This application note has been kindly provided by our friends at DEWESoft.
In order to pass the technical inspection for the formula student competition, all internal combustion engine cars need to pass a noise measurement procedure. In the unified rulebook it is stated:
IC3.3 Maximum Sound Level
At idle the maximum permitted sound level is 103 dBC, fast weighting. At all other speeds the maximum permitted sound level is 110 dBC, fast weighting.
IC3.2.1 The sound level will be measured during a static test. Measurements will be made with a free-field microphone placed free from obstructions at the exhaust outlet level, 0.5 m (19.68 inches) from the end of the exhaust outlet, at an angle of forty-five degrees (45°) with the outlet in the horizontal plane. The test will be run with the gearbox in neutral at the engine speed defined below. Where more than one exhaust outlet is present, the test will be repeated for each exhaust and the highest reading will be used.
IC3.2.4 Test Speeds
The maximum test speed for a given engine will be the engine speed that corresponds to an average piston speed of 914.4 m/min (3,000 ft. /min) for automotive or motorcycle engines, and 731.5 m/min (2,400 ft. /min) for “industrial engines”. The calculated speed will be rounded to the nearest 500 rpm. The test speeds for typical engines will be published by the organisers.
The test was normally performed by one person using a hand-held sound level meter. Each team needed to have a dash display or a computer connected to show the engine RPM during the test.
DEWESoft was one of the sponsors of the Formula student Austria 2017 competition at the Red Bull Ring in Spielberg, Germany. The event was also a promotion of the company and as such it was decided to do the noise test at a higher level.
- Sirius ACC with high dynamic range
- GRAS microphone (for example GRAS 146AE)
- Dewesoft X3
- Sound level meter plugin
SETUP FOR NOISE MEASUREMENT TEST
It is stated in the rulebook that the maximum permitted sound level is 103dBC for fast weighting at idle and 110dBC for fast weighting at maximum RPM. Using the microphone with an ACC input in IEPE mode, the sound pressure in Pa is measured. By using the sound level meter and selecting only the correct frequency and time weighting, the sound pressure level output is created.
The measurement was made more interesting by detecting the engine RPM only by using the microphone. For this, the FFT analyser was used with peak amplitude output. Array function “maxpos” were used to get the exact frequency of the rotating engine. Then this was divided by the number of cylinders, multiplied by two and converted form Hz to RPM.
In some engines, other unwanted frequencies were present which made RPM detection more problematic. An IIR band pass filter solved this problem. Cut-off frequencies needed to be set below idle and above maximum RPM. Caution needs to be taken, since the frequency depends on the number of cylinders in the engine. In the competition only 4-stroke engines are allowed, which have one firing per cylinder every two engine revolutions. If a 4-cylinder engine is measured, there are 2 combustion cycles each engine revolution. This means that at 6000RPM, the frequency of the engine is 200Hz (6000*2/60).
There is also an input field in the setup for determining the engine test speed. In the rulebook, chapter IC3.2.4, it is stated that the maximum engine RPM for the test is calculated from the mean piston speed, which depends on the engine stroke.
The setup that was prepared is very simple for the operator. The number of engine cylinders need to be selected, engine stroke entered and then the measurement can start. There are two big indicators showing if the idle and maximum RPM noise levels are exceeded.
The DEWESoft noise test raised a lot of interest at the competition and teams were impressed with the capabilities of the hardware and software. Organisers from other events saw a video from the DEWESoft noise test at the competition in Austria and invited DEWESoft to do the same test at the formula student competition in Brazil.
The Formula student event is a good promotion for DEWESoft and a great chance for people to start working in automotive and other areas to get to know DEWESoft. Every team that passed the noise test received a printed report and an USB pen drive with the datafile, software and product catalogues.
Dewesoft was one of the sponsors of the Formula Student race at the Red Bull ring in Spielberg 2017. Dewesoft team performed noise tests on all competing formulas. In idle mode, their sound pressure level must not exceed 103 dB(C) and at maximum allowed RPMs, their sound pressure level must not exceed 110 dB(C). Only the teams that passed the test were allowed to compete in a endurance race. Each team received printed report and a Dewesoft USB stick with a datafile.
For more information on the Noise Measurement Test conducted by DEWESoft, contact us.