Hi RAAL,
Thanks for the insights. Quite a lot!
This is very interesting, and would you please share more?
BTW, your posts are fantastically informative, but can you arrange them in shorter paragraphs? I'm dizzy in the crowded words
Thanks a lot
Thanks for the insights. Quite a lot!
This is very interesting, and would you please share more?
BTW, your posts are fantastically informative, but can you arrange them in shorter paragraphs? I'm dizzy in the crowded words
Thanks a lot
Hi guys
possibly better to split the discussion into genereal topics Aleksander is interested in and leave this thread
" On the directivity of dipole tweeters"
What you think?
I opend a thread for Aleksander and RAAL speakers here:
http://www.diyaudio.com/forums/mult...r-radisavljevic-raal-speaker.html#post2423174
Possibly one of the mod could add the pictures that got lost in the transfer?
Michael
possibly better to split the discussion into genereal topics Aleksander is interested in and leave this thread
" On the directivity of dipole tweeters"
What you think?
I opend a thread for Aleksander and RAAL speakers here:
http://www.diyaudio.com/forums/mult...r-radisavljevic-raal-speaker.html#post2423174
Possibly one of the mod could add the pictures that got lost in the transfer?
Michael
Thanks Aleksander for your answer.
I suppose I was a bit confusing in my last post.
I spent some hours listening to my Blindstone OBs: The BlindStone OB in various combinations with bass, mid and treble. Bass is two Eminenca Alpha15s up to 250 Hz, mid is 18Sound 6ND430 250 Hz - 2 kHz and treble is Neo3W from there.
Listening only to bass and treble I hear with my speakers a very clear definition and some ambience, not really very much of stereo I think. But you can get a picture of what the music sounds like anyway just from this bass and treble output.
But really most of the stereo effects and depth and imaging effects seem to lie with the midrange. Also only with midrange in place you can move around in the soundfield and get a picture of the sound dispersion.
/Erling
I suppose I was a bit confusing in my last post.
I spent some hours listening to my Blindstone OBs: The BlindStone OB in various combinations with bass, mid and treble. Bass is two Eminenca Alpha15s up to 250 Hz, mid is 18Sound 6ND430 250 Hz - 2 kHz and treble is Neo3W from there.
Listening only to bass and treble I hear with my speakers a very clear definition and some ambience, not really very much of stereo I think. But you can get a picture of what the music sounds like anyway just from this bass and treble output.
But really most of the stereo effects and depth and imaging effects seem to lie with the midrange. Also only with midrange in place you can move around in the soundfield and get a picture of the sound dispersion.
/Erling
Hi,
Power response is meaningless, unless you take into account the frequency dependent room absorption. I claim that none of the domestic living rooms have frequency independent absorption coefficient. Then why the power response should be constant, or frequency response? It's only an engineering conveniency (because amplifiers have constant response?).
Humans do not perceive a frequency response as such. Then why measure that?
In the forgotten past there was a time when people were not afraid to adjust the tone controls! How about a loudness button!? Anyone?
- Elias
Power response is meaningless, unless you take into account the frequency dependent room absorption. I claim that none of the domestic living rooms have frequency independent absorption coefficient. Then why the power response should be constant, or frequency response? It's only an engineering conveniency (because amplifiers have constant response?).
Humans do not perceive a frequency response as such. Then why measure that?
In the forgotten past there was a time when people were not afraid to adjust the tone controls! How about a loudness button!? Anyone?
- Elias
Fantastic post #140, Aleksander! 
I subscribe to almost everything in it (can't subscribe to omnidirectivity since I haven't used it in such consequent manner yet).
Things I would like to emphasize:
- If our room acoustics work well in normal life, our brain will not mix/confuse them with the acoustics of the recording.
- You can't expect to hear true stereo with different speakers left and right. In the same way you can't expect to hear real stereo with different room acoustics left and right.
- Don't attempt to correct faults of the speakers by tampering with the room acoustics. It will make things worse.
WRT the discussion on ILD, ITD and pinna cues:
Paul Hofman has a good summary of the interrelation between these effects in his doctoral thesis on pinna cues.
Chapter 1 (15 pages) will be sufficient to get the idea. "Although spectral cues may thus contribute to azimuth hearing, their relative weight compared to that of the interaural cues is nevertheless presumed to be small."
Rudolf
I subscribe to almost everything in it (can't subscribe to omnidirectivity since I haven't used it in such consequent manner yet).
Things I would like to emphasize:
- If our room acoustics work well in normal life, our brain will not mix/confuse them with the acoustics of the recording.
- You can't expect to hear true stereo with different speakers left and right. In the same way you can't expect to hear real stereo with different room acoustics left and right.
- Don't attempt to correct faults of the speakers by tampering with the room acoustics. It will make things worse.
WRT the discussion on ILD, ITD and pinna cues:
Paul Hofman has a good summary of the interrelation between these effects in his doctoral thesis on pinna cues.
Chapter 1 (15 pages) will be sufficient to get the idea. "Although spectral cues may thus contribute to azimuth hearing, their relative weight compared to that of the interaural cues is nevertheless presumed to be small."
