JBL CBT 70J-1 Review (Constant Beam Transducer)

[Rick Sykora]

From experience, my $1000 CBT24s easily provide a better listening experience for multiple listeners in my home theater than any monopole I have had or heard. I have heard the same from other owners. If their soundstage could be replicated in a smaller form factor at a lower price, I have little doubt they would dominate the home theater market.

Unless the CBT was able to get there (and still does not seem attainable today), can see why Harman left them in the sound reinforcement domain. With @Bjorn and others efforts, they may find a niche in the high-end market. Even with the current SOA tech, it still seems to be a stretch to get them packaged for broader market acceptance.

 

[yourmando]

So, he's essentially saying that CBT is still better for LCR, but the advantage is just smaller

I’ve found Floyd Toole’s comments in his book about the benefit of CBTs, in particular for side surrounds. P 279:

”JBLPro has a series of innovative free-standing CBT products, designed by Doug Button, that have found use in sound reinforcement and high-end cinema and home theater surround applications. The stability of both sound quality and sound level as a function of listening distance is striking, which makes such designs generally attractive and especially for use as side surrounds (see figure 14.4).”

He dives deeper on pp 393-394 where the primary benefit as surrounds is because of the less than inverse square law drop in levels. No comments on use as LCR, where this particular issue is not a big.

 

[yourmando]

Toole doesn't say anything about not using CBT speakers in general as LRC. Why would he do that when a well designed ground plane CBT measures generally much more even placed in the room compared to the most expensive Harman's speakers and has lower distortion as well?

He talks specifically about the CBT 70J-1 and the CBT 1000 which are free field CBTs and with some quite obvious weaknesses, which we also clearly seen in Amir's measurements of the CBT 70J-1. These do not avoid floor reflections, the on-axis response isn't great, there are diffraction issues, lobing of the 70-J-1 start fairly early in frequency, they don't offer much low frequency level and the horizontal directivity is quite uneven. In other words this type of CBT speaker simply has many disadvantages as a LRC speaker. However, this has nothing to with the CBT technology or CBT speakers in general. A CBT speaker can be deisgned in numerous ways.

Harman/JBL doesn't offer any ground plane CBTs which is the kind that makes sense to use as a LRC. Most likely they are not prioritizing it due to higher cost of both drivers and assembling or some other marketing reasons.

The best CBT surround speaker IMO would be one that is mounted against the ceiling and uses the ceiling as a mirror just like the ground plane version uses the floor as a mirror. That gives several acoustic advantages.

As you and Amir point out, a higher quality LCR would be welcome. I’d love to see the floor standing Keele design you guys are working on come to life.

However, I don’t think you’ll get much traction with surrounds as ceiling mounted CBTs (using it as a mirror). It simply would not work well with the latest formats that include a bed layer and a height layer. They need to be separated to sound like the mix intends, and the height layer needs to point toward the listeners—it can’t be perpendicular to the ceiling.

For the bed layer, the JBLPro CBT format seems ideal for placement flexibility because each pair of side and rear surrounds needs to be a bit higher as each row higher than the one in front.

 

[Bjorn]

As you and Amir point out, a higher quality LCR would be welcome. I’d love to see the floor standing Keele design you guys are working on come to life.

However, I don’t think you’ll get much traction with surrounds as ceiling mounted CBTs (using it as a mirror). It simply would not work well with the latest formats that include a bed layer and a height layer. They need to be separated to sound like the mix intends, and the height layer needs to point toward the listeners—it can’t be perpendicular to the ceiling.

For the bed layer, the JBLPro CBT format seems ideal for placement flexibility because each pair of side and rear surrounds needs to be a bit higher as each row higher than the one in front.

Perhaps my english was unclear. I still meant on the wall but all the way up towards the ceiling. If the height is sufficient of the CBT, it will also end being in more or less at the perfect height in regards to the surround standard. If the was placed on the ceiling itself pointing downwards, it wouldn't use the ceiling as a mirror.

 

[magic44ken]

For those of you who want to read more about the CBT technology and see different presentations, you can use the link below.
https://www.keele-omholt-technologies.com/papers.php

We (K&O Tech.) plan to release a ground plane CBT to the home market hopefully before summer but progress are slow these days with the corona situation.

