Determining Road Noise
Road noise inside the Sprinter was considerable because the cargo area was completely bare, consisting only of untreated steel structural members and body panels (see Figure 6). In addition, the cockpit area headliner and the A- and B-pillar panels were removed. It was of the utmost importance to determine the road noise spectrum to aid in the selection of appropriate damping and barrier materials.
Road noise inside the Sprinter at the passenger's listening position, which was believed to be identical to the road noise at the driver's listening position because of symmetry, was determined while the vehicle was driven on a freeway at a speed of 60 mph (96.6km/h). The PAA2 was programmed to record sound using A-weighting, the 50 to 110dB measurement range, and a 250ms response time. While an assistant drove the Sprinter, five measurements were taken at the passenger's listening position. The mean SPL was calculated for each of the 31 frequency bands.
The overall A-weighted road noise was 82.7 2.3dB. Figure 7 shows a graph of the A-weighted frequency spectrum of road noise. Besides the high-overall road noise, two large peaks, each about 75dB in absolute magnitude, were evident at 25 and 125Hz. A broad plateau in the road noise of about 70dB in absolute magnitude also occurred from 800 to 2,000Hz. (See Figure 7)
The Sound Dampening Process
For sound damping and barrier materials, I chose from Cascade Audio Engineering's line of finely engineered products. As a polymer scientist who specializes in the field of viscoelasticity, I believe Cascade Audio offers a unique and appropriately broad array of products, each specifically tailored to give the highest performance. I consider Cascade Audio's products essential to the sonic performance of my audio system.
The interior sheetmetal surfaces were treated with VMAX, a sound-damping pad constructed from an advanced, non-curing butylene rubber bonded to a thin layer of black aluminum. VMAX possesses exceptional damping properties and is easy to cut and apply. The entire treatment of the cargo area, for instance, was completed in one afternoon. The application of VMAX was intended to damp sheetmetal resonances at 200Hz and above. Figure 8 shows the cargo area of the Sprinter after VMAX was applied. Note that the floor was intentionally not treated at this point in the fabrication process because significantly more work inside the van will be required. Treating the cargo area floor with sound damping materials and barriers will logically take place after much of the cargo area interior is completed. In another article, we'll cover how VMAX was applied to other areas of the van such as the fire wall, dash, A-pillars, cockpit floorboards, doors, and driver- and passenger-side foot wells. (See Figure 8)