Automotive Acoustic Testing

Human beings are strongly affected by sound – a dozen or so notes elicit feelings of joy or sadness, excitement or tranquility. In the wrong context, however, our reactions can be irritation and exhaustion. The roar of a Ferrari sounds exhilarating to some, but will enrage others – automotive acoustic testing is therefore a vital part of vehicle development.

Generally, engines that purr quietly are associated with quality and reliability. No manufacturer today would try to market a car with the acoustics of the Reliant Robin, even if it were legal to do so. For those who spend hours behind the wheel, noise contributes to drowsiness, road rage and gradual hearing damage. Vibrations cause travel sickness in children and have been linked to low fertility in drivers. For families living near busy roads, constant noise can make life almost unbearable.

Today’s car makers mould a vehicle’s sound profile with the same care with which they mould its chassis and performance characteristics, but it is no small challenge. The overall volume and particular noises emitted by assembled vehicles are the result of complex interactions and vary during different manoeuvres in different conditions. Tuning out those bad vibrations requires sophisticated test equipment and expertise.

The abbreviation of these three challenges – NVH – has been adopted as shorthand for the specialist engineering field that addresses them. NVH engineers first need to isolate the acoustic environment so that all the issues can be assessed without extraneous interference. For this, a variety of soundproofed chambers come into play.

Often seemingly at odds is the need to model real driving conditions inside the controlled environment. For example, testing environments need rollers beneath the vehicle’s drive-wheels which are able to simulate and record road loading, a chassis dynamometer. Even wind conditions may need simulating.

The next stage is to model the sound profiles produced across the range of conditions using instruments such as force gauges, accelerometers, microphones and load cells. Managing and visualising the information collected requires a computer program equipped to perform appropriate statistical modelling (wavelet analysis, order and statistical energy analyses and Fourier transforms).

NVH engineers often use several kinds of chamber in the course of a single project. Two main ones are vehicle semi-anechoic chambers (VSAC) and hemi-anechoic pass-by chambers. In VSAC, the vehicle is retained on dynamometer rollers for measurements at a distance from the vehicle. The Pass-by, a more rigid test, is similarly on a dynamometer but with a different microphone size and distance configuration, to simulate the vehicle driving past.

VSAC chambers are fully lined with acoustic wedges, meeting the stringent requirements of reproducing an acoustically free field environment in accordance with ISO 3745.  Pass-by chambers allow relatively realistic road simulations, also designed to give a free field environment in accordance with ISO 362-3. A VSAC will be used for assessing ambient sounds, via computer-controlled multi-speaker arrays – particularly useful for exploring noises inside the cab.  A Pass-by will be used for assessing engine and tyre noise, acceleration and exhaust noises. Investigating ambisonic effects in software helps engineers to determine the best solution before laying a hand on the vehicle, shortening development time substantially.

Standard ISO 362-1 required noise tests to be conducted on outdoor tracks. This practice is inherently vulnerable to uncontrollable environmental factors such as weather and environmental noise. The 2016 update, ISO 362-3, allows indoor pass-by testing in facilities with suitable pass-by anechoic chambers. IAC Acoustics pass-by chambers already conform to additional specifications (ISO 16254) that address noise effects on pedestrians.

A major problem in any anechoic chamber is introducing HVAC services to control the temperature, humidity, pressure and air quality within the chamber. Special measures are taken to ensure that these services do not inherently contribute noise, nor interfere with acoustic reflections. Safety is also a paramount issue, so IAC’s professionally designed chambers have several layers of safety and security, including sensor/servo controlled dampers, valves and emergency shutdown capabilities.

Anechoic Chamber entrances also require large high performance acoustic doors.  Any testing suites with control rooms will similarly require acoustic doors, acoustic  windows and ventilation systems. Ancillary equipment such as cranes, vehicle lifts, lighting rigs, CCTV systems and other instrumentation also need effective integration.

Small anechoic chambers are also designed and installed around parts of the vehicle, to focus testing on particular components. In Powertrain Chambers, front and rear wheels sit on dynamometers outside this chamber, with drive shafts penetrating the chamber walls into the gearbox.

Automotive acoustic testing may already sound complicated, but leading NVH providers such as IAC Acoustics routinely deliver another crucial area of expertise: speed.

Acoustic testing environments don’t come flat-packed. Given their size and complexity, the ability to construct, fit-out and adapt test environments swiftly is crucial. Integrating them painlessly into the client’s production lines and project schedules is always a leading consideration. Experienced professionals such as IAC Acoustics can undertake the full life-cycle of a deployment, from strategic development and engineering planning, through to final commissioning. By providing a complete and integrated automotive testing solution, including ancillary equipment and services, IAC Acoustics is able to keep programme and costs to the agreed project scope.

With the advent of a new generation of road vehicles including hybrids, electrics and engines redeveloped for alternative fuels, development cycles will need to become shorter. Yet at the same time, new engines are prone to throw up less familiar acoustic problems. This means that automotive acoustic testing and acoustic modelling has an even more important role to play and should begin as early as possible in the design process – ideally while the vehicle is still on the drawing board.

Scott Simmons

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+44 (0) 1962 873018