The noise level created by the motor at any speed is in a fixed environment, take two motors same HP, Speed, Enclosure, and the applied voltage could be a factor of the noise, the installed conditions of a 1000 motors could vary from alignment to load, to piping connected to load, to actual load.
What are you or the customer looking for? One 5 HP motor versus another 5HP motor, one in a 50,000 sq foot plant, the other in a 500 square foot plant, while the motor under ideal isolated test conditions might be X, the noise generated from the motor in different conditions could be blamed on the motor.
Would be interested in the question broken down to specific reasons/needs. I know very few who shop based on noise at particular speeds, most either accept the sound levels, which range from pitch to volume to whatever, as an irritant.
Often shielding of the motor can contain any noise that might be a factor in other areas around the motor.
I am not a manufacturer of motors, except for the modification of specialty applications. For example we changed out several hundred motors for the National Weather Service contained in a dipole antenna body. Existing motor was a single phase permanent split capacitor synchronous motor, 110 volt, 1800 RPM DESIRED, due to a feedback tachometer mounted on the motor to verify the speed as these receivers accepted upper air feedback of weather conditions, from weather balloons launched two to three times a day. Location and tracking of the balloons were critical, if the tach feedback was off by one rpm [from 1800] the tracking electronics could not deal with the inconsistency.
I attempted to purchase motors for this application, but because the motor was mounted vertically in a solid cone, no ventilation, plus they were single phase, with induction synchronous rotors, voltage was a consideration, and the units were mounted from Hawaii to Guam to Florida, across the US and Territories.
I took the existing single phase PSC SYNCH MOTOR, which few ever had the torque, or would stay at 1800 rpm, or fail do to the heat.
While they only needed around 300 plus active motors, they needed half as many as spares, considering the past history of failures and the lack of ability to deliver accurate timely weather data over an exact path.
It was not a case of excessive NOISE, it was a case of perceived sound, it sounded different, so for those involved with any Governmental Agency knows that form, fit, function is their mantra and excuse to not accept anything.
We had several complaints of noise, turns out the noise was in no way a danger or at levels of any concern, just different.
While testing 4. 6. 8. 2 pole motors for “noise” in a controlled environment, is only data from those conditions, out in the wild west, those conditions are going to change, mounting, structure, all explained above will affect the motor’s “noise” levels, or perceived “noise” levels.
In the fact that no load, [NEMA] testing is not going to be exacting as other possible more exacting, different parameter type testing, if noise is a concern, is under full load, which again is a variable.
How many vanilla NEMA motors ever operate at “full load”?
Many run below the full load capacity, let alone service factor capacity, some operate slightly overloaded, few ever see the exact applied voltages, with changing of applied voltages during seasonal or daily changes in many variables effecting voltage supply.