Noise reduction was yesterday – Product Sound design is claimed in future

By Udo Wagner/Microtech Gefell/MTG

Microtech Gefell: E-mobility changes the world of acoustical engineering. The development of a new vehicle generation is going along with immense progresses in technologies of test bench systems and in the world of acoustic measurement.

The e-mobility-world presents a host of new challenges for acousticians. In the century of e-vehicles noise emission is as low as never was possible in the time of combustion engines.

But to low noise emission is dangerous, especially for pedestrians. Legislators in mostly 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 July 2019 in electrical, hybrid-electrical and fuel cell cars.

High-quality sound - a key factor for the development of e-mobility

Creativity of sound-engineers is not limited. Prof. Stefan Sentpali says “People have a given expectation, how sounds an object. … The sound has to be high-grade, must not creak or grind … people know how vehicle are sounding in science fiction movies. They will orientate themselves also in case of electrical vehicles. They are sounding up to two octaves higher, high frequency like a quiet pleasant whistle…” [(DPA), „Zwischen ´Star Trek´und Stille: Sounddesign bei E-Autos“/OTZ 15.12.2020].

MTG provides a wide range of measurement tools and equipment for sound-engineers, designing the noise of e-vehicles, including both single tools and turnkey indoor test benches for simulated pass-by and outdoor pass-by test bench systems.

In- and outdoor Pass-By measurement and e-mobility

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 design of e-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 indoor simulation pass-by test stands with up to 64 microphone channels in very large indoor facilities with acoustic elements. There are room-acoustic installations, which allow a measuring limit, covering a range from 40Hz to 20kHz and more. That requirement for the lower cut-off frequency came from combustion engines, but it’s usable for sound design in the e-mobility world too.

In opposite to the human ear, a measurement microphone is a sensor with a very linear frequency response. Only with filtering, e.g. conventional A-weighting or psychoacoustic evaluation in software systems, analysed signals come very close to the human sensation.

Already in case of the right microphone choose the changing measurement world becomes noticeable.

While traditional combustions engines exhibit their spectral noise dues in the frequency range lower than 3 kHz, electrical drive systems are audible in the 6 kHz-, 8 kHz- or 10 kHz range. Up to 3 kHz a mostly all high standard microphones, which are sound pressure receivers, have a very linear amplitude response. Upper 3 kHz it’s very 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, is important, because depending of the foam density, high frequency spectral dues are attenuated more or less. With windshields, which are not conform to manufacturer specification, the microphones fall out of the sound-level-meter or microphone tolerance-field at the lower limit.

The measurement microphone as a sound pressure receiver without a directional sensitivity for sound waves should have an exact omnidirectional characteristics. However, this is not quite correct in the frequency range above approx. 5 kHz, as the wavelength has the same dimension as the microphone itself.

That 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… up to its current version from 2017.

MTG makes use of the same effect by using microphones for pass-by in vertical position and random (diffuse) field microphones at the same time.

This microphone-type should usually ensure, that a microphone gets a linear frequency response in an echoing environment, because there are less reflection and pressure build up effects in that kind of sound-field. Besides 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 hand a vertical microphone optimally protected against rain, humidity and wind and on the other hand, the acoustical pattern is exact circular around its axis, which does mean, that 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 MTG-product, 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. With its frequency response and polar pattern it is allowed to be used with a sound-level-meter class 1 conform to DIN IEC 61672.

New - Sound power hemisphere SLH 112 for standard-compliant sound power measurements

What should come to a perfect acoustical result, needs much more development tools in the background. Sound-power is the metrological base of noise reduction investigation. But the same equipment can be used for sound creation of objects. On the classical way is measured the noise radiation through an envelope surface. Therefore are used standards DIN EN ISO 3744 for non-directional wideband sources and DIN EN ISO 3745 for the same and additional the “universal case”, what means, also more or less directional sound sources can be treated.

MTG has developed a completely new product for this - a sound power hemisphere (SLH 112).

Sound intensity probe SIS 190 double - one more professional tool for the sound designer

In addition to the hemispherical microphone arrangement, sound intensity probes/ SIS are also of metrological importance. The Sound intensity probes SIS 190 are two-microphone arrangements with exact selected phase responses and conform to standard DIN EN IEC 61043 and covers the complete sound frequency range from 40Hz to 12kHz. Sound-intensity probes can be used for location of partial components of sound emission.