Why does the polarity of a bass guitar recording vary?

  • #1
attaboy_jhb
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Sorry if this is the wrong place for this question. I am a musician and recording engineer trying to learn about why my bass guitar recordings sometime appear with the waveform going up first (positive polarity) and other times going down first (negative polarity) in my DAW (digital audio workstation). It seems to be related to how the sound is created with the instrument. For example: If I gently hit the strings down with the palm of my hand, the waveform appears positive to start with a positive polarity while if I play with my fingers normally plucking the notes, the polarity seems negative at first. Can someone explain why?

Also, can someone please explain why changing the polarity sounds different coming out of my studio monitors. Shouldn't flipping the polarity sound the same since all you are doing is flipping the sound wave upside down?
 

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  • #2
Andy Resnick
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For example: If I gently hit the strings down with the palm of my hand, the waveform appears positive to start with a positive polarity while if I play with my fingers normally plucking the notes, the polarity seems negative at first.

Just trying to understand your description: when you strike the strings in opposite directions, the waveform polarity also switches from positive to negative?
 
  • #3
attaboy_jhb
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Thanks for the reply. When playing the bass normally with my fingers the "leading edge" of the waveform is negative but before playing I actually did a test and hit down on the strings with the palm of my hand. The reason why I did this is because I wanted to see what the polarity was in my DAW and to test it with a normal bass note is harder because there is less of a transient. The more percussive sound you get when hitting the strings downward is easier to see the transient and if the waveform starts with a negative or positive cycle. I got the idea on how to check this from this website (https://www.johnvestman.com/polarity.html) which talks about the importance of having a positive polarity in your music recordings.

Does it maybe have to do with which way the strings are moving? When I pluck with my fingers I pluck the string upwards and inwards but when I hit the strings they tend to go down more towards the pickup... not sure here, so just guessing
 
  • #4
hutchphd
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I know that humbucker magnetic pickups are wound with multiple coils that alternate polarity (so any overall magnetic field interference will cancel) and I believe that bass guitars may alternate the polarity of that arrangement across the four strings. Do adjacent strings behave oppositely?
 
  • #5
attaboy_jhb
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They all seem the same to me. Here are 3 pics. one of the wave when slapping down on the strings you can clearly see positive leading edge but when plucking the strings it seems different. I will add the low E string waveform pic and the high G waveform pic so you can see
 

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  • #6
tech99
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Summary:: In my DAW looking at the waveform, it sometimes goes down before it goes up and sometimes the other way around

Sorry if this is the wrong place for this question. I am a musician and recording engineer trying to learn about why my bass guitar recordings sometime appear with the waveform going up first (positive polarity) and other times going down first (negative polarity) in my DAW (digital audio workstation). It seems to be related to how the sound is created with the instrument. For example: If I gently hit the strings down with the palm of my hand, the waveform appears positive to start with a positive polarity while if I play with my fingers normally plucking the notes, the polarity seems negative at first. Can someone explain why?

Also, can someone please explain why changing the polarity sounds different coming out of my studio monitors. Shouldn't flipping the polarity sound the same since all you are doing is flipping the sound wave upside down?
When the string vibrates it is compressing the air in the direction it is moving at any moment. The initial pressure increase is the beginning of the sound wave. If the string initially moves in the opposite direction, the phase of the sound wave is reversed.
Sound waves from the voice are sometimes asymettrical in shape, and maybe this occurs with instruments as well. The waves are not necessarily smooth sine waves. The two sides of the wave can be different shapes, but remember that the area under both halves must be the same so that the average is zero. The asymettry means that the peaks are higher for one polarity. This can affect audio if peak clipping is occurring.
In the case of digital audio, I don't think we can rely on the waveforms reproduced because the sound may be subject to vocoding and processing.
If it sounds different for the two polarities I can only suggest that either peak clipping is occurring or the audio has been subject to digital compression. Maybe try reversing the speaker connection and see if there is any subjective change. This would rule out a speaker issue.
 
  • #7
Andy Resnick
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I got the idea on how to check this from this website (https://www.johnvestman.com/polarity.html) which talks about the importance of having a positive polarity in your music recordings.

I interpret that URL discussion differently- the point seems to be that all the various tracks need to have the same polarity, to prevent interference effects from happening at the speaker (' They will counter-act each other and lessen the punch. ').
 
  • #8
attaboy_jhb
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I interpret that URL discussion differently- the point seems to be that all the various tracks need to have the same polarity, to prevent interference effects from happening at the speaker (' They will counter-act each other and lessen the punch. ').
How is that different to my interpretation? That is why i was recording my bass looking at the waveforms... to see the polarity
 
  • #9
attaboy_jhb
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When the string vibrates it is compressing the air in the direction it is moving at any moment. The initial pressure increase is the beginning of the sound wave. If the string initially moves in the opposite direction, the phase of the sound wave is reversed.
Sound waves from the voice are sometimes asymettrical in shape, and maybe this occurs with instruments as well. The waves are not necessarily smooth sine waves. The two sides of the wave can be different shapes, but remember that the area under both halves must be the same so that the average is zero. The asymettry means that the peaks are higher for one polarity. This can affect audio if peak clipping is occurring.
In the case of digital audio, I don't think we can rely on the waveforms reproduced because the sound may be subject to vocoding and processing.
If it sounds different for the two polarities I can only suggest that either peak clipping is occurring or the audio has been subject to digital compression. Maybe try reversing the speaker connection and see if there is any subjective change. This would rule out a speaker issue.
Thanks for the response. I don't understand what it has to do with the speakers. I record with a direct line from bass into my DAW and therein I was looking at the waveforms. Not sure what the speakers have to do with it

Do you know why waves are not symmetrical. I have noticed this too in my DAW but was never clear why sometimes the negative part of the wave seemed bigger than the positive part or vice versa
 
  • #10
hutchphd
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Does it maybe have to do with which way the strings are moving? When I pluck with my fingers I pluck the string upwards and inwards but when I hit the strings they tend to go down more towards the pickup... not sure here, so just guessing
This is exactly correct I believe. The magnetic pickups actually produce a signal proportional to the velocity of the string relative to the pole face. Moving towards and moving away will produce opposite voltages in the pickup. Notice this also means that vibrating perpendicular to the guitar face makes a somewhat different response than vibrating parallel. The mixture of those two modes can produce a complex waveform. Check this out:
https://www.acs.psu.edu/drussell/guitars/pickups.html

The issue of relative phases from various sources I think really matters only if the frequencies of the sources are similar. For a symbol crash or other percussive sound, many frequencies are involved, and the result of phase discrepancies is difficult to predict.

Of course the classic case is putting your stereo speakers out of polarity which produces an exact plane of cancellation equidistant from each speaker for all frequencies.
 
  • #11
Andy Resnick
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How is that different to my interpretation? That is why i was recording my bass looking at the waveforms... to see the polarity

I had thought you wanted to know why the signal polarity can be either positive or negative and how playing a stringed instrument different ways changes the resultant signal polarity.

Not sure what you mean otherwise, but I interpreted the URL discussion to mean that the polarity of a single isolated track can be either negative or positive with no clear resultant difference in audio quality; if different tracks played simultaneously have different polarities then they can interfere at the speaker resulting in a noticeable loss of quality.
 

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