Hi guys

I am looking for information about how a sound wave (human voice) from a speaker spread into the sounding environment? Like a graph, or a technical term I should be looking at?

I am working on a speaker phone intercom, I image this info can help me better understand how to minimize the feedback.

As usual, I may not be making any sense, please feel free to ask me to clarify it if this is the cause.

 

Basically, hard surfaces reflect the sound and soft surfaces absorb. Too much absorption and the room is "dead"; not enough and you have echoes and, of course, feedback. All this being said, you must avoid direct reflection back to the mike(s). In a performance setting, cardoid mikes are employed, among other things (speaker positioning for example) that take into account the room's acoustic properties. However, a phone system is a bit of a different animal. Noise gates may be of use in this area. Notwithstanding, here are a few sites for you to look over:

http://www.soundcontrolroom.com/design-considerations-for-recording-studios.php

http://www.audio-times.com/audiotim...-design-considerations-for-recording-studios/

http://www.astralsound.com/system_design.htm

 

Think of air pressure.
Sound is propagated as compression and expansion pressure waves.
At any given point in space the air pressure is changing at the same frequency as the electrical signal that is applied to the speaker (delayed, of course).

 

The high frequency (1 to 5 KHz) radiation pattern of a typical cone speaker is not that good. It generally holds a 3 db bandwidth over only 20 to 30 degrees around center line when used as a bare flat mounted radiator.

Here is a picture of a Bose 901. Notice that it is canted at about 30 degrees to improve distribution.

For even distribution and good frequency response you must look to cabinet and baffle design.

 

I should also toss in the fact that higher frequencies attenuate much sooner (and easier) than lower frequencies.

 

My favorite is a book called: Sound System Engineering by Don and Carolyn Davis. Even if you don't need to learn all of the math because you're not designing auditoriums, you can develop a very good model of how sound can be focused, directed, reflected, and absorbed, how the spectrum can be shaped and how sound disperses and diminishes in a 3 dimensional space.

 

I am thinking this when I ask my question in my OP, is that a graph or something I can look at? (imagine my drawing is in 3D )

 

That is a drawing of how the low frequency content of the back wave of a speaker cancels the front wave if it is not in an enclosure.

 

Maybe this will help. (As #12 said)

 

LOL- well bug13, ya asked for a speaker and got a recording studio, a PA system, classes in acoustics and sound dynamics, and Bose (for those who can afford 'em). You can't beat that. I guess we're bored.

 

This is a more accurate pattern.

 

Good find Les

 

As a final piece of confusion, bear in mind that the average human voice, when speaking, is between 85 and 255 Hz, harmonics notwithstanding.

 

That is a drawing of how the low frequency content of the back wave of a speaker cancels the front wave if it is not in an enclosure.

My drawing is just an example of what I am looking for (something I think it should look something like this), properly it is wrong.

 

This is a more accurate pattern.

So, how to I read this graph? Is there a technical term for this? (sound wave something something graph) so I can do more googling?

 

It's the sound pattern from a speaker cone (viewing from the sides or above or below. The degrees are the angle off center line, the decibels (dB) depict the strength/attenuation at various points.

 

As I talked about in my first post, a simple speaker does not have a smooth distribution of frequencies with respect to the aspect angle of the speaker. In other words, you can not simply mount a speaker on a panel and expect to hear the same sound directly in front of the speaker as you do 45 degrees from center. Volume and equalization set for optimal at 0 degrees will not work at 45 degrees.

 

So, how to I read this graph? Is there a technical term for this? (sound wave something something graph) so I can do more googling?

The correct term is radiation pattern, the term refers to the direction that most of the sound energy is concentrated into so you hear about direction or omnidirectional loudpeakers.

http://www.acs.psu.edu/drussell/demos/rad2/mdq.html

However as the sound permeates the room it is called the sound field and has two parts the near field and the far field.

The near field is basically only the sound that comes directly from the loudspeakers so the term direct field is also used.

The far field is the direct sound mixed in with the sound that bounces off the room walls. Because this process of bouncing is called reverberation the far field is also called the reverberant field.

An important quality of the room is called the reverberation time.

 

...I image this info can help me better understand how to minimize the feedback..

I can't imagine how. Sound is pressure waves propagating through air at the speed of ...sound, and what the listener (or microphone) hears is those waves as they strike directly and indirectly bounced off walls and such.

Even if I could calculate all the interference patterns, reflections, and so on, I don't see how that could be used to minimize feedback. Of course, my ignorance of audio engineering doesn't mean it can't be done. I'm just saying that you may be focusing on the wrong topic.

 

bug13 said: ↑
...I image this info can help me better understand how to minimize the feedback..

Well spotted wayne.

I also note that this is for an intercom, which has both a speaker and a mike the recipe for feedback.

The usdual way for intercoms to avoid feedback is the function switch, either listen od speak, but not both.

If you must have both then both the mike and speaker need to be highly directional.