Semi Open Speakers

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Joe_Limon

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Semi Open Speakers
« on: 9 May 2020, 04:02 am »
So, something bothers me about open baffle speakers and I haven't really read much on it or really have experience with it to know how much of an issue it is/could be.

With my current understanding on the theory of open baffle speakers it seems beneficial primarily to low frequency drivers that typically emit omni directional sound.  However when I picture the setup in my head for higher frequency drivers I have a pretty big hesitation when it comes to reflections off of the rear wall.  Wouldn't sound emit by the rear of a driver once reflected off of the rear wall and travel back to the front of the baffle end up creating positive and negative interference depending on distances and frequency?  Would it not be better to capture/collect these waves?  Perhaps not a full enclosure, but a partial enclosure where the driver can breathe freely and all higher frequency emissions from the rear of the driver can be mitigated.

I was thinking about combining the same geometry that is used for cones in anechoic chambers to trap tones not low enough to be omni directional.  So I came up with a few potential semi open enclosure layouts and wanted peoples opinions.

The first one is rear ported and has a bias to one side and will emit 2nd order or greater reflections.



The second one is side ported and I believe will not reflect any sound out.



And the third is front ported and does not have a bias to either side.



diyman

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Re: Semi Open Speakers
« Reply #1 on: 9 May 2020, 06:42 pm »
If you haven't experienced OB speakers how do you know that there is something that will bother you?

The reflection off the rear wall is part of the experience and is highly desirable provided the delay is at least 6ms. Some people think it should be a little longer.

Your suggestion sounds like it might be a solution in search of a problem.

Also whatever structure you build behind the driver will have some sort of resonances. One of the purposes of OB is to completely eliminate those resonances, because they color the sound.  The best box is no box.


FullRangeMan

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Re: Semi Open Speakers
« Reply #2 on: 9 May 2020, 06:58 pm »
Mids and treble dont need enclosure at all only the baffle, for the bass under 200Hz I prefer a Bass Reflex box that allow the use of just one woofer, usually 15''.

Joe_Limon

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Re: Semi Open Speakers
« Reply #3 on: 9 May 2020, 07:54 pm »
Ok but why?

RDavidson

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Re: Semi Open Speakers
« Reply #4 on: 9 May 2020, 11:42 pm »
To test your theories, couldn't you build a pair of OB speakers and experiment with sound absorbing material on the wall behind them? Maybe you could experiment with the amount and/or placement of it to control the amount and amplitude of reflected sound reaching the listening seat. Could be a very informative (and ideally fun) experiment.

FullRangeMan

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Re: Semi Open Speakers
« Reply #5 on: 10 May 2020, 12:02 am »

This image looks a vented enclosure without resonant duct.

Joe_Limon

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Re: Semi Open Speakers
« Reply #6 on: 10 May 2020, 02:32 am »
That would be pretty easy to experiment with. Any sound insulation panel behind the OB speaker will have an effect on the harmonics the listener hears.  Even placing solid shapes behind the OB speaker will have a similar effect.

The one exception might be the front ported design.

diyman

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Re: Semi Open Speakers
« Reply #7 on: 10 May 2020, 04:16 am »
That would be pretty easy to experiment with. Any sound insulation panel behind the OB speaker will have an effect on the harmonics the listener hears.  Even placing solid shapes behind the OB speaker will have a similar effect.

The one exception might be the front ported design.

Why would you want to do that?  The point of an open baffle speaker is to have the sound reflect off the wall behind the speaker and mix with the front wave at the listener location.

You can do that experiment and call it whatever you want.  But it's not related to OB.

Joe_Limon

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Re: Semi Open Speakers
« Reply #8 on: 10 May 2020, 06:29 am »
The point of open baffle is to add extra reflections to the sound?  I get that I do not understand open baffle mid and upper range drivers.  But what on earth is the point to that? That sounds like purposely leaving out damping in a closed enclosure so you can hear the internal reflections.

diyman

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Re: Semi Open Speakers
« Reply #9 on: 10 May 2020, 07:17 am »
The point of open baffle is to add extra reflections to the sound?  I get that I do not understand open baffle mid and upper range drivers.  But what on earth is the point to that? That sounds like purposely leaving out damping in a closed enclosure so you can hear the internal reflections.

The purpose is to add size, depth, and realistic imaging to the soundstage thereby increasing the impression that you are at the actual venue and not just listening to sound coming from a pair of boxes in your room.  You apparently have never heard open baffle, so perhaps you should do that before continuing.

timind

Re: Semi Open Speakers
« Reply #10 on: 10 May 2020, 01:13 pm »
I did love my Bose 901s back in the 80s.

But seriously, what's the major difference between open baffle and direct reflective sound? Many of the big speaker makers built speakers with a driver or two on the back to get better ambience.

Joe_Limon

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Re: Semi Open Speakers
« Reply #11 on: 10 May 2020, 02:22 pm »
The purpose is to add size, depth, and realistic imaging to the soundstage thereby increasing the impression that you are at the actual venue and not just listening to sound coming from a pair of boxes in your room.  You apparently have never heard open baffle, so perhaps you should do that before continuing.

