Eliminating Brake Noise

Our ear is a magical organ. The diaphragm in the ear is hit by the flowing air, and the tiny bones activate the nerves that transmit signals to the brain. How air, diaphragm, and nerve interact can mean the difference between talking or braking noise.

Suppose we put the ball joint separator on the floor. When the fingers touch the floor, they start to vibrate. When the fingers vibrate, they stimulate the surrounding air. The forward movement of the fingers pushes the air forward and compresses the air. When the fingers are pulled back, they draw the air back, creating a low-pressure area. If we were to graph the pressure change over time, it would fluctuate up and down like a wave until the finger stopped vibrating. These air pressure changes produced by sound waves will eventually enter our ears.

On the physiological and mechanical level, the ear is a simple device. The high-pressure and low-pressure areas of sound waves move the ear’s tympanic membrane in and out, which causes the structure of the inner ear to vibrate at the same frequency. These vibrations are then transmitted to the fluid in the inner ear, where they are converted into electrical nerve impulses. The brain receives these signals and interprets them as sounds.

Frequency and Pressure

All sound waves dissipate or lose energy within a given distance. The closer you are to the noise source, the louder the sound will be. Let’s add some engineering terms after what we just observed. Sound waves have two parts to form a cycle. These are called compression and sparse areas. The number of times the loop repeats in one second (previously called) is called frequency and is measured in hertz.

“Sound pressure” or “loudness” is proportional to the “amplitude” (height) of the sound wave. In other words, how far the sound waves can push and pull the eardrum. The strength of the sound wave depends on the distance from which it is measured. Sound pressure is measured in decibels or decibels.

In terms of frequency and sound pressure, your ears have a specific range of performance. In other words, the eardrum and the connected bones can only move so fast or so slowly (frequency/Hz). In addition, the tympanic membrane can only move so far (sound pressure). Ordinary people can hear sounds between 20 and 20,000 Hz.

Noise Control

If you are trying to eliminate a noise, you could try to stop it from moving the air around it. This is not possible with brakes because they do not operate in a vacuum and they cannot be separated from the vehicle.

The three available options are to change the frequency, eliminate the path of the vibration transfer or dissipate the noise by insulation absorption treatment.

How All Brake Noise Starts

Noise will occur on all brakes. When the friction material comes into contact with the rotor, the coupling will cause the brake pads and rotor to oscillate and vibrate. In engineering terms, this is called “force-coupled excitation”, and the components are locked into a combined system that will vibrate in the combined vibration mode of the system’s natural frequency.

The amount of excitation and the generated frequency will be affected by changes in the braking torque (or changes in friction coefficient) on the rotor surface. After the parts heat up, the rotor may form a hot spot, which will cause the rotor to have different friction areas, thereby generating different levels of braking torque. This friction coupling is where most of the braking noise is generated.

Looking at the Entire System

When you hear braking noise, it’s not just the sound of rubbing the coupling. The sound you hear will be the product of the structural transmission path of the entire braking system through the suspension components into the passenger compartment and the amplified noise in the reflecting wheel well as the reverberation echo chamber.

This is why we look at the entire system when diagnosing braking noise. Different components have different natural frequencies. Braking systems also have different components with different masses, shapes, and connection points, all of which affect the natural frequency and mode shape.

This will cause the combined natural frequency to produce a new frequency that is different from the original frequency.

Friction Materials

What factors make one friction material quieter than another? There are two parts of the answer here. First of all, if the friction material can better maintain a constant coefficient of friction under a wide temperature range and environmental conditions, it may be a kind of sound-absorbing mat. The friction material has a consistent braking torque under extreme humidity and temperature (-40°F to 500°F) environments, resulting in less vibration changes at the friction coupling.

Secondly, some friction material will leave or transfer a layer of friction material on the rotor surface. Some friction material companies claim that this layer can smooth the surface of the rotor, thereby reducing excitation and noise at the friction coupling. In addition, the transfer layer may be less sensitive to changes in braking torque caused by heat.


Many technicians condemned the brake pads in the noise problem, but in fact the source of noise is not the brake pads but the rotor. Most technicians blame the pad instead of the rotor because they believe that the rotor will not change under heat and extreme forces. But this is not true. Under the action of heat and braking force, the shape of the rotor can become “active” or “dynamic”.

When the mechanical force of the brake acts on the rotor, it can actually move and bend. Although invisible to the naked eye, this movement causes excitation and noise at the frictional coupling.

Calipers and Hardware

The caliper affects the friction coupling and can also be affected by the vibrations generated. As mentioned earlier, the source of most braking noise is the friction coupling. Variations in braking torque or friction coefficient can cause vibration and noise.

The calipers provide different mechanical forces to the friction coupling. With these different mechanical forces, the ability of the caliper to maintain a consistent “friction footprint” between the pads and the rotor surface will determine its ability to control noise.

The second must-have capability of a caliper is its ability to absorb and suppress vibrations. The heavier the caliper, the better it can dampen vibrations. But, like all automotive engineering and design issues, you can only place so much weight in the corners of the vehicle before it starts to affect unsprung weight. In some cases, engineers will transfer mass to certain areas of the caliper. If a component has too much freedom of movement, it will vibrate and make noise. But a closely fitting component, it is likely to vibrate and even excite other components around it. This effect is compounded when it can affect frictional coupling, for example in the case of a stuck brake pad. This is why it is necessary to ensure that slides, rails and clips allow movement, but not too much play. Many clips will lose tension over time. When spring tension is lost, it cannot isolate and dampen vibrations.

Brake Shims or Insulators

Brake pads can control noise in three ways. First, they control noise by preventing and reducing the transmission and amplitude of vibration forces, which can cause excitation of the calipers, brake pad components, and connection structures. This is achieved through the viscoelastic damping material within the layered structure of the gasket and the method of bonding to the gasket assembly.

Secondly, they can also increase the quality of the brake pads. The better the quality of the brake pads, the more it can suppress the vibration in the brake pads and calipers. They use an elastic interface coating on their surface to reduce the reaction force transmitted back to the brake piston.

Third, a good brake pad or insulator can act as a thermal barrier to ensure that the temperature of the entire brake pad surface is consistent. This helps ensure consistent braking torque.


Lubricating oil can reduce some of the brake noise. This is because when the caliper finger is lubricated at the point where it touches the brake pad, the lubricant forms a boundary layer that separates the vibration of the brake pad from the excitation of the caliper finger and the caliper. This is a way to solve the NVH problem, but it has its limitations.

Never use petroleum-based lubricants for brake assembly work, as this will cause the protective cover and seals to swell and fail. In addition, do not use ordinary “chassis grease” on the caliper sleeves and bushings, because it is very likely that the chassis grease will decompose and form a sticky mass within 500 to 1,000 miles, causing the caliper stuck.

All metal-to-metal surfaces should use high-quality molybdenum lubricants, and all metal-to-rubber parts should use high-quality silicone lubricants. Some silicone lubricants can be used on glass slides because they are formulated to withstand high pressures and high temperatures.