Old 01-04-2012, 07:09 PM   #1
mtthw
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Default mic newbie questions

I got a shure sm 58 plugged into a focusrite pre amp and I record into
reaper and I notice the bottom of the waveform tends to be louder than the top.

In other words, the phase - is greater than the phase +, I really don't know
how to phrase it.

But I would think it would be even, right? I wonder why it isn't even but
instead is lopsided.

Also, I wonder if clipping the input will hurt the pre amp or only the speakers if it blows the speakers. Will it mess up the pre amp too?

thanks for any info.
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Old 01-05-2012, 01:11 AM   #2
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Quote:
Originally Posted by mtthw View Post
I got a shure sm 58 plugged into a focusrite pre amp and I record into
reaper and I notice the bottom of the waveform tends to be louder than the top.

In other words, the phase - is greater than the phase +, I really don't know
how to phrase it.

But I would think it would be even, right? I wonder why it isn't even but
instead is lopsided.
As long as there is no DC offset, everything is fine. Vocals usually have an asymmetric waveform, especially male vocals.

Quote:
Also, I wonder if clipping the input will hurt the pre amp or only the speakers if it blows the speakers. Will it mess up the pre amp too?

thanks for any info.
Highly unlikely. Of course it is a question of how much you are clipping the input. Connecting a power amp to your mic input would be a bad idea
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Old 01-05-2012, 12:06 PM   #3
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Cool. Thanks!
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Old 01-05-2012, 03:07 PM   #4
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Quote:
Originally Posted by mtthw View Post
I got a shure sm 58 plugged into a focusrite pre amp and I record into
reaper and I notice the bottom of the waveform tends to be louder than the top.

In other words, the phase - is greater than the phase +, I really don't know
how to phrase it.

But I would think it would be even, right? I wonder why it isn't even but
instead is lopsided.
Based on what you're saying here mtthw, it sure sounds like DC offset to me. Positive and negative won't line up perfectly with a complex wave form but they should be somewhat equally divided.

I've never looked but there might be something in Reaper to get rid of DC offset.

Quote:
Also, I wonder if clipping the input will hurt the pre amp or only the speakers if it blows the speakers. Will it mess up the pre amp too?
Best to not clip either the inputs or outputs, if not to save them, just good practices.
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Old 01-05-2012, 04:02 PM   #5
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Thanks for the info/advice.

Maybe it is a llttle bit of DC offset. I shall have to wonder about this
and perhaps figure something out.

I have learned that the waveform represents the pushing and pulling of air, right?

So, what exactly does the middle line represent?

Anyway, it is only not very symmetrical, with below the line more than above. It sounds fine.

and I can phase invert it, and then the top is more than the bottom.



New to microphones, I haven't encountered this with soft synths, but it is an important topic, I guess. I have some books to look this up, too.
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Old 01-05-2012, 05:42 PM   #6
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As l0calh05t said, some acoustical waveforms are aysemetrical and this may be normal. What are you recording?

Quote:
So, what exactly does the middle line represent?
It represents "zero".


Quote:
I have learned that the waveform represents the pushing and pulling of air, right?
Right! Positive & negative* air pressure.

Think about how a speaker works... With no signal, it sits in the center "zero" position. Positive voltage pushes the speaker cone out creating positive air pressure. Negative voltage pulls the speaker in creating negative air pressure. A "normal" soundwave (like a simple sine wave) has a positive half-cycle and a negative half-cycle. And, the waveform crosses-through zero twice every cycle... Every cycle has a positive-going zero-crossing and a negative-going zero-crossing.

A microphone works the same way. Positive air pressure pushes the microphone's diaphram in, generating a positive voltage. Negative pressure (during the other half of the soundwave) pulls the diaphram out, generating a negative voltage. (The polarity on some mics & speakers is reversed, but this seems to be a psudo-standard.)

A couple of extreme examples - , if you are too close to the mic and you "pop your Ps", or if you put the mic near a drumhead, you can imagine that the waveforms might not be symetrical around zero.

Silence should be a flat-line in the center. With a mic & preamp, there will be some noise, but the noise should be centered around zero. If silence is not in the center, you have a true DC-offset. This is usually caused by a defect (or "weakness") in the interface (or soundcard).

