Subwoofer hum

Galisteo8
Galisteo8 used Ask the Experts™
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I have a Boston Acoustics speaker system on my PC.  Previously no hum; now, a bad hum.

I just moved to a new house.  At old house, no hum, even when everything was plugged in and operational.  Had the bass up to a nice, decent level -- no problem.  Unplugged everything, packed it, moved to new house a scant 2 miles away. Set it all back up again, changed NO settings... and now I have a hum.

Here's the funny part.  It has NOTHING to do with the little speakers; it has NOTHING to do with loose connections or a surge strip; it has NOTHING to do with being close to PC's power cord.  Here's how I know: I took ONLY the subwoofer to a bathroom, plugged it directly into an AC outlet by itself, near no other electrical devices, and it hums.  Badly.

Any thoughts or solutions?

--Galisteo8
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Hi Galisteo8,

The volume on the sub hasnt accidentaly bumped up to max has it? (assuming of course it has an independant volume control)

Other than that, it could have been damaged in the move...

Hope that helps,

-red

Commented:
The power lines in your new house have static (RF) riding on the AC and the filters in the power supply for that speaker aren't good enough to filter it out.
OAC TechnologyProfessional Nerds

Commented:
also try a surge protector that filters RF, may also be reforred to is RF isolation or something

Commented:
Surge protectors don't usually do squat for low levels of RF.
There may be some that do but they are built for large surges more than noise..

You can plug it into a UPS (battery backup unit) which would be a good idea for your PC anyway if you have noisy power lines.

You could also try a ferrite bead on the power cord.
- Not sure it would do any good there. But it might and it's easy.
- The best place to use those would be the DC output of the speakers power supply.

http://computer.howstuffworks.com/question352.htm
http://www.surplussales.com/FerBeaSle/FerBeaSle-1.html

You can scrounge ferrites out of old electronics (radios, TV's, computers) or from old cables as well just buy them.
They are cheap at electronics stores or you could buy a cable that has one and remove it. (Dollar store, thrift shop.)
.
Top Expert 2012

Commented:
You may have a ground loop in the circuit, caused by different voltages in the ground plugs.  Most often, this is caused by the power ground being attached to a cold water pipe and a cable TV ground being attached to a separate grounding spike (or whatever your source signal is grounded to).  You can fix this by using ground lifters - those three-prong to two-prong adapters - or by making all grounds attached to the same reference level.

Commented:
Clarification of Callandor's comment: (He has a good point)
If your grounds are not all grounded well there may be a small voltage if checked from one ground to another. ~ As in not all of them are zero volts in relation to -each other- even if they are zero in relation to what they are attached to. That situation is sometimes called a "floating ground". It can cause current to flow from ground to ground in a loop and that current may make noise in your speaker.
.
You might want to have the wiring checked in your house.
You may have improperly grounded (or NOT grounded) wiring.
.
Another cause might be a bad ground in the speaker's 'box'.
.

Author

Commented:
Regarding the move, the speaker would not have been damaged.  It was boxed up in its original packing, and transported by car 2 miles to its new home, where I lovingly carried it upstairs and gently nestled it onto the floor by my desk.

As for the groundin issue... If all systems -- PC, monitor, speakers, printer -- are all plugged into the same outlet, doesn't that negate the possibility that a floating ground would have any effect?

Also... a UPS.  Do *all* UPS's filter for RF?

Commented:
Yes but you have it on different wiring.
To hear a hum because of a grounding problem you need two ground related problems.
If you have a grounding problem (or poor design) in the speaker box you may not notice something on good wiring that you experience on bad wiring.

A UPS is a battery backup.
I would think that because of what they are that they are all filtered.
I haven't personally explored it this across all the brands to know this for a fact.
Mine have all been 'middle of the road' units. I haven't had high end or low end.
The more expensive ones actually take the power off the 'battery' through an inverter and charge the battery on 'the other side'. By it's nature it's a VERY effective filter.
Cheaper ones send power through and 'switch' to batteries (kept on a trickle charge) when AC power goes away. The ones I've had had AC recepticles labeld "filtered" or "unfiltered" where you plug into them. (The word may be 'regulated' instead of 'filtered', if it's regulated then it's filtered too.)
If you don't see a 'filtered' or 'regulated' output I would check the specs in detail before you buy.
.
OAC TechnologyProfessional Nerds

Commented:
Go downstairs and make sure that big thick wire is securely tightened to the water pipe. Follow the water lines in and look for a wire attached to the cold water line. Usually that's your ground.
try the following:

1. turn the power plug around, meaning   a||b  become b||a   || is your plug
2. add a ferrite core to the power cable. you can 'borrow' it from other appliance around the house.
3. on the audio input line, connect a ground cable to the outer ring (O) if it is rca. you can connect it to the metal body of your pc. if it is mini-stereo type, connect the ground to the outtermost ring  
]==>
 ^this one

Commented:
@chiingliang.. I dunno where you or Galisteo8 are at.
Where I'm at the plugs are more like |! than ||.
They only go one way.

