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Old 30th April 2012, 07:50 PM   #11
boris81 is offline boris81  United States
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Quote:
Originally Posted by puppet View Post
I've got a 2-way thing developing right now, so this will be a very interesting project to follow. Little if any of what I've done/noticed so far will apply directly to what you've proposed here but I'd still like to offer up a couple of points FWIW.

My bass is handled with a 0.47Qt driver (12") firstly on pure OB with minimal support below which evolved into full length wings @10" of depth, a narrower cap above for tweeter support and a bottom of roughly the same depth as the sides as the speaker is elevated off the floor about 10". Bass response was enhanced with the bigger wings ... wing resonance seemed to be confined to a small frequency area and was only really problematic at pretty high SPL.

The damping was tried as per various www resources (keysers thread, johnk, and many others) The thing, for me, that changed with all of the usual damping attempts .. just from a purely listening perspective now ... is that the speaker begins down the road to "sounding like a box". More damping, more box sound ... dead/lifeless .. especially in the mid range. Even just adding the full length wings changed the sound from pure OB to more like a box ... and it EQ's like that as well.

EGDE, at least, was not at all helpful in modeling an expected response with a "U" frame. My suspicion is that as side wings fold more toward the perpendicular ... the driver(s) sound characteristics, that you'd hear in a box alignment, become more "enhanced". I think the amount of damping might therefore become more room dependent than anything else as far as listening enjoyment is concerned. More of a "valve" for spacial level than anything.

Sadly, I don't have any hard data to share as I'm just getting deeper into this fascinating hobby. Good luck with your project boris81 !
Thank you for your input!
I'm afraid you are right that the amount of damping will have to be determined by experimentation. My idea is to start with a nice looking polar and continue from there. I have doubts about this method being able to hold a cardioid response for a wide bandwidth.

As for your design it sounds like you are working with a U-frame box. Note in JohnK's articles that a U-frame design develops a resonance above the dipole peak. He uses the U-frame only below the resonance point <100Hz. I doubt that U-frame will work well in the mids even when damped well. A damped lossy box with holes on the side behaves differently than a U-frame.

NaO U-frame
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Old 1st May 2012, 12:51 PM   #12
zmyrna is offline zmyrna  United Kingdom
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I have experimented with dipole rear wave damping in the past.
But my experience is limited to over 1500Hz, because the testing was done on my Heil AMT driver (Oskar Heil Syrinx speaker).
First of all reflections are not problematic for up to a meter of distance from the wall.
I think for lower frequencies you may go even lower.
At higher distance, perhaps because of reflection delay, the effect gets problematic.
Any damping that blocks the free air flow in the back of the driver has negative effects.
Best dipole damping that worked for me was a spheric shaped wool/sponge right behind the driver.
I am also thinking of wrapping my diy built by speaker cloth, giving it a pseudo-box look.
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Old 1st May 2012, 07:46 PM   #13
boris81 is offline boris81  United States
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Thank you everyone for the encouraging comments!

Let me address some of the concerns that were raised previously.

Quote:
Originally Posted by Radugazon View Post
But I am not so enthusiastic about the Le Cleac'h filter and it's implementation.

First, it's not a linear phase or whatever but only a "quasi optimal", more conceived by his author with in mind something easy to do and accessible to a majority of DIYers, than challenging the ultimate transient perfect.
Second point, finally it's not at all easy to do. There is a gap between the theory and it's application, the drivers have to be textbook linear, the Fc far from their natural roll offs.
In other words, the magic recipe of the spreadsheet needs a lot of measurements and a parametric EQ to comply the target...all this for at best a quasi optimal. This said for the sake of discussion, the important goal is only listening.

I have also the KX drivers on my PC, pretty amazing for a freeware. They are perfectly able to do the job.
The Le Cleac'h crossover is an topic on it's own and I would rather open a separate thread if there is enough interest. I made a 1st order Butterworth module for KX-Project, which cascaded with their LP or HP (at a Q-value of .71) gives a 3rd order Butterworth. Anyway, I have it working well already and it passes square waves but it's not perfect like you said. In any case, I'm flexible on the crossover ad I'd love to try different options as DSP makes it so easy.


Quote:
Originally Posted by RockLeeEV View Post
Love what you've got in mind.

