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I've noticed that if there's not a good connection between my jack and the port, like if it's dirty or half plugged in, then the sound is different, but there is still sound, and sometimes twisting it around makes it sound better.

Why is that?

I was looking at some schematics of a 3.5mm headphone jack, but it doesn't look like there's a "this part of the jack gets the treble" thing going on. You'd think that if there's a signal at all, it doesn't matter which way the jack is twisted since it's a ring inside a ring anyway.

Hydrangea
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The simple answer is: Changes in resistance/impedance at the connector or cable can change the actual or perceived frequency response that you hear from the speaker.

In the case of the actual frequency response: The connector and stuff inside the speaker form an RC filter (a low-pass filter). The connector is the R and the stuff inside the speaker is the C. As R goes up, the cutoff frequency of the low-pass filter goes up. Normally, the R is less than 1 ohm, so the cutoff frequency is very low (less than 5 Hz). But if the connector is bad or dirty then the R could be several hundred ohms, which could increase the cutoff frequency by several hundred times. The net effect is that the bass is all cut out.

And for perceived frequency response: Human hearing is very weird. At low volume levels, our ears simply don't pick up some frequencies very well-- usually the low and high frequencies. We're more sensitive to mid frequencies. So, as you reduce the volume level we perceive that the low and high frequencies are being cut out more than the middle frequencies. The "Loudness" button of a stereo attempts to compensate for this by boosting those frequencies and therefore making us hear a more balanced sound at low listening levels. If the connector is bad or dirty, less power will be making it to the speaker and causing a lower volume level. But due to human hearing, we would also hear less low and high frequencies-- making it sound like we're listening through a phone or something.

This is an over-simplified answer, but reasonably accurate for your purposes.

  • Aha, thanks. Particularly for the info about frequencies that get cut out. I hadn't realized that the power would actually affect the quality of the sound and not just the volume. – Hydrangea Apr 25 '11 at 20:10
  • I would expect the thing to make an RL-filter, as the speaker is mostly inductive (coil). – drxzcl Apr 25 '11 at 23:19
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    @ranieri. It certainly does, but the results are more unpredictable since it's dependent on the speaker impedance over the entire frequency spectrum. This effect is actually the biggest reason for using large gauge speaker cable, btw. –  Apr 26 '11 at 00:38
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    A coil which is rigidly mounted so it can't move will act as an inductor. A coil which is mounted so it can move (whether in a speaker or a motor) will accelerate in response to current fed through it, and will generate back EMF proportional to its velocity. In the absence of inductance, resistance, mechanical friction or boundaries, commutator effects, etc. the increase in back EMF in response to coulombs flowing through would mimic the response of an ideal capacitor. Of course, in practice the those effects make speakers and motors behave like highly non-ideal caps. – supercat Apr 26 '11 at 15:08
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For the case of it being dirty, turning the jack will many times help get a better connection. The reason it actually sounds poor is that the dirtiness causes noise to be added, signal to be cut, and a filter to be generated. There are different types of filters like low pass, band pass, high pass, etc. Depending on how the poor connection is made it can cause an effective filter. Also depending on how you have the jack turned there will be different characteristics of noise and signal.

For the case of it being half plugged in, you end up with things connecting to places they shouldn't be. Many times the Left and Right channel will get shorted together which can cause some phase cancellation, you can also end up with ground shorting the channels, but it can be a light short, which in turn will cause an effective filter as well.

Kellenjb
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The problem is more likely the common for both speakers becoming disconnected, putting both speakers in series across the L & R channel outputs. The sound will be in mono, AND, because the bass is mixed approximately equally between the 2 speakers, the 2 outputs are in phase & equal potential, so there is little or no voltage between them at bass frequency. What you will mainly hear is sounds mixed for the rear speakers on a surround sound system (equal & opposite phase).

The tip of the jack is the left channel, then the right, then the common, and, for earphones with extra functions like switches & a mic, the body is used for that. I don't know why the body isn't always the common. My suspicious mind tells me that Apple probably set that standard so you have to have Apple equipment to ensure they connect properly.

Peter R. McMahon
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