Measuring in Deafening Silence

E-vehicles in the lower velocity range and conventional vehicles with automatic start-stop system are particularly quiet. What pleases vehicle occupants is a major challenge for developers: they must find, analyze and eliminate all sources of irritating noise. In IAV’s new acoustics test cell in Gifhorn, passenger cars and commercial vehicles can be measured with precision. Because if its size, measurements can also be conducted outside the vehicle.

E-cars have the reputation of permitting silent mobility. But that is only true in part. In the lower speed range up to approx. 40 or 50 kilometers an hour, they are in fact very quiet. Above 50 kilometers an hour, however, tire, rolling and wind noises increasingly occur, making hardly any difference between them and a vehicle with conventional drive system. But cars with a combustion engine can be quiet too, for example when they are waiting at traffic lights and are equipped with an automatic start-stop system.

For vehicle occupants, the quietness inside the passenger compartment is most pleasant whereas it poses a major challenge for automobile developers: “We must prevent the silence from being disturbed by noises which mainly come from auxiliary units, such as compressor, vacuum pump or fans”, explains Oliver Olbrich, Team Manager for NVH Overall Vehicle at IAV. “To do this, we must be able to conduct precise acoustic measurements on vehicles.”

Soundproofing of 60 dB

This is where the new acoustics test cell at the Gifhorn Development center comes in. It has been available to IAV’s experts since July 2017 and satisfies quality class 1 under German standard DIN 3745. It is designed as a “room-in-room” structure, i.e. it consists of a double-walled chamber with a concrete foundation that is detached from the building’s foundation by a system of springs. The inside of the measurement chamber is lined with metal absorbers floating on melamine foam, mineral wool and broadband compact absorbers. “Soundproofing from the outside to the inside of the chamber filters out 60 dB“, reports Claus Brinkkötter, head of the Air-Conditioning department at IAV. “As soon as the doors are closed, ‘deafening silence’ prevails inside the chamber”.

The semi-anechoic chamber provides the capability of examining e-vehicles and combustion engine vehicles because in addition to room ventilation, the facility is provided with an exhaust emission extractor and diverse warning sensors for CO, CO2, HC and NOx. It is even big enough to perform measurements outside vehicles, for example measuring noise emission from stationary vehicles, idling comfort or sound transmission. It also provides a suitable environment for all relevant OEM test specifications.

Although it has only been available for a few months, the new test rig has already proven its worth in both internal as well as customer projects – such as in measuring coolant circuit acoustics of a conventional vehicle and the overall vehicle acoustics of an e-transporter. In deafening silence, the IAV experts can now measure, analyze and minimize all kind of different acoustic problems in passenger cars and light commercial vehicles.

Halbfreifeldraum nach DIN ISO 3745 der Güteklasse 1

• Dimensions (L x W x H): 11mx8mx4m
• Gates: two wings, 4 m x 3.25 m
• Test chamber structure: room in room; resonance absorber + broadband compact absorber
• Foundation: 70 t, concrete slab supported on springs (vertical supporting frequency < 10 Hz)
• Lower cutoff frequency: < 80 Hz
• Upper cutoff frequency: ≥ 12 kHz
• Soundproofing (from outside inwards): 60 dB
• Drive-in chamber for cars up to 3.5 t
• Ventilation: air exchanged 10 times an hour
• Exhaust emission extraction: 600 m3 per hour
• Electric mobility: wall box provided

Hardware, sensors and software

• 3-D scan & paint (noise source location)
• Point sound source
• Ultrasonic measuring system
• SQuadriga II (5 units)
• Zodiac-Data-Rec front ends
• PAK system
• MK2 (Müller-BBM)
• HEAD acoustics (ArtemiS)
• VBox (GPS system)
• Engine speed measuring system
• Pulse hammer
• HVAC and blower test rig
• Fan test rig
• A/C compressor test bench (R744)

Overall vehicle:

• External noise measurements on stationary vehicle
  Acoustic assessment of electric auxiliaries
  Air-conditioning acoustics
  Sound transmission measurements (insulation/damping)
  Seat adjuster and seat ventilator acoustics
  Transmission functions of force transmission points to the ear positions

Components/assembly:

• Coolant circuit acoustics (R134a, R1234yf, R744)
• HVAC and fan acoustics
• Flow acoustics (primary research)
• Acoustic tests on servomotors