In today’s society, with the continuous expansion of highways, railways, urban elevated roads, and urban expressways, traffic noise pollution has become one of the most pressing environmental concerns. As the most direct and effective infrastructure for controlling traffic noise, noise barriers are widely used in road noise reduction projects.
This raises a key question: exactly how many decibels can highway noise barriers reduce?
The noise reduction provided by sound barriers involves two key concepts: “laboratory sound insulation performance,” which refers to the inherent sound insulation properties of the material, and “actual on-site noise reduction,” which is the most commonly confused of the two. There is a significant difference between these two concepts, Below, LuoFu will address this question from these two perspectives.
Sound insulation performance of laboratory materials:
Testing the sound insulation parameters of noise barrier panels individually under ideal laboratory conditions serves as the core basis for evaluating their sound insulation coefficients: Domestically, these tests comply with national standards GB/T 19887 and GB/T 51335-2018; Internationally, we adhere to the EU acoustic testing standard for road noise barriers, EN 1793, and the laboratory sound insulation testing standard for building components, ISO 10140. These standards serve as the core basis for export trade and acceptance of overseas projects.
Sound insulation parameters for mainstream conventional noise barrier panels:
Metal acoustic barriers (perforated aluminum sheets/galvanized steel sheets): Standard sound insulation value ≥25–30 dB(A).
High-quality composite panels can achieve 30–35 dB(A) and are the mainstream materials for expressways and elevated roads.
Transparent PC weather-resistant panel noise barriers: Sound insulation value of approximately 20–28 dB(A), striking a balance between sound insulation performance and requirements for natural light and visibility.
Note: The above values represent the maximum sound insulation values under ideal conditions. In practice, sound waves undergo diffraction and scattering, making it impossible to achieve 100% sound insulation. Therefore, the actual perceived noise reduction will be lower than these values.
Actual noise reduction at the site: (Insertion Loss, IL)
What is Insertion Loss (IL)?
In all international highway standards—including ISO 10847, EN 1793-4, and the U.S. FHWA/AASHTO standards—Insertion Loss (IL) is the final and sole on-site acceptance criterion for evaluating the noise reduction effectiveness of sound barriers.
Formula for IL:
IL = L1 (original noise level without barrier) − L2 (noise level after barrier installation)
Units are uniformly expressed in dB(A) (A-weighted, reflecting the actual hearing sensitivity of the human ear).
The laboratory sound insulation value of a noise barrier represents the upper limit of the product’s intrinsic performance, while the IL (Insertion Loss) represents the final, actual noise reduction effect in the on-site environment after installation. Actual industry conditions: Conventional highway noise barriers provide stable noise reduction of 12–20 dB(A); elevated, irregularly shaped barriers can achieve 20–25 dB(A); and fully enclosed structures can reach up to 28 dB(A). The primary factors affecting IL are not the sound insulation of the panels themselves, but rather diffraction control, barrier height, site elevation differences, and construction integrity. This is also the fundamental distinction between professional acoustic barrier engineering and ordinary fencing.
Typical noise reduction ranges for highway projects:
Basic low-profile barriers (2–3 m) near low-rise residential buildings: 5–12 dB(A);
Standard highway acoustic barriers (3.5–5 m), mainstream engineering models: 12–20 dB(A);
Elevated curved or fully enclosed acoustic barriers (6 m or higher), fully enclosed viaducts: 20–25 dB(A)
Standard highway acoustic barriers (3.5–5 m), mainstream engineering models: 12–20 dB(A);
Elevated curved or fully enclosed acoustic barriers (6 m or higher), fully enclosed viaducts: 20–25 dB(A)
Differences in Noise Reduction Across Frequency Bands
Traffic noise is categorized into low, mid, and high frequencies, and the noise reduction effectiveness of sound barriers varies significantly across these ranges:
Low-frequency noise:
Low-frequency noise is relatively difficult to reduce. Below 500 Hz—such as truck engines and road vibrations—sound waves have longer wavelengths and are prone to diffracting over the top of the barrier, resulting in an attenuation of only 6–10 dB(A).
Mid-frequency noise:
The overall rumble of traffic. 500–2000 Hz: The primary noise reduction range is 10–16 dB(A), and this is the main frequency band targeted by conventional barriers.
High-frequency noise:
This is the most effectively reduced. Above 2000 Hz—such as tire friction and the shrill honking of trucks—it can be attenuated by 15–22 dB(A). This is also the type of noise that is most noticeably perceived.
Key Factors Affecting Noise Reduction Performance
1、Barrier Height (Most Important Factor)
The taller the barrier, the larger the acoustic shadow zone. Industry experience shows that for every 1-meter increase in height, the overall noise reduction improves by approximately 1.5 decibels; therefore, doubling the barrier height results in a 6-decibel improvement in noise reduction.
2、Distance Between the Sound Source and the Screen
The closer the sound source is to the noise barrier, the more effective it is; beyond 60 meters, the noise reduction effect decreases significantly. Therefore, the optimal protection range for highway noise barriers is buildings located within 0–50 meters on either side of the road.
3、Screen Structure
noise barriers are also classified into sound-absorbing and purely sound-insulating types. For example, using a composite barrier with a micro-perforated surface and a sound-absorbing core—such as a Metal Micro-perforated Noise Barriers—on the side facing the road results in a noise reduction of 4–8 decibels more than a single-layer sound-insulating panel, thereby reducing the accumulation of reflected noise.
4、Barrier Length
In design, the barrier must fully span the width of the residential area. If gaps are left at either end, sound waves will diffract from the sides, directly reducing the noise reduction effect by 5–10 decibels.
5、External environmental factors:
Topography and terrain: In open areas, noise reduction is relatively effective; however, in valley terrain, sound wave reflection and convergence may reduce noise reduction by 3–5 dB. When situated against a slope, the reflective enhancement effect can be utilized, resulting in an additional 2–3 dB improvement in noise reduction compared to flat terrain.
Weather conditions: Under strong tailwinds, sound waves are prone to diffraction, resulting in a reduction in noise reduction effectiveness of 3–6 dB; under headwinds, noise reduction effectiveness may increase slightly by 1–2 dB. In high-humidity environments, the noise reduction effectiveness of certain sound-absorbing materials may decrease by 1–2 dB.
By Sound Source Characteristics: For low-frequency noise generated by heavy-duty trucks, noise barriers can reduce noise levels by 5–10 dB; for high-frequency noise generated by passenger vehicles, the noise reduction effect is 10–20 dB. When vehicle speeds are high, the noise reduction effect diminishes, with the reduction being 2–3 dB less than at low speeds.
In summary, the noise-reduction effectiveness of highway noise barriers is influenced by a variety of factors, including the barrier’s height, length, structure, and materials, as well as the characteristics of external sound sources, topography, and weather conditions. For every 1-meter increase in barrier height, noise reduction can improve by 1 to 3 decibels. Different materials offer varying levels of sound insulation, while sound sources, topography, and weather conditions can also cause fluctuations in noise reduction effectiveness to varying degrees.