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Author Topic: Plane wave from subwoofer array  (Read 8365 times)

Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #20 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.  :)
« Last Edit: April 17, 2016, 01:09:46 pm by Luca Rossi »
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #21 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.
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Ivan Beaver
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #22 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?
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Luke Geis

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Re: Plane wave from subwoofer array
« Reply #23 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.
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Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #24 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.
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Luca Rossi

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Re: Plane wave from subwoofer array
« Reply #25 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.
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Jay Barracato

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Re: Plane wave from subwoofer array
« Reply #26 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

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Steve Bradbury

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Re: Plane wave from subwoofer array
« Reply #27 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.
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #28 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.
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Ivan Beaver
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Ivan Beaver

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Re: Plane wave from subwoofer array
« Reply #29 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.
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Ivan Beaver
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ProSoundWeb Community

Re: Plane wave from subwoofer array
¬ę Reply #29 on: April 18, 2016, 07:57:20 pm ¬Ľ


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