15 kHz or 20 kHz? Pick by part size, not by price

Buyers ask us this constantly, usually phrased as "which one is better". It is the wrong question. The two frequencies do different jobs, and the deciding factor is almost always the size of the part in your hand.

The physics you cannot argue with

An ultrasonic horn is not just a lump of metal. It has to resonate at the machine's operating frequency, which means its length is fixed by that frequency — one half of an acoustic wavelength.

Sound travels through titanium and aluminium at roughly 5,100 metres per second. Run the numbers:

That is the whole trade-off in one table. Lower frequency, bigger horn, more amplitude, more energy into the joint. Higher frequency, smaller horn, gentler action, less risk to fragile internals.

Where 20 kHz is the right answer

This is the general-purpose frequency and it is what most of our machines ship as. Reach for it when:

• The part fits comfortably inside a 100 mm circle. Electronics enclosures, mobile charger bodies, power adapters, bottle caps, connectors, impellers, eyelets.
• You are welding amorphous plastics — ABS, PC, acrylic, PVC, PS.
• You want one machine that will handle whatever the sales team brings in next month. Tooling for 20 kHz is the most widely available, and if you ever need a second horn in a hurry it is the easiest to source.
• Operator noise matters. 20 kHz is above most people's hearing range, so an unenclosed machine is far more pleasant to stand next to than a 15 kHz one.

Where you need 15 kHz

Once the part gets past roughly 120 mm, a 20 kHz horn physically cannot cover it in one shot. You either weld in multiple hits — which shows on the part and slows the line — or you drop to 15 kHz.

15 kHz earns its keep on:

• Automotive interior panels and door trim, where the weld line is long.
• Toys — large hollow mouldings, often in ABS but sometimes in PP.
• PP corrugated boxes and multi-head box welding. Polypropylene damps vibration badly, so the extra amplitude at 15 kHz matters as much as the horn size does.
• Geocell and geotextile welding, where you are joining long strips.
• File folders and stationery, where the seam runs the length of the part.
• Luggage, sports goods, rackets — thick sections, semi-crystalline resins, long joints.

The catch is noise. 15 kHz is right at the edge of what people can hear, and what they hear is an unpleasant hard tone. If the machine is going to run continuously next to an operator, put it in an acoustic enclosure. Not optional in our view — we have seen shops where the operators simply refuse to work near an open 15 kHz machine, and they are not wrong.

What about 30 and 40 kHz?

These exist for small, delicate work — medical disposables, tiny electronic components, thin membranes. The horn is small, the amplitude is low, and there is much less risk of shaking an internal component to bits.

They are a specialist purchase. If you are asking whether you need one, you probably do not. If you are welding something the size of a fingernail with a sensor inside it, then you do.

Wattage is a separate question

People conflate frequency and power, and they are independent. We build machines from 1,500 W to 4,200 W at both frequencies.

Frequency sets how big a horn you can run. Wattage sets how much work that horn can do per cycle. A 15 kHz 1,500 W machine and a 15 kHz 4,200 W machine take the same horn — but the 4,200 W one will drive a long, awkward joint in a filled polypropylene without tripping out on overload.

Rough guide for a starting point:

• 1,500–2,000 W: small amorphous parts, caps, eyelets, light-duty work.
• 2,000–3,000 W: the middle of the market. Most enclosures, chargers, battery packs.
• 3,000–4,200 W: large parts, PP and PE, long seams, filled resins, continuous production.

Undersizing wattage is a false economy. A machine that trips on overload three times an hour costs more in stopped production than the price difference ever saved.

What we would ask you

When someone rings us about frequency, these are the questions that actually settle it:

1. How big is the weld area, measured across the widest part of the joint?
2. What is the resin, and is it filled? Glass-filled PP behaves nothing like unfilled ABS.
3. How far is the joint from where the horn can sit?
4. How many parts per shift?
5. Is there anything fragile inside the part?

Answer those five and the frequency picks itself. Send us a photo of the part and the answers, and we will tell you which of our welding machines fits — including when the honest answer is the cheaper one.

Questions we get asked