Rudolf
Elias,
maybe "linear" was misleading
. What I wanted to know: Which of both responses would be the more important to you? And do you see, that changing one of them will automatically change the other? If neither frequency nor power response would be meanigful to you - what factor would you control primarily to arrive at a pleasing audio reproduction?Rudolf
Digital eq
If you use a digital source, a digital eq such as the Behringer DEQ2496 running digital in/ out or various software solutions, is essential. Do you want your system to sound like it measures flat quasi anechoic or do you want it to sound real. Trust your ears.
If you use a digital source, a digital eq such as the Behringer DEQ2496 running digital in/ out or various software solutions, is essential. Do you want your system to sound like it measures flat quasi anechoic or do you want it to sound real. Trust your ears.
Elias,
maybe "linear" was misleading
Rudolf
Hi guys,
My bad if i'm wrong, but can we say that in a room where the absorption coefficient is as regular as possible over the spectrum :
- if power response is constant
- if frequency response is constant
then have we an omni source ?...
Now, from the great paper linked by Rudolf (wow):
in a closed space with boundaries, distance information comes also from pinna related reverberance clues.
The reverberance clues provided by a record are by definition artificial ( mastering added reverb, and a mike is not a pinna).
Then, is a directional speaker the best way to go? (as it will give us only this artificial and unsatisfactory info)
I think, this little detail goes in the same direction than the unforgettable post #140
Staying with the Hofman paper, if we go back to the subject of this thread, what to do with this established fact:
between 1500 and 3000 (and a big peak at 2000), we have some trouble to localize a source
You can do the experiment immediately with a sinus generator: send the signal on one tweeter only and try to determinate if this comes from the left or from the right. In most of the cases you must be in trouble.
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We would need a speaker with "constant directivity" (CD). Omnis are CD speakers with a directivity which covers 360°. But there are CD speakers with less coverage too.
A "directional" speaker is a speaker with a narrow coverage? That does not mean that is has to be CD. But it can, of course.
With regard to reverberance clues let me tell a short story about my early ancestors:
They were chasing bears in the woods (or were they chased by the bears?). Anyway, when they heard a bear in a frighteningly short distance, his noises were reverberated by all the tree trunks around. One of my ancestors could not abstract the reverberance clues from the trunks. He had been late in evolution
. Guess who was eaten by the bear. My other ancestors were able to assign the clues not to the reflecting objects, but to the original source. They were able to pass their genetic advantage on to us . Hopefully you can distinguish the reverberance clues of your room from the reverberance clues of the recording. Otherwise you should stay well outside the jungles of Sulawesi
. And in the latter case you would need a "directional" speaker, yes.Luckily I prefer music which is richer in harmonics than a 2 kHz pure sinusbetween 1500 and 3000 (and a big peak at 2000), we have some trouble to localize a source
You can do the experiment immediately with a sinus generator: send the signal on one tweeter only and try to determinate if this comes from the left or from the right. In most of the cases you must be in trouble.
.
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Hi guys!
gee, am I yapping too much or what!
Michael, you're right, we should keep this thread on the subject.
Too bad someone always drops a bomb question to which an answer is not
within the topic of directivity of dipole tweeters.
CLS, me too! Short paragraphs from now on!
Scorpion, don't know about the others, but I assume you're right, even though
I haven't tried that one yet in that context. And if you're right, wide flush HF shouldn't hurt the imaging.
Elias, you nailed it right there, but you're pushing it with this linear response, and you know it! You're mean!
You know that direct sound should
arrive without being spectrally distorted, if we stand any chance in
preserving reality.
I should add to your questions, what is the difference in constant directivity of 40, 60, 70, 90 and 94.45 degrees?
Well, the difference is that some of those are naturally occuring with dipoles,
some with this or that horn flare, some with mistakes, and engineers loooove
constants! If everything else fails, like building a FR pulsating sphere, let's
introduce a fictious category of "the necessity for constant directivity" and
brag about it! It might sell good and it'll make our engineering look reeeel
smart!
As I've said before, the best thing about a dipole is that it helps in rooms with boomy bass, and it shoots the information to the wall in the back.
Having it's directivity constant, or this or that, will make the same difference
like whether my front firing speaker is constant or not.
We should all be aware that the constant directivity (CD) and smooth power
response concept was a necessity with PA and Sound Reinforcement
systems where you need to cover large areas of crowd. It helps when you
want everybody in the crowd to hear things pretty much the same and it
gives you a good up-front assessment of how many boxes you need to cover
some 2 acre meadow or a train station with this many dB, at that many +/- dB.
And I know how the concept of it, slowly crept into home audio, but that's another story, where it is almost meaningless.
My question to xy deg CD advocates in domestic use is:
why is it better than 360 deg CD (or omni), and what is the best CD angle?
30, 60, 90, 120 deg and WHY? WHYYYYYY!
I'm eager to know an I can't grasp it myself.
Rudolf, thanks! I bow to you in being able to summarize a few key points!
I could never do that!
