I am sound-sensitive. I hate entering a restaurant that blasts pop music, with harsh reflections bouncing off every surface. It’s exhausting. I also deeply appreciate when I enter a space that sounds right—a warm, soft welcome where reflections are gentle.
Think of an orchestra hall: the music sounds better because the space is designed for reverberation and clarity. Or think of the home: I don’t want to hear my partner flushing the toilet while I’m in the bedroom, or the clanging of pots in the kitchen, or cars roaring outside.
When we design buildings, sound matters. There are a handful of terms we use to measure sound:
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dBA (A-weighted decibels): Measures sound loudness as the human ear hears it.
30 dBA = quiet library
50 dBA = normal conversation
70 dBA = busy street traffic
90 dBA = lawnmower
120 dBA = rock concert (pain threshold begins)
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NC (Noise Criterion): Background noise rating.
NC 20 = whisper quiet, like a library
NC 40+ = noisy office, distracting
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STC (Sound Transmission Class): How well walls block airborne sound.
STC 25 = you hear normal speech clearly
STC 40–50 = good privacy between rooms
STC 55+ = excellent, apartment-grade separation
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NRC (Noise Reduction Coefficient): How well materials absorb sound.
Carpet = high NRC, soft, absorbs sound
Concrete/mirror = low NRC, reflects sound
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RT (Reverberation Time): How long sound lingers in a room.
Short RT = “dead” space
Long RT = echoey hall
Well-designed spaces balance it
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TL (Transmission Loss): Measured in decibels, how much sound is blocked by a material (lab test property).
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IIC (Impact Insulation Class): How well floors/ceilings reduce impact noise (footsteps, furniture drops).
Bare concrete slab = ~25 IIC
Carpeted floor = 65+ IIC
Let's discuss real life example:
Soundproofing Ceilings – The Structural Side
When it comes to noise between floors, the ceiling assembly itself makes a huge difference. Footfall noise, in particular, travels through the structure—not just the air—so you need more than insulation alone. The diagrams below show three common ceiling soundproofing strategies:
- Level 1 (Basic): Simply doubling up drywall. This adds some mass but leaves the structure coupled, so footsteps and impact noise still pass through (STC 52 / IIC 39).
Soundproof a ceiling: Basic (Level 1) (Source from: Soundproofing Company)
- Level 2 (Standard): Using decoupled drywall suspended below the joists. This breaks the direct vibration path and greatly improves airborne noise control (STC 66 / IIC 56).
Soundproof a ceiling: Standard (Level 2) (Source from: Soundproofing Company)
- Level 3 (Best): A fully treated assembly with triple gypsum layers, Green Glue damping compound, and decoupling. This treats both the subfloor above and ceiling below, making it highly effective against both airborne and impact noise (STC 76 / IIC 64).
Soundproof a ceiling – Best (Level 3) (Source from: Soundproofing Company)
These details highlight how structural choices—mass, damping, and decoupling—work together to shape the sound quality of your home. In practice, upgrading from a basic to a best-level assembly can transform a noisy upstairs neighbor into near silence below.
💡 Design takeaway: If you’re planning a bedroom, nursery, or home theater under a living space, investing in a Level 3 ceiling isn’t just a technical upgrade—it’s a lifestyle upgrade, turning everyday living noise into peace and quiet.
Home Theater (Source from: Houzz)
Sound transmission ratings for various wall assemblies. Note: 1+1 refers to 1 layer of 1/2-inch drywall on each side of wall; 1+2 refers to one layer of 1/2-inch drywall on one side and a double layer on the other side, and so on. （Source from: JLC）
This wall assembly diagram shows the STC rating of different wall types.
- A typical interior partition wall has an STC of about 35. That’s what you find in most homes—and yes, you can hear almost everything through it.
- Adding insulation inside the cavity only raises the rating slightly, from 35 to 38.
- If you use two layers of drywall (sheetrock) on each side, the rating moves from 35 to 39. That’s because drywall primarily reflects sound back into the room, instead of blocking it.
- With double drywall plus insulation, you can reach STC 41.
Wall Assemblies （Source from: West Coast Sound Solutions）
Sound is the physical vibration of air molecules. When the structure carries those vibrations straight through, sound transfers easily from one side to the other. The key is to break up the movement.
One of the most affordable and effective solutions is the resilient clip (also called a quiet clip).
- One side of the clip screws into the stud.
- The other side is slightly detached, holding the drywall.
- When sound hits the drywall, the clip dampens the vibration before it reaches the stud.
