Rotary Converter Noise: Each Maker's Published Figure

Rotary phase converter makers publish noise figures, and the lowest from the six we compare is 43 dB "from ten feet away." Phoenix Phase Converters prints it for all of its models. Phoenix says that's "the average noise in a room of people." On a list in the same Phoenix article, with no distances given, a whisper is 30 dB and a running box fan is 60 dB. The highest figure is "typically 65-75 dB at 3 feet," from a 2026 Phoenix guide. That figure covers rotary converters in general rather than one Phoenix model. Phoenix says it's about as loud as a washing machine on its spin cycle. The idler motor inside a rotary converter spins the whole time it's switched on, so you hear it between cuts, and all day if you leave it running. Once your machine is cutting, the converter may be hard to hear. American Rotary's CNC blog says that "In most cases, your three-phase machine will drown out the sound of the phase converter."

Of those six makers, three print a number. Phoenix prints three more figures between those two. Phoenix also sells the Whisper Series, a converter inside a cabinet built to cut the sound. That enclosed unit has its own figure: under 60 dB at 1 meter, which Phoenix calls "roughly the level of a normal conversation." Kay Industries (Phasemaster) prints 72 decibels or less at no load, with no distance given. Kay says its converter is about as loud as a motor the size of the one on your machine, or quieter. American Rotary prints 60 to 70 dB, with no distance given, and likens it to a bathroom exhaust fan running. North America Phase Converters (NAPCco), Phase-A-Matic and Capacitor Converters print none.

Those figures can't tell you which maker's unit is quieter. They were taken at different distances: ten feet, 3 feet, 1 meter, or a distance the page doesn't give. None of the pages that print a number names a test method, a standard or a meter. At Built for Production, we lay out every figure we found, with the distance and the document behind each one.

How loud is a rotary phase converter? Each maker's figure

Among the six rotary makers in the table below, published noise figures for rotary phase converters start at 43 dB "from ten feet away," and top out at "typically 65-75 dB at 3 feet." Phoenix prints both of those, and three more between them, on its own pages. Some are for its own units and some for rotary converters in general. The table says which. The pages don't say why the figures differ. Phoenix also prints a separate figure for its Whisper Series, a Phoenix converter inside a cabinet built to cut the sound: under 60 dB at 1 meter.

American Rotary prints two figures. Its FAQ gives 60 to 70 dB for every size of unit. Three of its blog posts give a separate figure for the idler alone: "less than 60 Db." They compare that to a box fan running. The FAQ names the idler's cooling fan as the main source of the noise. None of the pages says why the idler figure and the whole-unit figure differ, and neither the FAQ nor the three blog posts gives a distance. Kay prints 72 decibels or less at no load, with no distance given, on its FAQ page and in two PDFs. The table covers the six rotary makers we checked: Phoenix, American Rotary, Kay, NAPCco, Phase-A-Matic and Capacitor Converters.

Where a page prints no date, the table gives the year the site's own records show for the page's last update. We read every document in October 2026.

• American Rotary's FAQ says "All of our phase converters produce the same level of noise, no matter the size." So on American Rotary's own account, choosing a smaller unit for quiet gains nothing, and the size comes down to the motors the converter will run. Our phase converter sizing guide shows each maker's rule for that. Our sizing calculator applies the published rules of the makers it covers to your motor's nameplate numbers.
• On its Sound Levels page, Kay says its sound measurements follow a test standard, IEEE Standard No. 85. The same page offers "true untampered recordings of each converter measured at a distance of 10 feet." Kay's FAQ, which prints the 72 decibels or less at no load, says "Listen to our sound-level data and find out," pointing to those recordings. No Kay page says the 72 decibels was read at 10 feet, so that figure has no stated distance. The pages that print it name no method.
• Kay's is the only figure in the table that says whether the converter was idling or running a load: it's for no load. What load does to the sound is covered below, under "Where does the noise come from, and what changes it?"

Why can't you compare one maker's dB figure with another's?

The makers' figures differ in three ways: the distance from the unit, whether the meter was weighted, and the test method. Each one changes the number.

On distance, the OSHA Technical Manual's noise chapter says sound from a single source in open air drops 6 dB each time you double the distance. OSHA's example: 90 dB at 1 meter, 84 dB at 2 meters and 78 dB at 4 meters. Indoors the drop is smaller, because sound bounces off the walls, the floor and the machines; OSHA says the level then falls by less than 6 dB per doubling. So figures taken at ten feet, at 3 feet, at 1 meter and at no stated distance never line up against one another. There's no fixed number for turning a maker's reading at its distance into the level where you stand in your own shop.

