Where is the NVH?

Noise, Vibration and Harshness (NVH) have ailed the automobile since its first days, and will probably be with it as long as cars are built. Rattles, wind noise, vibrations and other annoying physical problems are all classified as NVH problems. To most vehicle owners, these problems are unacceptable, especially on a brand-new, just-delivered vehicle. Unfortunately, when you start talking NVH you’re dealing with a rather large group of problems that can range from a clunk heard when the vehicle crosses a rough road surface to a wheel vibration. And these problems are among the most difficult to locate and diagnosis anywhere on today’s vehicles.

Let’s take a closer look at NVH. First, we’ll review the definitions of basic NVH terms. Then we’ll go over some preliminary inspection techniques and road test procedures. Finally, we’ll calculate a tire/wheel frequency, which can be used to determine the type and source of an NVH problem.

NVH TERMS

Vibration – The repetitive motion of an object up and down, or back and forth; pitching, rolling, bounce and shake, shimmy and brake shudder are types of vibration.

Noise – An unpleasant or unexpected sound created by a vibrating object.

Harshness – Often associated with an abrupt thumping noise; harshness also refers to suspensions that react to road conditions by producing vibration; this vibration is usually in the range of 30 to 60 hertz (Hz).

Frequency – The measure of how many times a noise or vibration occurs in one second; expressed in hertz.

Sound – That which can be heard by the human ear; it has a frequency range of 20 to 20,000 Hz.

Shake – A repetitive motion that is horizontal or vertical with a frequency of 10 to 30 Hz.

Shimmy – A vibration that causes the steering wheel to oscillate with a frequency of 5 to 15 Hz. A high-speed shimmy occurs while driving at high speeds on smooth roads, whereas a low-speed shimmy occurs at low speeds driving across road bumps.

Resonance – The tendency of a system to respond to an outside vibration; for example, as an unbalanced tire frequency approaches the frequency at which other systems respond, the vibration becomes more noticeable throughout the vehicle.

Droning – An unpleasant sensation in the ears; also referred to as humming; 30 to 200 Hz.

Howling – A noise of a higher frequency than droning; usually above 300 Hz.

INSPECTION AND ROAD TESTING

Determining the cause of an NHV problem begins at the front desk when your customer brings their car in for service. The NHV Data Sheet, included with the December 2008 MasterTech Reference Book, provides a concise format to record all the pertinent information, from the type of vehicle to the speed at which the problem is occurring.

Obtain as much information as possible from the vehicle owner, such as the following:

• At what vehicle speed is the problem occurring?
• At what engine rpm is the problem occurring?
• On what side of the vehicle is the problem?
• Does the problem occur while braking, accelerating, steering or during steady speed driving?
• What accessories were in operation (such as the A/C)?

A thorough preliminary inspection should then be performed. This can reveal obvious items that might be causing the NHV problem. These can include an under inflated tire (Figure 1), a missing wheel weight, loose trim, panels or antennas, or an improperly stored jack that’s rattling around in the storage area. After you’re done inspecting the vehicle, take it for a ride.

During the road test, you should fall back on your experience of what is considered normal for that particular vehicle. If the symptom is obvious, verify that it is described on the repair order. If it is, then you need to answer some basic questions.

For example, if it is a noise, what type is it? Droning, also known as humming, is hard to hear as a sound. It is lower pitched than howling. Howling, in turn, is lower-pitched than a brake squeal. Is the problem harshness? It is often heard and felt as a thumping noise and a shock from the floor, steering wheel or seats.

If the problem is a vibration, how does it affect the vehicle? Does the vehicle pitch or bounce? Is it a shake or a shimmy? In some cases, you’ll have to determine the frequency. The Electronic Vibration Analyzer, or EVA (Figure 2) makes it easy to do this. This tool can measure frequency in 2-hertz increments as well as vibration strength. It is powered by the power outlet or cigarette lighter. The sirometer is a simpler tool that performs the same task. When a measured frequency matches the calculated frequency, the source of the noise or vibration has been found (more on that later).

The road test allows you the opportunity to learn about the possible causes of a noise or vibration. Always make note of the operating conditions under which the problem occurs. Most importantly, determine if the problem is related to vehicle speed or engine speed. Knowing this fact will narrow down the number of possible causes and reduce the number of calculations that you will need to do later.

Finding the location of the NVH problem is another important fact to determine from the road test. Not only does it help determine what components are responsible for the problem, it can help later when positioning test equipment sensors, if needed.

CALCULATING FREQUENCIES

As mentioned, frequency is the measure of how many times a noise or vibration occurs in one second. An example frequency is plotted in Figure 3. Every peak on the graph represents one occurrence of the noise or vibration. The amplitude is the strength of that event. For example, the higher the peak, the louder the noise.

For the purposes of this article, we will calculate the tire/wheel frequency. First, determine how many 5-mph increments there are at the vehicle speed related to the problem. At 70 mph, this number is 14 (70 divided by 5). Next, determine the number of times a wheel/tire assembly rotates per second at 5 mph (refer to a tire and wheel chart—not shown). The tire size is P225/60R16 and this size tire rotates 1.09 times per second. Next, multiply 14 (the number of increments) by 1.09 (revolutions per second) to determine the tire and wheel frequency. The answer is 15.26 (round to 15 for diagnostic purposes).

The order of a frequency is the occurrence in one cycle. A second order frequency occurs twice per cycle, and so on. In our example, the first order frequency is 15 Hz. To obtain the second order frequency, multiply the first order number by 2 (30); the third order frequency is obtained by multiplying the first order number by 3 (45).

For our example, the EVA measured a frequency of 54, which is the third order frequency. We can eliminate the wheel/tire assembly because a problem with that component is usually seen as first order (18 Hz). CV joints are either third or fifth order vibrations. Experience tells us that a third order tire and wheel frequency vibration (54 Hz) can result from a bad inner CV joint. Inspection of these components should reveal the source of the problem.