Quick answer: Lower frequency = greater amplitude and more energy per cycle, so 15kHz suits large or thick parts; 20kHz is the all-round choice for general plastic assembly; 35–40kHz delivers low amplitude for delicate, small or electronic components. The fastest way to decide: look at your part size, wall thickness and how far the joint is from where the horn can touch.
| Frequency | Best for | Typical amplitude | Part size | Common applications |
|---|---|---|---|---|
| 15 kHz | Large, thick or long parts; high power | 40–80 µm | Medium to very large | Automotive panels, appliance housings, large containers |
| 20 kHz | General-purpose plastic assembly | 25–60 µm | Small to medium | Consumer products, medical housings, most standard assemblies |
| 35–40 kHz | Small, delicate, tight-tolerance parts | 10–30 µm | Very small to small | Electronics, sensors, thin films, precise cosmetic welds |
Slotted aluminum bar horn with matching fixture on an ultrasonic welding press — slotting lets horn faces exceed the single-wavelength size limit.
What Frequency Actually Changes
Ultrasonic welding machines are usually described by their operating frequency — the number of mechanical cycles per second the stack (converter, booster, horn) vibrates. But frequency is not just a number on a spec sheet; it drives three things that decide whether your part can be welded well:
- Amplitude ceiling. Lower-frequency systems deliver higher achievable amplitude (the peak-to-peak movement of the horn face). More amplitude means more energy delivered into the joint per cycle.
- Weld depth and energy reach. High amplitude travels further through the part. This is why a 15kHz machine can weld a joint located far from the horn contact point, while 40kHz struggles with anything but near-field joints.
- Physical stack size. A 15kHz stack is physically larger than a 40kHz stack for the same power rating. Tight spaces and handheld tools favour higher frequencies.
15kHz: When Your Parts Are Big
Choose 15kHz when at least one of these is true:
- The joint area is wider than a standard 20kHz horn face can cover (~80 mm lateral limit) — 15kHz allows horn faces up to ~110 mm diameter, and slotted bar horns go larger still
- The wall section between the horn and the joint is thick (above ~3 mm)
- The joint sits far from where the horn contacts the part (far-field welding)
- The material is semi-crystalline and energy-hungry — PP, PA, POM and similar
Typical 15kHz work includes automotive interior panels, battery module components, appliance housings and large packaging. Power ratings at 15kHz commonly run from 2000W to 5000W, and dual-stack systems reach 2 × 5000W. Our 15kHz high-power pneumatic welder iAL delivers 5000W with a Φ100 mm horn face and 100 mm working stroke (time and energy welding modes), and the iALB dual-head configuration doubles that to 2 × 5000W for extra-large workpieces.
Watch out for: 15kHz is audible to the human ear (near the top of the hearing range). A well-maintained machine with a proper enclosure keeps noise acceptable, but plan for hearing protection in manual workstations.
20kHz: The Default Choice for Most Assemblies
If you have no special constraint, start at 20kHz. It is the industry’s general-purpose frequency for good reasons:
- Balanced amplitude range — enough energy for most joints, enough control for cosmetic welds
- The widest choice of tooling and machine options (servo, pneumatic, handheld)
- Compatible with nearly all common thermoplastics used in assembly
Around 20kHz you will find our widest product range: the servo-driven iSF precision welder for demanding assemblies, the digital pneumatic iBA series for stable repetitive production, and handheld spot welders for large-part tack welding.
Rule of thumb: if your part fits in two hands and the joint is within ~6 mm of a horn contact surface, 20kHz will almost always work.
35–40kHz: When Precision Matters More Than Power
Higher frequency means lower amplitude per cycle — and that is exactly the point for small parts:
- Very small weld joints on connectors, sensors and electronic housings
- Thin-walled or fragile parts that would crack under 15/20kHz energy levels
- Cosmetic welds where flash must be nearly zero
- Tight tooling spaces where a compact 40kHz stack fits better
The trade-offs: less energy reach (near-field joints only), smaller horn face, and generally lower maximum power. This is why 40kHz dominates in electronics assembly and rarely appears in automotive structural welding.
Near-Field vs Far-Field: The 10-Second Check
Before comparing frequency options, check your joint position:
- Near-field: the joint is within ~6 mm of where the horn touches the part → any frequency can work; pick by part size and precision needs.
- Far-field: the joint is more than ~6 mm away, or the energy must travel through a thick wall → you need lower frequency (15 or 20kHz) so the vibration survives the trip.
This single check eliminates most wrong-frequency purchases. One more physics warning: if the joint distance happens to equal about a quarter wavelength (λ/4) of the vibration in your part material, amplitude at the weld interface drops close to zero — the part will refuse to weld no matter how long you run it. Move the joint or change frequency.
Common Mistakes When Choosing Frequency
- Buying by machine price instead of part geometry. A 20kHz machine is often cheaper, but if your part is a 400 mm automotive trim panel, it will never weld it properly.
- Over-sizing the horn. Horn lateral dimensions are limited by the wavelength of the horn material — roughly λ/3. At 20kHz in titanium or aluminum that is about 80 mm; at 40kHz only ~40 mm; at 15kHz up to ~110 mm. Larger faces are possible with slotted designs or aluminum bar horns, but they need engineering support. A horn pushed past its size limit will crack.
- Assuming one machine covers all future products. If your product roadmap includes both small electronics and large housings, plan for two frequency lines or a multi-frequency platform such as the iSFT multi-frequency servo welder.
FAQ
Can one machine weld at multiple frequencies?
Only if the full stack — generator, converter, booster and horn — is designed and tuned for each frequency. You cannot run a 40kHz horn on a 20kHz generator.
Is higher frequency better quality?
No. Frequency must match part size and joint position. A wrong frequency gives bad welds regardless of quality level.
Why is 15kHz louder than 40kHz?
15kHz is at the edge of human hearing, so the vibration is audible; 40kHz is ultrasonic. Both are equally “ultrasonic welding” — you just hear one of them.
What about 30kHz machines?
30kHz exists mainly in specific regional markets and legacy tooling. 15, 20 and 35/40kHz cover virtually all new applications.
Ready to Match Frequency to Your Parts?
Send us your part drawings or samples — our engineers will recommend frequency, power and tooling, and run welding trials on your material.
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- Or contact our engineering team for a free application review