Noise Level Measurements of the DG-808B/C
As part of the certification procedure, there was a measurement of the noise level of the DG-800B.
The measurements were made according to Chapter 6 and 10; the values were 51.8 and 57.1 dB(A), respectively.What does this mean in a practical sense?
Due to increases in sensitivity of the citizenry, particularly in the neighborhood of airports, there has resulted increasingly stronger rules to enforce noise protection from aircraft. Until 1989, the upper limit in the entire ICAO for noise protection certification was 89 dB(A). Germany departed from this uniform rule in 1989 and dropped the value to 64 dB(A) as the normally required noise protection limit. On top of that, the term "elevated noise protection" was introduced, which decreased the allowed level another 4 dB(A) under the normal level. While protection at the "normal" noise level is today's requirement for certification of a new aircraft, meeting the even higher standards of the elevated noise protection requirements means:
- in some places such aircraft have lower landing fees,
- exemption from restrictions regarding operating hours, and
- that without such increased noise control, landing would not be allowed at all in certain places.
The measurements proceeded according to the provisions of "Chapter 6" of the ICAO. The noise level of an aircraft flying at full throttle over a start line is measured at a particular distance from the starting point. Naturally, not only the noise generation of the motor plays a roll but also the propagation of the noise and the rate of climb of the aircraft. In the DG-808C there is an unavoidably high noise level in the motor compartment but it is propagated upward. Because of the good climb rate, at the moment of the measurement the aircraft was already between 360 and 390 meters high and could be heard only weakly. The measured result of 51.8 dB(A) is an outstanding value and simply means the DG-808C is currently the quietest motorglider with storable motor that can be bought on the world market.
In order to ascertain how the human ear reacts to this noise, one must look a little bit at the physics of acoustics. Acoustic measurements are measured with the decibel scale. This is like the decimeter or the deciliter (important in determining the size of your beer glass!) and means nothing more than 1/10 of a Bel. (Named for Alexander G. Bell).
Further, one bel is a dimension less unit on a logarithmic scale. If a beginning value of "one" is defined then that means 1 bel or 10 decibels. If the noise level increases to 100 times the beginning value, then that is 2 bels. If 1000 times, that is 3 bels and 100,000 times would be 5 bels or 50 decibels. That sounds more complicated than it is. But the human ear has a roughly logarithmic response in that, from the softest leaf rustle to the loudest jet engine, every increase of the sound pressure level by ten is subjectively perceived as a doubling of loudness.
Result: a sound that is 10 decibels [one acoustic bel or 10 dB(A)] quieter than another is perceived to be only half as loud.
Expressed in another way, a measurement of 51.8 dB(A) for the DG-808C means that this aircraft is only slightly more than half as loud as the especially stringent requirements of the German elevated noise protection level. [51.8 : 60 dB(A), almost a 10 times ratio]. The sound of such an aircraft can hardly be heard against normal background noise.
For interested pilots, another story:
The latest measurements are to be made according to chapter 10, and that will soon be done. In this procedure the machine flies at 100 meters altitude to a measuring point and then goes into a full throttle climb. The noise level is measured at a definite distance behind the point where the climb began. Again, the advantageous design and construction of the DG-808C comes into its own. The noise generation, propagation direction, and climb rate play the deciding role. The result of this measurement is 57.1 dB(A). The German limit for normal noise protection according to Chapter 10 is 68.4 dB(A).
Curiously, the law writers have not established a level for an elevated noise protection. That leads to the situation that new machines (especially ultra lights) would be satisfactory with a possibly elevated noise protection level but, without a definition of the maximum allowable noise level, they could not get a certificate of noise protection which would have the negative results described above.
The DG-808C does not fall into this loophole in the law! At that time, the goal was set to achieve 5 dB(A) better than the noise level required by Chapter 10. That would mean a tighter definition because an aircraft that is OK according to chapter 6 [60 dB(A)] would normally be measured at about 65.4 dB(A) according to chapter 10. A limiting value for the elevated sound protection according to chapter 10 of 5 dB(A) below the normal value would be another tightening of the limits by 2 dB(A)!
This all leaves the pilots of DG-808B's cold.
Their aircraft fulfill all of the requirements with elan and with reserves for future restrictions.
- W. Dirks -
(Translated by David Noyes, Ohio, U.S.A.) 