fibre > foam for fire rating in commercial builds