A car in need of attention will be making many noises, some of these are so high in frequency they fall beyond the range of human hearing. This type of sound is known as ultrasound and is the high frequency sound above 20kHz, so high in fact that we can’t hear it unaided. When ultrasound is found it is often an early warning from the vehicle that needs to be acted on, if you can hear it normally it may be too late to take preventative action and results in a much more expensive remedy.
The history of ultrasound stretches back to 1924, but only recently has high quality microprocessor based digital technology become available providing a state of the art ultrasound diagnostic kit that is fully functional yet easy to use. This technology is now finding automotive applications and technicians are discovering that ultrasound tools are proving a valuable utility tool that can be used for a multitude of tasks and shortens the diagnostic process, which saves both time and money.
Ultrasound is generated by many different vehicle systems including mechanical, electrical, fuel and other high-pressure air, oil and water systems. Technicians are experienced in listening to the audible sounds made by cars to help locate and diagnose problems. Being able to detect, listen to and locate inaudible ultrasound provides a technician with a valuable addition to his diagnostic armoury.
Fact: The maximum range of human hearing includes sound frequencies from 20Hz to about 20kHz (a Hz is a cycle per second, a kHz is 1000 cycles per second). As we get older the frequency range decreases, especially at the upper limit to about 15kHz. Our normal voice range is about 500 Hz to 2kHz. Our ears are most sensitive at 2kHz to 4KHz. |
Ultrasound in a car is generated in three principle ways. They are pressurised leaks, friction and electrical anomalies. When a gas or liquid streams through a small-restricted hole such as a crack, tear or faulty seal, high frequency ultrasound noise is often generated. When mechanical components have insufficient lubrication, impact together or start to wear they generate ultrasound vibrations. When electrical circuits lose their insulation or start to break down, invisible arcing or tracking may occur and produce ultrasound. These are all Ultrasound clues that something may be amiss with a car, and demand further investigation.
Because ultrasound has a higher frequency than normal sound, its sound wavelength is shorter. Shorter wavelength sound is relatively directional, which makes locating and pinpointing the source of ultrasound much easier than normal audible sound, especially when there are other competing sounds. Ultrasound strength also drops of quickly with distance, further aiding location detection.
Listening to ultrasound for the first time may sound strange, as it’s a new sound. As you get used to the ‘signature sound’ produced by different components you soon learn to distinguish the different noise sources and more importantly the variation between bad from good.
There are two basic types of detection, the ‘sweep’ method used to detect and pinpoint vacuum and pressure leaks. A good quality ultrasound receiver will detect most pressurised leaks, including air, oil, water, electrical arcing, exhausts, air suspension, airbrakes, and refrigerant gas. The second is the ‘contact’ method used to find worn bearings, gears, valves, actuators and many other faults.
As a guide, a pressurised leak sounds like rushing or hissing air. A good bearing has a low level whirring sound, a bad and worn bearing crunches and crackles and electrical arcing or tracking will sound like crackling or scratching. Technicians will need to progressively learn the good and bad sound signatures.
A good quality ultrasound receiver will be able to help detect and accurately reproduce the ultrasound produced by bearings, valves, solenoids and other mechanical components. The ultrasound produced by these types of components will typically be mechanically generated. The contact method is used with a solid probe rather than an acoustic probe.
The acoustic probe is used to find leaks in electrical systems. The tip of the acoustic probe is touched onto the housing of the electric device and the gain adjusted to increase sensitivity until a clear sound is detected. Internal arcing, tracking and corona discharge will all give off a distinctive sound signature enabling the technician to trace a problem to its source.
Finding the source of wind noise and water leaks can often be difficult and time consuming using normal techniques. Using Ultrasound makes locating leaks easy.
When a car is driven a large air pressure differential is created between the inside and outside of a car body. The door and window seals are meant to keep the passenger compartment air in and outside air out. When these seals are damaged, a door is maladjusted or has worn hinges, hissing, whistles and other irritating noises occur. Water leaks can also be troublesome to find as where the dampness is visible may not be the entry point for the water. Water unlike air is influenced by gravity and will often end up at a lower point well away from where it came in.
Non-pressurised leaks cannot generate ultrasound therefore the transmitter has to be used. Placed inside the passenger compartment or vessel in which there is a leak, the transmitter literally floods the enclosed space with ultrasound, which can only escape through leaks and gaps. The technician then moves the receiver along seals, doorframes and other suspect areas until the transmitted sound is heard, indicating a leak.
Ultrasound tools have been proved invaluable for body shops when fitting replacement panels. It helps to ensure the accuracy of seals against wind noise and water leaks. |
An ultrasound tool kit, like the OmiSonic tester, will typically consist of a handheld receiver, a separate ultrasound transmitter, headphones and a range of probes. The ultrasound receiver accurately converts the high frequency ultrasound, which we can’t hear to a lower frequency we can.
The receiver should also have an ultrasound intensity meter as a comparative visual indication. Ultrasound like all sound can very enormously in sound level and so a sensitivity control is essential to adjust to the different sound level being detected. It must also have a volume control to adjust the headphone sound level.
Ultrasound vibrations typically travel through the air but can also be mechanically transmitted so a kit needs to contain both a solid and acoustic probe. The headphones must be robust to survive a garage environment and must also prevent all-extraneous noise from getting to the users ears, which will enable easy listening to low-level ultrasound. Your average home hi-fi headphones aren’t robust enough for a garage environment and don’t attenuate the background noise nearly enough.
An ultrasound transmitter is required to find non-pressurised leaks, the transmitter generates ultrasound artificially to enable the identification of non-pressurised leaks and holes. To work best and produce the strongest signal a transmitter and receiver’s ultrasound frequencies must be exactly matched. The transmitter has a constant tone but should also have additional modulated tones such as chirp or warble which will make detection much easier where there are competing ultrasound noise sources or when the sound signal path is in-direct.
Ultrasound technology is new to the automotive industry; in fact it is so new that many applications are still to be found. As a non-intrusive tool faults can be checked before components need to be removed i.e. alternators, coil packs, injectors and even tyres. Leaks can be detected without the need for smoke, water or dyes. Ultrasound tools are utilitarian as they are not vehicle specific and compliment other diagnostic equipment. As professional technicians we should embrace any technology that can help improve the diagnostic process.
