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Author Topic: Plane wave from subwoofer array  (Read 8297 times)

Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #10 on: April 16, 2016, 04:33:11 am »

Sorry-but I don't see any way that this makes any sense.

Of course mine is just an assumption, without right scientific basis. However i think that different sound fields may change the way we feel the wave intensity. I'd like to learn more in deep!  :)


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Steve Bradbury

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Re: Plane wave from subwoofer array
« Reply #11 on: April 16, 2016, 07:47:26 am »

The reason that none of this makes much sense is because everyone is talking in the wrong language with a severely restricted vocabulary. It is almost impossible to accurately describe the situation without using mathematics. If you dont understand mathematics, then you wont understand what is happening.

To compound the above, I have not found a quick, easy way to type out mathematical expressions on a discussion board.

I am assuming that in the original post PVL is the particle velocity. The ratio of pressure to particle velocity is the specific acoustic impedance (SAI). That no one has mentioned this is what lead me to say you are using a limited vocabulary.

In the case of a plane travelling wave The impedance is usually referred to as characteristic impedance (see equation 1 in diagram), which is a real term that is derived from the product of the density of the medium and the speed of sound in that medium (equation 2) If I remember correctly (should really have googled it) for air at 20 C the value is 407 N.s/m^3

For a diverging or standing wave the SAI becomes complex (equation 3). The real part is the acoustic resistance and the imaginary part the reactance. This should sound familiar to electrical engineers.

For a spherical wave the SAI can be written as equation 4, which when separated into the real and imaginary parts can be written as equation 5.

K is the wavenumber (also referred to as the acoustic wavenumber), it is 2 X PI / wavelength. r is the distance from the source.

Kr is the value that determines what you refer to as near or far field. Far field is when Kr is much greater than 1.

As you should see, with a large value of Kr in equation 5 the real or resistive part tends to the characteristic impedance (equation 2) and the reactive part tends to zero. This is why in the far field, as you describe it, a spherical wave can be viewed as a plane wave.

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Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #12 on: April 16, 2016, 03:18:34 pm »

Steve, your post is it very hopeful, thanks! ;)

Just another assumption:

In an active sound field the sound wave moves in one direction first compressing the air molecules on one side of the listener, then moving across the listener's position, and finally increasing pressure on the opposite side of the body. The pressure change is in ONE DIRECTION ONLY. As the pressure wave moves across the body, following compression of the air is rarefaction of the air a negative pressure zone. As this rarefaction of air creates a negative pressure zone, the listener is then "pulled" back toward the sound source.

So, positive pressure gives a linear "push" and it is then followed by a negative pressure which gives a linear "pull". Thus the listener is rocked back and forth as the sound wave passes by. This creates the physicality.

Now, take a subwoofer placed in a car or in a home room. When a tone is played, the wave will begin to propagate in one direction. However, it will then be met with reflections off the boundaries. So what actually ends up hitting the listener are bass waves from ALL DIRECTIONS. The air molecules are compressed and rarefied, which creates the SPL, but there is no directional change. That is the key. Integrated across the entire body of the listener, there is no net directional change in pressure. It pushes in all at the same time, then it sucks out (rarefied) all at the same time. As a result, the pressure goes up and down. SPL is measured, but there is no movement back and forth of the listener. No physicality, at least in some sense.

---

A further ramification of this is that various subwoofer designs could have more or less physicality depending on the extent to which they produce linear bass waves. Horns, for example, are frequently described as having more punch than direct radiators all other things equal. Within this model of physicality, this could be explained by the fact that a sound wave must travel some distance before exiting the horn, thus putting the listener effectively into the "far field" where bass waves are nearly planar even while standing directly in front of the horn itself.

The same might also be true of large radiators and/or large numbers of them relative to small diameter radiators, as the former creates something much closer to the planar wave of the far-field.
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #13 on: April 16, 2016, 05:10:40 pm »

Steve, your post is it very hopeful, thanks! ;)

Just another assumption:

In an active sound field the sound wave moves in one direction first compressing the air molecules on one side of the listener, then moving across the listener's position, and finally increasing pressure on the opposite side of the body. The pressure change is in ONE DIRECTION ONLY. As the pressure wave moves across the body, following compression of the air is rarefaction of the air a negative pressure zone. As this rarefaction of air creates a negative pressure zone, the listener is then "pulled" back toward the sound source.

So, positive pressure gives a linear "push" and it is then followed by a negative pressure which gives a linear "pull". Thus the listener is rocked back and forth as the sound wave passes by. This creates the physicality.

Now, take a subwoofer placed in a car or in a home room. When a tone is played, the wave will begin to propagate in one direction. However, it will then be met with reflections off the boundaries. So what actually ends up hitting the listener are bass waves from ALL DIRECTIONS. The air molecules are compressed and rarefied, which creates the SPL, but there is no directional change. That is the key. Integrated across the entire body of the listener, there is no net directional change in pressure. It pushes in all at the same time, then it sucks out (rarefied) all at the same time. As a result, the pressure goes up and down. SPL is measured, but there is no movement back and forth of the listener. No physicality, at least in some sense.

