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Open baffle speaker design

RayDunzl

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the dipole emission, unless placed in an anechoic chamber will create a maze of reflected wave, ie producing a reverberation effect,

I have dipoles - cross to the woofer is at 180Hz.

When I compare the impulse response of the dipoles with a more conventional speaker, the "maze of reflected waves" seems to be more a property of the conventional than the dipole.

This image infers about 10dB less reflected energy overall (blue) with the dipoles. At 27ms there is the double room length bounce. For the conventional, looks to me like reflections occur from many side surfaces.

When critically listening, I prefer the dipoles, as they present a much more focused image, loudest bounces from the direction of the "stage", without the extra high-level lateral reflections.


1544389250672.png


The room is a living space, counch TV, CD shelves, etc., not "treated".

This also shows in an unsmoothed frequency response - lots of cancellation hash on the conventional:

1544389774683.png


This is my interpretation.

@amirm will say "You can't hear that", my reply is "I hear something".
 
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Burning Sounds

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Spot on again, the dipole emission, unless placed in an anechoic chamber will create a maze of reflected wave, ie producing a reverberation effect, clearly distorting the original signal. Then, as people like harmonics added, some like reverb effects added to the music they listen. Question of taste, I guess.

You seem to be implying that only the dipole distorts the original signal. There are distortions added by box speakers - ported or sealed.
 

Don Hills

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... After explaining this, please help me understand the mechanism behind pressure-vessel bass boost below a room's lowest fundamental. Thanks! :)

Theoretically, "room gain" is simple. First, set some rules.
- The "room" is sealed and rigid.
- All frequencies are assumed to be below the lowest "modal" or resonant frequency of the room.

Under the above rules, SPL is simply the difference in air pressure in the room between the cone being "in" and the cone being "out". It can be calculated directly as the ratio between the volume of the room and the swept volume of the speaker - the amount of air it displaces. Therefore, a constant cone excursion results in a constant SPL, regardless of frequency. The attached Excel spreadsheet can calculate the theoretical SPL.

So where does the rising SPL as the frequency drops come from? Remember that in free space, to maintain a constant SPL as the frequency drops the cone displacement has to increase: 4x the excursion if the frequency halves. Under the above "in room" rules, 4x the excursion results in 12 dB more SPL. So a speaker with a flat free-air response will exhibit a rising response in a room.

Back to reality... real rooms aren't sealed and rigid. And they often have Helmholtz-style resonances at very low frequencies. So the amount of gain achievable may vary from almost none to quite a lot at some frequencies. There's also no hard and fast rule at what frequency it starts to become noticeable, beyond that in general the larger the room the lower the frequency.

In summary, "room gain" (what I call pressurisation SPL to separate it from above-modal SPL) is real, but don't count on it.
 

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Theo

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You seem to be implying that only the dipole distorts the original signal. There are distortions added by box speakers - ported or sealed.
That's not what I implied. Distortion level of speakers are the highest in the sound system. Bad closed/vented speakers will certainly distort more than a well designed dipole. For example, when a conventional speaker has a poorly engineered enclosure, it may vibrate in all directions... If the dipoles are focused enough, they may create less side waves than dispersive box speakers. It will also depend on the room itself. Only measurements can tell!
 

Cosmik

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Only measurements can tell!
Seems a bit random. Isn't it possible to have an intention and then design something to implement it, no measurements necessary?

Measurements are misleading - or at least ambiguous. If I set up a microphone to measure a speaker in a room, I'll get a certain imperfect result. I could then start to tailor that result by placing other speakers in the room, possibly wired in reverse phase. By manipulating the controls on a mixer, various filters, delay lines, I could probably get a perfect measurement from the microphone: ruler-flat response or perfect 'target curve' - whatever goal I have set for the measurement.

But it would sound terrible for the human listener - weird, with even weirder things happening in stereo and especially as they moved their head. This is only a slightly exaggerated thought experiment compared to the idea of metaphorically taking the backs off speakers to try to achieve the same thing.
 

DonH56

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Flat panels over most of their frequency range (above the deep bass -- depends on panel/driver size) generate a figure-eight pattern in front and behind with little radiation to the sides or top and bottom. To that extent they exhibit much less interaction with the room than conventional speakers. The caveat is that big honkin' back wave... And some designs, like Martin Logan, intentionally (by design) use a curved surface to add side reflections and provide a more "spacious" sound. The image is not a good but the overall sound is often preferred by listeners. Preference...

Historically flat panels were marketed as having lower distortion since the entire surface is driven, so a cohesive wavefront is presented with perhaps less modal artifacts in the drivers (panels), and excellent transient response due to the lightweight membrane. Whilst some of that has merit (I have been a Maggie owner since the late 1970's), their limited displacement means very high distortion with large excursions (10's of percent trying to produce loud deep bass IME), and those panels do exhibit modes (Magnepan uses those "Maggie Dots" to break up the modes; not sure what if anything ESLs do though Quad uses a concentric ring design to improve dispersion characteristics and such). I tend to think that for most listeners the magic is just in having such a huge radiating surface so the sound is more enveloping as they shift around. The lack of, or low-order, crossover is another plus when compared to many speakers that exhibit issues transitioning among drivers as you go through the crossover region, and some of that is simply the difference in dispersion characteristics with driver size.

