Acoustic Ratings Chart: STC, NRC, OITC, IIC, and NC Compared

A spec sheet lists a wall's rating, and the corridor behind it still sounds like a jobsite. The number on the sheet measured the wrong thing.

We build temporary walls for hospitals and airports, and several of our wall systems carry ASTM E90 laboratory testing, with the full reports available on request. Acoustic ratings answer different questions. STC rates how much sound a wall blocks, and NRC rates how much sound a surface absorbs.

Below, you'll learn what each rating measures and how lab numbers change in the field. You'll also get targets by space type.

Acoustic Ratings Explained

Acoustic ratings are single numbers that summarize how an assembly or surface handles sound.

Blocking ratings track how much sound an assembly blocks. Absorption ratings track sound that stays in a room. Background ratings track the noise level in a finished space.

The table below pairs each rating with its test standard and typical use.

STC Rating

STC rates how well a wall stops speech between rooms. Labs mount the wall between two chambers and measure the transmission loss at 16 frequencies. They then fit the results to a reference curve.

ASTM E90 specifies the lab procedure, and ASTM E413 converts the results to the STC number. The tested range runs from 125 Hz to 4000 Hz, which covers much of the speech range and skips low-frequency rumble. A wall with a high STC can still let bass through.

Every 10 points of STC halves or doubles the loudness the ear hears. The chart below shows what each rung of the STC rating sounds like through a wall.

Sources differ by up to about 5 points on where these descriptions land, so the rows reflect the two published tables that agree.

OITC Rating

OITC rates how well a façade stops transportation noise. ASTM E1332 builds the rating on a reference sound averaged from transportation sources.

Labs test façade specimens from 80 Hz to 4000 Hz and weight the result toward the low frequencies that trucks and jets produce. Two façades with similar ratings can still pass very different amounts of low-frequency sound.

ASTM E413, the classification standard that defines STC, excludes transportation noise and calls for the detailed frequency-band analysis that OITC provides. The STC vs OITC decision follows the noise source, with STC for voices and OITC for engines.

A hospital façade facing a truck route belongs under OITC. Sirens and horn-dominated rail noise can push OITC to demand more low-frequency isolation than the job needs.

NRC Rating

An NRC of 0.80 means a surface absorbs about 80% of the sound that reaches it. Labs average the absorption at 250, 500, 1,000, and 2,000 Hz and round to the nearest 0.05.

Standard suspended ceiling tile typically rates 0.55 to 0.75, so 0.80 or higher marks a strong absorber. Edge effects in the lab can push a reported NRC above 1.00.

Picture ceiling tile with an NRC of 0.90 in two offices split by a low-STC partition. Echo drops in each office, and conversations still carry through the wall. Use NRC to tune the room you're standing in, and STC to protect the room next door.

IIC and Delta IIC Ratings

IIC rates how much impact noise from footsteps and dropped objects reaches the room below a floor-ceiling assembly. A standard tapping machine drops five hammers 10 times per second, and microphones below record 16 frequency bands from 100 to 3150 Hz.

Delta IIC measures how many points a floor covering or underlayment adds to a concrete slab. Under ASTM E2179, labs test a bare 6-inch slab and then retest it with the covering installed.

The difference is the Delta IIC. A bare 6-inch slab rates about IIC 28, so a Delta IIC of 20 lifts it to about 48.

Temporary walls cover vertical separation, so IIC enters a renovation only when work happens above an occupied floor.

NC Rating

NC rates a room from its octave-band noise levels, 63 Hz to 8000 Hz. The rating equals the lowest curve, from NC-15 to NC-65, that stays above every band, so one loud band sets the number.

WBDG lists NC 30 to 35 for small private offices and NC 35 to 40 for general offices. The FGI guidelines cap background noise at NC 40 in hospital patient rooms.

Quiet rooms leave less masking noise, so conversation next door carries farther. A low NC target raises the STC you need in the wall beside it.

Acoustic Ratings for Temporary Walls

A temporary wall faces a different job on an occupied site, where noise arrives with dust. Temporary construction walls have to contain both.