Speaking of the third "rule" you extracted, a small correction of speaker's tone balance for better room acoustics integration is acceptable, but only
like smooth tilting of response, not creating dips or peaks, or anything that
relates to a specific point in frequency response.
Sendler, I'd go with measured flat, with the abovementioned corrections if necessary and a sizeable lateral and front wall speaker distance. If any room corrections are to be used, we should restrain the interventions to below 250
Hz, but that will be valid for only one speakers-listener placement, while
fixing room acoustics works for all positions.
Radugazon, we don't get and omni source if things are kept constant in much less than 360 degrees.
Recorded reverberant cues bare no pina cues, ase you well noted, so the height of tweeter sets the median stage height of the stereo image.
We can guess the recording room size by it's recorded spectral content, duration of spectral components and their onset time. We sift it through the database in our brain and interpolate the info with results what we have stored previously and guess the correct result by computational result in linear proportion to the database reference points. For example, strognly delayed HF reverb will be contributed to ceiling height and so on, so reverb info even without provided pina cues will be regarded as valuable. Evolution, after all, favorized redundancy in some key survival techinques.
Reverb may be totally artificial on Enya records, but not on vast majority of jazz and classical, even some one-take rock or what-have-you.
Telarc recordings are usually non-produced, like all Chesky bros. and many others. A great recent example is Iso-Mike recordings from Ray Kimber et al.
Now, even if the reverb is artificially injected, it belongs to the source, and if
you don't omni-flush the room, the source will be dismissed by brain as fake
source.
Who is to say that artist didn't want just that artificial recording space
effect that we will hopefully hear. Maybe somebody wanted to give an
impression of a singer laying down and singing upwards from the sarcofagus
in the great hall of the Great Pyramid, with listener f...ing the singer in
missionary position, who knows.
Ours is to try to fairly depict even situations like that, why not?
gee, am I yapping too much or what!
Michael, you're right, we should keep this thread on the subject.
Too bad someone always drops a bomb question to which an answer is not
within the topic of directivity of dipole tweeters.
CLS, me too! Short paragraphs from now on!
Scorpion, don't know about the others, but I assume you're right, even though
I haven't tried that one yet in that context. And if you're right, wide flush HF shouldn't hurt the imaging.
Elias, you nailed it right there, but you're pushing it with this linear response, and you know it! You're mean!
You know that direct sound shouldarrive without being spectrally distorted, if we stand any chance in
preserving reality.
I should add to your questions, what is the difference in constant directivity of 40, 60, 70, 90 and 94.45 degrees?
Well, the difference is that some of those are naturally occuring with dipoles,
some with this or that horn flare, some with mistakes, and engineers loooove
constants! If everything else fails, like building a FR pulsating sphere, let's
introduce a fictious category of "the necessity for constant directivity" and
brag about it! It might sell good and it'll make our engineering look reeeel
smart!
As I've said before, the best thing about a dipole is that it helps in rooms with boomy bass, and it shoots the information to the wall in the back.
Having it's directivity constant, or this or that, will make the same difference
like whether my front firing speaker is constant or not.
We should all be aware that the constant directivity (CD) and smooth power
response concept was a necessity with PA and Sound Reinforcement
systems where you need to cover large areas of crowd. It helps when you
want everybody in the crowd to hear things pretty much the same and it
gives you a good up-front assessment of how many boxes you need to cover
some 2 acre meadow or a train station with this many dB, at that many +/- dB.
And I know how the concept of it, slowly crept into home audio, but that's another story, where it is almost meaningless.
My question to xy deg CD advocates in domestic use is:
why is it better than 360 deg CD (or omni), and what is the best CD angle?
30, 60, 90, 120 deg and WHY? WHYYYYYY!
I'm eager to know an I can't grasp it myself.
Rudolf, thanks! I bow to you in being able to summarize a few key points!
I could never do that!
Speaking of the third "rule" you extracted, a small correction of speaker's tone balance for better room acoustics integration is acceptable, but only
like smooth tilting of response, not creating dips or peaks, or anything that
relates to a specific point in frequency response.
Sendler, I'd go with measured flat, with the abovementioned corrections if necessary and a sizeable lateral and front wall speaker distance. If any room corrections are to be used, we should restrain the interventions to below 250
Hz, but that will be valid for only one speakers-listener placement, while
fixing room acoustics works for all positions.
Radugazon, we don't get and omni source if things are kept constant in much less than 360 degrees.
Recorded reverberant cues bare no pina cues, ase you well noted, so the height of tweeter sets the median stage height of the stereo image.
We can guess the recording room size by it's recorded spectral content, duration of spectral components and their onset time. We sift it through the database in our brain and interpolate the info with results what we have stored previously and guess the correct result by computational result in linear proportion to the database reference points. For example, strognly delayed HF reverb will be contributed to ceiling height and so on, so reverb info even without provided pina cues will be regarded as valuable. Evolution, after all, favorized redundancy in some key survival techinques.
Reverb may be totally artificial on Enya records, but not on vast majority of jazz and classical, even some one-take rock or what-have-you.
Telarc recordings are usually non-produced, like all Chesky bros. and many others. A great recent example is Iso-Mike recordings from Ray Kimber et al.
