[Joe_Limon]

Here is my thinking going into that question.

For a given distance from the wall, the time delay on the sound will be the same, however, with higher frequencies, the next wave will change phase angle to different degrees.  While a 1" shift toward or away from the wall will work out to about a 1 degree phase shift at 20Hz, at 200Hz it will be 10.7 degrees, and 107 degree phase shift at 2kHz.

[diyman]

I understand what you are saying, but I believe you are overthinking this at the physical sound wave level.

The reality is that this is a psychoacoustic matter.  The integration of the two different signals, direct and reflected, occurs in the brain, not in the ears.  You might want to read up on the Precedence Effect.

[RDavidson]

Interesting stuff. This conversation has me thinking, if one bought OB speakers but couldn't move them out into the room far enough to get the proper reflective delay effect (with distance alone), could partial absorption behind the speakers help simulate the desired effect? I'm thinking about something like an open weave cloth curtain on the wall behind the speakers, not thick foam or anything like that.

[Rocket_Ronny]

I have placed pillows and comforters behind speakers and has helped.

Rocket Ronny

[diyman]

Interesting stuff. This conversation has me thinking, if one bought OB speakers but couldn't move them out into the room far enough to get the proper reflective delay effect (with distance alone), could partial absorption behind the speakers help simulate the desired effect? I'm thinking about something like an open weave cloth curtain on the wall behind the speakers, not thick foam or anything like that.

I don't think this would give you the effect desired from open baffle.  You don't want to get rid of the sound emanating from the back of the speaker and reflecting off the wall,  you simply want to delay it.

[Joe_Limon]

You could get the delay by creating a megaphone like folded horn.

[diyman]

Sure.  Along with a tremendous amount of distortion.

If you are really interested in open baffle there are plenty of well-established resources you can read to help you out.  I suggested a few earlier.

But it’s not really clear that you are truly interested in OB, or have even ever heard it.   So if you have a genuine interest in exploring OB for your own enjoyment purposes there are plenty of people here that I’m sure will help you out.

But if you just want to keep suggesting wild ideas I doubt that many people here are going to take you seriously or pay much attention.  Your call.

[Letitroll98]

Personally I like that Joe is coming up with crazy ideas.  It makes everyone think about what is actually happening and we're getting some good information.  And it's not like Joe is not listening to the answer when he asks a question, he asks until he understands, for the most part, then moves on to his next crazy question.  I say keep em coming, it helps all of us, and with the quarentines what are you doing anyway.

Which leads us to the folded horn.  No.  One, it's now no longer an open baffle design.  And two, the wavefront would still be measured from the front of the baffle no matter how long the signal bounces around inside the cabinet.

Lastly, when someone around here says "your ears" it's commonly accepted as referring to the ear/brain interface.  We all know ears don't process sound on their own.

[JLM]

Appreciate these thought experiments.  Please note:

All deep bass sources project in all directions (based on the laws of physics - size of sound waves, size of the baffle, and properties of air).  That's where the need for a baffle step compensation comes in, to boost bass output at lower frequencies where sound waves wrap around the cabinet. 

Only the highest frequencies "beam" enough to be thought of as a purely directional radiator. 

Diffusion is also based size of sound waves, distribution of the size of recesses, and sonic opaqueness of the diffuser material.  Most manufactured diffusers are not sonically opaque, and many have a poor physical design.  Diffusers that are up to 7 inches deep can diffuse down to roughly 1,100 Hz.

Most materials marketed as sound absorbent are ineffective, especially open cell foam.  Buy only after reviewing the data.  Different material absorb better at different frequencies.  Owens Corning 703, high density fiberglass is probably the best, and is even effective into bass frequencies (few are).  Thin sheets are useless, look for at least 2 inches. 

Agree with Letitroll98, the ear needs 11 millisecond (ms) delay to avoid imaging confusion.  That translates to 10ft extra travel distance (speed of sound is 1130 ft/sec), thus the 5ft rule for spacing speakers away from front wall. 

Do disagree with Letitrol98 about the distance of the folded (bass) horn.  The relative distance sound travels from each voice coil is what matters in reference to the needed 11 ms delay, so the route through the horn counts.  But we aren't discussing bass versus mid/treble.

If this news isn't acceptable to you, suggest using headphones.  Sorry it's just those pesky laws of physics and how the ear works. 

[Letitroll98]

Thanks for the reply JLM, but I'll continue to maintain that I'm correct about the folded horn.  But I'm going to start with an aside.  The delay we're talking about is a reflection off of a boundary to the listener's ear compared to the direct sound from the speaker to the ear.  So the delay is not measured back to the speaker, but rather to the listener, and is measured as the delay from direct sound from the speaker reaching the listener.   So back to the folded horn, you could bounce the sound around inside the speaker for two days and the boundary delay would be exactly the same.

[Joe_Limon]

First, this thread isn't about OB it is about semi open baffles.

Second, I know folded horn designs sound terrible due to how they distort sound.  This might not be an issue however as you don't need to create a horn shape for one, and secondly, this isn't modifying the direct sound waves you are interested in hearing. This is modifying reflected background noise.

Third. The folded horn would create nasty resonance patterns as sound waves reflect back to the driver... You would need to have the reflective surfaces occur at a gradient of distances to smooth out the harmonic resonances.

