Noise, Vibration and Harshness (NVH) Symptoms

Classify Noise, Vibration and Harshness (NVH) Symptoms 

For NVH concerns, it is necessary to classify the customer's concern into one of the 3 categories: noise, vibration or harshness. The reason for this is that a customer concern may consist of a combination of symptoms involving noise and vibration, or vibration and harshness. In cases where there are combination symptoms, determine which diagnostic path to follow: noise, vibration or harshness. For example, if a customer has a concern involving a noise and a vibration, and it is determined that it is vehicle speed-related, follow the vibration diagnostic path.

Noise Symptoms 

Once a symptom is classified as a noise, the particular conditions under which the noise occurs need to be identified. These conditions are identified and verified during the road test. For example, a noise may only occur while turning. The next step is to determine which systems on the vehicle are related to that condition. In this case, the steering system and wheel/tire system may be suspect. After identifying possible systems, a preliminary inspection of these systems should be done. If the source of noise is still unidentified, use a listening device (such as the ChassisEAR) to pinpoint the source. Once the source has been identified, determine if this source is related to the suspected system previously identified. If it is related, then complete the repair to resolve the customer concern. If it is unrelated, then it is possible that the source of the noise is a reactor to a noise being transmitted through a transfer path. If this is the case, repairing the reactor will not resolve the customer concern. The transfer path must be identified and a determination made if the noise is normal, but accentuated by the transfer path (conductor), or if the originator is the fault causing excessive noise to transfer to another component through a conductor. There is a relationship between systems identified as related to conditions and the noise transfer path. In some cases, the condition under which the noise occurs has nothing to do with the identified source. This relationship is important in the diagnosis of noise concerns. It is the first clue that the identified source of noise might be a reactor and that further investigation is needed to diagnose a possible noise transfer path concern. Based on the results from the road test, make a determination of which action in the symptom chart to take first.

Vibration Symptoms 

Most vibrations consist of movements back and forth or up and down that repeat. Every time the vibrating component goes through its complete range of motion and returns to the starting point is called a cycle. The rate at which these cycles occur within a given time is called the frequency. Frequency is measured in cycles per second or Hertz (Hz). One cycle per second equals one Hertz (Hz). Once the frequency of a vibration is known, calculations can be done to determine the system that is the source of the concern.

Order of Vibration 

The order of a vibration refers to how often the vibration is present in one revolution of the component. For example, a vibration that is present once each revolution of a component would be a first order vibration. A vibration present twice each revolution of the component would be a second order vibration. Vibration orders do not have to be whole numbers, they can have decimal values such as 1.5 order vibration or 3.08 order vibration.

The concept of order of vibration is important to remember when the measured frequency of a vibration does not seem to match the frequency calculations of any of the likely systems or components. As the order increases, the frequency of the vibration will also increase by a multiple of that number.

For example, vibration may be present where the frequency is measured at 14 Hertz (Hz). After doing the necessary calculations it is determined the first order tire and wheel frequency is 7 Hertz (Hz) and the first order driveshaft frequency is 22 Hertz (Hz). Based on this information it can be determined the vibration is most likely a second order tire and wheel vibration: 7 Hertz (Hz) (first order tire and wheel frequency) multiplied by 2 (second order) equals 14 Hertz (Hz) (second order tire and wheel frequency).

Relationship of Vibration Frequency to Order of Vibration 

After carrying out the road test as described, the vibration was determined to be either vehicle-speed related or electric motor-speed related. That determination will identify the vibration frequency calculations that should be done.

In calculating and using frequency readings it is important to remember the direct relationship between Hertz (Hz) and Revolutions Per Minute (RPM). One Hertz (Hz) is equal to 60 Revolutions Per Minute (RPM). This is easy to remember. Think of Hertz (Hz) as cycles per second. There are 60 seconds in a minute, therefore multiply the Hertz (Hz) reading by 60 to get Revolutions Per Minute (RPM). Conversely, divide Revolutions Per Minute (RPM) by 60 to get Hertz (Hz).

Use the Frequency and Revolutions Per Minute (RPM) Calculations Worksheet to calculate system/component frequencies. The worksheet provides the necessary steps to determine each system/component group frequency.