Yeah, CBT is fine in a room with a HIGH ceiling - it's really designed to overcome one of the limitations of a straight vertical array in a room WITHOUT a high ceiling!
But if you can build a floor to ceiling array, I'd not want a CBT.
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Bjorno, thanks, Hornresp has come a long way since it first came out! I need to get out of my cave more!!!
But if you can build a floor to ceiling array, I'd not want a CBT.
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Bjorno, thanks, Hornresp has come a long way since it first came out! I need to get out of my cave more!!!
The ceiling really didn't have anything to do with why the CBT was designed though it does have a smoother spectral content for ceiling reflections.
I wish I had a chance to talk to Keele. I've missed him at CES a couple of times now.
It's a long read, but this paper makes the case that the CBT is basically the optimum line array configuration:
http://www.xlrtechs.com/dbkeele.com...rmance Ranking of Loudspeaker Line Arrays.pdf
Lots of data in there.
I think there's a couple of things that may lead to a CBT sounding bad. IE, if you've heard a CBT, and didn't like what you hear, consider these:
1) On the horizontal axis, there is no pattern control. (Unless you made a CBT sphere! Which you could do.) Due to the lack of horizontal pattern control, you'll have to pay attention to the sidewalls. In addition to this, the optimum tweeter for a CBT is probably no tweeter at all. If you must use a tweeter, consider mounting it coaxially, like JBL does. If you mount it side-by-side, you're going to get horizontal comb filtering.
2) The sound radiated to the back wall will be focused at a single point. The CBT is basically a focused array when you look at it from the back side. So you'll really want to do something to treat that back wall.
In all other respects, it's pretty darn compelling. Particularly with DSP and amplification getting cheaper and cheaper. If I won the lottery, I'd probably take a crack at doing a straight CBT where the curvature is done via DSP.
One thing you learn from the CBT papers and the simulations is that it looks like there are no arrays that has been available publicly that cover the 65° arc that has the best results in the simulations.
But they do seem to offer a different type of a solution for what ails us in terms of coverage and soundstage.
Just not so easy of a cabinet to make.
But that's just how I likes it!
There is horizontal pattern control - just measure a CBT off-axis horizontally using different microphone heights. True, the drivers have their own effect on the dispersion so you may prefer some combinations over others. Tonally I find 2-ways to be much better with improved frequency extension and dynamics. Both of these far outweigh the off-axis response dip (which actually isn't that difficult to manage with the right crossover).
When I set up the DEQX for the original CBT-36 curves published by Don Keele he requested a 1K crossover point. With the 96dB slope it's very much an omni speaker. The tweeter is very small and shallow so it has virtually no pattern control and more prone to baffle diffraction. There are also some other factors that may play a role in that particular design so I don't think it's the best example of what a 2-way CBT can achieve. The desired price point made for some tradeoffs which also limited the overall performance.
I've been working with a DIY friend in Dallas to help him build a 2-way CBT. I'm looking forward to the comments on the design since he plans to take it to the MWAF in Dayton this summer.
I'm looking forward to hearing these. Don has been a frequent attendee at MWAF, and Tom Danley last year. Both were very approachable. I had a good discussion with Don about the new single driver CBT last year. It will be interesting to see who's judging this year. I've got a bunch of Qs for Tom.
Read the paper again.
There is no benefit to the CBT over a full floor to ceiling array.
(and ceiling "reflections" are a disadvantage to a CBT, not a benefit.)
If you think there is, then please cite it.
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Conventional loudspeakers produce spherical wavefronts
straight line arrays produce cylindrical wavefronts
One dimensional CBT arrays produce curved wavefronts
Two dimensional CBT arrays produce spherical wavefronts
I've listened to CBT arrays and straight arrays on the same day. The subjective difference to me is that the CBT is closer to the 'pinpoint' imaging that you can get with a good conventional loudspeaker. The straight line arrays tend to produce a big stage, but it's diffuse.
A lot of this will be subject to taste. I emailed someone who produced a well documented CBT, and he indicated that he'd opted to go back to straight arrays. Then again, his location is similar to the person that Rick said he's building a CBT for. So it's possible it's the same person and he went back to CBTs!
