| Country | Year reported | Noise level reduction |
| Belgium | 1981 | 8 - 10 dB |
| France | 1984 | 2 - 3 dB/3 - 5 dB |
| Germany | 1980 | 3 dB (50%) |
| Austria | 1988 | 3+ dB |
| Netherlands | 1988 | 2.5 dB |
| Canada | 1991 | Shown noise reduction |
| Arizona, USA | 1990 | 10 dB (88%) |
| California, USA | 1991-1993 | 3 - 7 dB |