Member
Joined 2003
Mige,
I agree OB imaging degrades with reduced distance to the front wall. I haven't tried anything as close as .75 meter, but I imagine it can be, as you say, a real mess.
Yes, it is true wool felt and it covers the entire rear of the speaker. A felt rectangle about 24"x36" is curved around the semicircular top, center, and bottom "rails". Though is is fairly stiff when formed in a half-cylinder, the felt is clamped in place by small aluminum bars bowed between the baffle sides. The bars seal the felt against the rails which separates the woofer and mids.
The concept is to produce a smooth transition (in terms of directionality, not just output level) to a forward firing 90 degree waveguide. In practice, the wool felt forms a low-pass filter with only minor effect at very low frequency...attenuation increases with increasing frequency. The felt tends to swamp path-length resonances so the overall baffle becomes more aperiodic. Though I haven't measured for it yet, it may produce a semi-cardiod pattern.
Lynn,
Yep, it would be relatively easy to add a rear-firing tweeter with both level and frequency contouring. My hunch is it would be most beneficial for 2-channel listening.
My 18N850's are "v1"...current production appears to be "v2". The v2 specs show a lower cone mass and higher Fs, but I believe you'd be happy with either version.
Paul
I agree OB imaging degrades with reduced distance to the front wall. I haven't tried anything as close as .75 meter, but I imagine it can be, as you say, a real mess.
Yes, it is true wool felt and it covers the entire rear of the speaker. A felt rectangle about 24"x36" is curved around the semicircular top, center, and bottom "rails". Though is is fairly stiff when formed in a half-cylinder, the felt is clamped in place by small aluminum bars bowed between the baffle sides. The bars seal the felt against the rails which separates the woofer and mids.
The concept is to produce a smooth transition (in terms of directionality, not just output level) to a forward firing 90 degree waveguide. In practice, the wool felt forms a low-pass filter with only minor effect at very low frequency...attenuation increases with increasing frequency. The felt tends to swamp path-length resonances so the overall baffle becomes more aperiodic. Though I haven't measured for it yet, it may produce a semi-cardiod pattern.
Lynn,
Yep, it would be relatively easy to add a rear-firing tweeter with both level and frequency contouring. My hunch is it would be most beneficial for 2-channel listening.
My 18N850's are "v1"...current production appears to be "v2". The v2 specs show a lower cone mass and higher Fs, but I believe you'd be happy with either version.
Paul
Hi
PaulW, with pop, yes but with classic it was superb – that's what puzzles me most. I wouldn't be happy to have a speaker that is picky about ( recording- ) style.
I found some hints that not the mere distance to the front wall is what counts but rather its relationship to the near field / far field transition distance which depends on both, room size AND reverberation time of the room. Maybe someone else has the in depth infos at what level of attenuation a phantom source close by is no longer prominent.
Anyway – manipulation of the radiation pattern, positioning and aiming of the OB, damping material on the wall to prevent from mirroring, rear tweeter "countouring", VVLOB – its quite a set of tools with pro's and con's we can choose from to optimise in room behaviour of OB's.
Greetings
Michael
PaulW, with pop, yes but with classic it was superb – that's what puzzles me most. I wouldn't be happy to have a speaker that is picky about ( recording- ) style.
I found some hints that not the mere distance to the front wall is what counts but rather its relationship to the near field / far field transition distance which depends on both, room size AND reverberation time of the room. Maybe someone else has the in depth infos at what level of attenuation a phantom source close by is no longer prominent.
Anyway – manipulation of the radiation pattern, positioning and aiming of the OB, damping material on the wall to prevent from mirroring, rear tweeter "countouring", VVLOB – its quite a set of tools with pro's and con's we can choose from to optimise in room behaviour of OB's.
Greetings
Michael
Several things:
- the bloated image size Paul experiences with the wrong rear reflection I believe has something to do not just with the rear wall, but also with Paul's large ribbons (if this is indeed the setup where you noticed it, Paul?). In my dynamic driver OB's I notice a collapse of the "openness" at 1m or less distance to to front wall, but no image size distortion. But my OB's have little rear radiation>1kHz because I use a dome tweeter, and the whole image perception etc largely comes in >1kHz.