The CBT papers use floor reflections for his own model while omitting those for the straight array.
In the papers where Keele does use floor reflections in the modelling of the straight array it is only 1.25 meter in height.
So, basically as always, his papers do not give a comparison to a straight floor to ceiling array compared to his curved one (that somehow always includes the 'perfect' floor reflection). Biased info if you ask me.
I'd also like to see an IR as measured at the listening spot from the CBT, never seen any published so far.
In the papers where Keele does use floor reflections in the modelling of the straight array it is only 1.25 meter in height.
So, basically as always, his papers do not give a comparison to a straight floor to ceiling array compared to his curved one (that somehow always includes the 'perfect' floor reflection). Biased info if you ask me.
I'd also like to see an IR as measured at the listening spot from the CBT, never seen any published so far.
Implying that the CBT did not have good imaging when compared with a "good conventional loudspeaker". That was my opinion as well. To me it is more important to control the horizontal directivity with a high DI than the vertical. The CBT does the exact opposite. Its wide horizontal directivity has too many very early reflections which mess up the imaging. Floor and ceiling bounces do have tonal effects, but lateral ones completely mess up imaging. Correct the floor and ceiling with absorbers or diffusors (like I do) and use high DI speakers and you will have great imaging and no tonal degradation from the floor or ceiling reflections.
I discussed this with Don many many years ago, but he didn't have an answer that I found compelling.
Correct.
To me, it's a nice solution for a living room or a bedroom, where the very narrow cabinet has high WAF but without the dynamic limitations of a conventional sound bar or wife-friendly speaker.
To my ears, waveguides are still the ultimate directivity controlling device. It's too bad they're so darn big!
To me, it's a nice solution for a living room or a bedroom, where the very narrow cabinet has high WAF but without the dynamic limitations of a conventional sound bar or wife-friendly speaker.
To my ears, waveguides are still the ultimate directivity controlling device. It's too bad they're so darn big!
I haven't been following this thread, so, I don't have a lot of background. Just responding to the two posts above this one.
Floor and ceiling reflections do have tonal effects, as Earl has noted, and they are pretty hard to treat in normal listening rooms. This is what the CBT and other home line arrays address. Reflections from walls are much easier to treat and control. At the very least, if you can control floor and ceiling reflections using absorbers and diffusors, you can definitely control side-wall reflections the same way.
Having uniform directivity, not constant directivity, through the frequency range leads to good sound, but having any particular kind of horizontal directivity (narrow or wide) is a matter of preference, not a requirement for good sound in listening rooms.
Floor and ceiling reflections do have tonal effects, as Earl has noted, and they are pretty hard to treat in normal listening rooms. This is what the CBT and other home line arrays address. Reflections from walls are much easier to treat and control. At the very least, if you can control floor and ceiling reflections using absorbers and diffusors, you can definitely control side-wall reflections the same way.
Having uniform directivity, not constant directivity, through the frequency range leads to good sound, but having any particular kind of horizontal directivity (narrow or wide) is a matter of preference, not a requirement for good sound in listening rooms.
I have done the direct comparison between CBT36 and Abbey several times and in different rooms as well. First of all, both measure measure very even above 1000 Hz in every room. The difference however, is below 1KHz. Here the Abbey measures like a roller coaster, while the CBT36 is still very flat down to approximately 300-400 Hz. The difference here is due to the fact that the CBT has constant directivity much lower in frequency.
The result of this is not only a flatter response but the speaker sounds much more natural even with untreated side walls. The reason for this is obvious; Abbey only avoids reflections high in frequency where the waveguide operates. Below you still get reflections and the result here is altering the spectral content. Besides, with the strong toe in there's an increase of opposite side wall reflections. But the important factor here is that the constant direcitivty doesn't go low enough in frequency. Hence, the reflections are only avoided in the highs and distribution isn't even. CBT36 on the other hand avoids floor reflections, have little ceiling reflections (dependent on the CBT design) and horizontally the coverage is wide but extremely uniform. So while you get side wall reflections, the spectral content isn't altered and it still sounds tonality correct.
