sound systems

[COMMUNIST FLISK]

stereos/speakers etc, use a vibrating plate, usually made of strong carboard, plastic etc, that vibrates, sending sound to your ear via air particles...

my question...
how does it produce more than one sound at once? (the vibrating plate i mean)

for instance, on a track, a drummer is hitting the bass drum, snare, and hi hat. a singer is singing, a bassist plucks and a guitarist is strumming all at the same time. all the sounds are differant amplitudes/pitches/frequencies...
so how does the plate achieve this?

 

[ebbixx]

Addition of waveforms. Do you need/want a picture?

 

[COMMUNIST FLISK]

yes please =]

 

[AndromedasWake]

The waveform that comes out of your speaker is very complicated. It's a superposition of many different frequencies, each with different amplitudes.

To separate them, you have to use Fourier transforms. For any sound which isn't a single frequency, the vibrational mode of your speaker is incredibly complicated.

Science. It's awesome.

 

[ebbixx]

Just in case

The mp3 for the file shown in the lower window is uploaded to http://www.questionscience.com/play_audio.php?audio=8. You should also be able to find it, as long as there's no deluge of audio uploads, under SineSaws at http://www.questionscience.com/audio.php?load=recent

 

[COMMUNIST FLISK]

ahh i see..
so physically the plate just does a differant pattern of vibration that produces a sound that is the two seperate sounds added together.....

 

[AndromedasWake]

Yes, and in the case of a song, you're listening to many many more than just two :shock:

 

[COMMUNIST FLISK]

second question. what is feedback?

 

[ebbixx]

AndromedasWake is right though, to separate out individual wave forms once they're combined DOES get very complicated, especially if the waveforms themselves are complicated, as live recorded individual tracks might be.

And I'm sure that the materials science aspects of speaker design, production and so on also add another layer of distortion and complexity to the real sound wave that hits your ear from any given set of speakers. That's why true audio engineers actually deserve the title, engineer.

My example is grossly simplified by taking just two types of waveforms and combining them in my DAW (Sony Acid). Live wave forms, containing live reverb, echos, overtones and harmonics (and more, I'm sure) would have been much harder to visualize and see how they combine to generate a single wave. But the process is still, at least as far as the signal that is fed to the speakers (perhaps oversimplified) one of adding the wave forms to each other. It's dependent on the nature of the speakers how well they manage to convert an electrical wave form like that back into compressive sound waves in the air.

 

[COMMUNIST FLISK]

so its like 1+2 = 12 in that rather than changing the individual sounds, the sounds dont interfere when they are added together?

 

[AndromedasWake]

second question. what is feedback?

Feedback is any situation where an output affects an input. In the case of a guitar, the output frequencies of your amp actually drive the input frequencies and you get a resonance feedback loop.

The sound will continue to exacerbate itself until you do something to remove the feedback, like muting your guitar or moving it far enough from the amp.

 

[ebbixx]

second question. what is feedback?

Usually feedback comes from "feeding back" the output (sound from speakers onstage for instance) into the input (usually a mic onstage that happens to pick up too much of the speaker output. that's why concert and PA speakers are usually placed to the side and in front of the performance space, to minimize the amount of amplified sound that reaches the various mics and pickups onstage.

It's addition of signals in a neverending loop, since Sound from Speaker A --> Mic B --> Amp C --> Speaker A --> Mic B ----- ad infinitum (or at least until someone cuts the power).

 

[COMMUNIST FLISK]

and im guessing the sound waves gain volume/frequency due to positive interference?

 

[ebbixx]

so its like 1+2 = 12 in that rather than changing the individual sounds, the sounds dont interfere when they are added together?

It's more like adding two functions together. Take two sine functions, for instance, that have different periods/frequencies, add the functions, and you can, in most cases see how they sort of overlay one another. If the value of both is positive at a given time, the additive value will be greater. Since the sawtooth wave was at 220Hz and the sine wave was at 880Hz in my illustration, you can see the slower sawtooth wave adding to and subtracting from the sine wave over each 4 cycles of the sine wave. Not sure that's clear, but looking at the pic I hope it is.

If one is positive and the other is negative, they could cancel each other out. In fact, I saw this once a few months back on YouTube, where someone had recorded something in stereo, but YouTube had encoded it to a mono audio stream. The Left and Right signals in the original were identical, but phase inverted (phase inversion means they were essentially mirror images of each other) so that the net when L and R were added to make one mono signal was that all values were 0. Thus the track was silent.

(However, in an actual studio or electronic music situation, you would also be limiting the overall amplitude of the composite signal, and that also happens when I add two wave forms together by putting them on separate tracks in my DAW.)

 

[COMMUNIST FLISK]

ahh i see. the water is clearer now. thankyou both

 

[ebbixx]

and im guessing the sound waves gain volume/frequency due to positive interference?

Not necessarily... that's usually where a mixing board (analog or virtual) comes in. Each individual waveform should, ideally, remain true to itself. But then things like walls, sympathetic vibrations, echoes, and harmonics come into the picture.

But in the case of feedback, yes, adding the same sound to itself, slightly out of sync and out of phase creates a mess that usually ends in very high amplitude and defeats the controls put in place to maintain control over the waveform being fed to the speakers, burning out or damaging the speakers if not caught in time (or if fuses or other limiting circuits do not cut off the signal before the diaphragm tears).

 

[COMMUNIST FLISK]

ahh i see, i work with a team of students who manage the sound and lighting for our school (im a lighting specialist) and its very fun, although we have these small clip on radio mics and the feedback with them is hurrendous

 

[ebbixx]

ahh i see, i work with a team of students who manage the sound and lighting for our school (im a lighting specialist) and its very fun, although we have these small clip on radio mics and the feedback with them is hurrendous

Where are the PA speakers located? Can you localize where on stage one can stand safely without getting feedback? I haven't worked a lot with lav. mics personally... there might be ways of positioning them better (or adjusting the PA speakers) to reduce the chance of feedback. But a lot depends on the pick-up characteristics of the the mics, I'd imagine. I have seen this a lot in stage show sound-checks, where the usual struggle is to find a gain setting that allows you to pick up the performer adequately without also picking up too much of the ambient sound.

Another problem is that since these are body mounted, and actors tend to move, they can easily turn themselves to aim directly at a potential source of a feedback signal, unlike a mic fixed to a mic stand or podium. Which is also a reason why speakers who can't resist the temptation to play with the mic tend to get punished frequently by feedback. I used to think it was God's judgment on their boring speeches, but then I grew up.