Door STC Ratings Explained: Sound Transmission (India 2026)

When a client asks why they can still hear the television through a closed bedroom door, the answer is almost always the door STC ratings nobody specified. STC, or Sound Transmission Class, is a single number that summarises how much airborne sound a door blocks — and in most Indian homes the door is the weakest acoustic link in the wall. A masonry wall might block 45-50 dB, but a hollow flush door in it blocks barely 20-25, so the whole partition performs only as well as that leaf and the gaps around it. This guide explains what door STC ratings mean, the typical STC of every door type, how doors are tested to ISO 10140 and ASTM E90, what STC you actually need for each room, and how mass and seals push the number up. It is the measurement-and-ratings companion to our product guides; if you want the how-to first, start with soundproof doors.

What STC and Rw actually mean

STC (Sound Transmission Class) is a single-figure rating, derived from the sound a door blocks across the 125 Hz to 4000 Hz speech range, fitted to a standard reference curve. A higher STC blocks more airborne sound. As a rough feel: an STC of 25 lets normal speech pass clearly; 30 makes loud speech audible but muffled; 35 reduces loud speech to a faint murmur; 40 means loud speech is barely heard; and 45 and above means most normal sounds are inaudible. Each 10-point rise is roughly a halving of perceived loudness, so the difference between a 25 and a 35 door is dramatic, not incremental.

Rw (Weighted Sound Reduction Index) is the ISO equivalent used in Europe and most Indian acoustic reports that follow international practice. For doors, STC and Rw track within a point or two, so a door quoted at Rw 32 is effectively an STC 32 door. The important caveat is that both numbers describe the door leaf tested in a sealed laboratory frame — the rating you achieve on site depends entirely on the perimeter and threshold seals, which is where most real installations lose 5-10 points.

Typical STC by door type

The single most useful thing a specifier can carry is a feel for what each door construction delivers. The table below gives indicative laboratory bands; treat them as a starting point, then verify against the actual product test certificate.

The lesson is that construction sets the ceiling and seals decide how close you get to it. A solid-core leaf rated 32 in the lab routinely measures 24-26 installed if the perimeter leaks; the same leaf with proper compression seals and a drop seal can reach its rating. For the seal contribution in detail, see acoustic door seals and door air-tightness, since an acoustically sealed door is also an air-tight one.

How STC is tested

Understanding the test stops you over-trusting a brochure number. STC is measured in a laboratory with two reverberation chambers separated by the test door. A loud, broadband noise is generated in the source room; sound-level meters measure how much energy reaches the receiving room across sixteen one-third-octave bands from 125 Hz to 4000 Hz. The transmission loss at each band is plotted and a standard STC reference contour is slid up until it fits within defined tolerances — the value where it lands is the rating.

• ASTM E90 is the American laboratory method for measuring airborne sound transmission loss; ASTM E413 then computes the STC number from those measurements.
• ISO 10140 is the international (and de facto Indian) laboratory method, from which the Rw single-figure rating is derived per ISO 717-1.
• Field-tested ratings (ASTM E336/E966 giving FSTC or NIC, and ISO 16283 giving DnT,w) measure the installed door in the real building and are typically several points lower than the lab number.

Because the laboratory and field numbers differ, a competent specification names both the target field performance and the lab-rated door needed to reach it after the inevitable site losses. As a rule of thumb, specify a door rated 5-8 points above your field target.

What STC you actually need, room by room

The right rating is a function of how quiet the room must be and how noisy the source is. Over-specifying wastes money; under-specifying defeats the purpose. The matrix below is a practical starting point for Indian residential and small-commercial work.

Notice that almost every meaningful upgrade involves seals, not just a heavier leaf. For most Indian homes the highest-value move is fitting a solid-core door and sealing it well, which lifts a bedroom from a leaky 24 to a private 34 at modest cost. Run a specific build-up through the acoustic door STC calculator to see the leaf-plus-seal result before you commit, and use the door air leakage calculator to quantify the gap losses that drag the rating down.

How mass and seals raise the rating

Two physical principles govern almost everything about a door's STC, and both are worth specifying to.

The mass law

For a single uniform leaf, sound reduction follows the mass law: roughly each doubling of the surface mass (kg/m²) adds about 5-6 dB of transmission loss. This is why a dense solid-core or solid-hardwood door beats a hollow one so decisively, and why thin metal sheet doors disappoint despite feeling solid. Adding mass — a denser core, a thicker leaf, a second skin — is the reliable way to lift the leaf's ceiling. The limitation is diminishing returns and the coincidence dip, a frequency band where stiffness lets a panel resonate and transmission loss sags; acoustic doors use damped composite cores to flatten that dip.

Sealing the perimeter

Sound, like air, will exploit any gap. An unsealed 3 mm perimeter gap can drop an STC 35 leaf to an effective 25 — the gap leaks the high frequencies that carry speech intelligibility. Compression seals around the jambs and head, plus a drop-down (automatic) bottom seal or a sealed threshold, close that path and let the leaf reach its rating. This is the single highest-return acoustic intervention and why the field and lab numbers diverge. For the seal hardware and fitting, see acoustic door seals; for the broader insulation picture, door sound insulation and the overarching door acoustic performance pillar tie the ratings into real-room results.

In the Indian market, be wary of acoustic claims without a third-party ISO 10140 or ASTM E90 test certificate — a marketing "soundproof" label means little without a number. Acoustic doors and their seals carry a premium (plus 18% GST), but the comfort and privacy payback for bedrooms, clinics and media rooms is real. The whole door decision threads back to the complete door guide.

Frequently asked questions

What STC rating does a bedroom door need in India?

For everyday privacy between a bedroom and a living area, aim for an installed STC of about 30-35. In practice that means a solid-core flush or solid-timber leaf fitted with perimeter compression seals and a drop-down bottom seal. A standard hollow-core flush door rates only 20-25 and lets normal speech pass clearly, which is why bedrooms feel un-private without an upgrade.

Why does my door perform worse than its rated STC?

Because the lab rating is measured on a perfectly sealed door. On site, gaps around the jambs, head and threshold leak high-frequency sound and can drop the effective rating by 5-10 points. The fix is rarely a heavier door — it is perimeter compression seals plus a drop seal or sealed threshold, which let the existing leaf reach close to its laboratory number.

Is STC the same as Rw?

They are different standards measuring the same thing and, for doors, they track within a point or two. STC comes from the American ASTM E90/E413 method; Rw is the ISO 717-1 single figure derived from an ISO 10140 test, common in Indian acoustic reports. A door quoted at Rw 35 is effectively an STC 35 door, so you can compare them directly.

How are doors tested for STC?

In a laboratory with two reverberation chambers separated by the test door. Broadband noise plays in the source room while meters measure how much reaches the receiving room across sixteen bands from 125 Hz to 4000 Hz. ASTM E90 (with E413) gives STC; ISO 10140 (with ISO 717-1) gives Rw. Field methods like ISO 16283 measure the installed door and read several points lower.

How high an STC can a single door realistically reach?

A single-leaf acoustic door with a dense damped core, full perimeter gasket and an automatic drop seal can reach roughly STC 38-45 in the lab. Beyond that you generally need a double-leaf acoustic door or a sound lobby (two doors with an air gap), which can reach STC 50-60 for recording studios and critical isolation, at a substantial cost premium.