- pop sounding bad: this might actually be an accurate reflection of what's on the recording: fairly simple sound effects and compressed dynamics, optimized for the tilted power response of typical box speakers. In my OB's I have also been disappointed in some pop recordings, but not all.
In the end this all harks back to the question, how much room reflections do you want added in via the radiation pattern, and from where? DSP created reverb can help but may also add artificiality. Radiation pattern created reverb (i.e., actual in-room reverb) is natural, but changes with the room and wall characteristics, and is fixed by the speaker positioning. But at least it doesn't change in effect when you move inside the room, becaue it is natural and belongs to the room.
So the decision in the end is, how directional should my speakers be? (assuming a smooth power response as a must in any case). Earl Geddes for instance believes in strong directionality above 1 kHz due the short delay of reflections, imposed by the geometry of small rooms.
If your room is large, wider dispersion may be better. I had no time recently to progress in my constructions, but I aim for 120 degree coverage, ideally matching the OB to a WG by morphing it into a damped rear a la Paul, to create the cardioid. Until this gets done I am actually happy with the OB/dome, as long as the SPL is not very high and the front wall is at >1.2m.
- the bloated image size Paul experiences with the wrong rear reflection I believe has something to do not just with the rear wall, but also with Paul's large ribbons (if this is indeed the setup where you noticed it, Paul?). In my dynamic driver OB's I notice a collapse of the "openness" at 1m or less distance to to front wall, but no image size distortion. But my OB's have little rear radiation>1kHz because I use a dome tweeter, and the whole image perception etc largely comes in >1kHz.
- pop sounding bad: this might actually be an accurate reflection of what's on the recording: fairly simple sound effects and compressed dynamics, optimized for the tilted power response of typical box speakers. In my OB's I have also been disappointed in some pop recordings, but not all.
In the end this all harks back to the question, how much room reflections do you want added in via the radiation pattern, and from where? DSP created reverb can help but may also add artificiality. Radiation pattern created reverb (i.e., actual in-room reverb) is natural, but changes with the room and wall characteristics, and is fixed by the speaker positioning. But at least it doesn't change in effect when you move inside the room, becaue it is natural and belongs to the room.
So the decision in the end is, how directional should my speakers be? (assuming a smooth power response as a must in any case). Earl Geddes for instance believes in strong directionality above 1 kHz due the short delay of reflections, imposed by the geometry of small rooms.
If your room is large, wider dispersion may be better. I had no time recently to progress in my constructions, but I aim for 120 degree coverage, ideally matching the OB to a WG by morphing it into a damped rear a la Paul, to create the cardioid. Until this gets done I am actually happy with the OB/dome, as long as the SPL is not very high and the front wall is at >1.2m.
Member
Joined 2003
The bloated image has been characteristic of all undamped OBs I've tried...at least 8-10 sets of baffles and drivers. Just my taste, but I tend to prefer precise imaging in the front channels along with very clear articulation...that just doesn't happen with a wide open OB rearwave firing into a close front wall. Pattern control has become a great tool in minimizing early reflections and, at least in my case, far more practical than room treatment countermeasures.
For now, my personal quest for the grail is to "perfectly" match a waveguide top end to a boxless midrange and bottom end. But I think there is no single answer...the trick is matching individual preferences to one's own listening room.
For now, my personal quest for the grail is to "perfectly" match a waveguide top end to a boxless midrange and bottom end. But I think there is no single answer...the trick is matching individual preferences to one's own listening room.
OK, I thought it was only with the line sources. I have not noticed the bloated image in my several OB iterations. Undamped rear wave in my case seemed to lead only to the resonance at onset of dipole rolloff, described by SL in detail, and if this is left uncorrected then I'd get muddy lower midrange.
Agreed on the other points - the room matching is the most important, and personal taste matters a lot here.
Another thing is that, sorry for the modelers, but I have had reasonable results only by building and measuring
. I found little real life value to The Edge, for instance.
