So, you want to decrease the noise in your vehicle while driving down the road? Or, get improved sound quality from your car’s sound system? Well first let’s discuss sound, including what it is and how it can be dampened.
Sound is a form of vibrational energy that propagates through a fluid (ex. solid, liquid, or gas) as a wave, with vibrations being the key component. Noise is, simply, undesirable sound, either because it is loud, unpleasant, or impedes one’s ability to hear properly. Within the auto world, reducing noise is beneficial to those that want a quieter drive or improved sound quality from their stereo system. In the case of RVs, campervans, cargo vans, old muscle cars, or trucks reducing noise while driving is commonly sought after. Lucky for you, many industries have investigated the topic of how to reduce vibrations, with the aerospace industry having done the most thoroughly documented research!
Here are some passive ways sound can be dissipated:
- Distance – Increasing the distance between the source and the receiver. As sound waves disperse, their energy density decreases.
- Reflection – Reflecting sound can change the direction the sound wave is traveling and will be different based on the material it hits.
- Diffusion – Diffusing sound will scatter the energy in multiple directions, decreasing the concentration of sound in any direction. Theoretically, if a vibration is fully diffused before a sound is formed, you will not hear anything. In the case of a vehicle, diffusion affects any sounds coming from the vibration of the vehicle’s body.
- Absorption – Converting sound energy into heat using materials that are porous absorbers or resonant absorbers. Absorption is, generally, dissipating airborne sound.
All of these methods have some level of overlap. Of course, increasing the distance isn’t really an option for a vehicle. Reflection may have minor improvements in reducing noise. But, because energy can only be transformed from one type of energy to another, you will see the largest improvement in noise reduction when you convert sound energy into an un-audible form. Both the diffusion and absorption of sound commonly convert sound energy to thermal energy. Because the loudest sounds in a van are coming from the body of the vehicle, it makes sense to target the reduction of sound through diffusion.
To understand the transformation of sound energy to thermal energy and, thus, how the diffusion of sound works, it is important to understand the physical mechanism of damping. When a sound wave’s amplitude of oscillation decreases with time, either due to friction or other form of resistance, the wave is said to be damped. In the case of sound diffusion in a vehicle, this vibrational energy is being dissipated as heat.
So, let’s go back to discussing why a van is noisy and discuss what can be done to reduce this noise. Metals are poor damping materials, resulting in their vibrations having high amplitudes. With a cargo van’s cargo area consisting of all metal, this makes for a loud ride. Structural damping is the preferred method for reducing this noise. Structural damping uses viscoelastic materials to store strain energy when deformed and dissipate some of this energy through hysteresis. Thus, structural damping dissipates vibrational energy before it is radiated as noise.
Now that you understand a bit about sound, ways sound can be dissipated and damping, let’s delve a bit deeper and talk about constrained layer damping. Below is a graphic summing up the previous section and where we are about to delve into: