Technology: what actually is sound pressure?
Following a number of meetings at FIM level, it appears that different noise standards will apply to the Supercross World Championship. From 2023, the sound pressure will be reduced to 109 dB. Previously it was 114 dB.
The other disciplines were also discussed, making it appear that the MXGP will also undergo this change. A year later, regular production off-road motorcycles will also become subject to this rule.
What actually is sound pressure?
Wikipedia tells us that sound pressure is a rapid local pressure variation in air or another medium around the prevailing static pressure. A local sound pressure is caused by a traveling or standing sound wave. So much for the theory. In practice, a sound source needs air to travel. For example, there is no sound in space because it is empty of air and therefore there can be no sound movement.
Compare it to a plastic bag that you blow full of air and burst with one blow. That sudden movement of air makes a sound. A modern four-stroke makes an average of 10.000 blows per minute. You have high and low pressure waves. It is the frequency of those waves, i.e. the speed at which they follow each other, that we hear as pitch. The volume of that pitch is determined by the sound pressure. The more pressure, the more volume. The sound pressure is expressed in decibels (dB).
Decibels are confusing
The decibel is a ratio on a logarithmic scale. Every increase of 10 decibels means an increase in power or energy by a factor of 10. An increase of 20 dB therefore means a factor of 100, an increase of 30 dB a factor of 1000, etc. In concrete terms, this means that a sound pressure of 120 dB is many times higher. is, for example, that of 110 dB.
Absorption damping
The dampers of our dirt bikes all use the principle of absorption damping. This method of damping is accompanied by low back pressure. A certain level of back pressure is necessary to ensure the proper functioning of an internal combustion engine. The exhaust gas flows at a high speed through a perforated tube so that part of the gases penetrates into the absorption material (damper wool) and another part gets a swirling character through the same holes.
These vortices cancel out certain high frequencies, which already dampens a large part of the sound. The sound waves then end up in the damping material, which absorbs the sound and does not reflect it back. This causes the sound wave in the main pipe to lose even more pressure and sound intensity.
The sound of Silence
You don't have to be a mathematician to understand that the noise of our dirt bikes is slowly killing the sport. The FIM's rule change is not without reason. From 114 dB to 109 dB is a big step in the right direction, but it could have been much greater. However, in the corridors we heard that the FIM, in consultation with the industry (engine and exhaust manufacturers), is gradually aiming for a final 98 dB sound pressure.
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