Just by adding clips, the wall’s STC rating can jump from the 30s and 40s all the way to STC 57.
If clips aren’t available, a staggered-stud wall (two offset rows of studs with a gap in between) achieves a similar effect: breaking the direct path of vibration.
An STC 57 wall is considered very high performance in terms of sound isolation. Here’s how to think about it:
🔊 What STC 57 Means
- STC 25–35: Normal speech is easily heard through the wall.
- STC 40–45: Loud speech is audible but not intelligible.
- STC 50: Loud speech is barely audible; TV/music is muffled.
- STC 55+: Almost complete privacy. Loud music or shouting is very faint or not heard at all.
So at STC 57, you’re in the range of apartment-grade or hotel-grade walls designed for privacy. Most normal household sounds (toilets flushing, conversations, kitchen clatter) will not carry through. Even loud noises like music or shouting are reduced to a dull, barely noticeable background.
🏠 Where You See STC 57
- High-end condos, apartments, or townhomes where acoustic privacy is required by code.
- Luxury hotels.
- Recording studios and home theaters (often combined with floating floors and acoustic doors).
In practical terms:
If you and your partner are in separate rooms with an STC 57 wall, you probably won’t hear each other at all under normal living conditions. Even if someone’s blasting music or yelling, it will sound distant and muffled.
Sound Quality in Space
Sound doesn’t just have a volume; it also has a quality. Think about the feeling you get from nails scratching a chalkboard—it’s not just loud, it’s unpleasant. The way a room sounds is shaped by the surfaces around you, and that comes down largely to the NRC (Noise Reduction Coefficient) of the materials.
I once tested this myself. I recorded my voice in the basement, where the walls are bare concrete, and compared it to a recording in my bedroom, where I have carpet. The difference was striking. In the basement, my voice echoed harshly, bouncing off the concrete walls. In the bedroom, the carpet softened the sound, making it feel warmer and less reflective.
That’s NRC in action. Low NRC materials reflect sound, creating echoes and a harsher quality. High NRC materials absorb sound, creating a calmer and more comfortable acoustic environment.
📊 NRC of Common Building Materials
| Material | NRC (approx.) | Notes | |
|---|---|---|---|
| Concrete wall (painted) | 0.02 – 0.05 | Very reflective, almost no absorption | |
| Glass (1/4") | 0.03 – 0.05 | Reflects sound, causes echoes | |
| Drywall (1/2" on studs) | 0.05 – 0.10 | Reflective, not absorptive | |
| Plywood / wood paneling | 0.10 – 0.15 | Slight absorption, mostly reflective | |
| Carpet (thin, on concrete) | 0.20 – 0.30 | Some absorption, better than hard floor | |
| Carpet (thick, with pad) | 0.40 – 0.60 | Good absorption, softens space | |
| Acoustic ceiling tile | 0.55 – 0.90 | Designed for absorption in offices/classrooms | |
| Mineral wool / Rockwool (2") | 0.80 – 0.95 | Excellent absorber, often used in wall cavities | |
| Fabric-wrapped acoustic panel | 0.70 – 1.00 | Very high absorption, used in studios/theaters | |
| Curtains (heavy drapery) | 0.35 – 0.60 | Varies with fabric thickness and pleating | |
| Open-cell foam (2") | 0.70 – 0.90 | Lightweight, excellent absorber |
🧾 Takeaway
- Low NRC (<0.10): Concrete, glass, drywall → harsh, echoey sound.
- Medium NRC (0.20–0.60): Carpet, curtains, wood → moderate absorption.
- High NRC (0.70+): Rockwool, acoustic panels → excellent absorption, great for studios or theaters.
So when you walk into a room and notice it feels harsh or cozy, what you’re really experiencing is the NRC rating of the surfaces around you.
The Problem with Glass Walls
Glass may look modern, but it’s one of the worst materials acoustically. With an NRC of only 0.03–0.05, it reflects sound inside the room and transmits it straight through to the next. That’s why in many coworking offices, glass “meeting rooms” feel anything but private—you can still hear every call from the other side.
Sound Reflection With Glass (Source from: AcounsticsFirst)
WeWork Working Space
Conclusion
Designing with sound in mind isn’t just about blocking noise from the outside world. It’s about creating comfort, privacy, and atmosphere inside. The right mix of mass (STC), absorption (NRC), isolation (IIC), and control (RT, NC) can transform a house into a true home, one that sounds as good as it looks.
So the next time you step into a room, pause for a moment—not just to see the architecture, but to hear it. The sound of a space may be its most underrated luxury.
