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Noise Control and Acoustic Insulation: Sound Sources and Frequencies

Noise Control and Acoustic InsulationAcoustic insulation absorbs, transmits, or redirects sound – a form of energy that travels in waves. Unwanted sounds, or noise, aren’t just unpleasant to hear. They can harm human health, jeopardize worker safety, and contribute to structural fatigue. The consequences of noise can be severe, but its characteristics are sometimes misunderstood.

In Part 1 of this series, Elasto Proxy shared a surprising example of two machines running at different decibel (dB) levels in the same room. In Part 2, we’ll examine some other sources of noise in the industries we serve. Importantly, you’ll learn why acoustic insulation must account for more than just a sound’s source. Frequency, the speed of the change of the sound, is a key design consideration for noise control.

Noise Sources

Noise can be caused by mechanical vibration, solid-to-solid contact, and fluid-solid interaction. Mechanical vibration induces resonance in structures such as vehicles, machinery, and equipment. Solid-to-solid contact includes both impact and friction. Fluid-solid interaction happens when air, a type of fluid, flows around built structures.

Mobile equipment and military vehicles are susceptible to all three noise sources. Mechanical vibrations from a vehicle’s engine bay can penetrate the cabin’s interior. Solid-to-solid contact occurs whenever a vehicle’s tires or tracks roll along roadways or off-road terrain. Friction happens during vehicle braking, especially with sudden stops. Air flow around a vehicle’s exterior can also cause unwanted sounds.

Designers of aircraft, infrastructure, and equipment face similar challenges. Airplane flaps, helicopter rotors, wind turbines, and fans have turbulent air flows around their edges. HVAC systems and light fixtures can induce mechanical vibrations. Solid-to-solid contact happens when an airplane’s wheels hit the runway. In the food processing industry, pouring ingredients into a hopper can also cause noise.

Frequency and Noise Control

Engine Bay Insulation

Frequency is an important feature of sound and, in turn, noise control. Hertz (Hz), the unit of measure that’s used for frequency, is the number of oscillations per second in a sound wave. Higher-frequency sounds have shorter distances between oscillations. Lower-frequency sounds have longer-distances. That’s nice to know from a scientific perspective, but what are the implications for acoustic insulation?

Physical barriers are less effective against lower-frequency sounds like the rumble of a diesel engine. When the source of the noise and its receiver are on different sides of a barrier, the goal is to block the sound. That’s why in mobile equipment and military vehicles, acoustic insulation is attached to the firewall that separates the engine bay from the cabin.

Sound absorption isn’t the only method of noise control. In buildings, physically separating the two sides of a wall makes it harder for sound to pass through. Known as decoupling, this soundproofing method uses acoustic insulation as the medium between two panels. Decoupling can be effective, but only at frequencies where resonance levels won’t induce mechanical vibrations – and more noise.

As noise control methods, sound absorption and decoupling are effective when the source and receiver are on different sides of a wall, screen, equipment enclosure, or other type of barrier. When the sound and receiver are on the same side, it’s important to limit the sound’s reflections instead. Since higher-frequency sounds radiate towards the ceiling, acoustic insulation may be installed overhead.

Choose Acoustic Insulation from Elasto Proxy

Acoustic radiation is complex. Product designers, engineers, and technical buyers need to know more than just the decibel level. Both the source of a sound and its frequency are important design considerations. Acoustic insulation can also be “tuned” to permit or prevent the passage of specific frequency ranges. Design assistance, custom fabrication, and testing services can strengthen your application.

Do you need custom acoustic insulation? Then it’s time to contact Elasto Proxy.

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