---

A further ramification of this is that various subwoofer designs could have more or less physicality depending on the extent to which they produce linear bass waves. Horns, for example, are frequently described as having more punch than direct radiators all other things equal. Within this model of physicality, this could be explained by the fact that a sound wave must travel some distance before exiting the horn, thus putting the listener effectively into the "far field" where bass waves are nearly planar even while standing directly in front of the horn itself.

The same might also be true of large radiators and/or large numbers of them relative to small diameter radiators, as the former creates something much closer to the planar wave of the far-field.
What you need to consider is the freq and the position at any one point in time of the listener.

The sound wave propagates through the air.  The freq of sound will determine the size of the waves (the length etc).

So with higher freq you will have more cycles in a specific distance between 2 points than with low freq.

Loudspeakers do not always produce a Positive pressure first.  They can produce a neg pressure first.  It just depends on the polarity of the signal applied to them and the type of cabinet and how it is wired.

So you are not first pushed and then sucked.  It could be the other way around.

The idea of "punch" is greatly misunderstood.

My limited testing has shown that if you take several subs, and listen to them and rate the punch.  for example which one has the most punch, the least punch etc.

Then you MEASURE the freq response, what you will find in that the one with the most "punch" has an exaggerated upper bass response.

When you apply eq so that all the cabinets have the same freq response (as a proper system should), all of the "punch" goes away.

So it is the basis of the cabinet design that produces a "non flat" response.

Now if you happen to like that-what that means is that you like a system that does not have a flat response.

The reason some horns do this, is that at the upper end of their response they have a lot of horn gain-which simply means they are louder up high than typical front loaded cabinets.
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Ivan Beaver
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Jay Barracato

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Re: Plane wave from subwoofer array
« Reply #14 on: April 16, 2016, 05:22:51 pm »

In general I seem to detect a misunderstanding of the difference between the propagation of the wave and the oscillation of the wave.

Sent from my XT1254 using Tapatalk

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Jay Barracato

Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #15 on: April 17, 2016, 05:15:35 am »

In general I seem to detect a misunderstanding of the difference between the propagation of the wave and the oscillation of the wave.

Sent from my XT1254 using Tapatalk


What do you mean precisaly? I still convinced that in small places, like rooms, bass frequencies don't propagate as well as outdoor. Even if the SPL  is very high, the net intensity is lower. This is due to boundary reflections, cabin gain, and slow deacay.

Do this simply experiment: Holds the SPL at says 90-100 db and try to measure particle velocity level at says 40-50 Hz in your sealed room or better in your car. Then do the same outdoor at 5-6 meters away from the subs, you will see the difference.
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #16 on: April 17, 2016, 08:59:55 am »


What do you mean precisaly? I still convinced that in small places, like rooms, bass frequencies don't propagate as well as outdoor. Even if the SPL  is very high, the net intensity is lower. This is due to boundary reflections, cabin gain, and slow deacay.

Do this simply experiment: Holds the SPL at says 90-100 db and try to measure particle velocity level at says 40-50 Hz in your sealed room or better in your car. Then do the same outdoor at 5-6 meters away from the subs, you will see the difference.
You are talking about different things.

How sound "propagates" is one thing.

How sound is reflected inside a room is a completely different matter.

Yes the sound is propagating in both cases, but the reflections will cause all sorts of interference and cancellations (and some additions) inside a room.

The sound is still moving in the same manner.

Now if you are talking about that the sound is more even outside-(due to no reflections-except the ground) then there is no argument about that.

But that is due to the lack of reflection, NOT the way the sound propagates.

But how it "hits your body" is still the same.
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Ivan Beaver
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Jay Barracato

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Re: Plane wave from subwoofer array
« Reply #17 on: April 17, 2016, 10:36:09 am »


What do you mean precisaly? I still convinced that in small places, like rooms, bass frequencies don't propagate as well as outdoor. Even if the SPL  is very high, the net intensity is lower. This is due to boundary reflections, cabin gain, and slow deacay.

Do this simply experiment: Holds the SPL at says 90-100 db and try to measure particle velocity level at says 40-50 Hz in your sealed room or better in your car. Then do the same outdoor at 5-6 meters away from the subs, you will see the difference.

All sound waves at the same set of conditions propagate at the same velocity regardless of frequency. The only way to change that is if the source, the listener, or the medium is in motion relative to the other two.

Sent from my XT1254 using Tapatalk

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Jay Barracato

Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #18 on: April 17, 2016, 12:25:30 pm »

All sound waves at the same set of conditions propagate at the same velocity regardless of frequency.

I'm not talking about the speed of sound...

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Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #19 on: April 17, 2016, 12:45:43 pm »

The sound is still moving in the same manner.

This is NOT true, because sound field property changes!


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ProSoundWeb Community

Re: Plane wave from subwoofer array
« Reply #19 on: April 17, 2016, 12:45:43 pm »


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