Weighting is the letter after dB. A meter set to the A scale, written dBA, comes close to how the human ear responds to different pitches. Hearing limits are written in dBA. OSHA writes a weighted level with its letter, dBA or dBC. The rotary makers print dB with no letter, so the page doesn't tell you whether the reading was weighted.

Method is everything else: the room, the floor, the load on the unit, the meter, and the person holding it. Of the six rotary makers, only Kay names a test standard, and the page that names it prints no number. So no printed rotary figure comes with a stated method.

What do the decibel numbers sound like?

Phoenix and American Rotary compare their figures to everyday sounds, and Kay compares its figure to a motor. Each comparison describes only the figure it's printed with. Here they are in one place:

• 43 dB at ten feet: "the average noise in a room of people" (Phoenix)
• Less than 60 dB for the idler, no distance given: "comparable to a box fan running" (American Rotary)
• Under 60 dB at 1 meter, Whisper Series: "roughly the level of a normal conversation" (Phoenix)
• Roughly 60 dB at three feet for the idler: "about the same as conversational speech" (Phoenix)
• Roughly 60 to 65 dB, no distance given: "about as much noise as a refrigerator" (Phoenix)
• 60 to 70 dB, no distance given: "comparable to a conversation" (Phoenix) and "similar to a bathroom exhaust fan running" (American Rotary)
• 65 to 75 dB at 3 feet: "About as loud as a washing machine during the spin cycle" (Phoenix)
• 72 decibels or less at no load, no distance given: "the same or less than a motor of the same horsepower as the load" (Kay). In plain terms: as loud as a motor the size of your machine's motor, or quieter

Phoenix's noise article also lists four everyday sounds with their levels. The table below sets that list beside sound levels published by OSHA and by the National Institute on Deafness and Other Communication Disorders (NIDCD).

Phoenix's rows carry no weighting letter and no distance. OSHA's and the NIDCD's are in dBA. Read the two sets side by side as a rough guide and no more.

A change of 10 dB is a big one. The Federal Highway Administration's Noise Fundamentals page says a change of 10 dB "will be perceived by an observer to be a doubling or halving of the sound," and it gives the example that 70 dB sounds twice as loud as 60 dB. American Rotary's FAQ prints 60 to 70 dB, with no distance given. By the FHWA's rule, the top of American Rotary's range sounds twice as loud as the bottom. A 3 dB change is barely perceptible and a 5 dB change is readily perceptible.

The same page has a table for adding two sounds together. When two sources differ by 10 dB or more, the louder one sets the total and the quieter one adds nothing to the reading. So when your machine is 10 dB or more louder than the converter, a meter shows about the machine's level alone. That fits American Rotary's point that a running machine drowns out the converter.

Where does the noise come from, and what changes it?

The idler motor and its cooling fan. American Rotary's FAQ says the noise "comes primarily from the idler motor's cooling fan." Kay's Sound Levels page says no rotating machine runs without some sound and vibration.

Idler speed. A slower idler runs quieter, by Phoenix's account: its noise article ties its units' "relatively low running speeds" to less noise. Kay prints its speed: "All Phasemaster phase converters operate at 1800 RPM."

Load. Three makers say what load does to the sound of their own units.

• Capacitor Converters' troubleshooting guide: "Some noise and vibration is normal, especially under no load conditions. As the rotary is loaded, it will run more quietly."
• Kay's FAQ: the level "will measure 72 decibels or less at no load and will decrease when load is applied." The FAQ gives no distance.
• American Rotary's AD(X) user manual: the converter "will run cool and quiet in the no load condition."

American Rotary's line covers no load only. Each is describing its own units, so none of the three tells you what yours will do with nothing on it.

The mounting surface. Four makers write about it, and they don't agree.

• American Rotary's AD(X) manual says to set the idler on the rubber isolation feet it ships with and, in capitals, not to bolt the idler motor down, because that can cause bearing failure.
• Kay's troubleshooting page says noise and vibration "are usually the result of a resonance caused by a mounting surface that does not support the converter well" and "also occur when the converter is bolted tightly to any surface." Its instruction: "place it on a substantial floor using the shock absorbing pads supplied with the converter. Do not bolt the converter to any surface."
• NAPCco's rotary installation manual calls the idler motor the idler generator, and says: "Do not bolt idler generator down solid. This can cause diminished generator life."
• Capacitor Converters says the opposite for its units: "Make sure that converter is securely bolted to the floor or surface it is resting on. A rubber mat or similar device may be placed under the converter before bolting it in place."

