Macbook External Speaker Buzz

January 9, 2008 8:58 AM

Why do my speakers randomly begin to hum/buzz?

I have a MacBook Pro 15" 2.15 Dual Core 2. I plug in, via 3.5mm cord in the headphone port, a JBL Creature 2.0 set of speakers.

Every so often a barely audible hum/buzz (lowish frequency), begins to increase in volume. It starts off really quiet, and gets pretty loud.

The weird thing is, when I hit the volume button to make it louder or softer, the sound completely stops. It seems as if any output of noise through the speakers stops the buzz. For example, if I play a song in itunes for a second and then pause it, the hum is gone.

Please help me! What is causing this and how do I prevent it? It is really weird.

posted by benji to computers & internet (9 comments total)

do you have a cell phone near the computer? turn it off or take it out of the room. It's probably interference.
posted by desjardins at 9:01 AM on January 9, 2008

This doesn't sound like GSM phone interference. That's more of a sudden clicking noise, not a rising buzz. It only lasts a few seconds, and isn't correlated to any other sound being played.

Anyway, this sounds more like a grounding or ground loop problem, which is harder to diagnose. Does it happen when using other speakers? Headphones? Tried a different cable going to the speakers? Made sure the cable is firmly plugged in to the computer and the speakers?

Also, is your computer running on battery or plugged into the wall? Is it plugged into the same outlet (or at least same circuit) as the speakers, or a different one?
posted by xil at 9:34 AM on January 9, 2008

It is definitely not GSM phone interference. I know what that sounds like.

Xil - wouldn't grounding create a constant buzz/hum rather than a sometimes silent, sometimes intensity-increasing buzzing sound? (I have no knowledge of this subject).
posted by benji at 9:37 AM on January 9, 2008

Do you have any lighting near the speakers? (in particular gas lighting like fluorescent tubing or backlit keyboards/accessories) The ballasts in these devices can cause the audible hum in speakers if they are not properly shielded. The increase in volume suggests that its regenerating that sound via a feedback loop. Have you tried disabling/muting your line-in and mic jacks? (I'm not familiar with macs, but in PCs there's also a loop device that you can mute)
posted by samsara at 9:40 AM on January 9, 2008

You're right that ground problems usually are fairly constant, but there are lots of stories of weird and unexplainable things happening. I'm no EE, so I won't even try to pretend I could explain in any detail.

In any case, you've got a fairly complex system -- computer, cable, speakers, each with their own power supply -- and you're going to have to isolate the individual parts in order to debug the problem. Did you try any of my suggestions above? Does it happen if the speakers are just sitting there, turned on but not connected to anything?
posted by xil at 10:40 AM on January 9, 2008

I think I've had the same thing on my MBP (approx. same model as yours) though I didn't notice any increase in volume. I kept thinking it was a jack/wire problem, but the fact that it disappeared after playing any sound makes that seem less likely. Could this somehow be a result of a lack of input, as in, the sound card/output being somehow "asleep?" I don't know, just spitballin' here...
posted by jessenoonan at 1:17 PM on January 9, 2008

this sounds like a feedback loop between the macbook pros microphone and your speakers. are your speakers pointed slightly toward your macbooks screen at all? see if it stops when you go into system preferences/sound and change your Input to something besides the internal mic.
posted by shmegegge at 3:28 PM on January 9, 2008

Alternative hypothesis: Motorboating, caused by poor poor power supply regulation in the amp? Commonly a "putt-putt" noise, but can be anything up to ~ 2x your AC line frequency (i.e. up to 100 / 120Hz)

I've seen it before with cheap speakers (& not so cheap; a AU$180 set of Altec Lansings for example). Basically something (turning the gain up, a bit of LF noise, etc) causes the amp & power supply to oscillate between themselves at a low frequency - amp draws a bit of current, PSU o/p dips, amp o/p dips, PSU o/p jumps back up, rinse, repeat. Playing a tone other than the oscillation frequency - e.g. the Mac "squick!" when you hit the volume buttons - stops it, until the next time something sets it off.

Cure? Decent power supply with better filtering. Sometimes adding a LF rolloff filter to the audio input can fix it, if the trigger is LF noise from the source.
posted by Pinback at 5:52 PM on January 9, 2008

Now there's a moneymaking idea - a "Computer Sound Enhancer" consisting of a box with a DC socket on one end, a suitable plug lead on the other, and a honkin' great capacitor inside. Better still, who needs the honkin' great capacitor? A couple of normal 470uF 30v electros + some lead shot for weight would do fine.

Wonder where I can get some ABS cases done up in iridescent blue with gold script stencilling?
posted by Pinback at 6:07 PM on January 9, 2008