Why audio speakers produce the same sound waves?

Just to give you a brush up on the working of speaker, I included this animation from how stuff works.

**broken link removed**

[Note] You need to have Adobe Flash Player 9 to see the animation, its free and can be downloaded.

Anywayz.I made a low power amplifier made around LM324 driven by +12 and -12V, I was driving a 8 Ohms Speaker with it.I used my Computer Audio Port and I played the song in my computer, then I amplified this signal with the LM324 with non-inverting amplifier and I was able to hear the Audio song on my 8Ohms speaker and I was also able to turn the Voice volume up and down by changing the gain of the non-inverting amplfier.All was good.

But I thought of making an inverting amplifier, as the output voltage would be 180' out of phase with the input voltage(Original audio). What would the voice be like? This was the question, which invoked me at first.

Then I made the inverting amplifier and I played the audio song which was 180' out of phase with the original audio song coming from the computer, the audio song was the same. There wasn't any difference at all. I was surprised, as why did this happen. This time the speaker was working differently.



Negative voltages implication means the speaker contracts from it's normal position and Positive voltages implication means the speaker expands from it's normal position.

If I get the same audio voice from the speaker, is this means the sound wave that is generated is same, whether it's made from condensation or rarefaction of air molecules.

Please have your say on that.

A sound from voices of music vibrates back and forth many times.
You cannot hear the first half-cycle to determine if it is condensation or rarefaction. You hear the vibrations instead.

An opamp does not have enough output current to drive an 8 ohms speaker. The sound level before clipping occurs will be extremely low.

Yes, the output volume was very low with the amplifier. It was not high, but I can clearly listen to the song.

Added after 1 minutes:

I am sorry, it was 16 Ohms speaker, I just checked this up. Actually I had two of them 8 Ohms and 16 Ohms. I just reconfirmed which one I was using. I was using the 16 Ohms.

Added after 10 minutes:

How can we check this thing? Can a high-speed camera detect the first half-cycle? and also what if the rare-fraction time and the condensation time is not equal. If I apply your concept to a sine wave then it's ok.Suppose I am listening to a sound on a speaker, which is a perfect sine wave driven at 1 Khz. So it doesn't matter to the speaker, whether the sine wave starts from the 0 degree or it starts from 180 degree. The sound produce will be the same, because condensation time and rarefaction time is same.The time the speaker contracts and expands is the same.

But when a song(Complex audio signal) is played, maybe the condensation time and the rarefaction time is not equal in a song. I mean if condensation occurs the longer and rarefaction occurs the shorter in original audio. and if this audio is inverted so rarefaction would occur the longer and condensation would occur the shorter. This should have distort the original audio signal.Maybe this happens but ears are not able to detect this.

Added after 16 minutes:

So Will this cancellation occur?
Suppose two speakers are fed with Audio Signal, one is inverted and other is not. If they both are placed in such an angle, that the sound wave of two speakers will collide with each other.



If the audio wave is inverse of each other, then no audio wave should have been present after combining of two sound waves. I should then hear the very low voice or no voice, because sound wave is cancelled with the other same inverse sound wave.

Sounds add or cancel depending on their phase. They add when their phase is the same. They cancel when their phases are 180 degrees.

Distance affects the phase shift of higher frequencies and reflections will will mix up the phase.
You will notice bass frequencies being cancelled if two speakers face each other and are close together (a 2cm space) and one speaker's wires are backwards. If the amplifier is stereo with a balance control and the sounds are mono then adjust the balance control to fine tune the cancellation.

One day an FM radio station played stereo music with one channel's phase reversed. In mono there was cancellation. I called the engineer at the station and he said it was recorded like that and he did not have a switch to correct the phase error.

Record then playback an explosion (gun shot?) outside where there are no sound reflections. Play it back so the speaker pushes the air to you the way it was real. Then reverse the speaker wires so the sound pulls air from you. They sound the same but maybe you can feel the difference.

umery2k75,
There are two issues involved. As umery2k75 stated, if you have a stereo system, and the two speakers are driven 180 degrees out of phase, then you will get undesireable cancellation. This is referred to in audiophile circles as "relative phase". The other issue is whether or not the absolute phase matters, e.g, in a stereo system with the 0 degrees relative phase, or a monophonic system. There is disagreement among audiophiles as to whether it matters whether there is a phase inversionin the playback system or not. Certainly for steady state sine wave tones, it does not. However, consider this situation: You have a drummer whack his bass drum once. In the live scene, the initial wave reaching the ear is a pressure (higher than ambient pressure) wave. This is then followed by a rarefication (lower than ambient pressure) wave. If the audio reproduction system has a 180 degree phase shift anywhere along the signal path, then the initial wave reaching the ear from the speaker will be a rarefication wave, followed by a presure wave. Does this matter? This is the question that creates controversy among audiophiles. Some say they can hear the diffrerence, others say no. Certainly for most music, you will not be able to hear the difference (my opinion). It would be interesting to perform a double blind test with the bass drum or some other percussion instrument as the musical source material.
Regards,
Kral

Yes Kral. I'm also thinking of doing some test to come to know about the difference, but I have to see how this test should be done, that will give the significance difference and how this must be done.Any idea would be appreciated on that

When a drum is hit, the surface that is hit moves in. But the surface on the other side moves out.