Each instruction is for that maker's own unit. The manual that ships with yours is the one that applies.

Will a rotary converter hurt your hearing or bother the house?

Most of the makers' rotary converter figures sit in the range the NIDCD calls unlikely to cause hearing loss. The NIDCD says sounds at or below 70 dBA are unlikely to cause hearing loss, even after long exposure. The lowest maker figure is Phoenix's 43 dB at ten feet. Phoenix and American Rotary both print 60 to 70 dB, with no distance given. Two figures pass 70 only at their top end: Kay's 72 decibels or less at no load, with no distance given, and Phoenix's "typically 65-75 dB at 3 feet" for rotary converters in general. Two caveats. The makers print no weighting letter, so their dB may not be the dBA the NIDCD means. And you may stand closer to the unit than the maker's meter did.

The next threshold is 85 dBA, the recommended limit for noise at work set by NIOSH, the US agency that researches workplace health. For a sense of that level, NIOSH says lawn mowers, vacuums and power tools can produce noise around 85 to 90 dBA. The limit is an average over an eight-hour workday. NIOSH says workers exposed at or above it are at risk of significant hearing loss over their working lifetime, and the NIDCD says long or repeated exposure at or above 85 dBA can cause hearing loss. NIOSH's 2016 article on noise exposure limits says the limit applies only to the workplace and isn't meant for noise from the general environment or from recreation. The same article says that protecting your hearing whenever sound reaches 85 dBA or more is good practice wherever you are.

NIOSH also gives a rough check that needs no meter: "Sounds that are 85 dBA often require you to raise your voice to be heard by someone 3 feet away." Stand by the running converter with someone 3 feet from you and talk at a normal level. If you have to raise your voice to be heard, the sound where you stand is often around 85 dBA.

Every rotary figure in the maker table is printed below 85. A converter adds hours of exposure, because it makes its sound the whole time it's switched on. The NIDCD names how long you're exposed and how far you are from the sound as factors in protecting your hearing. Its rule of thumb: "avoid noises that are too loud, too close, or last too long."

For the family upstairs and the neighbors, no maker publishes what carries through a floor or a wall. The EPA did publish levels for noise indoors and outdoors, in 1974. Its press release of April 2, 1974 identified 45 decibels indoors and 55 decibels outdoors as the levels that prevent "activity interference and annoyance." The EPA wrote its 45 and 55 decibels as averages of all the sound in a place over periods such as 8 or 24 hours. So a maker's figure for one running converter can't be checked against those levels as a pass or fail. The EPA meant them as a basis for state and local governments to set their own noise rules, so if a neighbor is the worry, your town's noise ordinance is the one to read.

Should the converter go in another room or an enclosure?

A rotary converter can go in another room or an enclosure, and American Rotary suggests both for quiet settings. Its FAQ says that in most shops with running machinery the converter noise is negligible, and for "quieter environments or offices" it suggests "mounting in a separate equipment room or using sound-dampening enclosures." Phoenix sells a converter that comes inside the cabinet. Its Whisper Series is a Phoenix rotary converter inside a cabinet built to cut the sound, on motor mounts that damp vibration. Phoenix says the unit inside is "Internally identical to the NL or NLT series for a given HP rating," two of its rotary lines. The series is "Available on select models," and the datasheet comes on request.

A box also holds heat. Phoenix's installation guide asks for a well-ventilated space because "the converter generates some heat during operation." The fan that makes the noise is the same fan that cools the idler, so boxing in the fan to cut the noise also boxes in the heat it removes. Before you build or buy an enclosure, ask the maker what airflow and clearance its unit needs inside one. Where the converter sits, and how the wire reaches it, are questions for your licensed electrician.

Is a digital converter or a VFD quieter?

A digital phase converter makes 3 phase power with solid state electronics instead of an idler motor. Every digital converter maker we read calls its digital units quieter than a rotary. Two of them print numbers. Phoenix's blog rates a digital converter at "< 40 dB (fans only)," with no distance given. That's close to the 30 dB whisper on Phoenix's own list. Phase Technologies puts its Phase Perfect units between 70 and 80 dBA, depending on which model and which document you read, with no distance given. On OSHA's chart, that runs from classroom chatter (70 dBA) to a freight train at 100 feet (80 dBA). Those figures come from its 2021 and 2022 documents. Its 2021 and 2022 product specifications are marked "Preliminary data; subject to change without notice." Its current documents print no figure.

