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Multi-Way Conventional loudspeakers with crossovers

Baffle Diffraction
Baffle Diffraction
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Old 8th January 2018, 05:07 AM   #1
Oneminde is offline Oneminde
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Default Baffle Diffraction

I am not going to go into depths of describing and talking about what baffle diffraction is, I am sure you are more than capable to do some basic research on the topic. The reason why I started this thread is do debate and get input on something I've been thinking about for quite some time. I just haven't gotten around to doing serious tests with it.

This is more a technical aspect of cabinets than a visual one, but a cabinet is more than a home for the drivers, its a furniture, its pleasure and escape to another world and so naturally, we want the loudspeaker to sound just right and this goal is achieved in different ways. Some with driver technology, some with size and numbers of drivers and some is done with the cabinet shape. Namely the baffle "interaction" with the soundwaves being produced.

Ideally, there is only direct sound. Ideally we listen only to the recording without reflective sound being added, and this is a huge challenge to accomplish for any loudspeaker producer. The best shape is a sphere with no section of the baffle outside of the driver itself, and while there are some manufacturers that use this to their benefit, aka produce more direct sound. Traditional cabinet solutions with a flat surface are not so lucky and usually deal with the issues by means of narrow designs - very little of the front outside the driver frame, others add beveled edges to smooth out the edge diffraction and while beveled edges is a good solution, you are still left with the flat front.

So lets attack the flat front and issue #1: Its made of materials that do not absorb sound. Its often made of hard materials and the latest trend is using aluminum which is from this perspective, even worse than wood materials. We could go with a cylindrical baffle but we are not, we are sticking to a flat surface because its easier and simpler AND cheaper, so now we arrive at the question of how do we address the diffraction for this surface.

Well, as I mentioned, you need to absorb the reflected sound. Just think about it. What do virtually all loudspeaker manufacturers do inside the cabinet, they use damping materials, why, to absorb internal reflected sound. Why, because the cone material is transparent to sound and to have two conflicting notes being played almost at the same time creates distorted and ugly sound. No one is questioning the use of damping materials on the inside, because it works. Yes yes, there are other hardware solutions to aid in this, but that is a different debate... remember, we are dealing with what and how the front is dealing with the sound.

So we accept internal damping because we need to - unless you are designing with open baffle, well, good for you, no need to brag about no issues ... LOL.

Are you paying attention ? can you see where I am taking this debate towards ? no / yes... Okay: Why not use adequate damping materials on the front ?

SAY WHAT NOW... yes, instead of a hard and flat surface that by no stretch of the imagination can absorb or heavily dampen the reflected sound, you create a soft surface, soft enough to take care of the critical section which is from Low Midrange to Upper Midrange (250-4kHz) and perhaps the Presence (4-6Khz), the bass is what it is, difficult to control.

My question to you is: Why are established manufacturers not doing this, is it a limitation of visual appeal ? Is it tradition ? Lack of imagination ? Lack of using correct materials ?

Now you go, what do you think.

- Oneminde -

Last edited by Oneminde; 8th January 2018 at 05:25 AM.
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Old 8th January 2018, 05:21 AM   #2
wintermute is offline wintermute  Australia
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Baffle Diffraction
I was planning on putting felt on the front of my mtm's under the grills. But never got around to it. Duntech do it (at least around their tweeters). I was going to use real wool felt.

Something to revisit!

Tony.
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Old 8th January 2018, 05:45 AM   #3
Mario Pankov is offline Mario Pankov  Europe
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Baffle Diffraction
Constraint number 1 should be aesthetics I think. Imagine a set of 40 grand speakers with some loose felt that look like a garage sale carpet at their front. I think a more doable approach is to use a baffle with irregular surface, like this Scallops - 3D Wall Panels (btw, thats a premade MDF sheet). Hard pressed wool may also look good but not sure how much it would absorb if made dense. Generally, it is much better to put effort in edge treatment as this is what creates smearing by additional diffractionary waves.

Last edited by Mario Pankov; 8th January 2018 at 05:47 AM.
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Old 8th January 2018, 06:34 AM   #4
Nico Ras is offline Nico Ras  South Africa
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In the 60/70/80s most speakers had acoustic foam under the grill cloth that lay against the baffle. Then came the naked speakers and looks like we will eventually return to how speakers where made then. There was a reason for properly designed grills, but the youngsters liked the hotrod look of open speakers rather than the sound.
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Old 8th January 2018, 07:05 AM   #5
LightBit is offline LightBit  Slovenia
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Because it is ugly.

Diffraction Doesn't Have to be a Problem
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Old 8th January 2018, 08:13 AM   #6
picowallspeaker is offline picowallspeaker  Italy
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Diffraction is a phenomenon that happens at the edges, be those the shape of the surround ( the suspension, if prominent ) or the basket, that's why most people prefer to mount the drivers flush. But that happens after the wave had formed, before we can see that the wave travels ( slips) on the surface, if present. I might add that other than spherical, a cilynder offers no surface to let the wave "grip" to it. Oh, but the measures tell that the impulse response is worse ...
And about felt, which is the only material in nature that has some real acoustic properties, it is not ugly at all ( of course if it's grey and it is modeled like that...whaaat ? it is cut at 90° angle !?! Didn't he consider the diffraction at angle ? )
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Old 8th January 2018, 09:16 AM   #7
planet10 is offline planet10  Canada
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Baffle Diffraction
Quote:
The best shape is a sphere
On the outside, but research since Olson has shown that a teardrop is even better. On the inside a sphere is a not good shape with 1 strong internal standing wave.