RealWall™ is STARC’s sound-blocking flagship, built to these specs:

• Laboratory STC of 21 under ASTM E90, with up to 50% noise reduction

• Up to 40 dB blocked

• Foam core with gasketed edges and interlocking joints

• 100 linear feet installed in an hour by a two-person crew

• ASTM E84 Class A rating, and exceeds ICRA Class IV and V

Its STC of 21 falls under the chart's first row, so we spec RealWall to contain construction noise and give occupied space a finished look.

Hourigan renovated the 5,500-square-foot Building One lobby at Richmond's James Center while tenants kept using it. RealWall panels kept noise down and looked like permanent walls. Director Scott Thomas said, "you walk by and think, 'That's quality.'"

In Phase 2, crews reused the same panels to build a secure temporary office in one hour and took it down in 30 minutes.

RealWall is a non-fire-rated wall, so jobs with a fire-resistance requirement need FireblockWall™. It covers renovations where fire safety and moderate noise reduction both apply. It carries a one-hour rating under ASTM E119 and a laboratory STC of 40.

Lab Ratings vs. Field Performance

A lab rating marks the ceiling for field results, since ASTM treats the tested assembly as an upper limit. Sound also travels around a finished wall through shared ceilings and ductwork.

Our telescoping panels push up to the ceiling and lock in place, and top-and-bottom gaskets close the perimeter. A partition that stops at a suspended ceiling leaves the plenum open to flanking.

Percent reduction and STC use different measurements, so match the figures before you weigh one wall against another. Request the E90 report for any temporary wall, and check whether the tested panel included its door.

Choosing Acoustic Ratings by Space Type

Each space type pairs a primary rating with its own target number.

Offices

GSA's federal office guidance sets STC 40 for private offices needing normal privacy and STC 45 for conference rooms and executive offices needing confidential privacy.

Video conference rooms climb to STC 53. Private-sector projects set their own targets.

Healthcare

The FGI's guidelines call for a composite STC of 45 between patient rooms on the same floor.

The stakes are financial too. Medicare’s Hospital Value-Based Purchasing program puts 2% of a hospital’s base operating payments on the line, and how quiet patients say the hospital is feeds the HCAHPS scores that program rewards.

Mortenson replaced sterilization equipment on the clean side of the central processing plant at Abbott Northwestern Hospital in Minneapolis, working behind our ICRA barriers. The barriers held ICRA Class IV and significantly reduced sound transfer. Field superintendent Peter Hauer said the staff never saw the renovations.

Hotels

IBC Section 1206 sets STC 50 by ASTM E90 between guest rooms and from public or service areas, and IIC 50 for floors.

A finished wall can meet NNIC 45 in a field test instead. Upscale brands often go higher, with STC 55 between guest rooms and STC 50 to corridors.

During a renovation, a temporary wall protects guests while the permanent walls take shape.

Specifying Temporary Walls by Acoustic Ratings

Acoustic ratings vary by system, so confirm each one against your project code before you order.

The Wall Wizard recommends a system after a few questions about your project. Our team recommends a wall faster when your quote request includes the linear feet of wall, ceiling height, target STC, any required fire or ICRA rating, and your phase and relocation plans.

Request a quote, and we'll confirm your specs and tailor one to your project.

Frequently Asked Questions

What does an STC 35 acoustic rating mean?

An STC 35 acoustic rating means a wall turns loud speech into garbled noise. It falls 15 points short of the STC 50 the IBC requires between hotel guest rooms.

Is an STC rating of 27 good?

No, an STC rating of 27 is poor, because normal speech stays understandable through the wall. It trails the IBC's STC 50 guest-room minimum by 23 points.

What does an STC rating of 40 mean?

An STC rating of 40 means a wall reduces loud speech to a faint murmur, and 10 more points would halve the loudness again. Our FireblockWall™ tests at STC 40.

What is a good Rw rating?

A good Rw rating, the ISO 717-1 weighted sound reduction index, runs 40 to 45 for offices needing privacy and 50 or higher between hotel rooms. Rw tracks STC within a few decibels, so US targets carry over.