New Windshield Improves Vehicle Interior Cabin Noise and Articulation Index

Sound and Vibration, Dec 2003 by Lu, Jun, Pyper, Jay

Acoustics plays an ever-increasing role in the automotive industry, particularly in the quest for a quieter passenger cabin. Consumers equate quiet with quality, as shown in the J.D. Power and Associates 2002 Vehicle Acoustics Study, and the OEM that can provide a quieter passenger cabin will gain a competitive edge in the marketplace.

One new factor contributing to consumer demand for quieter vehicles is the increasing use of telematics within the vehicle including voice-activated technology and cell phones. It is now important that OEMs examine all possible ways to reduce road and wind noise as well as structure-borne noise penetrating the cabin to levels where the human ear can easily and comfortably hear speech.

The intelligibility of speech is defined as the accuracy with which a normal listener can understand a spoken word or phrase. Speech has a dynamic range of about 30 dB in each 1/3-octave band from 200 to 6000 Hz, and the long-term rms overall sound pressure level at the speaker's lips is about 65 dB. The Articulation Index (AI) is a measure of speech intelligibility in continuous noise. It ranges from 0 to 1, corresponding to 0% and 100% intelligibility, respectively. AI determines a ratio between the received speech signal and the level of interfering noise. Speech intelligibility is based on this signal-to-noise ratio, which is determined by calculations within 18 1/3-octave bands or other appropriate methods described in ANSI S3.5. A higher ratio indicates greater speech intelligibility. For example, AI of 0.3 or below is considered unsatisfactory; 0.5 to 0.6 is satisfactory; and greater than 0.8 is excellent.

Within every acoustic environment, there is always a certain degree of ambient background noise present that interferes with the speech signal. This reduces the signal-to-noise ratio and the receiver must significantly concentrate on the speech in order to hear it with any degree of accuracy. Therefore, reducing noise entering the cabin that is contributing to ambient noise would be beneficial in enhancing speech intelligibility.

While many vehicle manufacturers inelude noise-reducing structures and packages in the production phase of the vehicle, a pervasive weak link in Noise Vibration and Harshness (NVH) solutions for vehicles is the glass. In particular, the vehicle's windshield. Previous efforts of the automotive industry to improve cabin NVH have rarely focused on designing the windshield for improved interior acoustics.

Acoustic energy can be transmitted rather easily through the vehicle's windshield compared with other passenger compartment boundaries. At high operating speeds, strong aerodynamic pressure fluctuations around the windshield cause the glass surface to radiate noise to the vehicle's interior. In addition, impinging airflows on panel edges and bends in the vehicle's exterior can generate acoustic noise with subsequent transmission to the vehicle interior. The transmission of airborne noise generated by adjacent moving vehicles and structure-borne noise created by structural vibrations of the car body contribute to the noise transmission of windshields. Given this information, automotive glazing now becomes a primary focus as a transmission path to the vehicle interior for road and wind noise, external airborne noise and structure-borne noise.

The standard laminated glass windshield was first introduced for the safety benefit of providing occupant retention in the event of a crash. It also has proven vibration damping characteristics. Laminated glass consists of a 'sandwich' of a tough, polyvinyl butyral (PVB) interlayer bonded between two sheets of glass under heat and pressure. The PVB interlayer damps vibrations in the glass and, in automotive applications, produces a significant reduction in road and wind noise. Remarkably thin, the 'sandwich' ranges between 3.8 to 5.2 mm in thickness, depending upon its application, and weighs about 11% less than tempered glass of similar thickness. This weight reduction becomes important in overall vehicle design given OEMs' concern with vehicle weight.

The typical approach for an acoustics solution with glass is to increase the thickness of the glass. Yet, this approach provides limited benefits in terms of attenuating cabin noise and is not a viable solution based on the weight and fuel economy concerns of the automotive industry. With the increasing demand for even quieter automotive interiors without adding weight to the vehicle, there is a strong demand for improved control of NVU via the windshield through product design at equivalent or lesser weight than that of standard windshields.

The acoustic performance of tempered glass and standard windshields made with PVB can be characterized by the Sound Transmission Loss (STL). The STL of tempered glass alone of varying thickness corresponds to the basic sound transmission behavior of a sound barrier with the response at low frequencies being determined by the panel's static stiffness. Since glass reacts best to excitation frequencies that match its natural frequencies, the low internal damping of tempered glass produces resonances that dramatically decrease STL. Sound transmission follows the mass law of acoustics above the resonant frequencies and is dictated by the mass or surface density of glass. In this mass-controlled region, the transmission loss increases approximately 6 dB by doubling its surface density or frequency.