I think you can do a good bit better. Dave Pellegrine at Parts Express has developed a waveguide for the CSS LD25X and some other tweeters for example. Love everything else though. The CA18RLY might just make more sense than the ER18RNX as a dedicated mid though.
Thank you for the suggestions!
I was not familiar with the CSS LD25X and after researching it, it looks like a great driver. Those waveguide experiments show some incredible results indeed! However, as "samadhi" already pointed out, it will not bring ground-breaking improvement over what I already have.
For all important parameters the CA18RLY and the ER18RNX are more or less the same. Except the ER18RNX is prettier. I believe the CA18RLY is targeted for sealed enclosures which makes no difference in my case. There are some very subtle differences that make the ER18RNX a better choice. For example the specs show better sensitivity for the CA18RLY but that's misleading because I don't have to do baffle step compensation and in my application the ER18RNX may gain a few dB.
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Old 1st May 2012, 09:22 PM   #14
boris81 is offline boris81  United States
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Default Vertical response

Quote:
Originally Posted by sumacSK View Post
Are you sure about those vertical reflections? Reading through some threads here at diyaudio left me with the impression that vertical reflections are worse than horisontal and always detrimental to sound quality (mainly in the lower mid bass region). OTOH delayed lateral reflections should be beneficial.. Maybe I should re-read the giant directivity thread again?!?
Quote:
Originally Posted by speaker dave View Post
No, I think you have it about right. It is much more important to deal with the veritical reflections than the lateral reflections, especially if the lateral reflections can come later in time. Floor ceiling and back wall reflections are hard for the ear to seperate out and end up being perceived as frequency response errors. Late lateral reflections add spaciousness to the sound.

David S.
There's definitely room for improvement with my design on the vertical axis. Let me elaborate my reasoning and the choices I made may become more clear.

Delayed horizontal(lateral) reflections add spaciousness and reinforce the stereo image but the off-axis frequency response must be similar to the direct one. This may or may not have to do with the fact that above 1600Hz we localize sound based on the level. In any case, successful Constant Directivity designs give evidence to that.

I'm not convinced that vertical reflections in that frequency region have a pronounced effect on sound perception. Evidence to that is the fact that almost all speakers out there are designed with severe nulls on the vertical axis. If the vertical reflections were indeed more important sideways D'Appolitos will be ruling the market

When speaking about lower frequencies I will agree that all reflections are detrimental. As the wavelength grows the room boundaries start to interact with the sound even before the waves are fully developed. The walls closest to the speaker begin to shape the sound. Reinforcements and cancellations develop both horizontally and vertically, at the same time. When I look at ripple tank simulations I find it incredible that we can hear anything at all!

Conventional speakers have omnipolar radiation at lower frequencies. They illuminate all 6 walls although the first 5 are arguably most important. In my design I'm making a priority to eliminate reflections from the floor and the back wall while reflections from the side walls are reduced. The most offensive reflections will be from the ceiling but I hope those will not be hard to equalize.
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Old 1st May 2012, 09:37 PM   #15
AllenB is offline AllenB  Australia
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We may not perceive vertical reflections well for imaging but they'll cause colouration, as will the horizontal reflections. Horizontal reflections usually are treated in a constant directivity design, by way of the directivity itself.

I don't agree that the ceiling reflection will be easy to equalise.
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Old 10th May 2012, 11:06 PM   #16
njoak is offline njoak  Sweden
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Hello Boris,

I usually don't write much on forums at all, but this thread is interesting enough to make me write my very first post here!

I am extremely tempted to try out the resistive-vents-approach to improve the horizontal directivity in an upcoming line array build, and in the course of researching this I found this thread. I will use 28 3.5" Vifa TC9FD18/08 as full range drivers in each speaker (cf. Roger Russels IDS-25), and would aim to match the radiation at frequencies below the beaming threshold, to the dispersion of the beaming region (or at least even out the differences). Hence, resistive vents from around 2 kHz and down.

A couple of questions:
Do you have any thoughts on box size/volume and effective vent area as of yet? Kimmo Saunisto seems to imply that he likes a vent area of about 1.2-1.3*Sd, but I have seen no discussion on the geometry of the vents, for instance how far back along the box the vented area should reach. I find it hard to understand the best geometry for wide-band venting. Have you looked at the midrange box of the Gradient Helsinki, by the way?

Also, I have to ask if you have thoughts on using felt material over the vents, again like Kimmo, or do you plan for regular stuffing?
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Old 11th May 2012, 05:05 PM   #17
boris81 is offline boris81  United States
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Quote:
Originally Posted by njoak View Post
Hello Boris,

I usually don't write much on forums at all, but this thread is interesting enough to make me write my very first post here!

I am extremely tempted to try out the resistive-vents-approach to improve the horizontal directivity in an upcoming line array build, and in the course of researching this I found this thread. I will use 28 3.5" Vifa TC9FD18/08 as full range drivers in each speaker (cf. Roger Russels IDS-25), and would aim to match the radiation at frequencies below the beaming threshold, to the dispersion of the beaming region (or at least even out the differences). Hence, resistive vents from around 2 kHz and down.

A couple of questions:
Do you have any thoughts on box size/volume and effective vent area as of yet? Kimmo Saunisto seems to imply that he likes a vent area of about 1.2-1.3*Sd, but I have seen no discussion on the geometry of the vents, for instance how far back along the box the vented area should reach. I find it hard to understand the best geometry for wide-band venting. Have you looked at the midrange box of the Gradient Helsinki, by the way?

