Familiarizing with Diffraction / An attempt to demystify an acoustic room property - diyAudio
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Old 12th June 2011, 04:34 PM   #1
mige0 is offline mige0  Austria
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Default Familiarizing with Diffraction / An attempt to demystify an acoustic room property

At current time, (as a result of a pitiful lot of non-understanding spread out prominently) diffraction is seen by many speaker builders as an ill behaviour to be minimized or best to be completely avoided - witch actually is a point of view that couldn't be put any more wrong.
(well, besides a hand full of very unrealistic but theoretically possible exceptions probably)

Leaving aside "dampening" for the moment, we have to understand “reflection” and “diffraction” as core and inherent acoustic room properties when dealing with solid boundaries.


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Old 12th June 2011, 04:34 PM   #2
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To get it out of the way, lets have a short talk about reflection first.
Reflection is most easy to understand as a mirroring effect when a wavefront hits any boundary.
The main outcome is that a "virtual (mirror) source" - with its origin behind the wall - gets created, which adds to our perception as ASAR pattern.

In our everyday experience quite anybody is familiar with this from floor/ wall bounce and the time domain equivalent to power response for example.
At this point it has to be reminded that boundary reflection is - of course - also happening at and within speakers - simply whenever a wavefront gets in contact with a boundary under non-parallel angle.


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Old 12th June 2011, 04:35 PM   #3
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Diffraction on the other hand happens whenever a wavefront propagates along a boundary and this boundary - at a certain point - gets curved (giving more space).

At any point of such curvature there is diffraction happening - meaning - not only at the curvature beginning, but all the way along any non straight boundary.
As some are of the errores thinking that diffraction is happening mainly due to 2nd derivation its worth to explicitly state that diffraction happens due to the reaction forces imbalance in the wavefront when the guiding boundary does not continue any perpendicular to the wavefront.
Hence diffraction is caused by 1st derivation of any guiding boundary.

The main outcome of this imbalance in reaction forces is the creation of a so called "second source" at any point of diffraction.
These "second sources" are actually true (real) point sources, hence radiating equally in any direction.
As those second sources also radiate back to where the wave front originates from, they actually provide a reflection mechanism - even without any reflecting surface being involved.

By constructive and destructive interference with the original acoustic source, some wiggles in frequency response are the result and - as all these source origins are fix in their spatial definition - a specific sound field is created.


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Last edited by mige0; 12th June 2011 at 04:41 PM.
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Old 12th June 2011, 04:36 PM   #4
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Lets slow down for a moment, take a breath and have a look back in history regarding diffraction:

To have a semantically precise handle on the subject, I strictly restrict “diffraction” to be a “term of cause” in the historically meaning of :

“bending around the corner”

meaning in the context of acoustics:

“bending of the wave front”

In German language its maybe more intuitively to do so, as “diffraction” in a technically understanding here is closely connected to “Beugung” = “bending”

Anyway - to my knowledge “diffraction”, as a word of science, prominently entered public awareness with the most famous photos of solar eclipse being taken to prove Einstein’s theory a long time ago.

The most important benefit to stay clear about that sharp definition (“diffraction” to be a “term of cause”) is that now we immediately see the big bunch of diffraction *effects* as a pure result of “diffraction alignment”.
Further more the point is easily made that “wave guide” oxymoron actually is pure nonsense in a strict technical / physical sense.
“Waves” simply can't be guided – what can be “guided”, so to speak, is the “wave front” only.


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Old 12th June 2011, 04:36 PM   #5
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Lets continue:
Beyond that, these second sources created by diffraction are actually a CMP behaviour - and of course result in a corresponding ASAR patten.
But - we deal here with a distributed effect - meaning - a multitude of acoustic sources that are distributed room wise and hence are distributed time wise as well
The ASAR pattern created at the point of listening hence, is quite of a different specific quality than the ones originating from more pronounced diffraction effects (like from an OB edge, from what results in "baffle step", or from a sharp edged horn "mouth" reflection for example)

Its easy and intuitively to understand that diffraction *effects* are the more pronounced the more the wavefront gets bent at short propagation time - meaning - the guiding boundary exhibits small radii or a sharp edge is happening.
But again, to stay clear: diffraction *effects* are related to the boundary radii in relation to wavelength - whereas diffraction itself is the *bending of the wavefront* - which for usual piston sources radiating into 4Pi, 2Pi or whatever, is always the same amount of diffraction (180deg, 90deg or however we want to put it).