If you want strong pin-pointing/imaging with CBT you simply treat the side walls. Something you have to do with a waveguide speaker like Abbey as well or where you are actually more dependent on it due to a collapsing polar too early in frequency. It takes a much bigger horn speaker to have a 90 degrees coverage or less low in frequency. But even than, you still have some side wall reflections. I have a horn speaker with a beamwidth of 70-90 degrees down to 250 Hz, where I don't completely avoid reflections either in my room and have to use some treatment on the walls.
Oh by the way, a CBT can produce close to perfect square waves! See the paper below.
AES E-Library Time/Phase Behavior of Constant Beamwidth Transducer (CBT) Circular-Arc Loudspeaker Line Arrays
The result of this is not only a flatter response but the speaker sounds much more natural even with untreated side walls. The reason for this is obvious; Abbey only avoids reflections high in frequency where the waveguide operates. Below you still get reflections and the result here is altering the spectral content. Besides, with the strong toe in there's an increase of opposite side wall reflections. But the important factor here is that the constant direcitivty doesn't go low enough in frequency. Hence, the reflections are only avoided in the highs and distribution isn't even. CBT36 on the other hand avoids floor reflections, have little ceiling reflections (dependent on the CBT design) and horizontally the coverage is wide but extremely uniform. So while you get side wall reflections, the spectral content isn't altered and it still sounds tonality correct.
If you want strong pin-pointing/imaging with CBT you simply treat the side walls. Something you have to do with a waveguide speaker like Abbey as well or where you are actually more dependent on it due to a collapsing polar too early in frequency. It takes a much bigger horn speaker to have a 90 degrees coverage or less low in frequency. But even than, you still have some side wall reflections. I have a horn speaker with a beamwidth of 70-90 degrees down to 250 Hz, where I don't completely avoid reflections either in my room and have to use some treatment on the walls.
Oh by the way, a CBT can produce close to perfect square waves! See the paper below.
AES E-Library Time/Phase Behavior of Constant Beamwidth Transducer (CBT) Circular-Arc Loudspeaker Line Arrays
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It is not about waveguide vs. arrays. It is about horizontal vs. vertical directivity. Earl is correct.
I made the same experience with a simple monaural experiment. I listened to music (and pink noise) over two long AMT drivers. These drivers are so long that their vertical directivity is extremely narrow. I used a switcher to switch between the drivers without delay.
The advantage of this arrangement is that it elminiates all other differences when comparing speakers. It is the same driver. The only difference between them is the orientation. One has a narrow vertical directivity and the other a narrrow horizontal one. Additionally I could rotate the whole arrangement to check if factory spreads between the two drivers exist. In short: they were not relevant.
I listened in different rooms and even in my large garden (practically reflection-free). The conclusion is pretty clear. These were the diference I heard in different rooms:
Horizontal narrow directivity and vertical wide:
- strong narrowing of image (point like)
- near head localization
- sound is a more "dull"
Vertical narrow directivity and horizontal wide:
- widening of image (in width and heights)
- more distant localization
- sound is brighter
I tried to visualize the imaging:
The better treated the room was (my home cinema) the less the differences were audible. But they only disappeared when I put the arrangement in the garden. Then the differences were completely gone.
Of course the conclusions may not be completely transferrable to stereo, because there were no phantom sources. I don't have 4 of the AMTs so I can not do this.
And btw. an equalized dome tweeter with wide dircetivity in both dimensions continued the trend in terms of wide image, more distant localization and brighter sound.
I think the explanation of the brighter sound is that lateral reflections arrive to the ear from a steeper angle. And if you take a look at the HRTF then it is cleae why they sound brighter.
In my opinion a few lateral reflections are good and help to create a more "natural" sound field. Even if they doesn't belong to the source they help to create a pleasing experience. But too many lateral reflections create a diffuse sound that many people like, but I don't. I don't know what compromise is best for my room. It is a long way to evaluate this.