I have very little experience with open baffle designs using conventional drivers. I piddled with a few way back when but, having Maggies already, never seriously. Probably Carver's ALS was the best example, or the upper end of the old Dahlquist DQ-10.
 
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I have very little experience with open baffle designs using conventional drivers. I piddled with a few way back when but, having Maggies already, never seriously. Probably Carver's ALS was the best example, or the upper end of the old Dahlquist DQ-10.

I suggest to audition some of the newer open-baffle dipole designs using conventional drivers.
The recent (last twenty years) offerings from Linkwitz in the Audio Artistry series and later in his own DIY-based systems would be good ones to evaluate.

Dave.
 

Soniclife

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I tend to think that for most listeners the magic is just in having such a huge radiating surface so the sound is more enveloping as they shift around.
When I demoed some MLs one of the few things I really didn't take to was how much the sound changed as I moved my head, something I have never noticed with normal speakers, I found it really distracting just leaning forward to pick up a cup of coffee for instance.
 

Sal1950

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When I demoed some MLs one of the few things I really didn't take to was how much the sound changed as I moved my head, something I have never noticed with normal speakers,
Really? Some are worse that others but they all change to some extent. IME
 

Soniclife

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Really? Some are worse that others but they all change to some extent. IME
Really. Even when I move from one part of the room to another, that measurements show a real change, I don't notice it, unless I'm really trying to hear it, and even then not much, my brain seems to process it out. I was surprised when I heard it on with the ML panels, as I was not listening for it.
 

Sal1950

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Really. Even when I move from one part of the room to another, that measurements show a real change, I don't notice it, unless I'm really trying to hear it, and even then not much, my brain seems to process it out. I was surprised when I heard it on with the ML panels, as I was not listening for it.
Were you speaking of tonal balance or imaging?
 

Sal1950

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If you listen to some music with heavy bass line you should be able to hear dramatic changes in balance as you move about the room. Changes may become less obvious as the frequency rises but again that will be largely dependant on the speakers dispersion character..
 

Soniclife

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If you listen to some music with heavy bass line you should be able to hear dramatic changes in balance as you move about the room. Changes may become less obvious as the frequency rises but again that will be largely dependant on the speakers dispersion character..
Unless I'm listening hard for it I don't hear it, same for the off axis rolloff from tweeters. My brain is lying to me, but in a beneficial way this time, hence I was so surprised to find the ML panels distracting when I move even small distances.
 

Burning Sounds

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Unless I'm listening hard for it I don't hear it, same for the off axis rolloff from tweeters. My brain is lying to me, but in a beneficial way this time, hence I was so surprised to find the ML panels distracting when I move even small distances.

Most likely due to the MLs polar response (and setup in the room) as @Sal1950 says.

As an owner of both panels (Maggies) and dynamic (LX521) dipoles one of the big differences seems to be in polar response where the Linkwitz speakers are IMO considerably superior. There is next to no shifting of image or tonality change when moving from the sweet spot. The Maggies still have it when it comes to sheer scale though, although not by much and that's really only the case if I use the LX521s bass units with the Maggies.

I've recently acquired a used Maggie CCN2 centre speaker so I now have a 5.1 Maggie setup - (MG2.5r fronts and my old MG1s as sides - these two sets of speakers have a combined age of almost 70! :D). Envelopment is definitely the correct term!
 
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A very complicated polar response on large panel speakers. Multiple lobes....adjacent lobes 180 degrees out....large acoustic source....etc, etc, etc.

The LX521 addresses many of the issues, but Linkwitz would have preferred to make the acoustic source even smaller. Unfortunately, the physics always stops good ideas, eventually. :)

Dave.
 

Soniclife

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Most likely due to the MLs polar response (and setup in the room) as @Sal1950 says.
Thinking again about this it might be the crossover from the cone bass driver to the panel, which is around 500hz in the models I was listening to, which would have very different polar responses.
 

RayDunzl

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MartinLogan - 7 positions across the couch, 1/3 smoothing, stereo output, mono sweep signal, two years ago:

Black - center
Gray - +/- 12"
Orange - +/- 24"
Green - +/- 36"

1544641841568.png



---

I'd probably assign the defect to comb-filter more than anything else.

Taking the two position +/- 12" from center (gray), 1/48 smoothing, center (red) for reference

Looks like classic comb filter from unequal distance to speakers as you move away from the sweet spot, not a dispersion problem (lobing dispersion of panels), in my opinion.

I'll have to repeat this with the more dispersant and scientifically approved little JBLs...

1544642183957.png


The 48Hz dip is a phase anomaly from the room - the bass goes out of phase at that frequency at the listening position. Bad setup, but, hey.

Comb filter:

1544642444992.png
 
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Sal1950

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MartinLogan - 7 positions across the couch, 1/3 smoothing, stereo output, mono sweep signal, two years ago:
Amazing how well acourate is smoothing in the 100 - 1K range for the center listening position!
 
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