Also, if anyone doesn't like thought experiments, you don't have to read the thread.

[Letitroll98]

I may be misinterpreting your diagram.  As drawn it appears there's an enclosure on the front of the driver (a).  So there's no enclosure at the front of the driver?   In that case dump the horn and move the speaker further into the room.

[Joe_Limon]

The last image is just a graphic I found showing how a megaphone works.  The thought was to reflect sound before emitting it in order to increase the phase delay behind the speaker.

[diyman]

First, this thread isn't about OB it is about semi open baffles.

Second, I know folded horn designs sound terrible due to how they distort sound.  This might not be an issue however as you don't need to create a horn shape for one, and secondly, this isn't modifying the direct sound waves you are interested in hearing. This is modifying reflected background noise.

Third. The folded horn would create nasty resonance patterns as sound waves reflect back to the driver... You would need to have the reflective surfaces occur at a gradient of distances to smooth out the harmonic resonances.

Also, if anyone doesn't like thought experiments, you don't have to read the thread.

There is no such thing as a Semi Open Baffle Speaker.  It is a term you have made up, but has no real meaning in the world of various speaker designs.

There are of course Open Baffle speakers, and the science behind them is very well understood.  I provided several references to papers by experts on the subject.  And there are more papers I can add if that’s not enough.

But you start out by immediately attacking OB without ever hearing it or really understanding the principles behind it.  Here is your very first sentence:

“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.”

You imply that there is some mysterious problem with Open Baffle as we know it today, despite the fact that many people enjoy it much more than boxed speakers.  And you haven’t really identified this mysterious problem, but nevertheless you are coming to the rescue with some new ideas.

I’m sorry Joe, but in engineering you first have to define the problem you’re trying to solve.  You haven’t done that at all.   So, as I said before, what you have proposed are really just possible solutions in search of a problem.   

If you are really interested in advancing the state of the art, then it seems logical that the first thing to do is experience OB and try to identify any problems that you can find.

[Joe_Limon]

If you are going to be pedantic could you at least try being constructive at it rather than straight up hostile?  Maybe suggest a better term? Or are you just salty for me suggesting this thread wasn't about straight up OB design despite the title and despite me accurately placing it in the enclosures circle?

The quote you are using is of me actively disarming people saying hey, I am not perfect. But lets talk about this subject.  I mean sure you can use it to criticize me, but those criticisms don't really mean much because I have already used them on myself.

As to semi open baffle speaker.  I still hold the term relevent because what I am discussing is novel, it is asking about what sorts of designs can be utilized to minimize/eliminate pressure build up behind the driver whilst dealing and understanding the interactions of the acoustic waves being emit.

[FullRangeMan]

Joe is not having any help with these latest provocative posts.

[diyman]

Joe,

If you think that you have a way to improve performance in open baffle speakers that’s great.   But I suggest that first you take the time to study and understand the current state of the art as presented in numerous documents by well-established experts in the field.

After that if you can present a cogent argument regarding your new ideas I think people will be interested.  But short of you identifying a specific problem or issue with OB, I see no value in considering your proposals as a solution to a non-existent problem.

Now if you just want to say here are some ideas for a new type of vented speaker, that’s different.  Go ahead and make your arguments why it’s better than any other current vented design.  People will probably be interested in hearing about it.

[Letitroll98]

As I noted before, it's fun to throw around off the wall ideas and see where it goes.  But Joe, you can't bitch when you get the answers.  No one's being pendentic, they're being pretty cooperative giving you good information.  It might be hard to find a store carrying OB speakers in your area, most are diy anyway, but there's a large magnaplanar dealer network and you can get a pretty good idea about how OB speakers sound by listening to planars.  I don't think you'll find the phase problems you're thinking about.

[Joe_Limon]

Letitroll98 yeah most people here were helpful and for that I am grateful.  My pointed response was toward diyman who seemed to get offended by the term semi open baffle.

Diyman I understand now. I offended you when I asked about OB speakers and questioned their efficacy in relation to mid and upper range frequencies.  However the questions I asked were logical and the answers that I got back were reasoned.  I suggested that reflections would mess with the frequency response and guess what, I was right.  The saving grace that I hadn't considered was that the time delay could be filtered out sub consciously provided the difference was great enough.

I am still unsure as to whether placing shapes behind the open baffle to direct sound waves could improve the sound or not. Further delaying the reflection does sound like it would make it easier to separate the background noise.  Alternatively one could also absorb or scatter or some combination of the three.  All without closing in the driver thus keeping it SEMI OPEN.

[RDavidson]

I don't think this would give you the effect desired from open baffle.  You don't want to get rid of the sound emanating from the back of the speaker and reflecting off the wall,  you simply want to delay it.

Right. Understood. That's why I'm curious if an open weave material would "simulate" the delay. It wouldn't absorb all the sound waves but would slow them down from reaching the back wall. Know what I mean? Again, my thinking is this could be useful to folks who want open baffle speakers but can't get them out into the room far enough from the back wall.

Let's keep the conversation rolling. It's good to have ideas about things even if you don't fully understand the science. Exploring and learning is always a good thing. Speakers everywhere have creative solutions to all types of acoustic problems. Some are better than others. Some are more science based than others. They run the gamut. I appreciate the creative thinking here which will always invite critical thinking with respect to applied science and engineering. People are very passionate about all this stuff. Stay calm.