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Old 12th June 2011, 04:37 PM   #6
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Having stated that diffraction is always the same for a given acoustic source radiating into a given acoustic room does not necessarily mean we are helpless in dealing with diffraction.
Quite in contrary - we are basically free in selecting the point in space where we want diffraction to happen - and - we are also free in selecting the amount of diffraction happening there (within given limits) - hence we certainly have some freedom in *alignment* of diffraction.

Coming back to horn/ WG/ diffaction alignment device (which is all one and the same) it is said that those improve impulse response - which is actually not any true at a closer look.
As shown, quite any diffraction causes CMP behaviour and hence the inherent CMP tail of ASAR patterns show up - meaning - *any* horn must perform sonically different to a nude direct radiator (for example).

Seen from a different perspective (and excluding the mechanism of reflection for a moment) diffraction is the underlying mechanism to fill *any* given acoustic room with sound - no matter how its shape may be.


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Old 12th June 2011, 04:38 PM   #7
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The overwhelmingly simple conclusion out of this is that

1 diffraction is a “term of cause”
2 diffraction cant be avoided – its simply a *must* in any real world acoustic environment
3 its a pretty silly attempt to aim after avoiding diffraction
4 diffraction is basically always the same for any given acoustic source / acoustic room combination
5 diffraction always results in specific sound fields and ASAR pattens
6 diffraction has to be “aligned” to optimize its effects towards whatever target
7 diffraction always results in degraded impulse response at the point of listening


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Old 12th June 2011, 04:38 PM   #8
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As an aside:
By speaking of wave front, sound field and acoustic source and acoustic room properties it may be worth to recall in this context that in reality (and more strictly seen) the usual assumptions regarding linear and adiabatic compression of air doesn't hold true.

This means that for one there is energy loss (= dampening) happening along wave front propagation and second that there is wave form deformation happening (= generation of side bands).

As both are “distributed effects” that culminate at the point of listening, they provide sort of "Propagation Path Conditions Memory" (PPCoM) and hence an additional, pretty specific sonic coloration.
Here especially the wave form deformation is interesting as this culminates differently depending on the actual SPL along its path - meaning - one and the same SPL created by constructive and destructive interference at a certain listening point can show different wave form deformation and hence provide the listener "clues" about PPCoM.


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Old 12th June 2011, 04:39 PM   #9
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To sum up:

All sonic differences in outcome between "guiding" contours are merely gradual - no big deal - beyond that is just sales speak.

As said - diffraction is a must - and besides that basically always of the same amount, not depending on contour selected, just depending on wave front generated and acoustic room to be finally radiated in
Certainly, diffraction is nothing to be avoided as an "ill behavior" (as popularized by Earl)

Also already pointed out: the term "wave guide" is an oxymoron.
The claim that an OSWG does anything fundamentally different than any other "horn" with respect to diffraction or loading does not and never did hold
Even “baffle step” is actually the same – diffraction related - thing.

Quite any "horn" provides "directivity" and "wave guidance" and "loading" (to stay in that terms) - compared to any direct radiator - and even these are usually not any free from "directivity" and "wave guidance" and "loading".

Also its a matter of fact that quite any "horn" / "WG" / "diffraction alignment device" exhibit the same “sound field and time domain impacts" / "CMP behaviour" / "ASAR patterns" caused by diffraction – again, differences are merely gradual and are subject to whatever alignment.


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Last edited by mige0; 12th June 2011 at 04:49 PM.
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Old 18th June 2011, 10:19 AM   #10
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After the lead in, lets jump right into what gives us a good grip on how diffraction effects come out with different “guiding contours” involved.

The most easy plots to grab are sound field plots (sonogram), as they immediately show defects / roughness caused by interference between the original source and any second sources.

The main disadvantage of such plots is that we are basically restricted to display the outcome at a single frequency each plot, nevertheless I find it very informative if supplemented with a directivity plot .



Obviously the most simple contour/ curve is a radius (part of a circle).
At the same time its the only guiding boundary thats 1st derivation is constant (and thats 2st derivation is zero).




First bunch is about 25mm / 1" diameter piston driver with different round over into infinite baffle



with 25mm / 1" radius / round over into infinite baffle

Click the image to open in full size.

Click the image to open in full size.



of radius 50mm / 2"

Click the image to open in full size.

Click the image to open in full size.




of radius 100mm / 4"

Click the image to open in full size.

Click the image to open in full size.



of radius 200mm / 8"

Click the image to open in full size.

Click the image to open in full size.




Michael

Last edited by mige0; 18th June 2011 at 10:38 AM.
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