Now, even if the reverb is artificially injected, it belongs to the source, and if
you don't omni-flush the room, the source will be dismissed by brain as fake
source.
Who is to say that artist didn't want just that artificial recording space
effect that we will hopefully hear. Maybe somebody wanted to give an
impression of a singer laying down and singing upwards from the sarcofagus
in the great hall of the Great Pyramid, with listener f...ing the singer in
missionary position, who knows.
Ours is to try to fairly depict even situations like that, why not?
I don't see any yapping in what you are writing. It is a most valuable discussion to me.
"Directivity" is on topic, of course. And directivity is strongly connected to issues like room response and the working of human hearing.
I'm not Elias, but isn't the difference the change in directivity index - the relation between direct and indirect sound?
A constant directivity speaker should have something like a "constant reflectivity" room as a counterpart. With a more directional speaker you can "switch off" parts of the room, which lack in constant or symmetric reflectivity.
In real life there is a difference between listening to a concert from the 5th row or the 30th row - the ratio between direct and reverberant sound. Most people would appreciate to sit in the 5th row instead of the 30th. Having a higher directivity index doesn't change the row in the recording of course, but it can help to look better into the recording details.
I don't think that a "best" CD angle can be defined in general. It's definition would depend on too many factors.
Rudolf
My question was more about fundamental reasoning behind the necessity of CD, as I mostly know what are the direct explanations of this or that angle being constant, but I find fundamental problem in it's conception.
The whole premise behind it, lies in two things:
1) Audiophiles have small rooms.
2) Audiophiles do not care about fixing their room acoustics, and let's not bother them with that.
No one, even dr Earl Geddes himself can tell me that I'm wrong with this as being the fundamental reasoning behind it, and no one can tell me that a CD speaker, has a better chance in sounding better than some other speakers that does not HAVE TO use a waveguide as a design necessity and it is used in a PROPER audiophile environment, i.e. the dedicated listening room, small or large.
CD is NOT a qualitative merit, but a merit of being usable in small, acoustically untreated rooms.
My car is better than Formula 1 'cuz I can go shopping with it!
Anyway, since I mention Dr. Geddes as the most influential CD proponent these days, here's an excerpt from his paper on CD, you can get it from his site.
I'll break his the original text with my comments in colour. My comments will be strongly flavored with my general view of how I think an audiophile should live and influence his/hers environment.
>>>>>>>>>
QUOTE:
6 of 14 A white paper by Dr. Earl Geddes, GedLee LLC
3.0 The Idea of an Ideal Polar Map
With the assumed familiarization with polar maps developed in the previous section and the kinds of things that
can be seen in them, the rest will be easy. First I want to talk about what a playback system in a real room should do. This
will by necessity be rather brief since this is a vast subject. Basically, I'm going state my beliefs about what the ideal polar
pattern is and why, but I won't give an elaborate proof for this position, although I believe that I could. However, for brevity
(and the sanity of the reader), I will simply state my position as my opinion and leave it there. In any case, it should be
appreciated that my opinions here are based on a lot of training, experience and data.
Many say that an omni-directional loudspeaker response is the ideal, but I do not accept this for a small room. That's
because the Very Early Reflections (VER) increase dramatically with this type of source because of its very wide polar pattern.
Because of the omni-directional response, this type of source cannot be situated such that it does not excite every possible
VER, and there simply is no way to fully absorb these VER without creating a room that is so dead as to be
acoustically lifeless.
It's been a general knowledge for decades that VER is managed with strategically placed absorption only in areas of first reflections. He later mentions the criteria of 500 Hz as practically feasible beginning of CD. Strategic absorption above 500 Hz is easy and this treatment does not produce "acoustically lifeless" rooms. Also, "full absorption" is no criteria at all at it is unfairly mentioned, since directivity discrimination can not be fully achieved. So, at how many dB dr. Geddes decided to alleviate the criteria of CD off-axis fallout(as much as horns can allow for it), that many he should allow for imperfect absorption. Also, better thing to do is to diffuse the VER or specular reflections and turn them into diffused sound field As the directivity gets narrower and narrower however, it should be intuitively obvious that I can orient the speaker in such a way as to avoid the nearer boundaries and thus minimize the VER to a much greater degree. In fact, it can be shown that the higher the directivity, the greater the average Reflection Free Time (RFT) immediately following
the direct sound. Ideally this would be about 20 ms, but that is never going to happen in a small room. Even 10 ms
requires some significant room modifications to achieve in typical sized listening rooms.SO WHAT?! Am I supposed to have my listening room designed by my wife's decorator? Or if my living room is my listening room, am I supposed to be ignored by my family in my audiophile needs? Little culture in using things properly and listening to the music will just help raising children into better people However, it is quite true that the
more that can be done to extend this RFT the better the image will be, with diminishing returns beyond about 10 ms.