A difference between a CBT like the JBL 70J-1and a ground plane version is the latter avoids floor reflections as it uses the floor as an acoustic mirror of the speaker. The result of the avoidance of the floor reflections, minimization of ceiling reflections and minimization of the vertical room mode is much more even frequency response when placed in a living room.

What is the speaker size when you say for home theater? For example, for traditional speaker a small tower uses 6.5 inch woofer and a mid tower uses 8 inch woofer, etc...

 

[yourmando]

Perhaps my english was unclear. I still meant on the wall but all the way up towards the ceiling. If the height is sufficient of the CBT, it will also end being in more or less at the perfect height in regards to the surround standard. If the was placed on the ceiling itself pointing downwards, it wouldn't use the ceiling as a mirror.

Yes, I see what you mean. It’s just a bit tricky with ceiling heights and shapes varying. Some have very high ceilings, my ceiling is high and is slanted, etc. Also clear separation from the bed layer and height/top layer is essential. Your idea could work work in certain rooms with low enough ceilings that are flat, but applications would be a bit limited vs the JBLPro design as surrounds.

 

[BenB]

Harman/JBL doesn't offer any ground plane CBTs which is the kind that makes sense to use as a LRC. Most likely they are not prioritizing it due to higher cost of both drivers and assembling or some other marketing reasons.

I don't see how a ground plane CBT would make any sense in most domestic rooms. The backward radiation onto the front wall would cause a very strong reflection. I recall reading about an installation where the owner ended up using multiple FEET of absorption on his front wall to try to mitigate it. I remember thinking how unfair it was that Don put together a white paper comparing his design to a B&W speaker. He had them both in a very large warehouse, and blasted the B&W for the uneven impact of the floor reflection on the measurements at the listening position. He didn't have any wall behind the speakers, that would cause similar issues for his speaker.

 

[HammerSandwich]

The Kii BXT claims to do the line array thing up to 250Hz. I guess that that's useful, but exactly how useful?

The BXT does produce some gain from floor mirroring, and it reduces floor bounce by varying the distances between each driver & ear. (IOW, each driver's cancellation is at a different frequency, so they all average out better than a single woofer.) That said, "line array" is probably more "explain the benefits easily" that objective accuracy.

Consider that the BXT is about 80cm tall. That's one wavelength at 430Hz. Could this offer useful line-array focusing below 200Hz?

 

[Bjorn]

What is the speaker size when you say for home theater? For example, for traditional speaker a small tower uses 6.5 inch woofer and a mid tower uses 8 inch woofer, etc...

I didn't say specifically for home theater. The version we're working on is a high-end CBT speaker. While it certainly can be used for home theater, I assume it will have a bigger market towards audiophiles and stereo. More information will come in a different thread.

 

[Bjorn]

I don't see how a ground plane CBT would make any sense in most domestic rooms. The backward radiation onto the front wall would cause a very strong reflection. I recall reading about an installation where the owner ended up using multiple FEET of absorption on his front wall to try to mitigate it. I remember thinking how unfair it was that Don put together a white paper comparing his design to a B&W speaker. He had them both in a very large warehouse, and blasted the B&W for the uneven impact of the floor reflection on the measurements at the listening position. He didn't have any wall behind the speakers, that would cause similar issues for his speaker.

Why does a ground plane CBT speaker cause a particular strong backward radiation?

Both the free standing and the ground plane CBT speaker has a baffle step like other speakers and depended on the baffle size, but with a line array it's different than traditional speakers and actually happens lower in frequency. Remember that the CBT36 and CBT24 have a narrow baffle.

 

[JustIntonation]

It may be worth mentioning that the CBT only has a wide dispersion at a certain height. Above and below that window, the dispersion is actually narrow. The horizontal directivity of "traditional" CBT speakers will vary quite a bit and depends on the design.

I think many tend to disregard the importance and influence of vertical directivity. Floor and ceiling reflections arrive very early in most living rooms and don't add anything beneficial psycoacoustically. Especially the floor reflection is also almost impossible to treat in a linear way. A rug will alter the spectral content greatly and no one wants thick and huge absorbers on the floor. The vertical reflections causes quite strong tonal deviations.

