How Lexus LS 600h Became the World’s Quietest Passenger Car

Automotive tests of the cabin/interior noise level when driving at different speeds seem to all conclude a clear winner. A winner that distances itself from the rest when it comes to offering a quiet car. I am talking about the Lexus flagship model LS 600h.

Clearly, putting together a car is a complex endeavor and the manufacturers need to strike an optimal balance between road handling, safety and comfort. I was curious as to how the LS 600h can deliver such a comfortable and quiet driving experience, so I asked Lexus/Toyota to let us in on what noise vibration reduction measures they have taken when designing and manufacturing the car.

People often seem to assume that it is the hybrid technology that makes Lexus cars quieter than other makes. A hybrid engine will rather reduce the engine noise, provided that there is also good soundproofing in the firewall; i.e. the wall between the engine and the cabin. However, the car may not be quiet overall. There are examples of hybrid cars that are fairly noisy at highway speeds due to lacking soundproofing, letting in both unwanted road noise and wind noise.

The story about the LS 600h and its holistic noise reduction measures is very impressive and I would like to share with you the details of what has been done to offer the world’s quietest passenger car.

“Sound absorbing and sound isolation items have been adopted throughout the body to achieve a high-quality interior space that is quiet both when the vehicle is stopped and while driving”, states Lexus.

See below an overview of key noise reduction measures taken and the desired effect of each:

For you to understand how extensive the work to reduce noise has been, see below a more detailed gross list of noise and vibration reduction measures:

Under Floor

• Sound absorbing and isolating structures are added to the under floor parts to enhance quietness.
  • Incorporated sound absorbing material in the floor under cover.
  • Incorporated sound absorbing material in the heat insulator of the exhaust pipe.
• The rear header area has been reinforced to reduce muffled sounds in the interior.

Measures to Reduce Wind Noise

• The shapes of the grille and fins have been revised to reduce wind noise.
  • To prevent cavity noise, consideration has been given to prevent the airflow from hitting the inner side of the lower grille bars.
  • Fins are installed to disrupt the airflow, helping to prevent cavity noise.
• The vehicle body features a wind noise reduction construction for enhanced quietness.
  • Foam insulation has been optimally placed within the closed cross section of the side member pillars and front header.
  • The step-up in the windshield moulding has been reduced.
  • Hood seal rubber is adopted.
  • Hood side seal rubber is adopted.
  • Fender seal rubber is adopted.
  • A weather strip is adopted to the gap between the front and rear doors.
  • A lip is added to the base of the glass run, and low settling material is adopted to the main weather strip.
  • A cap is adopted on the top end of the division bar.
  • Aero-stabilizing fins on the front door frame mouldings are also effective in reducing wind noise.
  • The outer door mirror visor has been downsized to reduce wind noise.
    • A power retractable unit is adopted to reduce motor noise when mirrors are being retracted.
• A roof moulding with rectifying shape, aero-stabilizing fin at the base of the front pillar, and newly shaped outer door mirror are adopted to rectify the airflow over the windshield glass towards the rear, helping to reduce wind noise.
• Noise insulating type laminated glass is adopted for the windshield glass.
• Noise insulating type laminated glass with an infrared cut feature is available for the front door glass, rear door glass, and rear door quarter glass.
• Front pillar base width has been increased to rectify the air flowing sideways over the pillar, helping to reduce rustling noise.
• The following changes have been implemented to the door inner weather strip, helping to reduce wind noise.
  • Front and rear end EPT (Ethylene-propylene Terpolymer) sealer optimized
  • Finer flock fibers on lip, and its material has been modified
  • Sound absorbing material positioned between lips
• Sound absorbing material has been added to the rear door service hole cover, helping to reduce road noise.

Noise & Vibration Reduction Measures in the Wheel Housings

• A rear wheelhouse liner with a resin coating is adopted on the inside of the rear wheelhouse.
• The rear wheelhouse liner contributes to quietness by insulating road noise, gravel noise, and splashing noise. The enhanced water repellence contributes to ice suppression performance.
• A hood silencer is adopted and the silencer layout is optimized. This enabled a reduction in
• transmitted engine noise, and class top level quietness has been achieved.
  • Adopted a melamine material in the hood silencer to contribute to a more lightweight design and enhance sound absorption performance. Consideration was also given to pedestrian protection performance.
  • Optimized the silencer layout in the engine room.
• Thickness of front fender liner sound absorbing material has been increased, helping to suppress engine noise.
• Rear liner sound absorbing shape and material have been changed, helping to suppress rear transmitted noise.

Hybrid Transmission

• An NV (Noise and Vibration) cover is installed on the outer area of the transmission to reduce noise generated from the powertrain.
• A dynamic damper is installed in the transfer to suppress the vibration transmission of the powertrain.
• The motor magnets are optimally positioned to reduce noise.

Engine

• Balance weights designed for weight reduction with hollow holes have been optimally arranged and the permissible unbalance has been reduced to lower vibration and noise.
• A dual mass type crank pulley equipped with a torsional damper and a bending damper is adopted to reduce vibration and noise.
• A 2-piece type oil pan consisting of aluminium alloy and steel is adopted. The oil pan, cylinder block, and converter housing are fastened in one unit to significantly reduce noise and vibration and achieve a more lightweight design.
  • A Teflon shaft seal has been adopted for the cylinder internal pressure seal, and an insulator has been adopted for the areas that the injectors contact the head to reduce vibration and noise.

Engine Mount Bracket

• A more lightweight design has been achieved and vibration and noise are reduced by adopting a no. 1 bracket made of aluminium alloy and a no. 2 bracket made of steel plate metal.
  • A dynamic damper is installed to enhance vibrations and noise levels.

Various Noise & Vibration Reduction Covers

• PUR (polyurethane) covers with high-level sound absorption are installed in each area to reduce engine vibration and noise.

Chassis

• A 4-point support structure is used for the fitting of the differential mount to the suspension member to reduce noise and vibration.
• Front Propeller Shaft
  • A low coercive force, cross-grooved constant velocity joint with excellent noise and vibration performance is adopted.
  • A 3-joint propeller shaft is adopted, which uses flexible coupling with excellent noise and vibration performance.
  • In addition to enhancing parts precision and balance performance, the mounting precision of the propeller shaft has also been enhanced to reduce noise and vibration.
  • The balance of the propeller shaft lengths has been optimized to reduce noise and vibration.

Front Drive Shaft

• A low coercive force, tripod constant velocity joint with excellent noise performance is adopted on the differential side.

Rear Drive Shaft

• A low coercive force, cross-grooved constant velocity joint with excellent noise and vibration performance is adopted.

Impressive list of measures, huh? So now you know that it took a lot of engineering excellence in order to make the world’s quietest passenger car a reality.

Graphics courtesy of Lexus/Toyota UK

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