Sound insulation principle of doors and windows

1. Sound Insulation Performance of Uniform Single‑Layer Materials

According to the mass law of sound insulation, the sound insulation performance of glass is determined primarily by its surface density (thickness × density).

At present, the density of glass in the domestic market is nearly uniform across all products. Therefore, increasing the glass thickness is the only practical way to improve its sound insulation.

Reference values for sound insulation of different materials.

The sound insulation values of various materials generally follow the mass law. For example, doubling the thickness of glass increases its sound insulation by approximately 3~5 dB(A).

Ordinary residential walls typically provide sound insulation in the range of 45–50 dB(A). The weakest link in residential sound insulation, however, is the doors and windows.

2. Sound Insulation Performance of Hollow (Insulating Glass) Structures

The sound insulation performance of double‑pane insulating glass is determined primarily by the total actual thickness of the two glass panes (e.g., for 5 + 9 Air + 5 insulating glass, the total glass thickness is 10 mm) and by the spacing between the two panes.

See Table 3 for sound insulation performance of insulating glass.

Coincidence dip in insulating glass: All insulating glass structures exhibit standing wave resonance, which can create a coincidence effect with sound waves in certain frequency bands. This allows sound waves in those bands to pass through the glass more easily.

Actual engineering data show that the coincidence effect bands of common insulating glass (e.g., 5 + 9 A + 5, 6 + 12 A + 6, 5 + 6 A + 5) and of hollow + laminated glass products on the market are concentrated between 150 Hz and 400 Hz. In contrast, the most bothersome frequency bands of traffic noise are concentrated between 125 Hz and 750 Hz.

Conclusion: Insulating glass exhibits a coincidence effect that reduces its ability to block low‑frequency traffic noise. Low‑frequency noise (the “hum”) penetrates easily into the room. Therefore, insulating glass is not recommended for buildings located near major traffic arteries.

Audio samples for reference frequencies: 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1,000 Hz, 2,000 Hz, 4,000 Hz, and 8,000 Hz. Does the noise in your home sound closer to 250 Hz or 500 Hz?

Note: Standard speakers cannot reproduce very low frequencies effectively. To hear these frequencies properly, use active noise‑canceling headphones or high‑fidelity headphones.

3. Analysis of Sound Insulation Performance of Vacuum Glass

Vacuum glass currently available on the market consists of two glass panes, each 5–10 mm thick. A large number of small metal or plastic pillars are placed between the panes to maintain the gap.

Atmospheric pressure at the Earth's surface is approximately 10 tons per square meter. Current technology cannot completely remove the air between the panes to create a true vacuum. A sufficient number of pillars must be used to keep the gap open.

Sound transmission requires a medium - gas, liquid, or solid. Because vacuum glass on the market contains a large number of pillars between the panes, it forms a solid sound transmission path (an acoustic bridge effect). This significantly reduces sound insulation, so the actual sound insulation performance is equivalent to that of single‑layer glass of the same total thickness.

Once vacuum glass is evacuated, sealing between the panes becomes much more difficult. Due to weather and atmospheric pressure changes, most vacuum glass products develop leaks within 3 to 5 years, leading to fogging and whitening of the glass. Sound insulation then degrades to a level comparable to insulating glass, including the same low‑to‑mid frequency coincidence effect.

Illustrated explanation:

Diagram 1 shows a complete hollow structure with no direct contact between panes (no overlap).

Diagrams 2, 3, and 4 show acoustic bridge structures (a fixed connection exists between the two layers of material).

Analysis: The complete hollow structure without overlap (Diagram 1) exhibits a low coincidence dip at 125 Hz, but its overall sound insulation is significantly better than that of the acoustic bridge structures shown in Diagrams 2, 3, and 4.

Conclusion: The actual sound insulation performance of vacuum glass is comparable to that of single‑layer glass of the same thickness. Some manufacturers mislead consumers by claiming that vacuum glass provides the best sound insulation. Others even falsely label insulating glass as vacuum glass. Buyers should be cautious.

4. Sound Insulation Performance of Laminated Glass

The sound insulation of laminated glass is determined primarily by three factors: the actual thickness of the glass, the thickness of the laminated interlayer, and the damping performance of that interlayer. For all materials, sound insulation roughly follows the mass law - surface density (material density × thickness) largely determines the overall sound insulation level.

All sound insulation materials exhibit a coincidence effect. The key to effective noise and vibration control is to ensure that the coincidence effect band does not overlap with the frequency band that needs to be attenuated. For laminated glass, the coincidence effect occurs at approximately 3,000 Hz - well above the low‑ and mid‑frequency bands of traffic noise. This makes laminated glass an ideal choice for buildings located near major traffic arteries.

Sound waves striking glass cause it to vibrate, which then radiates noise on the other side. Single‑layer, insulating (hollow), vacuum, and laminated glass all vibrate when exposed to sound waves. The interlayer film in laminated glass acts as a damping layer, effectively suppressing vibration and thereby improving sound insulation.

The better the damping performance of the laminated interlayer, the better the overall sound insulation of the laminated glass - improvements of 1~3 dB are possible. This is especially true for mid‑ and low‑frequency noise.

Sound insulation spectrum of laminated glass: The coincidence dip for laminated glass occurs between 2,000 Hz and 3,000 Hz - well away from the dominant frequency range of traffic noise.

5. Sound Insulation Performance of Double‑Window Structures

Description of double‑window structure

In standard insulating glass units, the gap between the two panes is typically only 6–24 mm. Increasing this gap shifts the coincidence dip of the insulating glass toward lower frequencies. When the gap reaches 80 mm, the coincidence frequency drops to approximately 63 Hz. Increasing the gap further moves the coincidence frequency even lower — farther away from the traffic noise band - resulting in higher overall sound insulation.

Double‑window construction: A laminated glass sliding or casement window is added to the existing window (which may be an insulating glass casement or sliding window). The distance between the two windows should be greater than 80 mm. This creates a double‑window sound insulation assembly.

Sound insulation principle of double‑window systems: Sound waves cause the first window to vibrate, which in turn excites the air between the two windows. The air layer — when the gap exceeds 80 mm - acts as an air spring, reducing the vibration transmitted to the second window. The greater the distance between the two windows, the more energy the vibrating air loses before reaching the second window, and the better the sound insulation performance.

6. Best Sound Insulation Options for Doors and Windows

For buildings located near busy traffic arteries, two window configurations offer the best sound insulation:

(a) A single‑window system using three or four layers of laminated glass, combined with extra‑thick frames and a fully sealed structure. This can achieve a measured sound insulation of more than 40 dB(A).

(b) A double‑window system consisting of an outer window with standard insulating glass and an inner professional laminated glass soundproof window, with an air gap greater than 80 mm between them. This air‑spring structure also achieves a measured sound insulation of more than 40 dB(A).

Important Notes (Precautions)

Single‑layer windows can never achieve better sound insulation than double‑window systems.

The discussion above focuses on sound insulation principles related to glass selection. The overall sound insulation of a complete window also depends on other factors, including frame profiles and seals.

Sound insulation follows the weakest‑link principle: the overall performance of the entire window is limited by its least effective component. (This is analogous to the way a wooden barrel's capacity is limited by its shortest stave.)