Are you (open) baffled yet?

[Svante]

Konnichiwa,



Well, I have been thinking about this (Norbert too and I cribbed some ideas from him).

First, if the baffle is in "free air", meaning with a substantial gap between baffle and floor, we will get fairly little effects from either sidewalls, ceiling or floor, simply because all these surfaces are in a "null" of the response. So only the sound radiated from the rear of the dipole becomes relevant to the room interaction.

Are they really in the "null"? The null is 90 degrees, but the possible path of reflection is maybe 45 degrees for the typical room. At 45 degrees the dipole has lost only 3 dB down. This would make a substantial contribution, IMO.

What will happen is that the sound will be reflected back into the room with virtually no attenuation at low frequencies.

The rear output from most cone drivers plumets above around 1KHz with a 2nd order function down to a shelf quite notably below the nominal output, perhjaps 12 - 20db lower or continues to drop (depends upon the construction of the driver). Further, the higher components of the rear output become progressively more directional, which together with a slight slant from the baffle and possible toe in ensure that reflected rear radition is quite a bit down at higher frequencies. So the first element for modeling would be a lowpass for the rear radiation at 1KHz.
[/B]

Ok.

Secondly, the distance to the rear wall will delay the rear soundoutput of the dipole and the added distance will further attenuate the outpuit. So if the rear wall is 50cm behind the Speaker and we measure at 1m distance the SPL ofg the rear radiation arriving at the virtual mike will be 6db attenuated compared to the direct radiation. As long as the distance in pathways is smaller than around 45 degrees or 7/8 wavelength (in our case 0.875cm or 400Hz) the rear radiation will re-inforce the front output with an almost full addition achieved. With 180 degrees or 1/2 of the wavelength (50cm or 690Hz) we actually subtract the sound output of the rear, but as this already 6db down it's not much of a suckout.

[/B]

Let me see if I get this:
I'd say that for LF the reflection will counteract (cancel at 0 Hz) the front radiation, at half a wavelength delay (wavelength=2m or 170Hz) they will add up, at one wavelength (1m, 345 Hz) they'd cancel and again at 1.5 wavelengths (0.67 m, 520 Hz) they'd add again and at 690Hz they'd cancel again. Given absorbtion at the reflection and a slightly longer pathway, the cancelation is probably not complete. Did I say the same thing as you?

Also, the relative importance of this rear wall would increase in the normal listening position, which is likely to appear beyond 1 m.

So all in all once we are far enough from the rear wall the rear output will merely cause some mild lift in the < 500Hz and put a few more wriggles above that. The LF effect should be easily predictable.

[/B]

And I'd say *around* 520 Hz.

Now to the floor. With the baffle extending to the floor and being nearly airtight connected to it we get out "floor image". This appears a second sound source, equal in level and polarity to the original. This actually effects both rear and front radiation, so if we superimpose this function on the front radiation and then derive the rear radiation with suitable delay and attenuation we acoount for it correctly.

[/B]

Yes. Actually it works the same way even if the connection to the floor isn't airtight. The difference between tight or not, will be the inclusion of the diffraction sources at the floor edge of the baffle.

I think a good way of imagining how this should be done is by covering the walls, floor and ceiling with (usual optic) mirrors. In any direction the speaker can be seen, we should pu a source. If we see the back of the speaker, the sign of this source should be negative. I'd like to do that to a room some time...
To this we should add the edge diffraction.

Now the floor image also has some delay. So again we will find that as long as the distance to the floor image (about 2 X hight above floor) is small compared to the wavelength (say 0.875 wavelength) the floor image output will addup, probably closer to 4db than the theoretical 6db, plus we will see the usual dip at 0.5 Wavelength = distance, again, this should theoretically go 0db but will likely be much less deep.

So, all in all there is a material LF boost from the floor and less so from the rear wall.
[/B]

Having little practical experience of dipoles, I'd like to ask you, if you move the dipole closer to the wall, does the bass appear to drop or get boosted?

PS, I had a long day at work, so while I have the logic clear in my brain I may have typed garbage by inverting a sign or making division a multiplication or such, so treat with care and check logic and math... [/B]

Don't worry, I always do...

