I noticed that sound of the engine of my car is different in summer, winter and the rainy season. Do you think that perhaps the percentage of moisture in the air could be the reason for this and if so, is the only reason?
As we have fuel-injected cars and they may experience a faulty sensor feeding the ECU incorrect information affected by air temperature, density, altitudes and humidity. These are some key contributors to engine performance, while fuel types and fuel quality also have an effect. So they can all affect engine performance.
Please check the attached table (*) for the attenuation of sound in air as a function of temperature, humidity, and frequency at the atmospheric pressure.
Attenuation shows a generic trend to typically increase with frequency while decreasing with rising humidity.
The effect of temperature is less clear, being more significant for higher or mean frequencies than for lower. For mean frequencies and dry air attenuation tends to decrease as temperature rises, but for higher humidity the opposite trend can be observed. For higher frequency attenuation tends to decrease for lower temperature, except for humid-cold air. Generally speaking, it can thus be said that the effect of humidity is stronger than that of the temperature.
Of course, higher humidity also tends to contribute for lowering the engine performance, what may lead to a generic tendency for increased power and noise if the same speed is to be maintained. However, the mean driving speed would tend to be reduced, along with engine power and noise, with raining weather, due to obvious safety concerns.
High air temperature may also contribute to decrease the engine's performance do to cooling difficulties, what may lead the average driver to reduce the mean speed and noise.
Reference:
(*) Cyril M. Harris (Ed.), Handbook of Noise Control, 2nd ed., McGraw-Hill, New York, 1979.
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