There are "offset correction" filters, or a high-pass filter (~20Hz) will remove the offset without affecting the sound (DC is zero Hz). But, if the offset is bad enough to cause clipping (distortion), the distortion cannot be filtered-out.


* The air pressure doesn't truly go "negative". It goes negative relative to the ambient atmospheric pressure. At some level (around 160dBSPL?) the negative-half of the soundwave creates a full vacuum and you get clipping because the air pressure simply can't go any more negative!!!!

Last edited by DVDdoug; 01-05-2012 at 05:51 PM.
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Old 01-05-2012, 07:20 PM   #7
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Thank you for such a great and informative reply to aide in understanding
how the sounds actually work.

I did a noise test as you suggested and it looks like my equipment is
functioning well.

I did more testing, and am finding different vowel sounds have more or less symmetrical waveforms over time, so I think the shape of the voice waveform
and how even it is has to do with formants, too.

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Old 01-05-2012, 10:17 PM   #8
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Quote:
Originally Posted by DVDdoug View Post
* The air pressure doesn't truly go "negative". It goes negative relative to the ambient atmospheric pressure. At some level (around 160dBSPL?) the negative-half of the soundwave creates a full vacuum and you get clipping because the air pressure simply can't go any more negative!!!!
An excellent post. It's not 160dBSPL, though, which is the kind of noise generated by a racing dragster. It's enough to cause permanently hearing damage (as well as temporary problems with vision and swallowing); however, this is still far from the point where air pressure "goes negative relative to the ambient atmospheric pressure", which is around 194dBSPL (at sea level, and on this planet, specifically, so your mileage may vary).

To put this in perspective;

The Who; 126dBSPL
Deep Purple; 130dBSPL
Leftfield at the Brixton Academy; 137dBSPL

Rifle discharges have been measured at around 168dBSPL to 171dBSPL from 1 metre.

Stun grenades generate 170dBSPL to 180 dBSPL

Between 190 - 195 dBSPL, human eardrums rupture 50% of the time. However, since this kind of noise is generally only generated by things like 1 ton of TNT at 60 feet (195.2), your rupturing eardrums might be the least of your problems.

Space shuttle exhaust has been measured at 215dBSPL (at a speed of 3mps)

The largest non-nuclear bomb used in WWII weighed 11 tons; 220dBSPL

198 to 202 dBSPL will cause human death from the shock wave alone.

The bigger volcanic eruptions have generated 310 -320 dBSPL.

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Old 01-06-2012, 02:41 AM   #9
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Atmospheric pressure: 100 kPa (actually closer to 101 kPa at sea level, but close enough)
0dB SPL: 20 µPa

100 kPa / 20 µPa = 5·10⁹

20 log10( 5·10⁹ ) ≈ 194 dB

Now you can calculate it for whatever planet you are on
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Old 01-06-2012, 07:04 AM   #10
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Now you can calculate it for whatever planet you are on
I could, if I understood that. I was sort of following you, up until µ ....
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Old 01-06-2012, 07:43 AM   #11
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Quote:
Originally Posted by Fex View Post
I could, if I understood that. I was sort of following you, up until µ ....
Uhmm... ok... is that up to

Quote:
Atmospheric pressure: 100 kPa (actually closer to 101 kPa at sea level, but close enough)
0dB SPL: 20
or up to

Quote:
Atmospheric pressure: 100 kPa (actually closer to 101 kPa at sea level, but close enough)
0dB SPL: 20 µ
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Old 01-06-2012, 09:13 AM   #12
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The second one. I know what kPa is. I Googled it.

I'm still not sure what the Kenya Ports Authority has to do with atmospheric pressure, but it's the µ that really stumped me.
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Old 01-06-2012, 09:35 AM   #13
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To mtthw.

Basically you are seeing what multiple frequencies "look like" when they co-exist (as in pretty much everything else you hear in real life). Sometimes certain frequencies at certain amplitudes will cancel out some portions of other frequencies when combined to make a noticeable difference in the positive and negative amplitude.

See below. First signal is 1000Kz sine wave at -20dB. Second is 500Hz sine wave at -20dB. Add them together and you have a non-symmetrical waveform. Nothing to worry about

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Old 01-06-2012, 05:56 PM   #14
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Thank you much, Shemp Awesome example/demonstration. Appreciate it.
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