Optimum Operating Frequency Of Ultrasonic Equipment

Generally speaking, the operating frequency of ultrasonic equipment should be the frequency that produces the best application effect. For example, welding of plastics, metals and consumables generally works better at higher frequencies, however this is not the only consideration.

Ultrasonic scalpels can provide optimal cutting rates at 20 kHz. However, the device may be too large for a surgeon to operate comfortably. Therefore, even if the cutting rate is reduced, a higher frequency can be selected to reduce the size of the device.

Because the low-frequency transducers are larger, allowing for a larger ceramic volume, these systems can deliver more power, which may be necessary for certain applications (eg, welding large plastic parts).

Lower frequencies have longer wavelengths, which result in lower stress at a given amplitude. Therefore, if the tool head must operate at high amplitudes where fatigue may occur, lower frequencies should be preferred.

Amplitude

If a high amplitude output is required, a lower frequency should be used. This is because:

The transducer output is higher, so less gain needs to be built into the other stack components. This provides more flexibility when designing these other stack components.

Since the stress is lower (above), it is possible to operate at higher amplitudes without fatigue failure.

Equipment availability

Ultrasonic applications are generally not very sensitive to frequency differences of several kilohertz. Therefore, it is common to find acceptable frequencies available with off-the-shelf equipment, usually in 5 kHz increments (eg 20 kHz, 25 kHz, 30 kHz, 35 kHz, 40 kHz). For example, an application might have an optimal frequency of 22.5 kHz, but work acceptable at 20 kHz or 25 kHz.

Cost

High-frequency equipment usually costs less than low-frequency equipment.

Healthy

Most people have limited perception of sounds above 18 kHz or even 20 kHz. However, there are exceptions. For example, an ultrasonic sewing machine originally designed for 20 kHz, but some female operators complained that they heard vibrations. Eventually, the frequency was increased by a few kilohertz to fix the problem.

Government regulations

Over time, there have been many government regulations regarding noise exposure. If ultrasound equipment operates at 18 kHz or less, a sound enclosure or personal hearing protection is often required to limit exposure. To avoid the need for sound protection, the operating frequency can be raised slightly, even though it may not be optimal for the application.

However, even some applications running at 20 kHz or higher still produce audible noise. For example, liquid handling applications can create cavitation, which produces a hissing sound. The plug-in application produces an audible noise when the horn bangs against the metal plug-in. Depending on exposure, a sound enclosure or personal hearing protection may be required.