HD- Beat frequency on 2 drives ?

[zjustice]

Posted a question a while ago - 2 drives working ok but a varying low level
hum

someone suggested beat frequency

moved them about , maybe a slight improvement but still there

an idea - both samsung 7200 sata drives - old one is 160gb , new one is
250gb

Q) Just wondering , would there be less problem if both 250gb ?

as any vibrations from the drive themselves would be the same
rather than a slight clash as maybe at the moment ?

thanks

 

[Rod Speed]

Posted a question a while ago - 2 drives
working ok but a varying low level hum

someone suggested beat frequency

moved them about , maybe a slight improvement but still there

an idea - both samsung 7200 sata drives -
old one is 160gb , new one is 250gb

Q) Just wondering , would there be less problem if both 250gb ?

as any vibrations from the drive themselves would be the
same rather than a slight clash as maybe at the moment ?

If its a beat, its the rotation rate of the drive
thats relevant, so the capacity is irrelevant.

 

[zjustice]

its like an alternating low level hum but deepish

only happens when both drives plugged in

i'd say its less now after alterations but could improve a bit more

 

[Rod Speed]

its like an alternating low level hum but deepish

only happens when both drives plugged in

Thats the most definitive evidence.

i'd say its less now after alterations but could improve a bit more

Its likely just something a bit loose in the 3.5"
drive bay, like how it attaches to the case.

You'd likely find that mounting one drive in a bay
kit in one of the 5.25" bays would fix it completely.

 

[Arno Wagner]

Posted a question a while ago - 2 drives working ok but a varying low level
hum

someone suggested beat frequency

moved them about , maybe a slight improvement but still there

an idea - both samsung 7200 sata drives - old one is 160gb , new one is
250gb

Q) Just wondering , would there be less problem if both 250gb ?

as any vibrations from the drive themselves would be the same
rather than a slight clash as maybe at the moment ?

This is called inteference. When you add (machanically in this case) two
sinewaves, you get the sum and the difference in the combined singnal
as well. So if one vibrates at 120Hz (7200 rpm) and one at 120.1HZ
(7206 rpm) you hear a signal wotn 0.1 HZ, i.e. repeating every 10
seconds. I have experienced this with various HDD pairs, but I also
experienced this strongest with a pair of Samsumgs.

The only way to make it go away is to exactly sync the two drives. Id
the difference in rpm gets smaller, the hum will just change slower
but be as strong.

The practical way to fight this is to avoid the mixing by
mechanically decoupling the drives. Personally I found that moving
is not enough and use decouplers. With the two Samsungs I use
one NoVibes by NoiseMagic and one Zalman ZM-2HC1. The NoVibes
is in a 5.25" bay and the Zalman is at the bottom of the case,
attached to the 3.5" bays. You just have to experiment.

I don't think that getting another Samsung will do anything,
they are very quiet but seem to produce pretty strong vibrations
(which are inaudible by themselves).

Arno

 

[George Pontis]

This is called inteference. When you add (machanically in this case) two
sinewaves, you get the sum and the difference in the combined singnal
as well. So if one vibrates at 120Hz (7200 rpm) and one at 120.1HZ
(7206 rpm) you hear a signal wotn 0.1 HZ, i.e. repeating every 10
seconds. I have experienced this with various HDD pairs, but I also
experienced this strongest with a pair of Samsumgs.

The sum and idfference terms arise from _mixing_, which requires some non-
linearity. Just adding the two sinewaves gives you two sinewaves.

I think what usually causes this low frequency vibration is a little different.
Each of the drives is slightly out of balance, and running at slightly different
rotational speed. At some point the out of balance condition on both drives is
adding to make a larger amplitude vibration, and later it tends to cancel making
the net vibration less. Your suggestions for ways to reduce the problem should do
the trick for the OP.