@DataDude.. Maybe 1 in 5 houses I've lived in even had stairs.
They also ground to pipes on outside faucets or near the power box.
They also use grounding rods near power boxes.
Older houses may not even be properly grounded (or at all).

All I'm saying is this site is global.
You have consider that not everyone has the same situation you do.
(Yup,, it's easy to forget that.........)
.
My Subwoofer also "hums" a little until it detects some input connected to it.
If you put an active audio input into it (even if it's just silence) does it still hums?

Author

Commented:
Wow, so many suggestions...  Not sure where to begin.

General question: What is a ferrite?

@chiingliang presciently deducted that my PC (and hence, my subwoofer) are upstairs...  :)

Carlos: Yes.
Top Expert 2012

Commented:
>General question: What is a ferrite?

http://www.palomar-engineers.com/Ferrite_Beads/ferrite_beads.html

You attach these to cables to reduce interference.  Most VGA cables have one on each end.

Author

Commented:
Why, yes it was, PCBONEZ...  Sorry 'bout that.  Having a brain fart.  :)
Hi Galisteo8,

An old rock 'n' roll sound tech's trick was to plug the 'hummer' into another power point -- one that's coming off another phase in the house (or venue). Houses have a few separate phases of power so as not to overload a single fuse or circuit breaker.
That way you break any ground loop that is created by two or more pieces of equipment working together.
I'm presuming that your power is earthed (3 pin) -- Please do not remove the earth pin (as was another old rock 'n' roll' sound tech's trick, which can easily result in death) -- because if there is a fault, your power will find the quickest path to earth -- through you or your loved ones.

Additionally there are 'hum eliminator' devices on the market. These these are inserted at line level before the input of any amplifier on the problematic piece of equipment (and really work).  
Jose ParrotGraphics Expert

Commented:
Hum with volume at zero:
defective filtering on speaker system power suply.
Solution: change or fix it.

Hum proportional to volume, distorced at high volumes:
defective cables, probably defective conector.
Solution: change cabling.

Hum not proportional to volume, never distorced:
Weak grounding for the computer.
Solution: to have a really good ground, pin "earth" really conected to earth.
Some wall outlets have the third (round) pin with no wires on it.

Hum is variable and appears at a certain volume control:
acoustic ressonance related to the environment.
Solution: lower the gain, move the booster to another place.

Jose
Wow....  I really appreciate all the input and suggestions everyone has provided.  It's been very educational.

Oddly, however, the hum has now disappeared.  Not sure what happened... nothing in the environment has changed.  I hadn't had a chance to try any suggestions that were posted.

By the way, just to clarify, as per Jose: The hum was somewhat proportional to the subwoofer's volume, although it did peak around mid-volume and then diminish a bit at higher volumes.  Of course, now I don't hear it at all.

I am not sure how to award points and close this question; so much great information, but I now have no way to test any of it...

Commented:
Here is a rare source of a hum for you..
It is usually seen in inverters for LCD monitors and is directly audible. (not through speakers.)
Occationally one of the transformer windings will loosen and develope an air space (room to physically vibrate) due to expansion and contraction during the heat-up and cool cycles from on-off uses. In that case you end up with an LCD screen that seems to be humming and sometimes it's loud enough to be very annoying.
I suppose a speaker winding could do something similar and create a noise in the speakers due to capacitance changes between the windings..
.
Isn't anyone going to go hummmmmm?
Jose ParrotGraphics Expert

Commented:
Just now I noticed the effect was in the bathroom...

So, most probable cause was ressonance.

Sub woofers works on positive feedbacking of filtered signals below some low audio frequency.
In other words, after amplification, the signal is sent to the speakers. To the sub woofer, the signal is sent after a low-pass filter, say just frequencies below 100 Hz are sent, in a 3dB atenuation per octave. (Numbers are samples). The box itself has a ressonance frequency, proportional to its sizes, format and interior forms. If you observe an acoustic guitar, you will notice that it has a ressonace box.

To amplify just low frequencies, the sub woofer subsystem uses something known as positive feedback. In few words, the filtered and amplified low frequency signal is sent back to the input, to be amplified again. This boosts the bass. An exemple of uncontrolled positive feedback is obtained by puting a microphone in front of the speaker of an ordinary amplifier. In this case, an acoustic/mechanical positive feedback. High frequency, due small size of the microphone.

Now, lets analyse the house's rooms.

All the rooms have a ResFreq. The frequency is inverse proportional to the sizes. The smaller the room, the higher the ResFreq. I am talking about sub sound frequencies.

What is special in the bathroom?
A bathroom, in general, is a small room, if compared to other parts of the house. A bathroom say, 3L x 2W x 2.7H m, has a ResFreq around 60 to 80 Hz (audible).
If the walls of such bathroom are hard and smooth (tiles, for instance), if there are no materials to absorb the sound (fabric, curtains, sofas), just hard materials, the sound waves reflect stronger. So, a bathroom can be a ressonance box.

