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When we talk about how "quiet" an electric motor is, a single decibel number is often quoted; yet to understand motor noise correctly two separate quantities must be distinguished: sound power level (Lw) and sound pressure level (Lp). These two measure different things and, when confused, mislead the purchasing decision. Sound power is the total acoustic energy the motor radiates as a source; it is independent of distance and is the motor's "own" quantity. Sound pressure is the value measured at a specific point, at a specific distance, and perceived by the ear; it changes with distance, environment and reflections. In this article we cover the concepts of Lw and Lp in motors, dB(A) weighting, the IEC 60034-9 noise limits, how Lp changes with distance, the pole/speed-noise relationship, the measurement method, quiet motor selection and environmental noise regulation.
Noise is not merely a comfort matter; it concerns worker health, regulatory compliance and is often an indicator of motor quality. Higher-than-expected noise may signal imbalance, a bearing problem, loose mounting or a weakness in magnetic design. That is why reading the sound values correctly is important both for choosing the right motor and for catching field problems early.
The Difference Between Sound Power (Lw) and Sound Pressure (Lp)
We can liken these two concepts to a heat source: sound power (Lw) is like the total heat a stove produces; as you move away from the stove the warmth you feel (sound pressure, Lp) decreases, but the total heat the stove produces does not change. The value given in a motor catalogue is most often the sound power level (Lw), because this is the motor's objective property independent of distance and allows motors to be compared fairly. What your ear hears in the field and what the sound meter shows is the sound pressure (Lp); this depends on the measurement distance and the environment.
In practice, at a typical measurement distance of 1 metre and under free-field conditions, the sound pressure level comes out about 7-12 dB lower than the sound power level (depending on motor size and measurement surface). So a motor whose catalogue lists 75 dB(A) sound power gives about 63-68 dB(A) sound pressure at 1 metre. When comparing two motors it is essential to compare the same quantity (both Lw or both Lp, at the same distance); comparing different quantities gives a wrong result.
Why Is dB(A) Used?
Sound levels are usually given in dB(A). The "A" here is a weighting curve that mimics the human ear's sensitivity across frequencies. The human ear is very sensitive to mid frequencies (1-4 kHz) and less sensitive to very low and very high frequencies. A-weighting corrects the measured sound according to this human perception, giving a number consistent with the "perceived loudness." That is why noise regulations and motor catalogues almost always use dB(A). The dB is not a linear but a logarithmic scale: every 3 dB increase means roughly a doubling of acoustic energy; a 10 dB increase is perceived by the ear as about twice as loud.
IEC 60034-9 Noise Limits
The noise of electric motors is regulated by the IEC 60034-9 standard. This standard defines the permitted maximum sound power level (Lw) according to the motor's power, pole count (speed) and frame size. The general rule: as the pole count decreases (speed increases) the permitted noise limit rises, because high-speed (2-pole) motors produce more aerodynamic and magnetic noise. The table below exemplifies typical maximum sound power levels by frame and pole based on the IEC 60034-9 logic (the values are typical magnitudes; the binding limit is the relevant table of the standard).
| Frame | 2-pole (3000 rpm) | 4-pole (1500 rpm) | 6-pole (1000 rpm) | 8-pole (750 rpm) |
|---|---|---|---|---|
| 90 | ~71 dB(A) | ~61 dB(A) | ~56 dB(A) | ~56 dB(A) |
| 112 | ~77 dB(A) | ~68 dB(A) | ~63 dB(A) | ~61 dB(A) |
| 132 | ~80 dB(A) | ~71 dB(A) | ~66 dB(A) | ~64 dB(A) |
| 160 | ~84 dB(A) | ~75 dB(A) | ~70 dB(A) | ~68 dB(A) |
| 200 | ~87 dB(A) | ~78 dB(A) | ~73 dB(A) | ~71 dB(A) |
| 250 | ~92 dB(A) | ~82 dB(A) | ~78 dB(A) | ~75 dB(A) |
Two conclusions emerge from the table: first, at the same frame the 2-pole version is the noisiest; second, as the frame grows (power increases) the permitted noise also rises. If quietness is a priority, when speed is free it is sensible to prefer a higher-pole (4- or 6-pole) version.
The Combined Noise of Multiple Motors
In a facility there is often not a single motor but several running side by side. A common mistake here is to add the sound levels directly; but because the decibel is a logarithmic scale, sounds do not add arithmetically. When two identical sources at the same level are combined, the total sound is only about 3 dB higher than a single source; four identical sources add about 6 dB. So two 70 dB(A) motors together produce about 73 dB(A), not 140 dB(A). This rule is important for correctly estimating the total noise load of a multi-motor facility. If one dominant source is 10 dB or more above the others, that dominant source practically governs the total noise; in that case quieting the noisiest motor delivers the biggest gain.
How Does Sound Pressure Change with Distance?
In a free field (reflection-free environment), as you move away from a point source the sound pressure drops by about 6 dB each time the distance doubles. So a motor measured at 70 dB(A) at 1 metre gives about 64 dB(A) at 2 metres and about 58 dB(A) at 4 metres. This "inverse square law" logic is very useful when planning motor placement and operator position: moving the motor away from the operator or placing a barrier in between significantly reduces the sound pressure exposure. However, in enclosed, hard-surfaced environments reflections reduce this drop; in a real environment Lp may come out somewhat higher than the free-field estimate.
