UK independent vehicle test, validation, and engineering services provider, Millbrook, is considering introducing acoustic holography as the next step in the further development of its new noise-measurement systems.

“Able to support the latest automotive technologies including electric and hybrid vehicles, our newly commissioned system provides a great opportunity to introduce more advanced test methods to reduce NVH, test to regulatory requirements, and facilitate investigation and development into improvements in vehicle refinement,” said Millbrook’s Principal Engineer, Ravi Bal.

He explains that the new system is ideal for (but not limited to) sound source localization and contribution analysis. Engineers will be able to measure acoustics and vibration inside a vehicle and sync it with data recorded outside, conducting a variety of objective measurements in a repeatable test environment.

The system that it replaces had been in service for 20 years, stated Bal: “Although everything was digital, there were limitations for efficient and thorough data analysis. So we have made a substantial investment in new equipment—both hardware and software—to expand on our test capability and to introduce more advanced test methods.”

The update has seen an advance from a two-channel setup to a 16 channel: “So we are looking at the introduction of acoustic holography, essentially an acoustic camera that can visually indicate sound sources and highlight potential problem areas. This requires multiple microphones built as an array, so our 16-channel capability allows us to do exactly that, with potential scope to improve spatial resolution by increasing the number of microphones used.”

Together with other upgrades, including Millbrook’s dynamometers, investment in its Portable Emissions Measurement System (PEMS), and increased capability within its test department, the new noise system provides the opportunity to test, investigate, and develop “way beyond” basic regulatory requirements, stressed Bal.

Millbrook’s work on noise was enhanced in 2012 with the introduction of an ISO 10844:2011 specification noise test surface, providing an advanced facility for investigating and understanding vehicle noise: “Whilst the anticipated update to European legislative type approval requirements did not go ahead as predicted in 2014, Millbrook remains at the forefront of the latest technology for test and development of noise solutions.”

Global noise test level

Efforts to introduce a global regulated legislative level are still under discussion, but it is difficult to say when the next legislative date will be.

Bal regards European pass-by limits on noise as being “fairly stringent.” They apply to all vehicles, including high-performance cars that are applicable to the European Community Whole Vehicle Type Approval (ECWVTA) framework directive. There is a fixed test procedure—ISO 362—first established in 1981, which was revised this year. Currently, though, the European regulatory standards use a 1998 version.

He explained: “With regard to high-performance cars, the market requires they have the ‘right’ exhaust note for their type—we all do! The current Type Approval test method varies according to the power output of the vehicle category and transmission type of the vehicle.

“As an example, a car with power output of 100 kW and manual transmission (categorized M1 by Millbrook), is tested in second and third gears at an entry speed of 50 km/h. At a specific point, designated Line AA, the car is accelerated on a “wide open” throttle for 20 m and then the accelerator is lifted. In second and third gears, a mathematical average is applied to the results and that must be below the limit applied to that particular class of vehicle. It should be considered for the majority of modern vehicles, that with electronic throttles, when a driver presses the accelerator pedal he is not making the vehicle’s speed increase but merely requesting that it should go faster. The limit is 74 dB(A).”

However, for test vehicles with a power output greater than 140 kW (and other stipulations), it is required to enter the test zone with only third gear selected, which typically tends to be quieter (lower engine speed than in second) but is given a 1 dB provision for its class, consequently raising the limit to 75 dB(A), which is a challenge.

The market emergence of electric and hybrid vehicles has brought a new dimension to noise testing. Bal explained: “We regard dB figures of mid to high 60s as the norm for these vehicle types based on our experience, and we attribute most of this to tire noise, with tire manufacturers required to declare the sound pressure generated by the tire—although this is tested via a different methodology to the whole vehicle.”

Reducing tire noise in electric vehicles increases the need to consider an artificial approach-warning systems for pedestrians. This is another aspect of noise testing. Bal wants to see limits that require minimum sound pressure levels at key frequencies: “These key frequencies should be established using data relating to a person’s perception of sound, i.e., can they hear it over a typical background noise? Therefore, creating a noise that is more than a constant drone, and distinguishable as an electric vehicle.” With a typical ambient noise level of around 46 dB, the situation of Millbrook’s noise site is conducive to achieving accurate measurement data.

Surface test

Although Millbrook has a control surface test area (ISO 10844), coupled with measurement equipment applicable to ISO 362 that can be used as a point of reference, tire test noise per se is not part of Millbrook’s work, but Bal says the possibility is under discussion. Potentially, it would involve a large investment and, again, varying standards across the world would present complications: “The European tire regulation R 117 has four elements: wet grip, rolling resistance, snow performance, and noise. We can assess noise by using a test trailer but it is quite an involved program. The tires have to be loaded to a set percentage of their maximum permissible load carrying capacity.”

If Millbrook did move into tire noise testing, it would also want to enter the other three categories, too. Bal added: “Rolling resistance is relatively straightforward, but wet grip would require further investment. We work in collaboration with Test World, the winter vehicle and tire test facility in Finland.” It facilitates access to snow all year round.

Bal hopes Millbrook will eventually test whole vehicle noise levels, embracing vehicle body structure, contact patch, aerodynamics, and powertrain contributions.

But he wants to see any new test regulations created via a different procedure: “At present, the perception is that legislators decide what is going to be required, but if a more pragmatic approach could be agreed, taking into consideration OEM and suppliers’ views at the earliest stage of decision making, a more appropriate conclusion for the industry could be reached.”