ProSoundWeb Community

Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Subwoofer Forum => Topic started by: Luca Rossi on April 14, 2016, 08:57:37 am

Title: Plane wave from subwoofer array
Post by: Luca Rossi on April 14, 2016, 08:57:37 am
Hi all, as topic subject, is it possible to get a true plane wave from a small-medium subwoofer array? I know that it is frequency, distance dependent..

I read from an acustic manual that in a front plane wave, PVL and SPL are in phase, so the intensity of the active field is maximum.

Thanks!
Title: Re: Plane wave from subwoofer array
Post by: Art Welter on April 14, 2016, 01:03:43 pm
Hi all, as topic subject, is it possible to get a true plane wave from a small-medium subwoofer array? I know that it is frequency, distance dependent..

I read from an acustic manual that in a front plane wave, PVL and SPL are in phase, so the intensity of the active field is maximum.

Thanks!
Luca,

A subwoofer array is a combination of individual omnispherical radiation patterns. An array small compared to the lower frequencies reproduced would not be a true plane wave. An array with a boundary larger than the low frequency wavelengths reproduced could be an approximation of a plane wave.

What is a "front plane wave" ?
What is "PVL"?
Is there a practical application behind your inquiry?

Art
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 14, 2016, 03:44:53 pm
I mean that when the distance from the omnidirectional source is far enough, the front wave becomes planar. This is called far field, so the sound pressure level is in phase with particle velocity level. For high frequencies, far field, is closer to the source than for low frequencies.

I think that for feeling more low frequency impact we need to get a true plane wave, regardless of a high spl. I would think that this can be obtained from a well sized subwoofer array. So we can get the intensity of the far field at closer distance.
Title: Re: Plane wave from subwoofer array
Post by: Stephen Kirby on April 14, 2016, 06:51:07 pm
Actually it's the other way around.  At the transition from near field to far field the wavefront becomes spherical.  Meaning if you had a large enough planar source (world's largest Magnepan?) it would be a planar wavefront in the near field, but in the far field it will become a spherical point source.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 14, 2016, 11:20:05 pm
As Art says- any subwoofer array is going to be a series of independent radiating devices.

Each of which has a spherical pattern.

What you want to end up with  is a spherical pattern-since that is the natural way sound propagates through the air.

THat is what will give you the best phase response.

When you have multiple devices arriving at a location, you will have phase interaction/cancellation etc.

This is not good for either the phase or the amplitude response.

A "flat" way can only be constructed by DESTRUCTIVE means.

Which means the output level will be lower-along with a lowering of the overall sound quality.
Title: Re: Plane wave from subwoofer array
Post by: Luke Geis on April 14, 2016, 11:34:04 pm
Stephen, I believe Luca is actually correct in his understanding of wave propagation in the near vs far field. In the near field sound waves are more spherical than in the far field from a point of incident standpoint.

Stoke's law for sound attenuation explains why lower frequencies attenuate less over distance than higher ones. In the sense of a perfect planar wave ( technically impossible to achieve ) it would never attenuate, but in order for this to be possible the planar wave source would need to infinite. This point only presents the possibility that having a planar wave " like " output for only the low end would not be ideal since it already has less attenuation over distance than that of higher frequencies. Hear are several links showing what I mean:

 Near Vs Far field waves
 (http://spiff.rit.edu/classes/phys312/workshops/w7a/sound2/plane_sphere.html)
 Stokes' law of sound attenuation
 (https://en.wikipedia.org/wiki/Stokes%27_law_of_sound_attenuation)
 Does it apply to all frequencies ?
 (http://physics.stackexchange.com/questions/194526/does-the-inverse-square-law-apply-for-all-frequencies-of-sound)
 Audio Engineers Handbook example 1.3.3
 (https://books.google.com/books?id=gDPJAwAAQBAJ&pg=SA1-PA31&lpg=SA1-PA31&dq=planar+waves+inverse+square+law&source=bl&ots=cbasBrriKe&sig=1yi3HefK2BqSaRRrQve493VlBi4&hl=en&sa=X&ved=0ahUKEwjy0oSBz4_MAhVU12MKHZD-AZ04ChDoAQghMAE#v=onepage&q=planar%20waves%20inverse%20square%20law&f=false)

No matter how you cut it, you will not get a true planar wave sub, ever....... And would probably not be ideal even it were possible.