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That's an interesting comparison FoLLgoTT. The slightly different placement of the drivers on the baffle may have contributed a little bit, but shouldn't be of any great consequences.
What you described, is what we already know well from psychoacoustics. Lateral contribution increases spaciousness. Vertical reflections are really best to avoid.
Whether people want side wall reflections or not may be a matter of taste, the room geometry and what music material one is listening to. However, the speaker really needs to be constant over the most sensitive area; meaning down to at least 500 Hz. And lateral contribution is basically desired if they are delayed sufficiently. So with a traditional CBT speaker, the closest side wall reflections would in most rooms be best to treat. While the opposite side wall reflections can be used as a lateral contribution if desired. The spectral content will be very similar to the direct signal.
What you described, is what we already know well from psychoacoustics. Lateral contribution increases spaciousness. Vertical reflections are really best to avoid.
Whether people want side wall reflections or not may be a matter of taste, the room geometry and what music material one is listening to. However, the speaker really needs to be constant over the most sensitive area; meaning down to at least 500 Hz. And lateral contribution is basically desired if they are delayed sufficiently. So with a traditional CBT speaker, the closest side wall reflections would in most rooms be best to treat. While the opposite side wall reflections can be used as a lateral contribution if desired. The spectral content will be very similar to the direct signal.
That's true. But as I wrote I rotated the whole arrangement to check that. The result was the same.
Yes, it is nothing new. But to hear it that clearly is a different thing than reading about it. It helps to develop a speaker with the directivity oneself likes best.
I never understood why Keele didn't try to control the horizontal directivity, too. I tried it with a waveguide, but then using more than one way is very hard to build. Seeburg did a good job with their GL-16.
Yes, experiencing it for ourselves is important. Great test!That's true. But as I wrote I rotated the whole arrangement to check that. The result was the same.
Yes, it is nothing new. But to hear it that clearly is a different thing than reading about it. It helps to develop a speaker with the directivity oneself likes best.
I never understood why Keele didn't try to control the horizontal directivity, too. I tried it with a waveguide, but then using more than one way is very hard to build. Seeburg did a good job with their GL-16.
If it's controlled or not is a matter of definition. It's controlled in the way that's very constant/uniform. So even with side walls reflections, it does sound really good. IMO better than a horn speaker, unless the horn/waveguide is large enough to keep it's coverage down to approximately 4-500 Hz. P.S. Obviously one should address some flutter echo with CBTs, so total naked walls on both sides would not be good.
However, if you define controlled as being similar to narrow, than it's not. But is that the right definition?
Either way, it's possible to minimize the horizontal beamwidth with CBT. It can be done in several ways and we have some plans to do it. The reason why Don didn't do it, was mostly likely due to the size or the cost. Without serious DSP and a lot of amps, a CBT with narrow horizontal directivity sufficient low in frequency becomes very wide. Plus the thinks a traditional CBT with wide coverage works really well, which I agree with.
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Treat the speakers you use and the room as one system. Look at what happens at the listening spot and measure what happens there...
The CBT will average out a lot of reflections but I'm guessing it still needs either a large room (with speakers free from boundaries) or use side wall treatment.
Someone with a CBT please post up the IR showing the first 20 ms at the listening spot. STEP too if possible.
The CBT will average out a lot of reflections but I'm guessing it still needs either a large room (with speakers free from boundaries) or use side wall treatment.
Someone with a CBT please post up the IR showing the first 20 ms at the listening spot. STEP too if possible.
A measurement from one room is not going give you any clear pictures as it will room dependent. Learning the behavior of the speaker, which I've described, will on the hand.
But here's one ETC from CBT36 from a room. You're better off looking at the paper for other measurements.
AES E-Library Time/Phase Behavior of Constant Beamwidth Transducer (CBT) Circular-Arc Loudspeaker Line Arrays
But here's one ETC from CBT36 from a room. You're better off looking at the paper for other measurements.
AES E-Library Time/Phase Behavior of Constant Beamwidth Transducer (CBT) Circular-Arc Loudspeaker Line Arrays