The Pole, Speed and Noise Relationship
Motor noise comes from three main sources: aerodynamic (fan) noise, magnetic (electromagnetic) noise and mechanical (bearing, imbalance) noise. As speed increases, the fan's peripheral velocity and thus the aerodynamic noise rise rapidly; that is why 2-pole (3000 rpm) motors are the noisiest group. 4-pole (1500 rpm) and higher-pole motors are markedly quieter. Magnetic noise is related to stator-rotor slot geometry and drive switching frequency; especially in drive-fed motors the switching frequency can create an audible hum. Mechanical noise, on the other hand, is a quality marker: a well-balanced motor with quality bearings runs quietly. We covered how to evaluate vibration and noise together in our article on noise and vibration: low-noise motor selection.
How to Choose a Quiet Motor?
- Where possible choose a higher-pole (lower-speed) version: 4- or 6-pole is markedly quieter than 2-pole.
- Compare the catalogue Lw value: Compare the same quantity (Lw) and the same operating point.
- Look for good balance and quality bearings: Mechanical noise is a direct quality indicator.
- Plan the placement: Distance and barriers reduce the Lp the operator is exposed to.
- Set the switching frequency in drive operation: A suitable frequency reduces the magnetic hum.
Most efficient motors are quieter thanks to an optimised fan and better balance; so when seeking quietness, high-efficiency-class motors offer an advantage. We covered the topic in our articles on quiet and low-vibration operation in IE4 motors and, on the synchronous reluctance side, noise and sound level (dB).
Noise in Drive (VFD) Fed Motors
In motors fed by a variable frequency drive (VFD) the noise topic is somewhat more complex. The drive applies a square-wave-like pulsed voltage to the motor; the switching frequency of these pulses can cause an audible magnetic hum (a high-pitched sound) in the motor's steel and winding. Raising the switching frequency often moves this hum to an inaudible level; but a very high switching frequency creates extra heat and loss in the drive. So a balance is struck between noise and efficiency. Moreover, when the motor runs at variable speed with a VFD, the fan noise also changes with speed; at low speed the motor becomes markedly quieter. This is an additional comfort advantage of variable-flow pump and fan applications. In drive operation the motor's winding insulation and voltage-spike management are also important; we covered this topic in our article on inverter-duty winding and du/dt.
Noise and Vibration: Two Sides of the Same Coin
Noise and vibration often come from the same root. Increased vibration in a motor usually shows up as increased noise too, because vibrating surfaces push air and produce sound. So a well-balanced motor that stays within the ISO vibration classes both vibrates less and runs quieter. We covered vibration classes and acceptance values in detail in our article on ISO 10816/20816 vibration acceptance values. The vibration and balance limits in super-premium motors are examined in our article on vibration and balance in IE4 motors (ISO 20816). A motor being noisier than expected is often also a sign of a vibration and balance problem; so evaluating the two quantities together is important for a correct diagnosis.
Measurement and Environmental Noise Regulation
Motor sound power measurement is done with standardised methods (sound intensity for sound power, or anechoic/semi-anechoic room methods) and the result is reported in dB(A). On the field side, workplace noise is limited by worker health legislation; above certain exposure levels hearing protection and measures become mandatory. Environmental (outdoor) noise is subject to separate limits according to residential and industrial zones. Therefore, in a facility where noise is critical, motor selection is a matter not only of comfort but of regulatory compliance. It should also not be forgotten that noise can be a fault symptom; higher-than-expected sound should be evaluated among the symptoms and causes of motor failures. To examine magnetic and mechanical noise sources in detail, our article on noise sources in asynchronous motors is a valuable resource.
Frequently Asked Questions
Is the dB value in the catalogue sound power or sound pressure?
In most motor catalogues the value given is the sound power level (Lw), because it is independent of distance and allows fair comparison of motors. Some catalogues also state the sound pressure (Lp) at 1 metre. When comparing two motors always compare the same quantity and the same distance; confusing Lw with Lp is misleading.
Why is a 2-pole motor noisier?
A 2-pole motor turns at around 3000 rpm; this high speed increases the fan's aerodynamic noise and the magnetic noise. At the same power, 4- or 6-pole versions run at lower speed and are therefore markedly quieter. If the speed is free for your application, you can prefer a higher-pole version for the sake of quietness.
How much does moving the motor away from the operator reduce the sound?
In a free field, when distance doubles the sound pressure drops by about 6 dB. So moving the motor from 1 metre to 2 metres gains about 6 dB, and to 4 metres about 12 dB. In enclosed, reverberant environments reflections reduce this gain somewhat; in that case barriers and sound-absorbing surfaces help.
At HEM Motor we offer quiet motor solutions in the right pole, frame and efficiency class for noise-critical applications, from wide stock with fast delivery. To assess together your application's sound power target, placement conditions and speed need and select the quietest right motor, request a quote; we are at your side with manufacturer stock advantage and fast supply.
