A cabin's noise level influences the vehicle's quality perception and affects driver fatigue. We put three vehicles to the test to see how they compare...

Perceived quality plays an important role in convincing potential car buyers to part with their hard-earned cash. This relatively subjective judgement call involves all the human senses, with sound – from the noise a door makes when it closes, to the clicking of the indicator – functioning as a substantial contributing factor. Cabin sound levels influence the vehicle’s perceived refinement and determines the ease of conversation between occupants.

To provide a quiet cabin, noise, vibration and harshness (NVH) engineers spend vast amounts of time in sound laboratories evaluating the selection of insulation and absorption materials, and the appropriate quantities. To see how effective – or not – their efforts are, we decided to choose three vehicles from different segments to evaluate their cabin noise levels.

In the past, CAR magazine has experimented with the measurement of interior noise level as part of the road-test regime, but because the analogue equipment failed to show significant differences between vehicles at the time, it was decided to abandon the test. Recently, however, we acquired a modern Casella digital sound-level meter and, to test its accuracy, we did some side-by-side assessment with state-of-the-art HEAD Acoustics equipment from Stellenbosch University to determine if we should once again include this procedure as part of our road-test findings.

The cars

We decided to choose three vehicles from three divergent automotive categories:

Crossover/SUV: Audi Q2 1,4T FSI Sport S tronic
Leisure bakkie: Isuzu KB300 D-TEQ DC 4x4 LX auto
Luxury vehicle: Mercedes-AMG S63 7G-tronic

Background

Sound is any pressure variation (in air, water or other medium) that the human ear can detect. The number of pressure variations per second is called the frequency of the sound, and the normal detectable frequency range of a healthy ear is between 20 and 20 000 Hertz (Hz). Sound travels at 340 metres a second (1 224 km/h) at standard atmospheric conditions. A sound’s magnitude is the amplitude of the pressure fluctuations and these can be measured in Pascal; however, because of human hearing’s extensive range, this results in very large numbers.

Therefore, the decibel (dB) scale, which is logarithmic (meaning not linear; higher numbers represent vastly louder sounds than low numbers), is used instead. The hearing threshold of 20 μPa (micropascal) is the 0 dB reference and the pain threshold is about 130 dB, which equates to a military jet taking off from an aircraft carrier. Perceived loudness is a complex field because the human ear is not equally sensitive at all frequencies (it’s most perceptive between 2 and 5 kHz, and less so at higher and far lower frequencies). That is why the A-weighting scale is applied by measurement equipment to evaluate the human ear’s perception of loudness and the unit is then dBA.

Test procedure

Each vehicle’s cabin noise was measured using two sets of equipment for 10 seconds during the following conditions: idle; 60 km/h; 80 km/h; 100 km/h; and 120 km/h. Note that all additional noise generators in the cabin, such as the audio system and climate-control fan, were switched off. The idling testing took place in CAR's garage (representing a controlled environment), with the vehicles’ stop/start systems disabled to keep the engine running. All doors and sunroofs were closed and the average sound level over 10 seconds was measured.

The noise-level testing at speed took place at our test strip in both directions to try and account for prevailing wind conditions (although the wind speed on the day was less than 10 km/h and had minimal effect on the results). The powertrains and suspension settings were all in comfort mode where possible and the automatic transmissions were left to their own devices.

The equipment

The Casella CEL 246 is a compact, easy-to-use, handheld, digital sound-level meter that can be used in a variety of applications. Its main benefit is that it gives the user an instant reading of the current sound level and can be configured for time weightings, measurement range, frequency weighting and display type.


The SQuadriga II mobile-recording and playback system from HEAD Acoustics is a state-of-the-art sound-measuring unit. In combination with headphone-based microphones mimicking the human ear’s performance and location in the cabin, it is capable of accurately measuring cabin noise to the manu­facturers’ specification.


NVH at Stellenbosch

The Sound and Vibration Research Group in the department of mechanical and mechatronic engineering at Stellenbosch University is overseen by Dr Annie Bekker. This group has established expertise, equipment and laboratory facilities to investigate noise and vibration problems for industry and the public sector.


Presently, PhD student Hannes Swart is investigating the enhancement of electric-vehicle sound signatures. Consumer perceptions of electric-vehicle sounds are evaluated through subjective playback tests where listeners are requested to judge enhanced and conventional vehicle sounds. These responses are statistically correlated to measurements that determine which acoustic features are likely to underpin the attributes that would constitute a safe, yet thrilling electric-vehicle sound.

Results

The cabin noise at speed is depicted below. We recorded a continuous 10-second reading with the earphone-based microphones connected to Stellenbosch University’s HEAD Acoustics SQuadriga II Mobile system, and we also took a manual snapshot reading with our own handheld Casella CEL 246 device (in white blocks).


Results explained

The idle test excluded the effects of wind and tyre noise, and focused purely on the level of engine sound reaching the cabin. Understandably, the S63 is not the quietest vehicle because the large-capacity (and delicious) V8 growl penetrates the cabin in true AMG style. On the other hand, the Q2’s 1,4-litre, four-cylinder turbopetrol is barely audible at idle, while there is no hiding the KB300’s turbodiesel clatter. When cruising at 60, 80, 100 and 120 km/h, tyre but especially wind noise became increasingly significant. The fact that all the vehicles had automatic transmissions meant that the highest feasible gear was automatically selected and therefore engine speed was kept to a minimum.

From the graphs (see below), we were surprised to note the Q2 has the highest cabin noise level of the three vehicles tested, closely followed by the KB300, with the S63 the quietest. We expected the big Benz with its double-glazed windows and superior sound insulation and absorption material to be victorious, but the Isuzu beating the Audi was a surprise. Comparing the snapshot readings on the CEL 246 device with the continuous traces of the SQuadriga II Mobile system, it is clear that you can misinterpret sound levels. The snapshot measurement depends on when it is taken, as it represents only a single point in time on the trace data. In future, our handheld device will be configured to measure average sound level over a 10-second interval, thereby eliminating the effects of varying sound intensity.

Conclusion

We found the idle test to show the most comparable results, as it is not influenced by wind, road surface or passing traffic. That said, as all three vehicles were tested on the same day in the same conditions, we are fairly certain about the relative results in this test. Because of the minor differences in absolute sound level, we are doubtful that conducting tests on other vehicles during different atmospheric conditions would be comparable with the results depicted here. What’s more, given the fact that the average human ear finds a 3 dBA difference just noticeable, it is clear that the idle test provides the most useful information when it comes to engine refinement. Therefore, we’ll add only this measure to our road tests.

Author: Nicol Louw