I would wonder more what creates the perception of impact with a sub speaker? Many feel that a horn loaded sub needs more distance for impact to be perceived. I have noticed that direct radiating subs have a similar effect too though. I think it takes time for the wave front to propagate from the source before it can be perceived truly. I am going to throw a spit ball, but my guess is that if it requires less than 1/4 wavelength between drivers to effectively couple, then why wouldn't it take at least 1/4 of a wavelength for a wave to begin to be perceived or recognized? If that is the case it may explain why you don't seem to get full frequency impact from a sub until you are several feet away?
Title: Re: Plane wave from subwoofer array
Post by: Merlijn van Veen on April 15, 2016, 01:37:08 am
Sunlight could be considered a planar wave by the time it hits Earth (150.000.000 km). The sun clearly is a point source. But the section of the arc of the wavefront that reaches us is only 0.002 deg and for all intends and purposes flat. Sunlight does not attenuate from Mt. Everest to sea level and casts parallel shadows proving that the light rays angles of incidence are virtually parallel to each other.

If you zoom in on the edge of a circle it eventually becomes flat.

Interesting topic.


Verzonden vanaf mijn iPad met Tapatalk
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 15, 2016, 03:24:10 am
Stephen, I believe Luca is actually correct in his understanding of wave propagation in the near vs far field. In the near field sound waves are more spherical than in the far field from a point of incident standpoint.

Yeah, that's what i mean. 
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 15, 2016, 03:29:49 am
I would wonder more what creates the perception of impact with a sub speaker? Many feel that a horn loaded sub needs more distance for impact to be perceived. I have noticed that direct radiating subs have a similar effect too though.

So yes, i would think horn loaded subs behave more like a plane wave, that's can be a reason why we get more impact, regardless of high SPL...

I think the goal is how the wave hits our bodies. Plane waves hit our body at the same time in all its size, while sphericals of course don't. Sure high SPL matters, but it is not the only.

Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 15, 2016, 07:04:30 pm

I think the goal is how the wave hits our bodies. Plane waves hit our body at the same time in all its size, while sphericals of course don't. Sure high SPL matters, but it is not the only.
So let's assume for a couple of seconds that you are correct.

Let's then look at the freq and the time periods associated with sub freq.

When you consider that, you will realize that the "different times that the sound hits your body" is EXTREMELY small when compared to the size of the sub waves.

And also consider that any "array" of subs is going to produce a different source of sound-at a physical different distance from you.

So what you have is MULTIPLE arrivals on your body (a different one for each driver), which is going to be larger/longer than a "planer" waver vs a normal wave.

Sorry-but I don't see any way that this makes any sense.
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi 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!  :)


Title: Re: Plane wave from subwoofer array
Post by: Steve Bradbury 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 donít understand mathematics, then you wonít 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.

(http://i65.tinypic.com/2w7er8h.jpg)
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi 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.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver 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.
Title: Re: Plane wave from subwoofer array
Post by: Jay Barracato 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

Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi 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.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver 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.
Title: Re: Plane wave from subwoofer array
Post by: Jay Barracato 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

Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi 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...

Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi 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!


Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 17, 2016, 01:00:03 pm
But how it "hits your body" is still the same.



No doubt that high SPL, flat phase-frequency response and fast decay are the first requirement for a good impact, however i'm still convinced that different sound field properties can change the way we feel it.  :)
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 17, 2016, 02:49:56 pm
I'm not talking about the speed of sound...
I guess we are not sure what you are talking about then.

Sound will move/propagate through the air depending on a number of factors.  But all freq will move though the air the same under the same conditions.

Yes things like air absorption will happen-mostly at the higher freq, but you are talking about subs freq.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 17, 2016, 02:51:14 pm
This is NOT true, because sound field property changes!
Can you explain what you mean a little better.

What exactly do you mean by "sound field property".

And what changes?
Title: Re: Plane wave from subwoofer array
Post by: Luke Geis on April 18, 2016, 02:05:23 am
From what I have heard and what I understand the nature of the question to be, I believe the answer lies in the time and virtual size of the element domain? While I imagine the empirical evidence to be sparse, to non existent, there seems to me evidence to support that theory? Let me try and explain.

In a room, if you turn the tops off and let a low bass note play out, you may begin to notice the note plays, but shortly after the rest of the room resonates and perhaps even rattles, but the hit that the room creates if pretty far away in time from the initial note that was played? In theory the room should rattle within milliseconds of the notes reproduction right? So why does it take half a beat or more for the room to resonate, or for you to really perceive that the note was played in that space? I think it has to do with the time it takes for that note to be fully experienced equally in either direction from a point of incident vs. the virtual size of the source. 

If you are outdoors in an open field and play a 30hz tone through the subs, my bet is that it will follow the " rules " until you get right next to and or near the subs. A pressure wave of sound is not unlike one in water. If you have a small piston and a large piston that vibrates at the same 30 hz frequency and with the same amplitude, the one with the larger piston will have a larger distance from the epicenter of the source in which the wave fully propagates. In other words, a smaller source creates a full cycle of the wave sooner at any given point.