 

[ThorstenL]

Konnichiwa,

Spotted my own mistake....

What will happen is that the sound will be reflected back into the room with virtually no attenuation at low frequencies.

I forgot to account for the fact that rear output is inverted polarity!!!! So we will get a null at 360 degrees for the rear weave (and at 0 obviously) but a boost at 180 degrees.

All thes rest holds pretty well though....

Sayonara

 

[ThorstenL]

Konnichiwa,

Are they really in the "null"? The null is 90 degrees, but the possible path of reflection is maybe 45 degrees for the typical room. At 45 degrees the dipole has lost only 3 dB down. This would make a substantial contribution, IMO.

But at 45 degrees the path length becomes long and with a toes in baffle you will be receiving a substantially diffused and attenuated wavefront. In the area where normally the biggest problems would occour the output is drastically reduced. If not all we get is a bit more LF energy, of which you cannot have enough with a dipole anyway... ;-)

Having little practical experience of dipoles, I'd like to ask you, if you move the dipole closer to the wall, does the bass appear to drop or get boosted?

When hearing this with ESL's, I noticed that close to rear wall positioning had a better balance, with them becoming very lean far out in the room. Of course, the full range rear output and lack of acoustic opaqueness of the diphragm played havoc with anything else, meking them unlistenable close to walls...

Sayonara

 

[Nuuk]

Having little practical experience of dipoles, I'd like to ask you, if you move the dipole closer to the wall, does the bass appear to drop or get boosted?

When I experimented with my OB's, I found very little difference in the bass levels when moving them further forward of the rear walls. But the sound is better overall with them more than 60cm into the room. The depth of the sound stage is particularly good.

 

[Rudolf]

@svante regarding diffract.exe

I did some experimenting with diffract.exe
Very interesting tool. But it took me some time before I discovered the scaling function and baffle shaping with the red dots. Still wondering, what the Window function (and its Rectangle and Hanning features) is all about. Is it like a hole in an infinite baffle? Could I simulate a Karlson coupler with it?

Wouldn´t it be a good idea to implement some guidance in the Help menue?

Just wondering

Rudolf

 

[Svante]

Re: @svante regarding diffract.exe

I did some experimenting with diffract.exe
Very interesting tool. But it took me some time before I discovered the scaling function and baffle shaping with the red dots. Still wondering, what the Window function (and its Rectangle and Hanning features) is all about. Is it like a hole in an infinite baffle? Could I simulate a Karlson coupler with it?

Wouldn´t it be a good idea to implement some guidance in the Help menue?

Just wondering

Rudolf

The window tab is an *attempt* to create an "optimum" baffle shape in some sense. The idea is that the edge reflection impulse response should be smeared in time like a hanning window. The spectrum of such a pulse lacks higher frequencies, more or less, and this would minimise the ripple at higher frequencies. The result is the heart shape you see when you apply the hanning window.
Don't take it too seriously, though. Even though it the shape that the program produces reduces the ripple, I have discovered that it doesn't actually correspond to a hanning window, I hae the math slightly wrong.
The response always correctly reflects the shape, though, it is just the shape that is wrongish.
I wrote about this in a separate thread somewhere. Hmm...
http://www.diyaudio.com/forums/showthread.php?postid=324212#post324212

 

[Svante]

Re: @svante regarding diffract.exe

Still wondering, what the Window function (and its Rectangle and Hanning features) is all about. Is it like a hole in an infinite baffle? Could I simulate a Karlson coupler with it?

Wouldn´t it be a good idea to implement some guidance in the Help menue?

Just wondering

Rudolf

...and no, it has no extra holes or anything, all the predefined functions on the tabs are just to set the shape of the (flat) baffle.

What is a Karlson coupler?

 

[Svante]

Re: @svante regarding diffract.exe

Wouldn´t it be a good idea to implement some guidance in the Help menue?

Rudolf

I really should take the time to read your post completely, sorry for the multiple postings...
Help file, well, the original intent with the program was just to educate myself. But since I have had some 150 unique IP downloads, this means that there was an interest for this little hack. So maybe I should rethink about that.

 

[Aksioma]

Question for Quei!!!