It is also true that VERs and a short RFT do add to the perception of spaciousness, which is considered to be, by many,
a good thing, but just as certainly it is know that they detract from the ability to image the content of the source because of
the confounding influence of the VER. Toole is a strong proponent of a large amount of VER because of its increase in the
spaciousness effect. He appears to discount the negative aspects of this on imaging however. (Dr. Toole, does not make
many statements about “image”, perhaps being concerned over its loose definition.)
It seems to me that if I can create “spaciousness”
without increasing the VER, then I can achieve the best of both worlds. This can in fact be done by making the
room fairly reverberant, particularly behind the listener, which will improve the feeling of spaciousness through the multitude
of lateral and rear reflections that will occur. However, the use of this technique with wide directivity speakers is not
going to yield a very good image due to the VERs from the nearer walls, and the speakers should have a fairly constant frequency
response in all directions, i.e. the power response, otherwise the sound quality will be colored. If the speakers do
have a narrow directivity then the frontal VERs have been lowered, hence improving the image, and yet I can still retain the
feeling of spaciousness because the room itself is fairly reverberant. In fact, if most of the reflections are coming from the
sides and rear, as opposed to the front, then the quality of the spaciousness is know to improve. Hence a narrow directivity
lowers the VERs and extends the RFT yielding good imaging, while the rooms high reverberation yields good spaciousness. Room treatment does the same thing.
It is most curious that this is quite often the opposite of what is done in many rooms. That's because this and every other DIY audio forum does not have discussions on room treatment, nor the industry bothers to emphasize the importance of it, as it will lead to less sales when people realize that the quality gain with treated room is higher than investing the same money into the new gear.
Another factor in favor of the high directional response is the possibility of increasing the so-called sweet-spot of the
loudspeakers through judicious use of the directivity fall-off with angle.This is not a merit of quality, neither it is important in usage. My guests and I listen sitting behind one another and both maintain the perfect symmetry to the system as well as to the room. This can be a little hard to follow, but I'll give it a
try. It is easiest to start with a discussion about why there is a sweet-spot. In normal piston loudspeakers, the directivity is
getting narrower at the higher frequencies and so it is usual to design the speaker such that the listener is on the axis of the
speaker. This has been the sole reason why the “axial response” has carried so much weight, because it has been assumed
that the listener is always on-axis. But what then happens if the listener moves off axis. In general the response from the farthest
loudspeaker will fall in level and there will be an increase in the time delay relative to the nearer speaker. This “double
whammy” of a lower level and a greater delay to the farther speaker, will immediately pull the image to the nearer loudspeaker.
In most piston source loudspeakers the image collapses very quickly as the listener moves away from dead center.
Now, if somehow we could cause the nearer speakers level to fall while the farther speaker's level actually increased
then we could partially offset the subjective effect of the image collapsing to the nearer speaker. (The farther speakers time
delay will increase, but its level will increase as well.) Nothing can be done to correct for the time delay differences, but if
we can make the level differences great enough then we might be able to achieve the effect that we want. Keep this highly
desirable feature in mind as we look at directivity requirements on the polar maps.
The next thing that we need to talk about is Low Frequencies (LF) in small rooms. This too is a massive subject that I
will only touch on here. Basically from the perspective of the main speakers, the LF directivity is not very important. This
is because the nature of the sound field in the modally dominated frequency region that exists at LF in a small room is such
that the directivity does not even enter into the picture. Since when? Rotating my dipole woofer in the same point of placement, around its vertical axis, I can set the desired excitation of diagonal vs, longitudinal modes and that a fact. BTW, by rotation, the omni top-section works just the same. With the use of multiple subs the mains directivity at LF is not an
issue at all. More speakers to buy. Certainly above some frequency it starts to become an issue, but clearly this is not 200 Hz, that's far too low,
and just as clearly by 1 kHz we need to have the directivity well under control since, by this frequency, the ear is beginning
to get quite sensitive to reflections and timing aberrations. Hence, somewhere between 200 Hz and 1 kHz the directivity
needs to start to become narrower - down to the design angle - and then remain at that angle for the remainder of the audio
bandwidth.
It might be said that lower is better as regards the point at which the directivity begins to narrow, but this comes at some
cost in terms of the loudspeakers size and its actual cost to produce. The sizes and costs will approximately quadruple for
every octave that one wants to push the directivity control down in frequency. A reasonable sized and priced system can
achieve about 800 Hz and a large and expensive system might get down to about 500 Hz. But going below this gets
extremely big and expensive as might well be anticipated with a quadrupling of these factors just going to 250 Hz. Not with a direct-radiator cardioid, if dipole isn't acceptable for low front VER criteria. For the
purposes of this paper I will assume 500 Hz as a practical limit.
END QUOTE
>>>>>>>>>>>>
What a room treatment can do, especially diffusion, to VER and RFT, is beautifully explained in this paper attached, by Peter D'Antonio of RPG.
Look carefully at the graphs and you'll see what CD is aimed against. Also, keep in mind that CD will not correct the room problems, it will ignore them to an extent, and that will not make the system and the room sound excellent, only better. ONLY the room treatment and proper placing will make things sound excellent, something known only to people with treated rooms. If you haven't ever been in one, make the effort and visit one, you owe it to yourself. It will put you on the same plane and level of understanding to the people with treated rooms, that find it hard to talk audio to people that don't use them.