Just because the sound wave is moving forward does not mean that the " space " behind it is stationary. There is now a wave that you have equal opportunity to hear in either direction from you. One part of the wave is moving away from you while the other towards you. If the wave is just starting, you can only hear the forward side of it, as the 180* phase of the wave has not been formed yet. My guess is that you can't fully experience the frequency of the sound until the wave is equidistant in peak to trough at your point of incident? This would explain why a room won't resonate until well after the note has been played. It has taken that long for the note to actually have been fully produced AND have traveled to a distance equidistant from a point in the room that resonates at that frequency. My theory is that the smaller the virtual source of audio that reproduces the sound, the sooner that sound can be perceived. I think it has to do with relative impedance on the air? The larger the source, the more time is required for a full wave cycle to be reproduced from any given point in space. 

In the case of a horn loaded sub there is a different impedance on the air in which the speaker is affected, making it seem like a larger point source. A horn loaded sub has a higher impedance imposed on its element, which makes its pistonic output bigger in relation to a direct radiating sub with the same number of elements. A direct radiating sub has its elements directly coupled to the open air in front of it which lowers its impedance on the air, making it seem like a smaller source in relation to a horn loaded sub. If that is true and my theory is true ( I'm not a physicist and am again only guessing ), then that might explain why a direct radiating sub seems to have more " impact " in less distance than a horn loaded sub? The answer would then be to make subs that are physically smaller in size and have less atmospheric impedance to create a more immediate " impact " in the near field.
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 18, 2016, 03:18:30 am
In the case of a horn loaded sub there is a different impedance on the air in which the speaker is affected, making it seem like a larger point source. A horn loaded sub has a higher impedance imposed on its element, which makes its pistonic output bigger in relation to a direct radiating sub with the same number of elements. A direct radiating sub has its elements directly coupled to the open air in front of it which lowers its impedance on the air, making it seem like a smaller source in relation to a horn loaded sub.

This is what i think too.

...then that might explain why a direct radiating sub seems to have more " impact " in less distance than a horn loaded sub? The answer would then be to make subs that are physically smaller in size and have less atmospheric impedance to create a more immediate " impact " in the near field.

However this is a bit weird conclusion, becouse i always felt that horn loaded subs have more impact then direct radiators... and i would think this is not only due to a better impedance match with the air wich raises the efficiency. Of course the horn is increasing the path lenght, the greater traveled distance makes the front wave more similar to a plane wave.
Title: Re: Plane wave from subwoofer array
Post by: Luca Rossi on April 18, 2016, 03:34:38 am
Can you explain what you mean a little better.

What exactly do you mean by "sound field property".

And what changes?

In a sealed room, at very low freq, the soundfield will approximate the same all across the entire room, and the velocity part will be low.
There will, however, be differences as you move in very close to the source, and more for increasing frequency.
Depends on the room, but I also agree that even very close to the source, in fact even inside a horn mouth, there is significant contribution from room reflections.

In the near field close to a source pressure and velocity are 90 degrees out of phase.
Near field means that the distance to the source is small or comparable to the wavelength, the wavefront is spherical, so that the velocity potential kind of 'leaks' sideways.
In the far field pressure and velocity are in phase, as the wavefront now approaches a plane wave.

However, for a standing wave in a room the net intensity is zero, and pressure and velocity are 90 degrees out of phase.
The active intensity is zero, but the reactive intensity is high.

In all practical situations in real rooms reflections will affect both phase and amplitude of the velocity potential relative to pressure, so that intensity will be highly influenced by the room and QUITE different from the free field situation.

Clothing and skin may be more sensitive to velocity, and velocity even out of phase with pressure may work.
If there is no driving pressure, the air particle velocity will just disappear when the wave hits something with higher resistance, such as the floor.
 
Mid and upper bass impact, which works more on the solid parts of the body, would then require more intensity - there has to be pressure, as well as velocity.
A sound field with high intensity is a plane wave, in free field.
Such conditions we find outdoor, at distances > wavelength.
Title: Re: Plane wave from subwoofer array
Post by: Jay Barracato on April 18, 2016, 10:49:19 am


No doubt that high SPL, flat phase-frequency response and fast decay are the first requirement for a good impact, however i'm still convinced that different sound field properties can change the way we feel it.  :)

Not if they are all calculated from the same parameters.

If you are using sound particle velocity in the textbook definition, it is usually calculated from the same variables as SPL and therefore will vary systematically with SPL.

Since the textbook definition refers to the rate of the migration of the particles of the medium from an equilibrium position, I would not expect this calculated value to correspond with human response to sound.

I really think that what you are getting at is in the domain of impulse/response.

