Tech Briefs

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A HEAD in NVH analysis

HEAD acoustics' SQlab II Mobile Laboratory for in-vehicle noise and vibration studies.

HEAD acoustics is known in the NVH industry for their pioneering efforts in artificial-head technology and psychoacoustic workstations that are considered the standard for vehicle-sound-quality evaluation. The HEAD measurement systems allow binaural recordings that provide "virtual reality" when played back. Listening to binaural recordings while analyzing data on the company's ArtemiS analysis system guides the operator in troubleshooting noise problems and choosing the best metrics for noise evaluation. HEAD acoustics has expanded from this base to offer tools that allow more general-purpose NVH studies.

In response to the growing demand within the automotive industry to do more testing and analysis "in the computer," HEAD acoustics has developed simulation tools that bring computer models to life and allow engineers to experience design variations before vehicles are built.

The four primary simulation tools offered by HEAD acoustics include the Sound Simulation System, Binaural Transfer Path Analysis, Binaural Transfer Path Synthesis, and Binaural Synthesis.

• Sound Simulation System (H3S): A real-time, calibrated acoustic subsystem developed primarily for driving simulators. It generates and modifies realistic cabin sound environments for different driving situations in a wide range of vehicle and engine types. The simulation includes both airborne and structure-borne sound excitation and can include presentation of vibration data. Aurally-accurate simulation of vehicle interior noise is achieved by calling up short, binaurally-recorded or modeled segments from noise databases. Real-time simulation of rapid dynamic changes in driving modes such as sudden application of full throttle, gear change, road type change, etc. are performed under the control of an external vehicle dynamic model or may be handled using a simple control program networked via TCP/IP to the H3S PC. The simulation combines engine, tire, wind, and background noise to create a realistic sound impression. An innovative synthesis algorithm responds to changes in drive modes, avoiding the perception of periodicity of any of the elements creating the sound field.
• Binaural Transfer Path Analysis (BTPA): An analysis method that dissects the spatial sound field heard by the driver into its individual path contributions.
• Binaural Transfer Path Synthesis (BTPS): A tool permitting the listening to and analysis of each path independently for identification of those paths that contribute the most to objectionable noise attributes. It also permits superposition and audition of the spatial sound field path by path; for example, BTPS results can be "driven" by the H3S.
• Binaural Synthesis: Generation of external, non-stationary sound sources in the virtual environment, such as other vehicles on a road. An online binaural convolver generates directionalized binaural sound sources from single-channel source data. Calculation includes not only primary sound components, but also reflections, reverberation, and Doppler shift. (Instantaneous geometry and velocity vector relative to the listener are calculated for sources according to their spatial travel paths.)

Since 1992, HEAD acoustics has developed techniques for BTPA and BTPS and has completed many field-based tests using the technology. Results from several dozen vehicles have been used to demonstrate how the vibration and airborne excitation of the powertrain, engine mounts, and body affect noise quality in the vehicle cabin.

Of special relevance are the methods HEAD acoustics has developed for identifying the individual paths responsible for transmitting noise in vehicles. These methods can be used in situ, without disassembly or removal of vehicle components. Another advancement is a technique for transposing data measured on a test stand into the actual operating environment.

Typical configuration of an acoustic simulation system showing H35 installed in a passenger car, enabling interactive modification of the sound scenario. Click to enlarge

Acoustic development of today's vehicles demands the ability to predict sound quality, but calculating predicted sound quality using numerical values is generally insufficient. Reliable evaluation requires listening to, or "auralizing," predicted sounds. This requirement has resulted in the development of the Binaural Hybrid Model based on a combination of actual airborne and structure-borne signals in an acoustic simulation environment. Use of this model is particularly appropriate for troubleshooting and sound-design tasks.

The presence of annoying noise components in the cabin of a vehicle is generally the driving force behind NVH troubleshooting studies. BTPA enables quick identification of the relevant sources and transfer paths, revealing the root cause of a particular noise problem. It also offers cost reductions in the NVH refinement process by identifying the source of sound problems faster, avoiding unnecessary testing and trial-and-error fixes.

Simulating interior noise characteristics requires simulating not only spectral patterns but also time structures. The calculations required are based on complex airborne and structure-borne transfer paths. Aurally accurate spatial playback based on artificial head measurements enables the reliable evaluation of sound quality.

Structural modifications to the body of a vehicle are minimized by measuring a complete vehicle, including the installed engine and powertrain. Three-dimensional measurements are taken to determine the transfer functions of the engine mounts, the input impedance of the body, and the structure-borne transfer functions. The final step is to identify the transfer paths from the engine compartment to the ears of the driver or passenger, taking into consideration the "room" acoustics of the engine compartment. Because the removal of major components such as the engine are often required in conventional transfer path measurement, the transfer behavior typically changes over certain paths. The number of iterations and likelihood for erroneous results are minimized with BTPA evaluation of a fully assembled vehicle.

In addition to systems developed for effective evaluation of vehicle sound quality, HEAD acoustics has developed a line of telecommunication test systems known as ACQUA for evaluating speech quality.

A major area for application of HEAD acoustics telecom analysis is hands-free telephony, especially in the burgeoning field of automotive telematics. The ACQUA telecom analysis system, especially in conjunction with the H3S real-time simulation engine, permits objective as well as subjective evaluation of hands-free telephone systems and voice-activated accessories inside automobiles under interactive driving conditions.

- Jean L. Broge

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Prodrive answers drivetrain dilemma

Efficient packaging is an essential element of modern car design, but achieving it can become a major problem when platform economics demand that a vehicle with a front-wheel-drive transverse engine be converted to one with four-wheel drive and a longitudinal gearbox. The positioning of a longitudinal gearbox may demand significant re-engineering of the vehicle structure and a complicated power transmission path. The difficulty may be compounded if the car has a short, steep hood line.

Prodrive has designed a system to overcome those problems. Its Compact Diagonal Transmission (CDT) takes power from the transverse engine to the clutch in the conventional way, then back through a hollow shaft to a bevel gear. From there it is transmitted, via what Prodrive terms a "unique" diagonal transfer shaft at some 20° to the longitudinal axis of the car, to an inline gearbox. Prodrive claims that what has prevented the successful application of such a system in the past has been the difficulty of establishing an efficient and reliable link between the diagonal transfer shaft and the inline gearbox. Skew gears are a possibility, but their downside includes severe wear and excessive heat generation.

Prodrive, working with gearbox specialist Hewland Engineering, explained that its solution was to create a novel type of compound gear using a new gear-cutting technique to achieve the required angles and tolerances. From this compound gear, power goes to the integrated center differential that sends torque back to the rear differential and forward to the front differential. The CDT can be integrated with the vehicle manufacturer's gearbox or with a wide range of Prodrive gearbox technologies.

Complementing its gearbox work, Prodrive has also introduced a supplementary pair of onboard constant velocity (CV) joints to allow an angled driveshaft to connect to the front wheels via the main CVs. This innovation significantly reduces the size of the differential pack, allowing the engine to be moved rearward, if required, without changing the original driveshaft positions, thereby improving the vehicle's weight distribution.

- Stuart Birch

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