Everybody is welcome to my room, if ever passing here.
The whole premise behind it, lies in two things:
1) Audiophiles have small rooms.
2) Audiophiles do not care about fixing their room acoustics, and let's not bother them with that.
No one, even dr Earl Geddes himself can tell me that I'm wrong with this as being the fundamental reasoning behind it, and no one can tell me that a CD speaker, has a better chance in sounding better than some other speakers that does not HAVE TO use a waveguide as a design necessity and it is used in a PROPER audiophile environment, i.e. the dedicated listening room, small or large.
CD is NOT a qualitative merit, but a merit of being usable in small, acoustically untreated rooms.
My car is better than Formula 1 'cuz I can go shopping with it!
Anyway, since I mention Dr. Geddes as the most influential CD proponent these days, here's an excerpt from his paper on CD, you can get it from his site.
I'll break his the original text with my comments in colour. My comments will be strongly flavored with my general view of how I think an audiophile should live and influence his/hers environment.
>>>>>>>>>
QUOTE:
6 of 14 A white paper by Dr. Earl Geddes, GedLee LLC
3.0 The Idea of an Ideal Polar Map
With the assumed familiarization with polar maps developed in the previous section and the kinds of things that
can be seen in them, the rest will be easy. First I want to talk about what a playback system in a real room should do. This
will by necessity be rather brief since this is a vast subject. Basically, I'm going state my beliefs about what the ideal polar
pattern is and why, but I won't give an elaborate proof for this position, although I believe that I could. However, for brevity
(and the sanity of the reader), I will simply state my position as my opinion and leave it there. In any case, it should be
appreciated that my opinions here are based on a lot of training, experience and data.
Many say that an omni-directional loudspeaker response is the ideal, but I do not accept this for a small room. That's
because the Very Early Reflections (VER) increase dramatically with this type of source because of its very wide polar pattern.
Because of the omni-directional response, this type of source cannot be situated such that it does not excite every possible
VER, and there simply is no way to fully absorb these VER without creating a room that is so dead as to be
acoustically lifeless.
It's been a general knowledge for decades that VER is managed with strategically placed absorption only in areas of first reflections. He later mentions the criteria of 500 Hz as practically feasible beginning of CD. Strategic absorption above 500 Hz is easy and this treatment does not produce "acoustically lifeless" rooms. Also, "full absorption" is no criteria at all at it is unfairly mentioned, since directivity discrimination can not be fully achieved. So, at how many dB dr. Geddes decided to alleviate the criteria of CD off-axis fallout(as much as horns can allow for it), that many he should allow for imperfect absorption. Also, better thing to do is to diffuse the VER or specular reflections and turn them into diffused sound field As the directivity gets narrower and narrower however, it should be intuitively obvious that I can orient the speaker in such a way as to avoid the nearer boundaries and thus minimize the VER to a much greater degree. In fact, it can be shown that the higher the directivity, the greater the average Reflection Free Time (RFT) immediately following
the direct sound. Ideally this would be about 20 ms, but that is never going to happen in a small room. Even 10 ms
requires some significant room modifications to achieve in typical sized listening rooms.SO WHAT?! Am I supposed to have my listening room designed by my wife's decorator? Or if my living room is my listening room, am I supposed to be ignored by my family in my audiophile needs? Little culture in using things properly and listening to the music will just help raising children into better people However, it is quite true that the
more that can be done to extend this RFT the better the image will be, with diminishing returns beyond about 10 ms.
It is also true that VERs and a short RFT do add to the perception of spaciousness, which is considered to be, by many,
a good thing, but just as certainly it is know that they detract from the ability to image the content of the source because of
the confounding influence of the VER. Toole is a strong proponent of a large amount of VER because of its increase in the
spaciousness effect. He appears to discount the negative aspects of this on imaging however. (Dr. Toole, does not make
many statements about “image”, perhaps being concerned over its loose definition.)
It seems to me that if I can create “spaciousness”
without increasing the VER, then I can achieve the best of both worlds. This can in fact be done by making the
room fairly reverberant, particularly behind the listener, which will improve the feeling of spaciousness through the multitude
of lateral and rear reflections that will occur. However, the use of this technique with wide directivity speakers is not
going to yield a very good image due to the VERs from the nearer walls, and the speakers should have a fairly constant frequency
response in all directions, i.e. the power response, otherwise the sound quality will be colored. If the speakers do
have a narrow directivity then the frontal VERs have been lowered, hence improving the image, and yet I can still retain the
feeling of spaciousness because the room itself is fairly reverberant. In fact, if most of the reflections are coming from the
sides and rear, as opposed to the front, then the quality of the spaciousness is know to improve. Hence a narrow directivity
lowers the VERs and extends the RFT yielding good imaging, while the rooms high reverberation yields good spaciousness. Room treatment does the same thing.
It is most curious that this is quite often the opposite of what is done in many rooms. That's because this and every other DIY audio forum does not have discussions on room treatment, nor the industry bothers to emphasize the importance of it, as it will lead to less sales when people realize that the quality gain with treated room is higher than investing the same money into the new gear.
