PWM noise is a function of 3 components:
o Bearing thrust & slippage noise from PWM on/off pulses
o Motor IC noise from PWM "average" voltage re rise/fall
o Harmonics as a result of the PWM frequency
1) Bearing thrust & slippage noise from PWM on/off pulses:
o PWM frequency applies voltage as "1 0 1 0 1 0" re on/off
o So there is a resulting mechanical kick to the bearings
o This will result in some noise from acceleration
o Similarly, some noise from deceleration
o The net result is a mechanical addition to rotation noise
PWM relies on mechanical slippage to work - that the rotor-fan
& magnets keep on turning during the "off" period. Thus the
fan rather than using a continuous analog signal is using a
digital signal whose *average* voltage (like r.m.s to a.c.) is
less that the peak voltage of "1" (12V, 24V, 36V, 48V etc).
2) Motor IC noise from PWM "average" voltage rise/fall
o Motor ICs for all DC fans are self-starting
o Many self-starting circuits trigger at a certain voltage
---- and involve a cut-out + delay + re-start process
o This inevitably results in noise varying by fan & PWM specs
---- varying by fan re form of self-start circuit
---- varying by PWM frequency
---- varying by PWM voltage (not all are Vs re fan rating)
---- varying by PWM duty-cycle re 50-70%
o Motor ICs are also designed for V-continuous not chopped
---- the chopper frequency can affect motor-ic quiet-operation
---- quiet hard drives phase head movement into start/move/stop
---- a similar technique is applied to "new low noise motor ic's"
This is usually one of the loudest noise inducers:
o Because self-start circuit involvement results in mechanical noise
o Which has it's own amplitude, frequency & natural frequency
Vis., some fans perform terribly under PWM.
Taking Papst System 3000 PWM 12V controller & Papst 6212NM:
o This sucks re noise quality at lower PWM settings
o The fan will *growl* at a volume *louder* than it's full-Vs use
o The "PWM growl" negates the benefit of the PWM at <80% duty
Most systems aim to run fans at 70% duty cycle, since that is a
good realisation of cfm & static pressure, whilst at the same time
having a disproportionate benefit in terms of bearing longevity.
Taking Papst System 3000 PWM 24V controller & Papst 6224NM:
o Blissful smooth operation from 50-100% duty cycle
o No PWM noise, but no "halving of dB(A)" at 50% duty cycle
o Reason being that the fans have very large ball-bearings
This feature of very large fans isn't unusual:
o Noise is a component of rpm - and also bearings
o Big fans like Comair Rotron Caravel don't half dB(A) at 50%
---- taking a 115/230vac model 450cfm & 45dB(A)
---- at 50% duty cycle you will not see 22.5dB(A) but 30-35dB(A)
Can't recall the figure off hand as I don't use it, since PWM gets
a different dB(A) for reasons 1-2-3 on that fan. However, on the
very big fans there is a base-level noise due to bearing rumble.
It's small - and not unpleasant - just something that shows up in
the dB(A) figure in absolute levels, and subject to preference.
3) Harmonics as a result of the PWM frequency:
o This can be a biggee
o PWM frequency of 1-2kHz on most fans means run
---- they motor and *amplification thro fan mechanics* will growl
---- specifically they will emit a 1-2kHz noise during PWM
o PWM frequency of 10-20kHz is more preferable
---- and tends to increase the number of fans PWM "works" with
o Some work is being done into ultrasound
---- however noise damage in watts in ultrasound is *tiny*
---- so whilst we can't hear it, it can damage us - so vital research
As a function of this PWM frequency, you have bearing interaction.
Overall, there is a critical path of PWM frequency, motor ic type
& self-start circuitry, bearing type & size, bearing/magnet
mechanical characteristics re slippage, and mechanical slippage
of the overall system. Each varies according to each fan. Some
fans seemingly identical bar voltage *sound* very differently.
This is why PWM implementation in industry ends up specifying
a fan which has been proven - another can be unpredictable,
and why PWM is disliked in that production lacks substitution.
Personally I prefer:
o Proven fan/controller combination = PWM if cost permits
o Variable fan in use/replacement = use 2-voltage operation
---- this requires a consideration of hysterisis re control
Hence for the latter I use 24M1A at 12/24V operation, since for
a 128 fan application the PWM array to control it is >400$US. In
comparison a 12/24V controller using industrial DIN 12V PSU in
either series of parallel (designed to permit it) is cheap, 6$US.
My comments were & are re PWM.
However, the bigger fans do have bigger bearings - whilst the
rpm may be far lower (just 1350rpm or less for 254mm fans), so
more of the base-level-noise can be from the bearing themselves.
This becomes especially true into the bigger (300mm+) EBM fans.
The noise, however, may not be intrusive or annoying.
Some people even like the sound of very big industrial A/C, it is
quite loud - but sufficiently white-noise to be easily masked. A
lot of smaller-scale fan "white-noise" has low-period harmonics
in it and these are more easily selected by human perception.
Within about 7-10yrs fans will integrate PWM as standard, not
as present forms, but completely serial-port programmable; may
only be possible for distributors, but is an industry end-goal. In
terms of temperature profiles, low-temp operation & so on.
Biggest gripe over PWM is quiet enough at low temp operation,
yet enough airflow at high temp operation without jumping there
too fast. This is obviously load dependent - and why a quiet PSU
with PWM control may be quiet on one PC yet noisy on another.
In such cases, the noisy PC often has higher thermal output and
often is using the PSU fan as per the ATX spec & profit-remit to
cool not just the PSU but the entire PC - hence high cfm & noise.
If 38cfm is needed at 35dB(A) for a PSU-PC-cooled solution then
offloading it to 2 fans allows 36cfm at a combined silent 17dB(A).
Nothing wrong with larger fans, but ideally one should redesign
a PC case to house them (or multiple PCs in a box similarly). Very
easy to pick up flight cases on castors, portable, rugged, dense.
Then make the PC structure out of nylon & ABS, or aluminium. A
perforated T3R5 alloy (3mm hole, 5mm pitch) is pegboard PC.
Such solutions avoid obsolescence (AT ATX BTX etc), and can be
very economic if used to house multiple PCs in self-made racks.
Vertical racks allow narrow form-factor, passive 1U heatsinks &
use high airflow from larger fans running as required. Keep the
air resistance low (blow bottom to top, blow-thro-cases) and it
can be very quiet whilst very easily and cheaply made.
BTX cases are a very short-term solution, Intel trying to play
catch-up to the Apple Dual G5 - and quite a clumsy nasty design.
That is case manufacturer/integrator/OEM profits speaking tho.
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