Noise reduction was yesterday – Sound creation makes its claim for the future

Electrical mobility changes the world of acoustical engineering. The development of a new vehicle generation is unveiling immense progress in technologies for test bench, and measurement systems.

Acousticians should be excited. After many years of investigation, and preparation now we are living in the century of electrical mobility. Noise emission is low which was never possible in the time of combustion engines, both gas and diesel.

But low noise emission is dangerous, especially for pedestrians. Legislators in most countries demand a minimum noise in the speed-range up to 20km/h and in reverse mode (European directive no.540/2014). These systems are called AVAS (Acoustic Vehicle Alerting Systems) and are obligatory since 1st of July 2019 in electrical, hybrid-electrical and fuel cell cars.

Creativity of sound-engineers is not limited. Prof. Stefan Sentpali says “People have a given expectation, of how an object should sound. The sound must be high-grade, must not creak or grind … people are familiar with vehicle sounds used in science fiction movies. They will quickly familiarise themselves with the sounds of electrical vehicles. Sounding up to two octaves higher, high frequency like a quiet pleasant whistle…” [1].

There are tools and equipment for sound-engineers, who develop the noise of the electrical vehicles. Pass-By measurement conform to DIN ISO 362-1:2017-10 is a standard method for all approved vehicles, but the same installed test benches are suitable for the creation of electrical vehicle sound

There are real pass-by test benches in weatherproof outdoor installations with two obligatory measurement microphones on one hand and on the other hand there are simulation pass-by test stands with up to 64 microphone channels in large indoor facilities with acoustic elements

It’s difficult to imagine, but there are room-acoustic installations, which allow a measuring limit, covering a range from 40Hz to 20kHz and more. That requirement for the low frequency limit came from combustion engines, but it’s usable also for developments in the electrical-mobility-sound-world.

Our human ears are sensitive in a frequency range between 16 Hz and 20 kHz, while the loudness sensation extends from 0dB, which is a sound pressure from 20µPa, to 130dB. The lowest limit is called hearing threshold, the upper limit is called pain threshold. Frequency- and loudness sensation in nature is anything but linear and that fact is important, when we want to create a sound, which is “Music to my ears” [2].The opposite to our human ear, a measurement microphone is a sensor with a very linear frequency response. Only with filtering, for example conventional A-weighting, or psychoacoustic evaluation in software systems, do the analysed signals come anywhere close to our human sensation. Now through the use of the microphone the changing world becomes noticeable. While traditional combustions engines exhibit their spectral noise dues in the frequency range lower than 3kHz, electrical drive systems are audible in the 6kHz-, 8kHz- or 10kHz range. Up to 3kHz almost all high standard microphones, which are sound pressure receivers by their physical characteristics, have a very linear amplitude response. Within the upper 3kHz it’s especially important to choose the right type of frequency-response-type for a microphone – free-field or random field type. Also, the influence of the foam-windshields, which should lower the acoustical disturbances of wind turbulences around the microphone body, are important. With windshields, which don’t conform to microphone manufacturer specification, the microphones fall out of the sound-level-meter or microphone tolerance-field at the lower limit.

The microphone as a sound pressure receiver without a directional sensitivity for sound waves should have an exact omnidirectional characteristic. This is not completely true in the acoustical frequency range in the upper 5kHz, because the wavelength has the same dimension as the microphone itself. This fact causes a difference in sound-level measurement results, for example in a pass-by measurement, if microphones of a special type, like free-field with an exact linear amplitude response on axis, are installed vertical or horizontal in respect of their own axis. In other words, the important question is, if the membrane looks to the heaven or to the moving vehicle. That fact is not described exactly in the pass by standard ISO 362… in its current version from 2017.

Measurement equipment manufacturers like Microtech Gefell utilise the same effect, by installing microphones for pass-by measurement in a vertical position with simultaneous use of random (diffuse) field microphones. This type of installation should usually ensure, that a microphone gets a linear frequency response in an echoing environment, because there is less reflection and pressure build up effects in that kind of sound-field. Such a microphone type exhibits an exact linear class 1 response in 90 degree off-axis during application in free-field conditions. So, it is an ideal instrument to do pass-by measurements, because one the one hand it is a vertical microphone optimally protected against rain, humidity, and wind and on the other hand, the acoustical pattern is exactly circular around its axis, which means, the position of the car while driving by is not causing a wrong measured noise level value because of the polar pattern effect and the angular position.

The newest product, which is developed by Microtech Gefell GmbH, is called WME 980 CN (community noise) includes exactly these acoustical and practical contexts and is a perfect tool for standard conform measurements as well as modern sound design (– WME 980 CN). With its frequency response and polar pattern, it is approved for use with a sound-level-meter class 1 conforming to DIN IEC 61672.

To achieve a perfect acoustical result, more development tools in the background are needed. Sound-power is the metrological base of noise reduction investigation. But the same equipment can be used for sound creation of objects. The traditional way is measuring the noise radiation through an envelope surface. Therefore, the standards DIN EN ISO 3744 for non-directional wideband sources and DIN EN ISO 3745 are used and additionally the “universal case”, which means, directional sound sources can be treated.

Microtech Gefell has developed a new product SLH (Schallleistungshemisphere), which allows sound-power measurements using the standards 3744 and 3745. It is a hemispherical microphone arrangement with a 2m or 4m diameter, which can be installed with 6 or 12 bars and which allows the use of 10 or 20 pieces of high standard measurement microphones conforming to the standard DIN IEC 61094-4.

The same microphone arrangement can be used in a perfect application to record the sound of components of electrical vehicles. We remember at the beginning of this explanation, that their sound is one or two octaves higher and according to physics, such soundwaves are much more directional. These and more facts are playing a significant role, indeed even BMW have enlisted the help of the famous sound-designer Renzo Vitale who worked with composer Hans Zimmer on sound design for the Vision M Next concept car…” [2], to develop their exclusive sound experience.

Another application for sound investigation of acoustical components through use of hemispherical microphone arrangements is the measurement of tire noise. While engine and gearbox noise have been important parts in the “combustion era”, tire noise is more and more important with electrical cars and motorbikes. That part of noise emission was covered by the higher noise of the traditional elements. There are existing special tire noise test benches, where moving tires are installed on a separately driving roller dynamometer, surrounded by a microphone-hemisphere.

In addition to a hemispherical microphone arrangement there are sound-intensity probes, which are two-microphone arrangements with exact selected phase responses. These instruments are used for sound-power measurements and conform to standard DIN EN IEC 61043. Likewise sound-intensity probes can be used for location of partial components of sound emission. It goes without saying, that Microtech Gefell’s SIS 190 double, which covers the complete sound frequency range from 40Hz to 12kHz, is another professional tool for the sound designer.

Facit

This brings us to the conclusion that the future sound development in the time of electric mobility comes closer to the composition of music. The high professional Microtech Gefell microphones are bringing together our experiences in technical acoustics and recording. What is visible is also audible in the wide range of its studio microphones: “We in Gefell are measuring, what you are hearing AND are hearing, what you are measuring”.

by Udo Wagner, Director at MIcrotech Gefell

Sources

[1] Ostthüringer Zeitung, Freitag 25th of December 2020 issue (DPA), „Zwischen ´Star Trek´ und Stille: Sounddesign bei E-Autos“.

[2] Automotive testing technology international, November 2020 issue, page 062ff, “Music to my ears”.

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