Another factor in favor of the high directional response is the possibility of increasing the so-called sweet-spot of the
loudspeakers through judicious use of the directivity fall-off with angle.This is not a merit of quality, neither it is important in usage. My guests and I listen sitting behind one another and both maintain the perfect symmetry to the system as well as to the room. This can be a little hard to follow, but I'll give it a
try. It is easiest to start with a discussion about why there is a sweet-spot. In normal piston loudspeakers, the directivity is
getting narrower at the higher frequencies and so it is usual to design the speaker such that the listener is on the axis of the
speaker. This has been the sole reason why the “axial response” has carried so much weight, because it has been assumed
that the listener is always on-axis. But what then happens if the listener moves off axis. In general the response from the farthest
loudspeaker will fall in level and there will be an increase in the time delay relative to the nearer speaker. This “double
whammy” of a lower level and a greater delay to the farther speaker, will immediately pull the image to the nearer loudspeaker.
In most piston source loudspeakers the image collapses very quickly as the listener moves away from dead center.
Now, if somehow we could cause the nearer speakers level to fall while the farther speaker's level actually increased
then we could partially offset the subjective effect of the image collapsing to the nearer speaker. (The farther speakers time
delay will increase, but its level will increase as well.) Nothing can be done to correct for the time delay differences, but if
we can make the level differences great enough then we might be able to achieve the effect that we want. Keep this highly
desirable feature in mind as we look at directivity requirements on the polar maps.
The next thing that we need to talk about is Low Frequencies (LF) in small rooms. This too is a massive subject that I
will only touch on here. Basically from the perspective of the main speakers, the LF directivity is not very important. This
is because the nature of the sound field in the modally dominated frequency region that exists at LF in a small room is such
that the directivity does not even enter into the picture. Since when? Rotating my dipole woofer in the same point of placement, around its vertical axis, I can set the desired excitation of diagonal vs, longitudinal modes and that a fact. BTW, by rotation, the omni top-section works just the same. With the use of multiple subs the mains directivity at LF is not an
issue at all. More speakers to buy. Certainly above some frequency it starts to become an issue, but clearly this is not 200 Hz, that's far too low,
and just as clearly by 1 kHz we need to have the directivity well under control since, by this frequency, the ear is beginning
to get quite sensitive to reflections and timing aberrations. Hence, somewhere between 200 Hz and 1 kHz the directivity
needs to start to become narrower - down to the design angle - and then remain at that angle for the remainder of the audio
bandwidth.
It might be said that lower is better as regards the point at which the directivity begins to narrow, but this comes at some
cost in terms of the loudspeakers size and its actual cost to produce. The sizes and costs will approximately quadruple for
every octave that one wants to push the directivity control down in frequency. A reasonable sized and priced system can
achieve about 800 Hz and a large and expensive system might get down to about 500 Hz. But going below this gets
extremely big and expensive as might well be anticipated with a quadrupling of these factors just going to 250 Hz. Not with a direct-radiator cardioid, if dipole isn't acceptable for low front VER criteria. For the
purposes of this paper I will assume 500 Hz as a practical limit.
END QUOTE
>>>>>>>>>>>>
What a room treatment can do, especially diffusion, to VER and RFT, is beautifully explained in this paper attached, by Peter D'Antonio of RPG.
Look carefully at the graphs and you'll see what CD is aimed against. Also, keep in mind that CD will not correct the room problems, it will ignore them to an extent, and that will not make the system and the room sound excellent, only better. ONLY the room treatment and proper placing will make things sound excellent, something known only to people with treated rooms. If you haven't ever been in one, make the effort and visit one, you owe it to yourself. It will put you on the same plane and level of understanding to the people with treated rooms, that find it hard to talk audio to people that don't use them.
Everybody is welcome to my room, if ever passing here.
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It's just coincidental, I suppose, that the frequency below which Geddes finds directivity unimportant is . . . the frequency below which his speakers no longer maintain "controlled directivity". And while his "cross fired" approach may broaden the "imaging area" when close to the loudspeakers, it also substantially *increases* the same VER (from the side walls) that he otherwise says we should avoid, without doing anything to improve imaging when more that a few feet further back. In other words it's a technique for very small rooms (very small listening areas), at best.
OTOH my dipoles, when placed 5 ft. from the diffusive front wall (behind the speakers) has VERs that fall almost entirely outside the 10 ms "window" (probably most first reflection comes from the wall/ceiling junction and is 15-20 ms delayed), and the side walls are in the dipole null . . . but they do increase "spaciousness", without any negative effect (and maybe even with some positive effect) on "imaging". The result (combined with a relatively dead back wall) is a relatively large listening *area* (perhaps 8 ft. x 8 ft.) with acceptable-to-very-good "imaging" and overall very pleasant sound, and fairly uniform sound throughout the room as a whole. All without the in-your-face brashness that comes from sitting (almost) in the mouth of a horn.