If you try, you will find the ResFreq of your bathroom, by singing the word "ohm" in low frequencies notes. Please don't do this with people around, they will think you are mentaly disturbed...

So, probably what happened was:
The ResFreq of your subwoofer is be very close to an harmonic of the ResFreq of the bathroom. The positive feedback boost the effect. If your woofer is on a box or on a chair of hard plastics, it could amplify even more...

Jose
Hi Jose,

Gee you've really opened a can of worms here, Mate.

Can you  clarify these comments? <the filtered and amplified low frequency signal is sent back to the input, to be amplified again.>

It doesn't appear that the Boston Acoustics sub operates in this manner. Most subs don't, because they don't have microphones at the input to re amplify the signals.
I'm not saying that you haven't found some device that operates in this manner, I'm just saying that all the subs that I have dealt with don't. But please point to your example.

Also: <An exemple of uncontrolled positive feedback is obtained by puting a microphone in front of the speaker of an ordinary amplifier. In this case, an acoustic/mechanical positive feedback. High frequency, due small size of the microphone.>

The physical size of a microphone is not the cause of high frequency feedback. Small microphones often have wide frequency responses. Even a vocal microphone can reproduce 50Hz.


Cheers




Jose ParrotGraphics Expert

Commented:
Hi Fred,

The main subject is on ressonance. And a clarification of how and why the misterious hum appeared.

First, you are right:  no subwoofers (SW for short) have microphones in it.  But, please note, I NEVER saw that.

My example with the mike made a confusion, when my idea was to help in understand the theory, by comparing an ACOUSTIC feedback with an ELECTRICAL feedback.
So, lets forget the mike example, and try again.

POSITIVE FEEDBACK, PF for short, is a method used on dynamic subwoofers (Passive ones didn't have that feature). I understand that Galisteo8's SW is a dynamic one.

PF for SW consists on the following actions:
1. The ELECTRICAL signal containing audio frequencies is filtered by a Low-Pass filter, that is, only frequencies lower than 200 Hz (or around that) pass through it.
2. The SW internal amplifier amplifies the filtered electrical signal to some desired electrical amplitude.
3. Part of the signal (say, 1%) is sent back to the input of such internal amplifier. This is the PF. (That 1% is obtained by using a resistive voltage divider, made with a 99 ohms resistor and a 1 ohm resistor, for exemple).
4. There, at input, the PF is added to the normal input, resulting in a higher signal.

An "intelligent" circuit limits the PF amplitude. If there is no such "intelligence", the output was turned 101% of the original, and again 101+1%, again 102.01+1%, in an endless loop, resulting in pure distortion.

The described method boosts the sounds, in our case, the low frequency sounds.

The "intelligence" I cited is designed for normal living rooms, home theater rooms, not bathrooms, that are ressonace boxes as well.
Please, note: these are just ELECTRIC signals.

Now, lets talk on the microphone.
The effect I described is called microphnism, a usually undesirable property of some electronic circuits or components in which MECHANICAL vibrations of a component affect the signal being transmitted through the circuit.
This is not a pure electronic effect, but a mixing of electronic and mechanical effects instead.

You are right again: even small mikes are capable of pick (or capture) 50 Hz sounds. Please, note: I NEVER saw the opposite to that. I saw another thing: the microphonism, the typical one, is a consequence of a ressonance effect.

Here you have pointed an error of mine. The frequency of the microphonism is also a mixing of ressonance and cutting frequency of analogical processing of the PF by the internal ciicuitry, that has a delay (actually it is not a pure resistor divider, it has a capacitor too). Such delay determines the bandwitch of such effect. Thanks for that.

Anyway, the mike exemple is not part of the explanation about the hum.

I would like to know if Galisteo8 can try to repeat the hum at the bathroom, keeping in mind the subjects discussed here.

Regards,
Jose



Commented:
I think since there's an open can of worms we should just go fishing and forget about the speakers..
Jose ParrotGraphics Expert

Commented:
Hei, Bonez, weak joke. I assume, have really opened the can... but the motivation is the same of all of us, in helping the colleagues. Hope all of you do the same for me too, even with open cans.

Commented:
Well JoseParrot..
If you'd read the last Author's Comment before you posted all that you'd have seen there is no longer a problem and it's time to go fishing...
But thanks for your effort...

Author

Commented:
Sure, let's PAQ and refund.

I appreciate all the responses.  Don't know what happened.... The sub hummed in the office (when plugged in by itself). The sub hummed in the bathroom (just the carpeted sink area).  It still hummed in the office... Then I just noticed the other night that it no longer hummed.

Hmm...

:)
Galisteo8
It's the funniest thing! Now the hum is in my bathroom.
I tried singing Ohm but it wasn't until I sang the full mantra, "Jai guru deva om" that I got in tune with the resonant frequency -- Still got the hum though, but it's only when I hum. So I've started whistling instead.

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