Diffraction

[454Casull]

How bad is it really, e.g. on a speaker baffle with sharp 90 degree edges? Let's assume that the response where the listener is located is flat - does diffraction still negatively affect sound quality?

And why does diffraction not seem to affect the Pluto's woofer, even though centered-on-cylinder has one of the most egregious levels of diffraction?

Pluto specs

Is it because the measurement distance is too small? With a woofer that size, somewhere just over 1 kHz the response should show more pronounced peaks and dips... which are not visible.

[planet10]

Just found this Understanding Cabinet Diffraction.pdf - File Shared from Box.net - Free Online File Storage (not read it yet)

Edge diffraction has different properties as the frequency goes down. At high frequencies there is time delayed reradiation transitioning to baffle step loss at low frequencies. What is high or low frequency is a function of edge sharpness (round over) and baffle width. Even at the baffle step around over or champher size affects the ripple in the baffle step transition.



For some real world measures of how big to get how low: "Acoustic Diffraction: Does It Matter?" James Moriyasu (Feb 2003 aXp). Its been a long time since i read it, but what i took away was that the bigger the better and that a champher is almost as effective.

As a 1st hand example, I have both Mar-Ken12t and CGR Mar-Ken12 here. Basically the same box in different formats, the latter with a big champher, and the former with a token champher. It is pretty easy to hear the difference.

dave

[454Casull]

I have seen this before - I understand the shape affects the response. But, if the response is then flattened, how does the speaker sound?

[planet10]

What do you mean by flattened? It is not possible to EQ out edge diffraction, one might be a bit more successful with the ripple at the BS but i doubt it.

dave

[6.283]

Originally Posted by 454Casull

And why does diffraction not seem to affect the Pluto's woofer, even though centered-on-cylinder has one of the most egregious levels of diffraction?
Is it because the measurement distance is too small? With a woofer that size, somewhere just over 1 kHz the response should show more pronounced peaks and dips... which are not visible.

Look at the 3rd response plot from the top.

Pluto electronics

There is your baffle step and diffraction ripple. But that is measured from the top (0°). All this diffraction does not even ocurr at 90° (listener direction), which is not shown here. But that is typical for diffraction. The greatest response changes are mainly on the main sound propagation axis.
As a result, the listener is only exposed to it in vertical direction where we are less sensitive and the baffle step peak is even already in the stop band (>1KHz) where the response is down > 10dB.
Here is another good read.

Frequently Asked Questions

[whgeiger]

The figures shown here come from [1] and the paper [2] by the same author.

Regards,

WHG

[1] Acoustical Engineering,” Harry F. Olson, D. Van Nostrand Co., Inc., 250 4th Street, New York 3, NY 1957
BKPA1 - Acoustical Engineering

[2] Direct Radiator Loudspeaker Enclosures
Even though a direct radiator loudspeaker mechanism produces a uniform acoustical output with respect to frequency the diffraction effects of the enclosure may introduce wide variations in the frequency response. To determine the diffraction effects, a small direct rdiator loudspeaker mechanism was designed and constructed to deliver uniform sound output with respect to frequency. This loudspeaker mechanism was mounted in twelve enclosures of different shapes and the frequency response characteristic was determined experimentally. The experimental results agree with the theoretically predicted performance in the cases where the performance can be determined by theoretical considerations. A cabinet has been developed which exhibits a smooth frequency response characteristic and still retains an exterior which is acceptable from the standpoint of appearance.
Author:Olson, Harry F.
Affiliation:RCA Laboratories, Princeton, NJ
JAES Volume 17 Issue 1 pp. 22-29; January 196
AES E-Library Direct Radiator Loudspeaker Enclosures

[sreten]

Originally Posted by 454Casull

And why does diffraction not seem to affect the Pluto's woofer, even though centered-on-cylinder has one of the most egregious levels of diffraction?

Pluto specs

Is it because the measurement distance is too small? With a woofer that size, somewhere just over 1 kHz the response should show more pronounced peaks and dips... which are not visible.

Hi,

Its because to see it in action requires a small source compared to the
baffle, such that path lengths to the edges are quite similar and reinforce.
(Why the Olson studies are as symmetrical as possible, with a small source.)
(Why you offset midranges and treble units, to vary the path lengths.)

A large pistonic source on a similar sized baffle has very varied path lengths
which smooths the effect leaving the 6dB baffle step effect + usually a mild
hump of around 2dB above that before settling to +6dB.

rgds, sreten.

[Charles Darwin]

I was under the impression that baffle edge diffraction and baffle step compensation are two different problems with different solutions.

Baffle edge diffraction can largely be fixed by rounding over the edges. It affects tweeters.
http://www.silcom.com/~aludwig/images/diffdem.gif

Baffle step compensation affects bass or low mids because the speaker goes from full space to half space radiation.
Can be fixed in the xover or by soffit mounting the speakers.

[badman]

One note, often missed, on roundovers-

They're useful even when they're smaller than ideal. The reason is, trace the rays- the further from 90 degrees the angle of incidence to the baffle, the longer a given roundover/bevel takes to complete. a 1" roundover could look like 3" if it's the portion on the bottom of a floorstanding 6.5" "average" tower (bottom being far from the midwoof)

[planet10]

Originally Posted by whgeiger

The figures shown here come from [1] and the paper [2] by the same author.

Don't know about the paper [2] but the images i posted were scanned from pages 22 & 23 of one of my copies of Acoutical Engineering.

dave