Types of Noise and Vibration

NVH stands for Noise Vibration and Harshness and is an industry term associated with the treatment of vibration and audible sounds. Harshness usually refers to treatments of transient frequencies or shock. Noise denotes unwanted sound, hence treatments are often to eliminate these sounds and vibrations, but occasionally products are engineered to magnify sound and vibration at particular frequencies. The human ear can generally detect frequencies in the range of 20 to 20,000 Hz (Hertz, or cycles per second.) Vibrations above and below this range may not be detectable to your ear, but may still require treatments for improved product performance and longevity. The frequency of the noise is quintessential, as it dictates which method of treatment or what material will work best. Multiple frequencies may require multiple treatments. Folks who first become interested in noise and vibration generally have confusion about how noise and vibration propagate and even more confusion about how to treat it. While the subject has endless subtleties, here is a short primer on the subject so you can start to get "your feet wet" without wasting time.

There are 2 types of detectable noise: Structure borne and Airborne

Structureborne is vibration that you predominantly "feel" like a deep booming base sound from the car’s radio next to you at the street corner or the eerie vibration you feel when a train comes down the tracks right next to your house. These are typically low frequency vibrations that your ear may be able to hear, but are primarily "felt" through your sense of touch. Structure borne vibration is strictly classified as vibration that is transferred through a solid or semi-solid medium from the source (ie a vibrating mechanism like a train or a tractor or even an earthquake!) to a receiver (like you or some other sensing device). These structure borne frequencies are generally classified as less than 1000 Hz. Structural vibrations can be treated 2 ways: Damping or Isolation

Airborne noise is the kind of sound that most people think of as noise and travels through gaseous mediums like the air. Some people might classify your voice as noise, but we like to think that things like your air conditioning or even the hum of your computer are better examples of unwanted noise. These vibrations are detected by your ear and may be impossible to detect with your sense of touch. There are 2 ways to treat unwanted air borne sound: Barriers and Absorbers

Damping and Isolation

Damping is defined as a treatment of vibration to reduce the magnitude of targeted vibrations. (The term "dampening" is also used occasionally, but most experts in the field of NVH refer to dampening as the act of sprinkling water on something...) All damping materials have different characteristics at different frequencies, so choose your treatment intelligently or recruit a consultant to assist you.

Isolation is defined as a method of detaching or separating the vibration from another system or body. Isolation by definition does nothing to reduce the vibration magnitude, it simply seperates the vibration from the system you wish to protect. Some materials may actually perform damping and isolation functions at particular frequencies, but the words are very different and often confused. All materials for isolation perform differently at different frequencies, and if chosen incorrectly, may worsen your problem, not improve it.

Barriers and Absorbers

Barriers perform a blocking function to the path of the airborne sound. A closed door is a simple example of a noise barrier. Barrier performance is strongly correlated to the openings or air gaps that exist after the barrier is employed. If your door is only closed half way or there is a large gap under and above the door where sound may travel through, then your door will be far less effective at blocking sound. If there are no air gaps when a barrier is employed, then weight becomes the dominant factor in comparing barriers. Doubling weight typically reduces sound transmission by 6 dB based on the Mass Law. Barrier performance is dependant on frequency and is most fruitful with high frequencies.

Absorbers reduce sound by absorbing the energy of the sound waves and dissipating them as heat. Carpet is a common example of an absorber. Absorbers are ranked by their ability to absorb sound that otherwise would be reflected off its surface. By definition, an absorber does not block sound transmission, although many materials are designed to block and absorb sound. Good absorbers generally contain complex geometries to trap incident sound waves, rather than reflecting them back into the air. Like all other NVH treatments, absorber performance varies with frequency.