So they cancel and you hear nothing. Correct?

Of course you hear the drum. But you don't know if the first wave is pressure or vacuum.

When a drum is hit, the surface that is hit moves in. But the surface on the other side moves out.

Click to expand...

I think this happen because they both are dealing with other spaces.



When drum is hit, the compression occurs, on the left side of the drum and on the right side of the drum the rarefaction occurs. They both are seperated by the Drum sheet, that' why I think they don't cancel out, because they couldn't get mixed up because there's a sheet of drum. I think if drum sheet is just removed instantly when the rarefaction and condensation is made within the air molecules, they might cancel out and no voice would be generated. I think it's the material that seperates the addition of rarefaction and condensation on the two sides of the drum, or two sides of the speaker, that's why they don't cancel out.

The enclosure for a speaker keeps the sound waves at the front and back from cancelling.
The enclosure size matches the spec's for the speaker so that it resonates a little but not a lot at bass frequencies.

Most drums have 2 sides with vibrating skins except a kettle drum.
A bongo drum is different with only a single skin and a tube that enhances the resonance of the skin.

The audible effect of phase reversing one channel of stereo audio depends on the speaker distance and also on the enviroment. The strongest effect is in open air, where the speakers are effectively radiating free field waves (except for ground reflections). In a room, most of the sound waves reaching your ears are from reflections. So the phase of a particular frequency component observed at your ear is not directly related to the phase at the speaker. At low frequencies, where the wavelength is large compared to speaker distance and room dimensions, you will usually observe a cancellation, also in a room.

Regarding the interesting question of audibility of absolute phase mentioned by Kral, I think, it is audible with suitable respectively critical signals and certain speaker properties. There are probably several candidating phsysical effects, but an obvious one is this: When the speaker diaphragm moves towards the hearer at the front of e. g. a bass drum beat, it causes phase modulation (doppler frequency shift) of other frequencies radiated at the same time.

umery2k75,
Here's how I would proceed with the experiment:
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First, record a very short musical passage that has a strong percussion content.
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Set up a playback scheme that will allow you to repeat the passage quickly and easily.
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Set up a scheme that allows you to quickly change the absolute phase. A double pole double throw switch can be wired to reverse the input connections to the speaker.
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Play back the passage in pairs, identifying the test number only to the listeners. For each pair, the first playback will be either normal or reversed, selected at random. The second playback of the pair will be either normal or reversed, again selected at random. The listeners don't know what they are hearing. They are instructed to record on a test sheet whether the playback passages in each pair sound alike or different. Only the testor knows whether they are the same, and whether the first playback is normal or reversed.
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Use as large a number of listeners as you can round up. Use as many playback pairs as possible in the test.
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After completion of the test, recorde the number of correct and incorrect answers.
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Perform an analysis of variance (ANOVA) on the results to determine whether the results are statistically significant or not.
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Remember, only the testor knows what combinations (normal or reversed for each playback) comprise each test,
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Regards,
Kral

If the amplifier is stereo with a balance control and the sounds are mono then adjust the balance control to fine tune the cancellation.

Click to expand...

Is that the reason why there's balance adjustment between two speaker. In my car there are two speaker, one is installed in right door and other on left front door.I don't think they give the balancing just because of phase cancellation inside the car. Even in the Hi-fi stereo system I have at home, has two big speakers, there's also balance between the two. I think those manufactures don't give balancing for these phase cancellation.In my TV there are two speakers, even my computer speaker set has two speaker and my monitor has two built-in speakers.Car space is small inside and room is big. It's hard to believe for all those past days ,I used to balance the speaker adjustment , just because the voice when reflected back cancel out the other one. So I was used to fine tune this balancing.

The speakers on a stereo are supposed to have the same phase, not opposite phase. Then phase cancellation does not occur.

In my demonstration of phase cancellation I said to connect one speaker backwards, then it has opposite phase with the other speaker.

A balance control is provided to "balance" the volume from each channel because maybe you sit closer to one channel's speaker than the other.

In my demonstration the balance control "fine tunes" the volume adjustment of each channel so the sound cancels because the speakers have opposite phase.