Sound absorbing mats like dynamat do work. Expensive, of course. Surround any
direct path for sound from the MoBo to the outside world with that and you
won’t hear it. Just allow some angled hallway of airflow to keep the box cool.
It doesn’t have to be close. A big box with indirect airflow lined with
acoustic mat will hide lots of fairly noisy components. This is my favorite
option as dynamat etc. is beneficial to any sound rig.
If you are going to try something smaller and closer to the source, like goo on
the ferrite block, then thermal properties become as important to acoustic
properties. Go for the thermal pads like Gavrik mentions or black potting
compound (thermal epoxy). I think the acoustic output is from the green board
itself though, no matter what is driving it. So if you’re trying to isolate
the sound close to the source, you’re likely going to be putting pads on top
and bottom of the MoBo to try to catch the vibration. This sounds problematic
/ messy in this situation.
Also, I have used 12v and 19.5v inputs to the barrel connector on my soekris
board. Soekris sells 12v power supplies, but the board is rated to deal with
6v to ~36v as valid input. The tone of the sound that I could make out changed
a lot with different input voltages. You may want to try a laptop power input
instead of the 12v input to see that moves your noise problem outside of the
human hearing range.
Good luck.
ED.
> On 2016, Jan 26, at 11:22 PM, gavrik peterson <[email protected]> wrote:
>
> It appears that C427 and C418 are 2.5V electrolytic capacitors. Their low
> voltage rating suggests that they are associated with a switch-mode regulator
> that supplies power to the CPU. Notice that the inductors, L30 and L31, are
> very close to the place you identified as the source of sound. I suggest that
> it is far more likely that the noise is coming from one or both of these
> inductors. This because the ferrite that forms the core of these inductors is
> a material that exhibits magnetostriction. That is, the material responds to
> changing magnetic flux by changing its dimensions. While the dimensional
> changes are small, it is enough to produce sensible levels of sound. I have
> never experienced this sort of behavior from electrolytic capacitors.
>
> At this point a question remains. Is the regulator circuit associated with
> these capacitors and inductors doing something irregular that is producing
> this noise? Or, is the regulator behaving properly, and this noise is simply
> due to that fact that the CPU current demand is changing (CPU current is very
> dynamic) and this is causing the sound? I am thinking that the second
> possibility is most probable. As I don't have a schematic, or a board to
> examine, I can only guess. But most likely the two inductors and two
> capacitors are the output of a two-phase buck regulator that reduces the
> voltage from the external power adapter to something like 1.5 Volts for the
> CPU. If this is the case, one would expect the variation in CPU supply
> current to cause the inductors to produce sensible sound.
>
> If it is the inductor(s) that is producing the sound, I am not sure what one
> can do about it. Perhaps some non-conductive material can be placed over the
> inductors to absorb the sound. Some rubbery stuff that is acoustically lossy
> might work. One thing that comes to mind is the materials that are used to
> remove heat from electronic components. This material is sort of gooey and is
> very soft and conformal, so it can be applied to irregular surfaces. This
> stuff is referred to as "Thermally Conductive Gap Filler Pads". One supplier
> of the material can be found here. And more specifically, the HCS10 material
> is here. If you are interested in this solution you might call the company
> and ask about acoustical properties. Usually suppliers will send free samples
> for evaluation. Notice that the HSS10 material is very soft and is available
> in up to 0.20 inch thickness.
>
> -- Gavrik
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