I am looking for the best electrical motor in terms of low noise ('naturally silent" no need for isolation or special bearing ...). thank you !

Power/speed requirements I suppose would be a starting point with more questions to follow.

the targeted power range is around 1kW and for speed above 5000rpm, thank you.

At that speed wind noise is going to be a challenge. Brushless, Fully potted stator.
Questions: Truly silent, or merely silent within the human hearing range? Fixed speed or variable?

thank you , I was looking for a ranking for most silent (human hearing range) electrical motors. speeds can be above 5000 rpm. what about IPMSM , SPMSM?

IPM is probably not the way to go. The reluctance forces cause a little torque ripple, even with good closed-loop control. This will be audible.

I'm not sure if PMSM or induction motor would have more potential for silent operation. Induction motors have the advantage of near-perfect sinusoidal flux, although you'd have to watch out for saturation of the stator and rotor, at least for closed-slot designs.

Do not use a fan.

Is this for wind application? if it is... we can throw the conventional induction machine out of the wind(ow) (i know... it's a generalization). Now, for today's new applications, we are left with two 'real' options: PMSMs or DFIGs. As far as noise creation on the electromagnetic part, torque ripple (also referred to as pulsating torque) will be naturally present in both machines. However, note that since PMSMs have PMs on/in the rotor part, the issue of cogging torque (which is a component of the overall torque ripple) will always be present, even when the machine is open-circuited. so in that respect, maybe DFIG is better, but this is only one part of the problem.
Overall, i think that for the application engineer, if torque ripple is indeed an issue; i would make sure that some physical consideration was taken when designing the machine's geometry in such a way so that torque ripple will be minimized (i.e. skewing of the stator teeth, introduction of dummy slots, etc.)
Hope this helps.

Abdelhadi, the noise comes from well known point, but there is no miracle, if so everybody will do such motor. You have mechanical noise from bearing, flow noise from fan, and elecrtical noise coming from VSD, whatever the technology, you'll get noises. What is needed to know is what is asked by the local norms, what is the exact application and regulation you want to do. Then we could give you idea about product type...

thanks a lot for all your comments,
One application is for home electrical appliances. There is more and more demand on silent equipments but silent motor is not enough for example the body of a ventilation system brings noises during operations.

In my experience the most silent motor is the surface magnet brushless dc with the sinusoidal flux distribition on the magnet poles, the slotless winding configuration, no saturation in magnetic circuit and the sinusoidal drive .

Aleksandr's recommendation sounds plausible! surface magnet brushless dc will omit the creation of torque ripple due to salience. Sinusoidal flux distribution will reduce torque ripple due to the matching of the stator excitation and the B-EMF. Slot-less winding configuration will reduce the affect of cogging torque.
Good suggestion A.N!