Jaguar says its new diesel-powered XJ luxury sedan achieves "world-class smoothness and quietness" because of sound-deadening technology it developed with suppliers, according to Automotive News Europe.

Jaguar credits an innovative sound pack from German supplier Carcoustics and acoustic laminated side glass from Sekurit Saint-Gobain of France. In addition, active engine mounts for the diesel XJ developed by British supplier Avon Rubber cancel 90% of idle vibration, a frequent source of irritation on diesel cars.

The XJ is Jaguar's first diesel in the upper-premium segment and is designed to compete with diesel models from higher-volume German competitors: the Mercedes S class, BMW 7 series and Audi A8.

"We wanted the [XJ] V6 diesel to match the V6 [petrol] model for interior noise at idle," said programme chief engineer Dave Mitchell. The raw noise emissions of the diesel engine at idle are three times greater than those of the petrol V6, said Mitchell.

A key goal was eliminating noise in what Jaguar calls the "cackle region," full-throttle engine loads between 2,000 and 4,000 rpm, Mitchell said.

Working with Carcoustics, Jaguar developed an under-bonnet sound pack that met noise goals and was 20% lighter than the petrol model.

The Carcoustics package includes a double bulkhead liner, tunnel liner and insulation on the engine compartment sides and the wheel arches. The fibreglass bonnet liner has integrated seals to close all gaps and prevent the escape of radiated noise. It also has a multi-layer undertray seal (using acoustic materials similar to those in anechoic chambers) under the vehicle, Mitchell said.

Jaguar's specifications for the diesel XJ were challenging, said Roland Bischoffs, head of marketing for Carcoustics.

"The target was superior acoustical comfort: the dynamic of a high-powered diesel and the absence of any diesel-specific noises," he said. "The package was developed with substantial support from our tech centre in Leverkusen, Germany."

Sekurit supplied 5mm thick, five-layer acoustic glass for all openings except the rear window and the rear quarter lights, giving a 7dB reduction over standard laminated glass.

Jaguar says the combined effect is a new level of diesel refinement. Idle vibration measured at the steering wheel is half that of competitor models, claims Mitchell. The XJ's 40dB interior noise level at idle matches the petrol XJ - and beats the 43 dB of the comparable BMW diesel 7 series, he said.

Hearing experts say each 3dB represents a doubling of sound pressure level, but is at the lower end of what humans can perceive. But a 5dB difference is easily perceptible.

"Generally our car is 5 dB quieter than the competition during driving, especially in the cackle region," said Mitchell.

Jaguar has adapted the diesel sound pack for use on petrol XJs, carrying over the Sekurit acoustic laminated glass and Carcoustics sound deadening except for undertray and wheel arch liners.

Auto market intelligence
from just-auto

• Auto component fitment forecasts
• OEM & tier 1 profiles & factory finder
• Analysis of 30+ auto technologies & more