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phantom image with loudspeaker array

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Old 01-24-2013, 09:44 AM   #1  
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Default phantom image with loudspeaker array

Hello everybody,
I am a newbie, I hope I am addressing the correct post.

I have a loudspeaker array of 4 small speakers (each one as big as a Rubik's cube), linearly equally spaced by 2 cm (0.8 inches). I would like to play a sound source (pink noise) from the first to the last speakers, to give the illusion of a moving sound source.

One trivial solution would be, for right-to-left movement:
0 0 0 1 at time t
0 0 1 0 at time t+n
0 1 0 0 at time t+2n
1 0 0 0 at time t+3n

But this obviously gives me the impression of sound "jumping" from one speaker to the following.

Another solution would be updating the intensities twice more frequently:

0 0 0 1
0 0 0.5 0,5
0 0 1 0
0 0.5 0.5 0
0 1 0 0
0.5 0.5 0 0
1 0 0 0

but I am not sure it is the correct way.

The question is: which sound intensity should I set the speakers so that the sound is "smootlhy" perceived as moving?
I guess this is related to how to create a phantom image.
Please help!

Thanks a lot!
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Old 01-24-2013, 10:32 AM   #2  
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Location: Ohio
Posts: 14,237
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Quote:
Originally Posted by chepotenza View Post
Hello everybody,
I am a newbie, I hope I am addressing the correct post.

I have a loudspeaker array of 4 small speakers (each one as big as a Rubik's cube), linearly equally spaced by 2 cm (0.8 inches). I would like to play a sound source (pink noise) from the first to the last speakers, to give the illusion of a moving sound source.

One trivial solution would be, for right-to-left movement:
0 0 0 1 at time t
0 0 1 0 at time t+n
0 1 0 0 at time t+2n
1 0 0 0 at time t+3n

But this obviously gives me the impression of sound "jumping" from one speaker to the following.

Another solution would be updating the intensities twice more frequently:

0 0 0 1
0 0 0.5 0,5
0 0 1 0
0 0.5 0.5 0
0 1 0 0
0.5 0.5 0 0
1 0 0 0

but I am not sure it is the correct way.

The question is: which sound intensity should I set the speakers so that the sound is "smootlhy" perceived as moving?
I guess this is related to how to create a phantom image.
Please help!

Thanks a lot!
What you are attempting to do is not create a phantom image but illustrate the doppler effect.
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Old 01-24-2013, 05:20 PM   #3  
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Thanks for your reply, but the Doppler would be a minor issue. The thing is that I would like to give the user the impression that the sound is coming from the middle of two speakers, as the array would be composed by a moving sound source, and not by a discrete array of static sources.
I am happy even if the source does not move (say, I can just play the "phantom" source between speaker 2 and 3), but I must be able to render it., and I don't know how to do that.

What would you suggest?
Thanks again!
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Old 01-25-2013, 07:44 AM   #4  
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Location: Ohio
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I don't understand
Quote:
I would like to play a sound source (pink noise) from the first to the last speakers, to give the illusion of a moving sound source.
If it is playing form 1st to last the sound source is indeed moving.

also
Quote:
linearly equally spaced by 2 cm (0.8 inches)
I don't know what you plan on accomplishing placing speakers less than a inch apart - it is practically pointless as the ears ability to discriminate isn't that precise.

As far as 'phantom source' it sounds like what you are trying to do is in essence create a soundstage or have imaging. You need to record in stereo with good equipment and then have speakers with excellent sound dispersion. If you listen to my HT just plying 2 ch stereo recording (front L and front R) you would swear that my center and surrounds are on as well even though they aren't as the sound field dispersion is so good you can pick out the location of musicians and instruments.
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Old 01-25-2013, 10:57 AM   #5  
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Location: Idaho
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If you have a number of speakers and simply change from one to another, then the sound will appear to "jump". Decreasing the level of one speaker as the level to an adjacent one is raised, will produce a more realistic "motion". The more steps used (increasing an adjacent speaker by a factor of 0.1 versus 0.5 - with more steps), and the more speakers involved (not just two), the more realistic the effect will be. Unfortunately, this involves some computing power (if the audio is digitized), and total control over the volume of each speaker. It's an ideal application for a DSP chip, but you need to know how to use one and how to write the instructions. Also, to make it truly realistic, phase changes need to take place in each speaker output as the sound "moves" across the sound-field. All this happens naturally when a recorded sound is captured by multiple microphones, but to synthesize the effect, it's more difficult to get it right though a good approximation can be obtained.

Also, as mentioned, the spacing on your speakers is quite close. Move them further apart unless you're trying to produce a miniature sound stage of some sort (model railroad???).

The lower the frequency of the sound, the less you'll be able to localize it (it will appear to spread out and not come from a point source). Your small speakers and close spacing would be wasted if you try to do this with a low frequency.

Last edited by RBTO; 01-25-2013 at 05:53 PM..
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