The printed numbers can't settle which type is quieter. Phase Technologies' figures are in dBA and give no distance. The rotary figures carry no weighting letter, and only some give a distance. Without the same weighting and the same distance, the two sets can't be lined up.

In the makers' words: Phoenix's comparison article rates a rotary "Moderate (idler motor hum)," its GPX digital line "Very Quiet (fan only)" and a static converter "Silent (no moving parts)." The silence has a cost: a motor on a static converter gets only part of its rated horsepower, which our static converter guide covers. NAPCco describes its digital units as "No moving parts, slight electrical hum." Phase Technologies' Phase Perfect web page says "Near silent" with "no moving parts." Phase Technologies' Gen4 manual for the Phase Perfect says the PT models have enclosure fans.

ABB's fan-cooled drives, at 50 to 62 dBA, sit around OSHA's level for a conversation at 3 feet (60 dBA). WEG's under 60 dB sits below the 60 dB box fan on Phoenix's list, and neither ABB nor WEG gives a distance.

A VFD has no idler. The motor a drive runs can whine. ABB's ACS355 manual says a higher setting for the drive's switching frequency lowers that motor noise, and a Danfoss drive application note says the same of its drives. The setting is in the drive's manual, and the drive maker's support line is the one to ask about it. The trade-offs between the three types are in our rotary vs digital vs VFD guide.

Is my rotary converter's noise normal?

Some running noise from a rotary converter is normal. Phoenix's noise article says "it's normal for a phase converter to create some noise" and that excessive noise can point to a problem with the unit. Capacitor Converters' troubleshooting guide says "Some noise and vibration is normal, especially under no load conditions." Kay's FAQ puts normal at "72 decibels or less at no load," with no distance given.

A change in the sound is what matters. American Rotary's AD(X) manual says that if the converter doesn't sound like it's running smoothly, turn it off and call American Rotary's technical support. NAPCco's manual says to turn the converter off if excessive noise or vibration occurs. NAPCco's troubleshooting table also lists checks for a converter that vibrates, runs loud or has a squealing bearing noise. One is to make sure the converter isn't bolted down solid. Another is whether the converter is new and still breaking in. NAPCco says new converters "may need" a break-in period, and its table gives that as 25 to 50 hours of running. The rest are electrical checks. A grind, a squeal, a scrape or a hum that wasn't there last month: turn the unit off and call the maker, with the model and serial number in hand. We don't give repair steps.

Can a rotary converter run all day?

A rotary converter can run all day. American Rotary's manual says its converter "may operate continuously with or without a load," and NAPCco's FAQ says the same of its units. American Rotary's FAQ adds that "Most users leave converters running during work hours since the idling consumption is minimal." The noise runs as long as the idler does, and so does the power use. Our cost to run calculator has the published idle power figures for the units it covers: American Rotary and Ronk rotary converters, Phase Perfect digital converters, and ABB and WEG drives.

How do you check your own converter's noise level?

NIOSH's Sound Level Meter app is free to download and runs on iOS devices only. NIOSH rates it accurate within plus or minus 2 dBA and says it can't verify the accuracy of Android apps. Then:

• Take a reading with the converter off. That reading is the background sound of the room. Switch the converter on and read again; the converter is what lifts the second reading above the first.
• Stand at the maker's distance: ten feet for Phoenix's 43 dB, 3 feet for its 60 dB and 65 to 75 dB figures, 1 meter for the Whisper Series. American Rotary and Kay give none, so pick a distance and write it down with the reading.
• Set the app to A weighting, which is what the NIOSH and NIDCD figures use. A maker's figure printed without the letter may or may not have been weighted, so treat a match between your reading and the maker's figure as rough.
• Take a reading with the converter idling and nothing on it, then another with your machine running. Idling with nothing on it is the state Kay's figure describes: 72 decibels or less at no load, with no distance given. If the machine-running reading is 10 dB or more above the converter-only reading, the machine drowns out the converter. The machine's level is then the one that matters for your hearing. If the two readings are closer than 10 dB, you're hearing both.

If your reading at the maker's distance is far from the maker's figure, send the maker the reading, the distance and the weighting.

If you're weighing a rotary converter for a garage or basement shop, call (855) 923-2610 or email contact@builtforproduction.com, and we'll go through the maker's figure for the unit you have in mind, conditions and all. When the answer belongs to the maker or your electrician, we'll say so. Our contact page has the number and the email.