Th eproblem with lining the damping material is that it needs to be “deep”. With an ideal material the sound needs to travel thru something like a quarter wavelength of it to be effective.

Further, diffraction has different behaviours as the frequencies change. At higher frequencies we take about edge diffraction. At lower frequencies we talk about baffle step (diffraction). It is a case of how big the discontinuity is compared to the wavelength.

Damping can be effectve at higher frequencies — felt can often be seen around tweeters, but at low frequencies not so much.

dave
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Old 8th January 2018, 09:37 AM   #8
Juhazi is online now Juhazi  Finland
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Two quick and dirty simulators

- The Edge Tolvan Data for Windows
- BDBS Loudspeaker Design Software requires Excel

Edge is very simple and uses flat baffle. Baffle shape is free, number and diameter of drivers is free, measuring distance and angle is free (up to 89¤). Closed box or open .
BDBS can simulate floor and front wall too. Only one driver in a rectangular baffle/box or open baffle.
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Last edited by Juhazi; 8th January 2018 at 09:39 AM.
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Old 8th January 2018, 10:27 AM   #9
kimmosto is offline kimmosto  Finland
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Quote:
Originally Posted by Juhazi View Post
...and angle is free (up to 89¤).
..but the result is very wrong - such as with The Edge. Both have diffraction model which is valid for axial response, about +/-15 deg. window. Please try to remember this while advertising quick & dirty tools.
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Old 8th January 2018, 10:42 AM   #10
TMM is offline TMM  Australia
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My understanding (someone correct me if wrong):

Baffle edge diffraction is caused by the abrupt change in acoustic impedance that occurs at the edge of the baffle. As a result, the sound wave propagating across the baffle is both reflected and diffracted when it meets the edge of the baffle, with the ratio of reflection/diffraction at any given point being related to the geometry of the baffle, the location of the speaker driver on the baffle and the frequency. The result is 6dB attenuation at low frequencies due to total refraction and nulls occurring at high frequency where almost total reflection occurs. Depending on the frequency, the reflected sound can be in-phase with the directly radiated sound from the speaker driver resulting in 0dB at the listening position. Otherwise the reflected and direct sound will have a phase and amplitude difference - at certain frequencies they will be 180degrees at of phase and cancellation will occur resulting in a null in the frequency response.

A circular baffle with the driver mounted dead centre and the listener positioned dead centre on-axis is the worst possible baffle configuration because the reflected sound from every part of the edge will have the same phase, ensuring the maximum possible nulling. A rectangular baffle is better because now you have different distances from the driver (and listener) to various points on the edge of the baffle, thus the baffle diffraction effects from different parts of the baffle become randomised and cancel each other out, resulting in a smoother response. Mounting a driver off-centre on the baffle or listening off-axis can smooth the response for the same reason. A program like
'The Edge' can simulate this. That said, the response achieved, even if smooth will only be valid for one listening position. If you change the listening position the baffle diffraction effects change and you may have a rough response again.

Rounding over the edges of the baffle makes the change in impedance more gradual thus reducing the amount of reflection. The radius of the round-over must be sufficiently large compared to the wavelength thus a really big round-over is required to reduce the severity of nulls at lower frequencies. Generally the bigger the radius the better. The most extreme case of a roundover is spherical enclosure, which can be considered a circular baffle with the round-over having the same radius as the circular baffle. Roundovers can also be aesthetically pleasing. Chamfers/fillets are also quite effective, such as the ones seen in a lot of Troels Gravesen's cabinets e.g. Ellam-FLEX

Attaching foam/felt to the baffle between the driver and the baffle edge adds attenuation so that the amplitude of the soundwave is smaller when the reaches the edge of the baffle thus reducing the effects of edge diffraction. A small amount of reflection and refraction will also occur at the location of the foam since it itself is an acoustic impedance discontinuity. Like a roundover the effect of the foam is related to it's thickness and the length of the sound wave so it is only effective at high frequency. The problem with foam is that it looks ugly!

One final solution which may not be very practical, is to just make the baffle HUGE. Imagine that your baffle is so large that the distance between the edge of the baffle and the listener is many times the distance from the speaker driver to the listener. Since sound from a point source propagates as a spherical wave, 6dB of attenuation occurs which each doubling of distance. If the sound propagating from the edge of the baffle towards the listener is much attenuated compared to the sound coming directly from the speaker driver then it won't have much effect on the frequency response at the listening position. If we consider 24dB to be sufficient attenuation (a maximum null of around 0.5dB) then the speaker driver must be 2^(24/6) = 16 times closer to the listener than the edge of the baffle. I use this effect to make measurements of speaker drivers on a test baffle. The distance between the microphone and the closest edge of the baffle is around 60cm, and the distance from the microphone to the speaker driver is only around 5-15cm. The result is that I get a frequency measurement which is most influenced by the driver itself and not so much the test baffle. How close you can 'listen' to a speaker driver depends on how large the cone/dome is. If you get too close now you have distance variations between the listener and different parts of the cone, and that results in phase differences therefore cancellation/nulls.

As a final note, remember that a speaker designer may have compensated for a baffle effect in the crossover. So by altering the baffle of a speaker you may need to alter the crossover to suit.

Last edited by TMM; 8th January 2018 at 10:56 AM.
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