In addition, the dipoles maintain "controlled directivity" down far enough that they don't excite the lateral and vertical room modes (which remain a problem with horns-in-boxes) . . . a clear advantage to maintaining controlled directivity" over a wider range than just the tweeter.
Of course I'm "lucky", and have an inherently "good" room to work with/in . . .
OTOH my dipoles, when placed 5 ft. from the diffusive front wall (behind the speakers) has VERs that fall almost entirely outside the 10 ms "window" (probably most first reflection comes from the wall/ceiling junction and is 15-20 ms delayed), and the side walls are in the dipole null . . . but they do increase "spaciousness", without any negative effect (and maybe even with some positive effect) on "imaging". The result (combined with a relatively dead back wall) is a relatively large listening *area* (perhaps 8 ft. x 8 ft.) with acceptable-to-very-good "imaging" and overall very pleasant sound, and fairly uniform sound throughout the room as a whole. All without the in-your-face brashness that comes from sitting (almost) in the mouth of a horn.
In addition, the dipoles maintain "controlled directivity" down far enough that they don't excite the lateral and vertical room modes (which remain a problem with horns-in-boxes) . . . a clear advantage to maintaining controlled directivity" over a wider range than just the tweeter.
Of course I'm "lucky", and have an inherently "good" room to work with/in . . .
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Flip your premise around and maybe you could say that Geddes designed directivity down to 500Hz because it wasn't necessary to go lower.
As for the significant toe-in, the increase in VER depends on the room's size and aspect ratio.
Yes, you could say that, but if you've got any experience with loudspeakers that *do* retain constant directivity below 1000 Hz. (and Geddes's speakers don't actually maintain constant directivity even down to 500) you probably wouldn't. And if you consider the size of the necessary horn (to stay CD) a potential . . . obstacle . . . might suggest itself (one that might lead to a paraphrase, like, "necessity is the mother of rationalization" . . .
Or not, of course. It could, as noted, just be a fortuitous coincidence that horns become impractical just when it "doesn't matter", and "baffle step" has never really been a problem after all (we just thought it was, all those years).
Aleksandar,
a whole lot of things to comment, but where to begin ...?
When I talk about CD, I think of a wider application than Earl. In my mind a properly done dipole would be as well CD as Geddes Summa or an omnipole.
If you increase the CD angle, you have to increase the control over room acoustics, yes. And yes, getting the room right is a problem for many people. And no issue for a lot of other people (the "let's have a party" part for instance).
I can't understand the difficulties to fight VER. The nulls of my dipoles are aiming roughly at the sidewall reflection points (like reported by "dewardh"). Measured SPL in that direction is 6-9 dB down (0.2- 4 kHz) compared to on-axis. Add the additional distance to the ear and we are below the researched detection threshold for image shift/spreading of speech in a live room (Toole, Sound Reproduction p.85).
I acknowledge that there is a certain reverberant "noise" floor in my room - the background is not as "black" as it could possibly be - but I don't experience a loss of imaging precision that would bother me.
I have heard those "old" planar dipole panels, where "apparent source width" could be better translated as "apparent mouth width", because the singers mouth was as wide as the stereo basis. But the directivity of those panels must have been a far cry from what I've got here.
Regarding RFT: Isn't this a relict from the times when real music was banned from loudspeaker and room measurements? Doesn't todays research into speech and music processing in the brain (Auditory scene analysis) advocate the contrary: Every reflection can contribute to the intelligibility of a sound and the ability to localize it?
This is not an request to ban all absorbers and diffusors from the listening room at home, but to use them as sparse as possible. I believe this is somewhat in line with some thoughts of David Moulton.
Rudolf
a whole lot of things to comment, but where to begin ...?
When I talk about CD, I think of a wider application than Earl. In my mind a properly done dipole would be as well CD as Geddes Summa or an omnipole.
If you increase the CD angle, you have to increase the control over room acoustics, yes. And yes, getting the room right is a problem for many people. And no issue for a lot of other people (the "let's have a party" part for instance).
I can't understand the difficulties to fight VER
. The nulls of my dipoles are aiming roughly at the sidewall reflection points (like reported by "dewardh"). Measured SPL in that direction is 6-9 dB down (0.2- 4 kHz) compared to on-axis. Add the additional distance to the ear and we are below the researched detection threshold for image shift/spreading of speech in a live room (Toole, Sound Reproduction p.85).I acknowledge that there is a certain reverberant "noise" floor in my room - the background is not as "black" as it could possibly be - but I don't experience a loss of imaging precision that would bother me.
I have heard those "old" planar dipole panels, where "apparent source width" could be better translated as "apparent mouth width", because the singers mouth was as wide as the stereo basis. But the directivity of those panels must have been a far cry from what I've got here.
Regarding RFT: Isn't this a relict from the times when real music was banned from loudspeaker and room measurements? Doesn't todays research into speech and music processing in the brain (Auditory scene analysis) advocate the contrary: Every reflection can contribute to the intelligibility of a sound and the ability to localize it?
This is not an request to ban all absorbers and diffusors from the listening room at home, but to use them as sparse as possible. I believe this is somewhat in line with some thoughts of David Moulton.
Rudolf