# Massive 50Hz bump, whats causing it ?!



## CamZH (Jan 3, 2013)

Hi All,
Finally got around to taking some readings in the bedroom last night.

I've got a littleish 2.1 system with some DIY 5" 2way boxes and a DIY powered 10" sub.

I've noticed that music doesnt always sound so great, and figured that the sub was poorly tuned.
That is, sometimes the bass/kick in some tracks is very loud (sounds ok on other hifis)

and this is what I've plotted.









I've tried plotting with the sub on and off and I still get a massive spike at 50Hz.
Same happens with the sub only, and no full range drivers.
I'm not sure if this is actual or just measured.

I'm using a Galaxy CM140 mic, into a M Audio Fast Track pro.
I've used the same amp before without seeing the hump.
And tried adjusting the bass controls.

Room resonance ?
Something else strange ?


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## primetimeguy (Jun 3, 2006)

Something in your measurement rig is not working or setup properly, that plot is not showing what you are hearing, something is very amiss. Sorry I can't help with a solution but others here surely will.


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## fusseli (May 1, 2007)

The huge spike and lobes around 50Hz is noise from your AC mains. The shape of your measurements suggest to me lots of clipping is going on in your measurements, so definitely check your levels. You might also try moving around your outlets or trying a different power strip, or something to that effect. Then retry your measurements.


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## CamZH (Jan 3, 2013)

If i get some time today I'll try this again in a presentation room we have at work.
Can use exactly the same gear and see if the symptom still exists.

Might need to do another feedback test / calibration on my M-Audio box too.


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## dougc (Dec 19, 2009)

How close are the mains to a wall, corner, or floor? Most of the designs I look at building won't work for me since I have to stick them in a corner close to the wall. My current center is too close to the wall and there is a spike from 50-70.


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## EarlK (Jan 1, 2010)

That doesn't look like a viable acoustic capture .

It does look like the noise spectra of a soundcard ( with nothing feeding the mic input ) .

:sn:


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## CamZH (Jan 3, 2013)

Well it looks like i need to get back to the drawing board a little.
Havent had a chance to test out the measurement gear in another room.

I have had issues in the past running the M-Audio box from my laptop. it seems to be very noisy.

In this set up the only 50Hz interface i can see is from an Alarm clock sitting right on top of the M-Audio Box.
The main amp is DC powered (switch mode from a laptop supply)
And my PC was running from batteries, and the M-Audio powered from the laptop.

Then again maybe the lack of reference to ground is the issue...


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## fusseli (May 1, 2007)

Could have been an issue with where the amp in your 2.1 system was plugged in vs. where the laptop was. Usually running the laptop on batteries solves ground loop issues ... :scratch:


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## CamZH (Jan 3, 2013)

The Laptop was running on batteries.
So should be avoiding ground loops from that persepective.

It could be something strange between the sub amp and the main amp.

With any luck i'll have some time soon to get back into testing. but doubt it'll be this weekend coming


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## primetimeguy (Jun 3, 2006)

The 50hz bump is the least of your worries. There is no way your high frequencis are 40db higher than low frequencies. This is not measuring your room, something is not connected or working properly.

Sound card issue possibly?


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## CamZH (Jan 3, 2013)

^ Thats a great observation.
I didn't even notice as i was looking for something I was hearing.

But sure says that its not working correctly.

I'll need to record some audio with the mic next time to make sure it is working as expected.

All i did was use the visual indicator of the signal/clip LED's on the audio interface, and assumed it was working...

But i guess in this instance I'm the only one being made a fool of


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## CamZH (Jan 3, 2013)

Ok,
so I had a chance to get some more measurements done on the weekend.

This time using a Shure SM57..

This is from the training room we have at work.
But at least it looks a bit more realistic...


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## sdurani (Oct 28, 2010)

CamZH said:


> This is from the training room we have at work.


Is one of the dimensions of that room around 18-19 feet (6 meters)?


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## CamZH (Jan 3, 2013)

yeah, i'd say its about that wide.
I'm not too worried about that room, it was just a place to test my gear and make sure it was operating correctly.


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## sdurani (Oct 28, 2010)

I know you were just testing the measuring gear in that room, but my eye caught the dips at 30Hz, 60Hz, 90Hz, etc. To confirm they were due to room modes, I took a guess at one of the dimensions.


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## JohnM (Apr 11, 2006)

Looks like you might want to go through the REW SPL meter calibration so the plot is drawn at the correct level, and scale the graphs to have a 60 dB vertical span (e.g. 45 dB at the bottom of the graph, 105 dB at the top, per the graph posting advice.


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