Hi Quei, I am waiting impatiently for your answer about Prometheus loudspeaker. We are representative seller for Slovenian market, have not so much experience with oppen baffle sistems, but first impressions are very positive. But you know, one problem is vith Prometheus loudspeakers- its in some cases disturbing nasality. As I saw in your posting, you have cured this problem. Please, give us an adwice!!!

Regard
Iztok Gligoreviè
Aksioma_president
www.aksiomahifi.com
gligo01@yahoo.com

 

[ThorstenL]

Re: Question for Quei!!!

Konnichiwa,

Hi Quei, I am waiting impatiently for your answer about Prometheus loudspeaker.

I'm pushed for time, your e-mail sits waiting to be answered. In short, try removing the upper rearward facing panel and see what it sounds like with the driver mounted from the front. Make sure you have burned in the drivers well....

Sayonara

PS, at a recent party in my place the Prometheus where driven by a 50W PP Valve Amp and my PC, including waves MaxxBass and Waves Renaicance EQ. Made an excellent dancefloor system with surprising impact and all, despite being run without Subwoofer and equalised for dipole rolloff!!!! No nasality either....

 

[Aksioma]

Prometheus

Konnichiwa Kuei,

I do not understand, what you mean, I have driver mounted from rear side of baffle, so as in origin plan suggests. Prometheus is well burned in and powered with Sugden A21 stereo amplifier with 25 Watts pure A class. Please, could you give me detailed explanation?!
Regard Iztok

 

[ThorstenL]

Re: Prometheus

Konnichiwa,

I do not understand, what you mean, I have driver mounted from rear side of baffle, so as in origin plan suggests.

I did this too at first, I liked the result better front mounted. Try it (perhaps with a test baffle). Also, the panel above the Driver in the rear can cause colorations. TRy covering it with sound absorbing felt or leave it off.

Please, could you give me detailed explanation?!

No more detailed than this....

Sayonara

 

[Nielsio]

(Kuei,)

What would you prefer (open baffled):

1. Tangband W3-871S (or Tangband W3-926S) crossed to Gradient W 210/8

or

2. Ciare CH250

It's a DIY, on the cheap, system for my sister. I also have a 197? Marantz 1060 for her (with 3x EQ: bass, mid, highs).

or 3. Any other suggestions?

 

[ThorstenL]

Konnichiwa,

What would you prefer (open baffled):

Never heard either or, so no real idea, but on gut feeling - the 10" Ciare probably with supertweeter.

Sayonara

 

[Aksioma]

Prometheus

Konnichiwa Kuei,

My answer was deleted or lost by admin I suppose, so I will post another one. Onesmore thank you for yours answers, what I need more, are detailed datas of your condensator values for tweeter and resistor values for woofer. Please, give me exactly types of both.

Best regards
Iztok

 

[ThorstenL]

Re: Prometheus

Konnichiwa,

My answer was deleted or lost by admin I suppose, so I will post another one. Onesmore thank you for yours answers, what I need more, are detailed datas of your condensator values for tweeter and resistor values for woofer. Please, give me exactly types of both.

I use NO resistor on the woofer and overall around 2 Ohm in series with a M-Cap supreme 0.22uF parallel with 1uF M-Cap QS.

Sayonara

 

[GeorgeBoles]

This is totally new to me!!

I was actually looking for information on a quick (my first) subwoofer DIY box for this Easter School holidays, and then was thinking about a sonotube type design, also for this Easter break, and I came upon this site!

So now I am thinking about building some two way "open baffle" speakers with my new soon-to-be subwoofer for the bass.

But what size do you make the baffles? Most look pretty small really, especially the really simple ones which look like just some cut down ply sheets sitting on simple stands. (Of course, some are BEAUTIFUL works of art too.)

Is there more science to it? Or could I use the door I took off the storage room, (perhaps a bit flimsy) or maybe the door off the old shed in the back yard may be a bit more substantial and have fewer resonance.

This all LOOKS so simple, that anyone could do it ... I have followed links to baffles made only of STYROFOAM!!!