Agreed on the other points - the room matching is the most important, and personal taste matters a lot here.
Another thing is that, sorry for the modelers, but I have had reasonable results only by building and measuring
. I found little real life value to The Edge, for instance.
I'm with you guys on wall distance. Too close kills the image.
My OBs are open only below 800Hz (2nd order), from there up it's horn.
For casual listening the baffles are about 60 cm (2 feet) from the wall. Sounds OK, but not great. At 120 cm, there is a definite improvement. The image detaches from the speakers and extends behind the speakers. Bass at 120cm from the wall is bigger and has more punch, too. Drum benefit greatly.
Too close to the wall means a lumpy FR and the sound sticks to the speaker, never detaches. I don't seem to hear "image bloat" at any position.
A test of how close is too close might be fun. Easy to try. And then maybe some rear damping.
My OBs are open only below 800Hz (2nd order), from there up it's horn.
For casual listening the baffles are about 60 cm (2 feet) from the wall. Sounds OK, but not great. At 120 cm, there is a definite improvement. The image detaches from the speakers and extends behind the speakers. Bass at 120cm from the wall is bigger and has more punch, too. Drum benefit greatly.
Too close to the wall means a lumpy FR and the sound sticks to the speaker, never detaches. I don't seem to hear "image bloat" at any position.
A test of how close is too close might be fun. Easy to try. And then maybe some rear damping.
Just for inspiration - u might wanna look at;
http://www.troelsgravesen.dk/OB7.htm
Vintage high efficiency OB speaker
http://www.troelsgravesen.dk/OB7.htm
Vintage high efficiency OB speaker
Wall damping ?
http://www.advancedacoustics-uk.com/highendaudio/Photophonic_Panel.html
or plain
http://www.advancedacoustics-uk.com/highendaudio/Signature_Acoustic_Panel.html
Baffle sides ?
http://www.advancedacoustics-uk.com/Problem_Solved/Tamel_Ceiling_Tiles.html
Cheers ....... Graham.
http://www.advancedacoustics-uk.com/highendaudio/Photophonic_Panel.html
or plain
http://www.advancedacoustics-uk.com/highendaudio/Signature_Acoustic_Panel.html
Baffle sides ?
http://www.advancedacoustics-uk.com/Problem_Solved/Tamel_Ceiling_Tiles.html
Cheers ....... Graham.
Graham Maynard said:
Ha! LOL! I just about fell of my chair. Those are great.
You see, I'm a fine art printer. So last night at work, while taking a canvas print off the printer, I was thinking about wall damping behind the OBs . "Why not print a nice image onto open weave fabric and stretch it over a sound panel" thinks I.
My wife would love it, as she likes to cover every square inch of wall space with art or "stuff".
I had already done this with speaker grill cloth, to limited success. A wonderful idea. =)
Brett,
You can find them here. The shipment cost from Melbourne is acceptable.
I am getting 4 large panels (1.2m x 2m) soon. The trouble is that I have not found the high resolution photos to be printed on them!
www.essentiallyart.com.au
www.acousticaldesign.com.au
Regards,
Bill
You can find them here. The shipment cost from Melbourne is acceptable.
I am getting 4 large panels (1.2m x 2m) soon. The trouble is that I have not found the high resolution photos to be printed on them!
www.essentiallyart.com.au
www.acousticaldesign.com.au
Regards,
Bill
Has anyone done an OB with the same size speaker or smaller cousin mounted magnet to magnet behind the OB?
Seems as if the rear speaker has many options available for controlling the rear wave; volume, phase, frequency range, etc.
Another option is two identical speakers mounted isobaric clam shelled, one on each side of an OB. This should have already been done.
Zene
Seems as if the rear speaker has many options available for controlling the rear wave; volume, phase, frequency range, etc.
Another option is two identical speakers mounted isobaric clam shelled, one on each side of an OB. This should have already been done.
Zene
You guys might like this...
current experiment with vintage drivers,
http://www.troelsgravesen.dk/OB7.htm
C
current experiment with vintage drivers,
http://www.troelsgravesen.dk/OB7.htm
C