Choosing the "best" operating frequency
Health should be a primary concern, especially considering government regulations. Most ultrasonic health issues are related to hearing.
Most people have limited perception of sound above 18 kHz or even above 20 kHz. However, there are exceptions. For example, one ultrasonic sewing machine was originally designed for 20 kHz but some of the women operators complained that they could hear the vibrations. Ultimately, the frequency was increased by several kilohertz to solve this problem. Thus, even though an application may perform best at a certain frequency, this may be overridden by health concerns.
There are many government regulations related to noise exposure over time. If ultrasonic equipment is operated at 18 kHz or below then sound enclosures or personal hearing protection are often needed to limit the exposure. To avoid the need for a sound protection, the operating frequency may be increased somewhat even though that may not be optimal for the application.
However, even some applications that operate at 20 kHz or above still generate audible noise. For example, liquid processing applications generate cavitation which produces an audible hiss. Inserting applications produce audible noise as the horn hammers the metal insert. Depending on the exposure, these may require sound enclosures or personal hearing protection.
Without other considerations, the operating frequency should be that which produces the best application results. For example, for plastic and metal welding, delicate parts often weld better at higher frequencies.
An ultrasonic surgical knife may give optimum cutting rate at 20 kHz. However, this device may be too large for a surgeon to comfortably handle. Thus, a higher frequency may be chosen to reduce the device size even though the cutting rate is reduced.
Alternately, a plastic part may be too large for a 20 kHz horn so a 15 kHz system may be needed.
Because lower frequency transducers are larger and therefore allow greater ceramic volume, these systems can deliver greater power which may be needed for a particular application (e.g., welding large plastic parts).
Lower frequencies have longer wavelengths which results in lower stress at a given amplitude. Thus, if a horn/probe must be run at a high amplitude where fatigue may occur then a lower frequency may be preferred.
If high amplitude is required then a lower frequency should be used. This is because —
- The transducer' amplitude is higher so less gain needs to be built into the other stack components to achieve the required output amplitude. This allows greater flexibility in designing these other stack components.
- Because the stresses are lower (above), higher amplitudes are possible without fatigue failure.
Ultrasonic applications are usually not very sensitive to frequency differences of several kilohertz. Thus, an acceptable frequency can usually be found for which off-the-shelf equipment is available, typically in increments of 5 kHz (e.g., 20 kHz, 25 kHz, 30 kHz, 35 kHz, 40 kHz). For example, an application may have an optimum frequency of 22.5 kHz but may, instead, operate acceptably at either 20 kHz or 25 kHz.
Higher frequency equipment generally costs less than lower frequency equipment.