What Is Sound?
Sound is a mechanical wave—a disturbance that travels through a medium (air, water, or solid materials) by causing molecules to vibrate and transfer energy from one location to another.
The Physical Process
Key insight: Sound cannot travel through a vacuum because there are no molecules to vibrate. This is why if you can eliminate or disrupt the medium (air gaps, isolation), you can stop sound transmission.
Frequency & Wavelength
Not all sounds behave the same way. The frequency (pitch) of a sound determines how it travels, how easily it's blocked, and what materials are effective at stopping it.
Why low frequencies are hardest to block
- Long wavelengths diffract easily: A 100 Hz wave (11 ft wavelength) easily wraps around door frames and wall gaps.
- Requires massive barriers: Thin acoustic foam does essentially nothing against bass.
- Penetrates materials: Low frequencies transfer energy into solids, causing walls to vibrate and re-radiate sound.
Decibels (dB)
Decibels measure sound intensity on a logarithmic scale, where each 10 dB increase represents a perceived doubling of loudness.
| dB | Example |
|---|---|
| 10–20 | Rustling leaves, breathing |
| 30 | Whisper, quiet library |
| 40 | Refrigerator hum |
| 50–60 | Normal conversation |
| 70 | Vacuum cleaner, traffic |
| 80 | Alarm clock, garbage disposal |
| 90 | Lawnmower, power tools |
| 100+ | Motorcycle, nightclub |
Understanding the logarithmic scale
Airborne vs. Structure-Borne Sound
This is the most critical distinction for effective soundproofing. The two types require completely different treatment approaches.
Airborne Sound
Sound that travels through air until it strikes a barrier (wall, door, window). The barrier vibrates and re-radiates sound on the other side.
- • Add mass (drywall, MLV)
- • Seal gaps (weatherstripping, caulk)
- • Absorption (insulation in cavities)
- • Decoupling (resilient channels)
Structure-Borne (Impact)
Sound created by direct physical impact on the building. Vibration travels through solid materials at 10× the speed of airborne sound.
- • Source treatment (carpet, rubber pad)
- • Decoupling (isolation clips)
- • Damping (Green Glue)
- • Foam panels don't work!
Key difference
STC Ratings
Sound Transmission Class (STC) is a single-number rating that measures how well a partition reduces airborne sound. Higher STC = better sound blocking.
| STC | Quality | What you hear |
|---|---|---|
| 25–35 | Poor | Normal speech clearly understood |
| 38–42 | Fair | Loud speech heard but not understood |
| 45–50 | Good | Loud speech faintly heard |
| 50–55 | Very good | Very loud sounds barely heard |
| 60+ | Excellent | Shouting barely audible |
Common construction STC values
- Hollow-core door: STC 20–25
- Single stud wall, no insulation: STC 33
- Same wall + fiberglass: STC 39
- Double drywall + insulation: STC 45–48
- Decoupled wall: STC 50–55
STC limitations
- • Only measures airborne sound, not impact
- • Tested in ideal lab conditions
- • Doesn't account well for low-frequency bass
- • Real-world often 5–10 points lower
- • Small gaps drastically reduce actual STC
The Four Principles of Soundproofing
All effective soundproofing uses one or more of these four mechanisms. The best solutions combine all four.
Mass
Adding weight
Heavier materials are harder to vibrate. Mass resists sound wave energy, preventing transmission.
Absorption
Dissipating energy
Porous materials trap sound waves, converting acoustic energy to heat through friction.
Decoupling
Breaking connections
Separating two sides of a barrier so vibrations can't mechanically transfer. The most powerful technique.
Damping
Converting vibration to heat
Visco-elastic materials prevent panels from resonating and amplifying sound.
The Golden Formula: M-A-D-D
Combine all four principles for maximum soundproofing. A properly built wall can achieve STC 60+.