Sent from my XT1254 using Tapatalk

Title: Re: Plane wave from subwoofer array
Post by: Steve Bradbury on April 18, 2016, 06:50:37 pm
Impact and punch are subjective terms, most texts that discuss acoustics wonít use such terms. It is also likely that punch comes from percussive sounds with a fast initial transient. Spend some time on the speakerplans forum and you will find many threads dedicated to what they refer to as kick bins or cabinets. They cover the 80Hz to 150Hz range and as their name suggests designed to emphasise or add impact/punch to the kick drum. To me, even a very loud 40Hz sine wave wonít have punch.

Returning to acoustics, most terms have specific meanings. Sound intensity, for example, is the average rate at which sound energy is transmitted through a unit area perpendicular to the specified direction at the point considered. The unit is W.m^-2. For a spherically radiating source the area is proportional to the square of the radius, so if you double the radius (your distance from the source) the area increases four times and the intensity will decrease by a factor of four or 6dB.

A point source is one where the source is small compared to the wavelength of the transmitted sound. You canít have a large point source. Except at low frequencies no PA loudspeakers are point sources. For a horn to be effective it must be too large to be a point source. Manufacturers may call their products point sources, but if they exhibit directivity they are not.

Also, the fact that things are out of phase is not always a problem. If a source is causing the pressure to vary sinusoidally as cos 2.PI.f.t the particle displacement will be out of phase and varying as sin 2.PI.f.t. The particle velocity is, however, the time derivative of the displacement and so it must be varying as sin 2.Pi.f.t. This is how waves propagate.

If you canít understand that, consider a loudspeaker diaphragm. Say it is connected to a signal generator producing a sine wave. With the generator switched off the diaphragm is at rest at the centre of its travel limits. When the signal is turned on the diaphragm moves forward. At some point, depending on the amplitude of the signal, the diaphragm stops and then starts to move backwards. It passes through the centre position and continues until it reaches maximum backward displacement. It again stops and then starts to move forwards, once again passing through the centre position.

Every time the diaphragm reaches maximum displacement the velocity is zero. It has to be because it changes direction. The maximum velocity occurs as the diaphragm passes through the centre position or the displacement is zero.

The displacement and velocity are therefore 90 degrees out of phase. If you drew a graph of the displacement, the velocity would be represented by the slope of the graph (Mathematically this is the derivative of the displacement). The fact that the two parameters are out of phase is just how the thing works and how we describe it.

Coming back to your near field thingÖ

When the radius of a spherical acoustic source is small compared to the wavelength the specific acoustic impedance near the surface of the sphere is highly reactive. This reactance is the result of the radial divergence of the acoustic wave and represents the storage and release of energy because successive layers of the fluid must stretch and contract circumferentially altering the outward displacement. This acts as a mass-like reactance of the specific acoustic impedance.

As a result, at long wavelengths the pressure is almost PI/2 out of phase with the particle velocity. For a constant velocity the intensity is proportional to the square of the frequency and depends on the fourth power of the radius of the source. It can be shown that any source, when small relative to the wavelength can be considered spherical, this is why small sources (compared to the wavelength) are poor at radiating acoustic energy.

Interestingly, at the other end of the system, a velocity sensitive microphone only increases in output at 6dB for each halving of the distance when the sound source is reasonably far away from the microphone. As the source gets closer the increase tends towards a 12dB increase for each halving of the distance leading to an exaggeration of the bass frequencies. It is usually referred to as the proximity effect.

Iím not sure why you think that the room effects should start within 1msec of the sound source. The speed of sound in air is finite and near enough 344m.sec^-1. The wave only travels 34cm in 1msec. Unless you have set your loudspeaker very close to a wall the wave wonít be aware of the room within 1msec.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 18, 2016, 07:49:47 pm


In a room, if you turn the tops off and let a low bass note play out, you may begin to notice the note plays, but shortly after the rest of the room resonates and perhaps even rattles, but the hit that the room creates if pretty far away in time from the initial note that was played? In theory the room should rattle within milliseconds of the notes reproduction right? So why does it take half a beat or more for the room to resonate, or for you to really perceive that the note was played in that space? I think it has to do with the time it takes for that note to be fully experienced equally in either direction from a point of incident vs. the virtual size of the source. 



No.

The reason is that it takes time for the sound to travel.

All freq will travel at the same rate for the same conditions.

It takes time for the sound to come from its source (either a loudspeaker or a voice or an instrument or any other sound source) to any other surface.

It will start to rattle "within milliseconds" of the start of the sound.

How many milliseconds depends on the distance.

A rough estimate is 1 foot/ millisecond.
Title: Re: Plane wave from subwoofer array
Post by: Ivan Beaver on April 18, 2016, 07:57:20 pm
Of course the horn is increasing the path lenght, the greater traveled distance makes the front wave more similar to a plane wave.
I don't see how a longer path would change the shape of the wave.

It starts out spherical-and keeps its shape.

In some cases the size of the exit of the horn is the same (or smaller) than some front loaded subs.

So the "shape/size" of the wave is the same.