 

[zigipha]

I get this same effect on my laptop - a low-mid frequency hum that goes
up and down in amplitude every 2-3 seconds. I called dell and they
diagnosed it as a disk going bad and sent a replacement. But the new
one had the same sounds. It comes and goes day to day. Any thoughts?

 

[Timothy Daniels]

(e-mail address removed) says...

The sum and idfference terms arise from _mixing_, which requires
some non-linearity. Just adding the two sinewaves gives you two
sinewaves.

I think what usually causes this low frequency vibration is a little different.
Each of the drives is slightly out of balance, and running at slightly different
rotational speed. At some point the out of balance condition on both drives
is adding to make a larger amplitude vibration, and later it tends to cancel
making the net vibration less...

And so where is the "non-linearity" which causes the "mixing" so that
the beat frequency appears for the out-of-balance condition?

*TimDaniels*

 

[Rod Speed]

And so where is the "non-linearity" which causes the "mixing" so
that the beat frequency appears for the out-of-balance condition?

He's saying that isnt happening with the hard drive situation and
that what is happening is the second effect he listed. He's right.

 

[Rod Speed]

I get this same effect on my laptop - a low-mid frequency hum that
goes up and down in amplitude every 2-3 seconds. I called dell and
they diagnosed it as a disk going bad and sent a replacement. But the
new one had the same sounds. It comes and goes day to day. Any thoughts?

Thats a different effect to what the OP is seeing, because you only have one
drive.

It likely is a beat frequency in your case, but not with the hard drive.

Does it have more than one fan internally ?

 

[zjustice]

i have one of those "quiet pc" models and it has a sort of custom hd cage at
the bottom

it has rubber bungs inside and outside on the cage and custom screws to go
thru all this into the drives

so i'm thinking the zalman wont fit

mind u it backs up what other people have said about something loose

thing is the case is really solid and strong , not really "floppy"

all ideas welcome !

 

[zjustice]

quick thought on this

Q) Is this just an irritation or is the vibration damaging the drives ?

which is obviously the most important thing

 

[Rod Speed]

quick thought on this

Q) Is this just an irritation
Yes.

or is the vibration damaging the drives ?
Nope.

which is obviously the most important thing

Sure, but I agree that a silent PC is very desirable.

If you want one, one obvious possibility is to put one
of the drives in a bay kit in one of the 5.25" drive bays.
That should eliminate the noise.

 

[J. Clarke]

The sum and idfference terms arise from _mixing_, which requires some non-
linearity. Just adding the two sinewaves gives you two sinewaves.

No, it gives you a sine wave that varies its peak amplitude periodically.

Interference is the correct technical term for this phenomenon, and the rate
at which the peak amplitudes occur is called the "beat frequency".

There is a decent discussion of this at
<http://www.physicsclassroom.com/Class/sound/U11L3a.html> and nice
demonstration at

I think what usually causes this low frequency vibration is a little
different. Each of the drives is slightly out of balance, and running at
slightly different rotational speed. At some point the out of balance
condition on both drives is adding to make a larger amplitude vibration,
and later it tends to cancel making the net vibration less. Your
suggestions for ways to reduce the problem should do the trick for the OP.

Which is exactly the phenomenon that Arno was describing.

 

[Timothy Daniels]

(e-mail address removed) says...

The sum and idfference terms arise from _mixing_, which requires some non-
linearity. Just adding the two sinewaves gives you two sinewaves.

The sum of two sine waves gives the sum and the difference of the
two sine waves. The difference is commonly known as the "beat frequency".
If the two frequencies are similar, the sum is usually limited in amplitude by
characteristics of the medium, but not by the math. In AM radio, the sum
of a RF carrier wave and an audio wave (widely different in frequency
and assumed for simplicity to be sine waves) gives sine waves having
the sum and the difference of the two frequencies - known as sidebands.
No non-linearity is required. See:
http://hep.physics.indiana.edu/~rickv/Beats.html
This all falls out of Fourier analysis, basic college sophomore physics/math.