I get the hand wavy bit, but lack the direct logic as to how and simply gaining listening experience won't help with the logic side.  For example, if first order and second order reflections help build soundstage.  Why do people go to lengths to absorb or diffuse first and second order reflections?

Letitroll98

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Re: Semi Open Speakers
« Reply #12 on: 10 May 2020, 03:57 pm »
The difference, and what you're not conceptualizing, is time.  Early reflections distort and muddy the sound because the ear mixes them with the direct sound.  Later arriving reflections, over 10ms, don't confuse the ear-brain interaction and add to spaciousness.   So the goal is to minimize or eliminate early reflections.  The two main strategies for that are controlled directivity and room treatment.  For long period reflections on open baffle speakers you need only to move the speakers far enough away from the front wall to delay reflections longer than around that 10ms range because side wall reflections are largely cancelled by design.  Enclosed drivers operating above the Schrodinger frequency need to deal with side wall reflections.  Thus the advantage of OB mid and high frequency drivers.

Don_S

Re: Semi Open Speakers
« Reply #13 on: 10 May 2020, 04:11 pm »
Joe,

Which OB speakers have you heard?

nickd

Re: Semi Open Speakers
« Reply #14 on: 10 May 2020, 04:22 pm »
Quote
Why do people go to lengths to absorb or diffuse first and second order reflections?

The idea is to trick the ear/brain interface into thinking you are not in your smallish listening room but in a live or actual recording studio environment. Not an easy task. Just like when you look at a photo. Your eye can easily see lack of dimension and knows it’s a facsimile of a real event.

Absorbing reflection and positive bass room nodes are just a couple of tools. Diffusion breaks up reflecting sound and spreads the time arrival. Just tricking the ear/brain that that wall or surface may or not be there. The idea is to make the room disappear. Not easy to do. Open Baffle speakers with proper space behind them (3’ min.) and some diffusion are a powerful tool. The illusion can become very enjoyable.

Joe_Limon

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Re: Semi Open Speakers
« Reply #15 on: 10 May 2020, 05:29 pm »
Awesome thank you Letitroll98 that makes a lot more sense.

Rocket_Ronny

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Re: Semi Open Speakers
« Reply #16 on: 10 May 2020, 05:40 pm »
Yes. it's the 10 ms delay that is needed. Estimate each ms to 1 foot. So 10 ms would translate to 5' off the back wall. I personally like at least 8'. Going 5' or less, I would treat the wall so as to either dampen, or deflect, the rear wave reflections.

Another way to deal with open baffles close to a rear wall is to sit close to the speakers and have them spread out wide with a serious toe in. That way the rear wave is not bouncing straight off the wall directly to you, but shot around the room in a longer fashion. Also, since the sound on the sides of open baffles is nulled to 0 the wall in between the speakers will see next to no direct back wave.

Kind of like this picture of my system, just imagine the speakers 3' off the back wall.

Rocket Ronny





Here is a system set up and sound samples.

https://www.youtube.com/watch?v=BqzB6JkDBA8

https://www.youtube.com/watch?v=zUxOBPcw5uY&feature=youtu.be

https://www.youtube.com/watch?v=ocJnvay2c7Y&feature=youtu.be

https://www.youtube.com/watch?v=KdBHXtTEFq0&feature=youtu.be

Joe_Limon

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Re: Semi Open Speakers
« Reply #17 on: 10 May 2020, 06:27 pm »
Is the 10ms just a rule of thumb or is it also dependant on frequency? I.e. the higher the frequency the closer to the wall.

Is 10ms based more around the minimum phase angle shift at the lowest non omnidirectional frequency?

Rocket_Ronny

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Re: Semi Open Speakers
« Reply #18 on: 10 May 2020, 07:11 pm »
A general rule. But don't get hung up on all the different audio rules, experiment yourself.

It's more the mid and high frequencies that I find muddle the sound from early reflections.

What some people do with open baffle, and planar, speakers is to place them close to the wall for general living and pull them way out into the room for serious listening. I love open baffle speakers, and what I have right now is simply killer. Very few speakers interest me now a days.

Rocket Ronny

diyman

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Re: Semi Open Speakers
« Reply #19 on: 10 May 2020, 08:33 pm »
Is the 10ms just a rule of thumb or is it also dependant on frequency? I.e. the higher the frequency the closer to the wall.

Is 10ms based more around the minimum phase angle shift at the lowest non omnidirectional frequency?

It has nothing to do with frequency.  It's based on the speed of sound, which is entirely independent of frequency.

There are many papers on the subject and not everyone thinks the delay has to be 10ms, although that is certainly safe.  Some think that under the right set of conditions 6ms is adequate.   

The easiest paper to read is "Delaying Room Reflections: Constraints on Room Size and Loudspeaker Placement" by Richard Taylor, May 2013.

At the other end of complexity is Siegfried Linkwitz's very thorough AES Convention Paper "Room Reflections Misunderstood?" from October, 2007.

I suggest you start with Taylor, however.   It's much shorter and easier to read.  Both are very good.