What does one lose, apart from box resonances, if one goes for open baffle? Some efficiency? How much efficiency do you lose? If open baffle, then why doesn't cutting a hole in the wall (right through into the next room) and sticking the speakers in there work ... or does it?!

Maybe the question should be: "How much science is there in box design, or perhaps there is too much?" Is escaping from boxes a simplifying and edifying undertaking?

If there are any specific design principles to be learned, is there a compact "beginner's site" to visit, or is this it?

Lots of questions all thrown together in my 'flu-fuelled haze, and they are not meant to be demeaning or derogatory, but rather meant to be in a genuine information-seeking manner - in search of affordable, beautiful sound.

Regards,
Mark Walsh.

 

[Nuuk]

What does one lose, apart from box resonances, if one goes for open baffle? Some efficiency? How much efficiency do you lose? If open baffle, then why doesn't cutting a hole in the wall (right through into the next room) and sticking the speakers in there work ... or does it?!

That's infinite baffle - another approach and like all loudspeaker systems, has its advantages and disadvantages.

Maybe the question should be: "How much science is there in box design, or perhaps there is too much?" Is escaping from boxes a simplifying and edifying undertaking?

You are learning speaker building is a combination of science and empericism.

If there are any specific design principles to be learned, is there a compact "beginner's site" to visit, or is this it?

Not really a beginners site. Read as much as you can on diyAudio using a search for 'open baffle'. The things is, open baffle design is so simple, it doesn't really need a tutorial as such. If go with the the 'single driver' approach, you just look at what other people have got and experiment with a piece of cheap baffle, measure the response and then build a 'pretty' baffle of the correct size (for you).

One thing I will tell you is that once you have got used to open baffle sound, you will find it very difficult to ever enjoy a box type speaker again!

 

[Roscoe Primrose]

Re: This is totally new to me!!

If there are any specific design principles to be learned, is there a compact "beginner's site" to visit, or is this it?

Not a beginner's site, but try
Linkwitz Lab

Peace

 

[ThorstenL]

Re: This is totally new to me!!

Konnichiwa,

So now I am thinking about building some two way "open baffle" speakers with my new soon-to-be subwoofer for the bass.

But what size do you make the baffles?

Sufficiently large to make acoustic sense.

Is there more science to it?

Of course. To optimise an open baffle is as complex and involved as optimising any other speaker. You have one bonus though, open baffles (dipoles actually) with conventional cone drivers cause less room interactions with your listening room than other systems.

This all LOOKS so simple, that anyone could do it ...

Most things look that simple.

I have followed links to baffles made only of STYROFOAM!!!

I believe such "designs" tend to be early attemps "just to see how it works easy & cheap".

What does one lose, apart from box resonances, if one goes for open baffle?

The ability to use enclosure resonances to reinforce selected frequency ranges (eg low frequencies). What your driver does not do on it's own will not be compensated for by enclosure resonances creating output.

If open baffle, then why doesn't cutting a hole in the wall (right through into the next room) and sticking the speakers in there work ...

Well, if the room next door is yours this is indeed the near ideal solution. Sadly in my case it belongs to the neigbours house....

Is escaping from boxes a simplifying and edifying undertaking?

That it is, always, not only with Monkey Coffins.

If there are any specific design principles to be learned, is there a compact "beginner's site" to visit, or is this it?

No real beginners site. Just two axioms:

1) Unless you are willing to use substantial amounts of equalisation your driver must have a Qt of at leat 0.5 preferably higher than that at 0.7 or more up to around 1.2

2) Open baffles are sized inversely proportional to the frequency, the lower the desired LF cutoff, the bigger the baffle. For an LF extension similar to a normal 2-Way mini monitor requires around 6-7 Squarefoot with a driver having a Qt of 0.7 & an Fs around 50 - 60Hz. Go lower in terms of LF cutoff or drivers Qt the bigger the baffle becomes. Each drop in Qt by 1.4 (eg from 0.7 to 0.5) or reduction in LF cutoff (eg from 60Hz to 30Hz) quadruples the required baffle surface.

All limitations can of course be overcome by electronic means, if you are prepared to throw loads of power at the speaker system (and use drivers that can handle such power levels) and equalise the response.

Sayonara