*TimDaniels*

 

[Eric Gisin]

Correct.

The sum of two sine waves gives the sum and the difference of the
two sine waves. The difference is commonly known as the "beat frequency".
If the two frequencies are similar, the sum is usually limited in amplitude by
characteristics of the medium, but not by the math. In AM radio, the sum
of a RF carrier wave and an audio wave (widely different in frequency
and assumed for simplicity to be sine waves) gives sine waves having
the sum and the difference of the two frequencies - known as sidebands.
No non-linearity is required. See:
http://hep.physics.indiana.edu/~rickv/Beats.html
This all falls out of Fourier analysis, basic college sophomore physics/math.

You don't have a ****ing clue. Adding two sine waves produces no others.

Amplitude modulation is NOT adding two signals.
It is multiplication of the carrier by a biased signal.

 

[Timothy Daniels]

You don't have a ****ing clue. Adding two sine waves produces no others.

Amplitude modulation is NOT adding two signals.
It is multiplication of the carrier by a biased signal.

<hee hee> You're cute when you're offensive.
When you have time, read the web page.

*TimDaniels*

 

[Eric Gisin]

<hee hee> You're cute when you're offensive.
When you have time, read the web page.

You are a ****ing moron, Timmy. Neither of you have a clue.

You cited a bird course in the physics of sound and music.
Come back when you have read some radio electronics.

 

[Timothy Daniels]

You are a ****ing moron, Timmy. Neither of you have a clue.

You cited a bird course in the physics of sound and music.
Come back when you have read some radio electronics.

<hee hee> "Radio & Electronics". Wasn't that a magazine?
Or was that a correspondence course? Seriously, though, folks,
Fourier analysis will show that any waveform can be reproduced
by a combination of sine and cosine waves of various frequencies
and phases (most times containing an infinite no. of components),
and that such a combination of sines and cosines are indistinguish-
able from the original waveform. As a matter of fact, they are identical.
In the case of two sine waves, they are indistinguishable from a
combination of two sine waves that have frequencies that are the
sum and the difference of the two original sine waves' frequencies.
Just ask any undergraduate physics student.

In the case of two hard drives spinning at slightly different
speeds (and who says that hard drives spec'd at 7200 rpm
must spin at exactly 7200 rpm?) there could easily be a beat
note that is audible. The question is merely why the rotation
of the platters is coupled to the air - is there an inherent platter
imbalance built into the product that causes vibration in the drive
housings? I'd guess that the vibration is from bad bearings in
both of the hard drives.

*TimDaniels*

 

[Rod Speed]

<hee hee> "Radio & Electronics". Wasn't that a magazine?
Or was that a correspondence course? Seriously, though, folks,
Fourier analysis will show that any waveform can be reproduced
by a combination of sine and cosine waves of various frequencies
and phases (most times containing an infinite no. of components),
and that such a combination of sines and cosines are indistinguish-
able from the original waveform. As a matter of fact, they are
identical. In the case of two sine waves, they are indistinguishable
from a combination of two sine waves that have frequencies that are
the sum and the difference of the two original sine waves' frequencies.
Just ask any undergraduate physics student.

Irrelevant to what happens with a beat frequency between two hard drives.

In the case of two hard drives spinning at slightly different speeds (and who
says that hard drives spec'd at 7200 rpm must spin at exactly 7200 rpm?) there
could easily be a beat note that is audible.
Yes.

The question is merely why the rotation
of the platters is coupled to the air

They are actually coupled by the drive bay stack metal.

Thats why damping mountings work and why having
one drive in the 3.5" drive bay stack and one in the
5.25" drive bay stack eliminates the problem.

- is there an inherent platter imbalance built into the product that causes
vibration in the drive housings?

There is always some vibration with anything rotating at that speed.
Its actually the best test for a drive spinning up, feel the drive.

I'd guess that the vibration is from bad bearings in both of the hard drives.

You'd be wrong.