NVH in Electrical Vehicle (NEV)

With the introduction of electric motors, new unfamiliar sound that is often tonal and whining, and in some cases, such as from power electronics, are very high-pitched and annoying and not connected to the driving situation. In winter climates, new situations occurs such as an increased proportion of noise from the combustion-powered car heater and range extenders, increased share of studded tires, audio, frequent transitions between electric and internal combustion engine, lack of masking of weaker sounds as ESP, squeaks and rattles, etc.

Today's electric vehicles are often modified standard cars with well-insulated engine compartment and the relatively high weight. For future electric cars the requirements for low weight cars will increase, making the importance of NVH in electric cars to increase over time. Car companies therefore need new experiences and test procedures to systematize and track the various NVH sources.

Project Background

The technology used in hybrid/electric vehicle concepts is significantly different from conventional vehicle technology with consequences for noise, vibration and harshness (NVH). Each hybrid/electric configuration brings unique NVH demands that result from a number of sources. In general, sound and vibration levels in the car's interior are significantly lower for EV's. However, tonal components due to electromagnetic forces in the electric motor are in varying degree present and provide audible feedback of operation during driving.

Aim

Which are the customer preferences for electric motor sound and what measures and methods are needed to fullfill targets?  

In order to answer the aim the following underlying research questions need to be addressed:

 Which is a correct way of specifying target vibration for attatchment points of an electric motor to the receiving structure? Can frequency response based substructuring be an approach for translating test rig vibration levels to complete vehicle sound pressure levels?

Industrial relevance

Electric propulsion is in line with the product strategy of Volvo Cars. Customers will have great expectations on an electrified premium car; it should be quiet and refined but also have the right sound character. Therefore it is important to secure NVH competence in this area. Knowledge in psychoacoustics and sound of electric motors will be increased and engineering methods will be further developed. The outcome of this work will provide guidelines for complete vehicle, system and component target setting for electrified Volvo cars.

Approach and findings so far

• As an initial study, investigations were carried out to determine how the test environment influences the evaluation of the sound perception in an EV. Headphone evaluation with playback of binaural head recordings in both listening lab and a vehicle demonstrator "sound car" were compared to the real perception in a production-like electric car. Although there was no difference in mean rating for the same sound stimuli between the three conditions, the variance in within-subject differences between the in-car and the two laboratory conditions is less for the subjects who assessed the in-car condition first compared to those who started evaluating in the laboratory.

• The follow-up study was conducted as a master's thesis work with the purpose of providing metrics and targets to the perception of electric vehicle motor noise. The objective was to investigate the perception of tones with varying levels in a constant masker consisting of road and wind noise. The relationship between the subjects' ratings and an objective measure called prominence ratio were studied. The tones in the study below 1 kHz were harder to detect than the tones above 1 kHz. frequencies above 1 kHz were rated as more annoying than tones below at a significance level of 95%.

• Ongoing studies covers spatial mapping of airborne transfer functions (ATF) of an ERAD. Investigations on how the ATF from different positions over the ERAD surface to a fixed response position varies in strength.

A licenciate thesis will be presended during the autumn of 2013.

Publications within the project

1. Sound Quality Evaluation of Electric Cars – Preferences and Influence of the Test Environment, Aachen Acoustic Colloquium 2011.

2. The influence of the acoustic transfer functions on the estimated interior noise from an electric rear axle drive. (SAE Paper, submitted 2013)

3. Determination of radiated sound power from an electric rear axle drive in situ and its contribution to interior noise (SAE Paper, submitted 2013)

4. Tones in Noise - A study of the perception of motor noise in an electric vehicle. Masters thesis at Volvo Cars 2013