A vibration caused by a vibrating force and amplified by resonance causes the O-ring of the exhaust pipe and engine mounts to vibrate, transmitting a noise to the cabin. The passage which transmits the vibration or noise is called the transmitting system.
2.Vibrating element (sound generator)
Vibrating element (sound generator) is the portion which generates vibration or noise. In a vehicle, they could be the vehicle body, steering wheel, and floor panel, etc.
When expressing vibration, we generally say “large”, “small”, “rough”, “fine”. The magnitude of a vibration is indicated as the amplitude, and its fineness is referred to as its frequency.
Even if the amplitude is the same, depending on the frequency, we usually feel a vibration differently. Therefore, the magnitude of a vehicle vibration is expressed as its acceleration.
Acceleration means the change in speed within a specific period of time, and for the unit, m/s2 is used.
Gravitational acceleration (G)
“G” is a unit of vibration acceleration based on gravitational acceleration (9.8 m/s2)
1G = (9.8 m/s2)
1.Method for indicating noise
A sound is indicated by the amplitude and frequency of waves, in the same way as vibration. The tone of the sound is decided by the shape of the waves.
Amplitude = If the amplitude becomes larger, the sound becomes louder.
Frequency = If the frequency becomes higher, the pitch of the sound becomes higher.
Shape of waves = If the shape is different, the tone is different.
The pitch of the sound can be indicated by the frequency. But the sound pressure level is sensuous, so it is impossible to indicate the sound pressure level by only using the amplitude (the energy of sound). Therefore it is necessary to use various compensations. For its ease in applying compensation, the unit called dB (deciBell) is used for applying the sound pressure level to the human hearing sense.
Deci means one tenth. So deciBell is the unit which indicates one tenth of a Bell. Bell is the unit which compares two sound levels, and indicates the rate of them in logarithm.
Sound pressure level indicated by dB
2.Measurement of sounds
The measurement of the human hearing sense varies by the frequency. For this reason, a sound level meter measures sounds compensating hearing sensitivity for sound frequency.
In A characteristics, measured sounds are compensated in the way that the result is the closest to the human hearing sense. This type of characteristic is used commonly to measure noises of vehicles. Measured values are indicated by the unit dB(A).
This is a nearly flat type of characteristics. Because it defers from the human hearing sense, measured values are higher than the sensitivity of the human hearing sense especially in low frequencies. Generally, dB values are higher than at A characteristics. Measured values are indicated by the unit dB(C). Measurement in setting A characteristics and C characteristics