Is It Useful to Put Soundproofing Cotton in the Trunk?

[Barbosa]

Generally, placing soundproofing cotton in the trunk is not useful. While installing soundproofing cotton in the trunk can absorb some noise energy and convert it into heat, thereby dissipating it, the effect of merely using soundproofing cotton to block noise is minimal. This is because most noise is transmitted into the car through the vehicle's body frame structure, and adding soundproofing cotton does little to address this. Here are some potential drawbacks of soundproofing the trunk: 1. Soundproofing the trunk requires removing the trunk liner, which is often secured with clips. These clips are very prone to damage during removal. 2. If the technician performing the removal is not skilled, it can easily lead to issues like abnormal noises during later use of the vehicle.

[Quintero]

As an audio enthusiast who has modified over a dozen car sound systems, I can confirm that adding sound insulation to the trunk is indeed effective. Tire noise and exhaust resonance transmit from the rear wheel arches into the trunk, creating a dull humming sound. In my own car, I applied a dual-layer solution: first adhering butyl damping sheets to suppress metal panel vibrations, then topping it with aluminum-faced insulation foam to absorb stray noises. The cabin becomes noticeably quieter at highway speeds, especially with the reduction of that gritty noise on concrete roads. Two important tips: 1) Avoid cheap foam materials as they emit odors under high temperatures; 2) Pay special attention to the inner surfaces of rear wheel arches - that's the primary noise source. Also check your tailgate seals, as air leaks will significantly reduce the insulation's effectiveness.

[OBryce]

Last time I helped my neighbor deal with the noise issue in his car. His old car had an empty trunk, and when the speed exceeded 70 km/h, there was a constant buzzing sound in the ears. We tried laying a full sheet of sound-absorbing foam on the trunk floor, and the actual test showed a noise reduction of about 3 decibels, with noticeable improvement in low-frequency rumbling. It's important to note that different materials vary greatly in sound insulation effectiveness—ordinary felt has limited effect, so it's necessary to use automotive-specific flame-retardant sound-absorbing foam. During installation, the spare tire and tools must be removed, and the edges must be trimmed to fit snugly; otherwise, rattling noises can become even more annoying. Finally, I suggested he add a layer of material to the back of the rear seats, as the sound was coming through the gaps in the rear seat backrest.

[VonRiver]

From a materials engineering perspective, the principle of trunk sound insulation foam lies in its porous structure absorbing sound wave energy. When a sedan is in motion, noise levels near the wheel arches can reach 70 decibels, while thin steel plates provide virtually no sound insulation. Tests show that using 2cm-thick sandwich-structured sound insulation foam (butyl rubber + aluminum foil + fiber layer) can reduce noise transmission by 5-7 decibels. However, two critical factors must be considered: first, the foam must completely cover the metal surface; second, all gaps must be sealed. Many vehicles have unprocessed wiring harness holes near taillights and rear seat fasteners, which significantly diminishes the effectiveness. A reminder for EV users: the area below the trunk contains the battery pack - be sure to avoid covering any cooling vents during installation.