Lend me your ears

Techniques developed in the auto-industry allow engineers to orchestrate machinery sounds to the ‘taste’ of the user.

Your car may be deceiving you. That roaring sound you enjoy, apparently emanating from under the bonnet, may have little to do with the engine. It does, however, have a lot to do with psychologists, software, and exciting new materials technology.

Along with performance, safety and visual appearance, sound is playing an increasingly important role in the purchase of a car. And this does not mean eliminating as much of it as possible. On the contrary, the ear also participates in the overall driving experience and has its own predilections when it comes to creating an aural impression; a sports car should sound like a sports car even if it really produces no more noise than a family estate. In fact, the same engine is often used across a range of vehicles but tuned differently for a performance signature to suit the car’s usage — or image.

At Daimler Chrysler’s research centre in Berlin, psychologists are assigned the task of improving the character and quality of the sounds heard from within the vehicle, with a view to meeting customer requirements and enhancing acoustic comfort. To this end they first assess the significance drivers attach to various sources and causes of sound. With the help of comparative acoustic tests, they then determine the subjective sound qualities of various vehicles.

Questionnaires carried out among drivers have demonstrated to the psycho-acoustic experts that almost 80% of the participants want to hear the sounds produced by the vehicle, especially engine noise, while at the wheel of the car. And for 60% of drivers, the turn indicators come a fairly close second. The ratings for other sources of acoustic information, on the other hand, such as braking and tyre noise, lie well under 20%.

There are some sounds which the driver wants to hear and must hear. But people assess the quality of a sound differently. Preferences and prejudices, experience and expectation determine the hearer’s perception of a sound and its subjective qualities.

The Winchester-based automotive test facility specialist, IAC, completed the installation of a sound quality suite for Jaguar at its Whitley Engineering Centre. The listening room provides an environment in which the Jaguar team and its customers assess the quality of both live and recorded vehicle sounds relayed through headphones or speakers. Listening ‘juries’ may be asked to comment on anything from tyre and engine noise to the whirr of an electrically operated seat.

The results of these comparisons ultimately find their way into automobile development. In a second phase, acoustic measuring technology engineers create a synthesis between subjectively perceived sound quality and measurable acoustic characteristics. Finally, design engineers devise parts and vehicle components with vibration characteristics which give rise to these subjectively perceivableand objectively measurable acoustic effects.

But how? The use of sound quality engineering tools has been mainly restricted to the refinement and troubleshooting of physical prototypes. Currently designers have few tools to help them listen to their ‘virtual’ models. However, imagine if engineers could use a computer’s mouse to listen to the engine note from within the virtual car’s interior as the car accelerates.

They could then modify an engine mount thickness to tune the car for the desired sound. Traditionally engineers have studied noise, vibration and harshness (NVH) by testing physical prototypes late in the development process — when design changes are expensive and time consuming to implement.

Advances in Frequency Domain Transfer Path Analysis (TPA), an accepted tool to model or troubleshoot lower engine orders for in-vehicle noise, allows the contribution of each vibration from the engine to be evaluated for both the amplitude and phase of the sound field’s partial pressure at the driver’s ears. From this quantitative analysis the sound paths can be ranked and critical elements of the design can be identified. In other words TPA can map a sound with its mechanical source.

Usually, such analysis is made on a stationary or slowly varying speed change, where the goal is to accurately characterise the pathways, rather than the sound quality perceived by the driver. However, there is little difference between a diesel and an ordinary petrol engine in spectral content, but the perception of sound quality is very different. Similar software developments by LMS International have led to specialised vibro-acoustic simulation software. Recently launched Sysnoise 5.5 from LMS uses patented ATV software which can predict the sound inside a cavity, estimate the sound field around a structure, or calculate the structural response to an acoustic load. As such, it is suited to solve problems in acoustic analysis of passenger compartments, acoustic radiation prediction of an engine and powertrain, and acoustic transmission analysis of wall partitions, doors and seals.

Damping

Working alongside Sysnoise is LMS’s Violins which predicts the vibro-acoustic response of 3D multi-layered absorptive panels by modelling the interactions between the vibrations of the elastic layers and the airflow through the pores of the absorptive layers. The user can select the sequence of elastic, porous or acoustic layers and can assign material properties to each layer. VIOLINS offers a detailed analysis of the fluid flow within the porous layers and of the displacements of the elastic layers, including energy densities. Additional results include surface admittance and panel transmission loss.

This recent ability to fine tune the design of absorptive layers has helped development of two successive damping technologies as Pace Award finalists. Last year saw Rieter’s Ultra Light pads go on to win the award.

The Ultra Light acoustic vehicle treatment absorbs sound efficiently enough to reduce the weight of automotive acoustic treatments by as much as 50%, or between 10 and 30 kg per vehicle (22-66 lbs.).

The key to this approach is shifting the emphasis from insulation and reflection with heavy barriers to absorption. This has been achieved with the application of a new class of multi-layer composites and lightweight materials developed and patented by Rieter.

The Rieter system is now being incorporated in programs by a growing number of OEMs on both sides of the Atlantic.

Audioguard, an alternative to pads, is a sprayable vibration damping epoxy which is becoming popular in automotives. ‘The key characteristic,’ says Dr Ray Schappert of PPG Industries, ‘ is that it converts vibrational energy into heat in its molecular structure – so it dissipates that energy and it does not get translated into the interior vehicle compartment. In general after laboratory testing it’s equal to or slightly better in sound damping performance than pads. It brings a couple advantages to the automotive customer – as it is sprayable you can target more effectively the exact location where you want to apply the material.’

What this means is a potential reduction in vehicle weight because the acoustic engineer can now target the high energy spots of the floor pan or the dash pan.’Pads are pretty labour intensive to insert into the vehicle and the spray applied material allows a more precise location. When you look at the number of pads on a vehicle, every pad has an inventory part number and that is a cost to carry all those parts. This simplifies the inventory management system by converting all those parts into one.’

The Audioguard system has been adopted by Daimler Chrysler, Ford and Audi.