Also, I have to ask if you have thoughts on using felt material over the vents, again like Kimmo, or do you plan for regular stuffing?
Hi,
unfortunately I can't answer any of your questions. I have no experience with this type of speakers and I'm perusing this project solely based on data published by Kimmo, Keyser, JohnK and others. Progress relies entirely on the outcome of my experiments. At this point I don't know if my proposed design will work as intended.

I have even less grasp on the resistive vent approach. If I understand correctly the resistive vent is not meant to absorb the sound energy but to restrict air-flow. In that application the box acts somewhat like a soft spring on the back of the driver (as opposed to a sealed box which would act as a stiff spring). I have no idea what the polar response of that configuration will be.

For my application I'm strictly after a cardioid response and I'm not set on any particular way to achieve it. I may remove the side and rear panels and not use a box enclosure. I'm gravitating towards something like this:
Adventures in cardioid

I wish I could give you more relevant information but I don't know much on the topic of absorption materials and vent sizes. What I would do in your case is start with a small-scale model and experiment with different configurations. Measurement equipment is essential but thankfully not difficult to acquire.
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Old 11th May 2012, 05:54 PM   #18
njoak is offline njoak  Sweden
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Quote:
Originally Posted by boris81 View Post
Hi,
unfortunately I can't answer any of your questions. I have no experience with this type of speakers and I'm perusing this project solely based on data published by Kimmo, Keyser, JohnK and others. Progress relies entirely on the outcome of my experiments. At this point I don't know if my proposed design will work as intended.

I have even less grasp on the resistive vent approach. If I understand correctly the resistive vent is not meant to absorb the sound energy but to restrict air-flow. In that application the box acts somewhat like a soft spring on the back of the driver (as opposed to a sealed box which would act as a stiff spring). I have no idea what the polar response of that configuration will be.

For my application I'm strictly after a cardioid response and I'm not set on any particular way to achieve it. I may remove the side and rear panels and not use a box enclosure. I'm gravitating towards something like this:
Adventures in cardioid

I wish I could give you more relevant information but I don't know much on the topic of absorption materials and vent sizes. What I would do in your case is start with a small-scale model and experiment with different configurations. Measurement equipment is essential but thankfully not difficult to acquire.
Hi,
And thanks for replying. I have the same feeling like you do, that it might take a lot of experimenting to tune something like this. In my case that might be exactly what prevents me from doing it. On the other hand, the fine results in the threads you link to stem from seemingly simple and straightforward means.

In any case, my intuition tells me the boxless/muffler approach is qualitatively equivalent to the box-with-sidevents approach, maybe with edge diffraction playing some role in the difference? Topologically they seem equivalent to me, at least. The effective distance the air needs to travel between front of membrane to vent of course also determines the dipole step frequency; i.e. the highest frequency at which you can expect to "cardio-ize" your response -- but I cannot quite get a feel for bandwidth versus vent geometry (yet).

Anyway, my goal would be cardioid dispersion as well, just like you. Hope you make progress, I would be quite interested to see it. Love your idea and your box design looks great! The thread is subscribed to.

Cheers,
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Old 11th May 2012, 06:48 PM   #19
boris81 is offline boris81  United States
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Correct, when it comes to a cardioid response the boxless/muffler approach produces similar results to the box with side holes that is stuffed with absorbing material.

I was thrown off by the term "resistive vent" because that is used in an Aperiodic Loudspeaker Enclosure Design for bass tuning. With resistive ports on the side of the box an aperiodic enclosure should theoretically produce some type of cardioid response. Unfortunately, I'm not familiar with any research that specifies how different parameters can be optimized to arrive at a desired response.
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Old 11th May 2012, 07:28 PM   #20
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Quote:
Originally Posted by boris81 View Post
I'm not convinced that vertical reflections in that frequency region have a pronounced effect on sound perception. Evidence to that is the fact that almost all speakers out there are designed with severe nulls on the vertical axis. If the vertical reflections were indeed more important sideways D'Appolitos will be ruling the market
Not following your logic here. I think you are saying all would be well if the floor reflections have flat response, or that if speakers send messy response towards the floor that proves it must be inconsequential. That isn't true. Floor and ceiling bounce energy, no matter what the spectrum, will arrive at the ear just enough delayed to give very significant comb filtering.

The issue is one of psychoacoustics. We are inundated with reflections from all directions. If they come from the sides our binaural hearing lets us separate them very well from the frontal sound. We perceive them as added spaciousness or image spread. Anything that arrives on the median plane (the plane halfway between the two ears) we will have a hard time seperating out. It will be combined with the direct sound as a nonflat, comb filtered composite.

Vertical directivity of any form is useful. Lateral reflections, if sufficiently delayed, are pleasant and add spaciousness. Toole writes a lot about this, you should read his book.

D'Appolitos should be vertical!

David S.
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