It's possible to design a CBT with a narrow horizontal directivity. Or with the ability to change the horizontal beamwidth for that matter. Many possibilities but cost will substantially increase the more complex it's designed. Personally I lean towards horn speakers if a truly narrow directivity is desired. Because it can be done at a much lower cost. But it becomes big if it's going to be great sounding. No free lunch as they say.

Yes that is what I meant.
This CBT has a very narrow vertical dispersion yet a very wide horizontal dispersion.
So it appears to me that Amir's listening preference (for his preference of sounding "tonally correct") is not for a narrow or sloping overall off-axis but more specifically for a narrow or sloping vertical off-axis.
I say this because he consistently rates speakers that have both a broad vertical and horizontal off-axis (and as a result have an "in room" fr that does not have much of a slope) as being bright. And he rates speakers that have a narrow vertical and horzontal off-axis as sounding tonally correct (waveguided designs). Now here we have a CBT that has a wide horizontal off-axis yet a narrow vertical off-axis and he still rates it subjectively as sounding tonally correct. So it appears the vertical off-axis is leading for his subjective experience in his room.
So perhaps a good rug and/or ceiling absorber (cloud) would have fixed his subjective experience of tonality for the speakers he found to sound "bright" yet which showed a smooth off-axis and a flat on-axis fr.

btw, for what it's worth I've personally always found a good rug or carpet to sound better than a fully reflective floor.

 

[BenB]

Why does a ground plane CBT speaker cause a particular strong backward radiation?

Both the free standing and the ground plane CBT speaker has a baffle step like other speakers and depended on the baffle size, but with a line array it's different than traditional speakers and actually happens lower in frequency. Remember that the CBT36 and CBT24 have a narrow baffle.

Narrow baffles combined with small drivers that don't beam until high in frequency is a recipe for a lot of sound wrapping around the baffle and radiating backward. Secondly, by effectively eliminating a floor bounce, and reducing the ceiling reflections, the reflection off the front wall becomes a higher fraction of the sound reaching the listener. Third, to whatever extent the speaker "defies the inverse square law", the sound from the virtual source behind the wall is louder at the listening position than a traditional speaker that does follow the inverse square law. Lastly, these types of speaker are advertised as being "room friendly" and mitigating undesirable early reflections, so the customer base is skewed toward those who care about reducing unwanted early reflections.

 

[Bjorn]

Yes that is what I meant.
This CBT has a very narrow vertical dispersion yet a very wide horizontal dispersion.
So it appears to me that Amir's listening preference (for his preference of sounding "tonally correct") is not for a narrow or sloping overall off-axis but more specifically for a narrow or sloping vertical off-axis.
I say this because he consistently rates speakers that have both a broad vertical and horizontal off-axis (and as a result have an "in room" fr that does not have much of a slope) as being bright. And he rates speakers that have a narrow vertical and horzontal off-axis as sounding tonally correct (waveguided designs). Now here we have a CBT that has a wide horizontal off-axis yet a narrow vertical off-axis and he still rates it subjectively as sounding tonally correct. So it appears the vertical off-axis is leading for his subjective experience in his room.
So perhaps a good rug and/or ceiling absorber (cloud) would have fixed his subjective experience of tonality for the speakers he found to sound "bright" yet which showed a smooth off-axis and a flat on-axis fr.

btw, for what it's worth I've personally always found a good rug or carpet to sound better than a fully reflective floor.

Take note that a CBT doesn't has to be designed to be wide horizontally. So we can't make general rules, it will depend. One could design a CBT with super narrow horizontal dispersion.

I agree that a rug in front of speakers often improves compared to a reflective floor and it minimizes flutter echo. But it's a very bandlimited treatment which primarily absorbs above 3 KHz and leaves the other frequencies untreated. So compared to broadband treatment it's quite poor when you've done the comparison. Everything is relative.

 

[Bjorn]

Narrow baffles combined with small drivers that don't beam until high in frequency is a recipe for a lot of sound wrapping around the baffle and radiating backward. Secondly, by effectively eliminating a floor bounce, and reducing the ceiling reflections, the reflection off the front wall becomes a higher fraction of the sound reaching the listener. Third, to whatever extent the speaker "defies the inverse square law", the sound from the virtual source behind the wall is louder at the listening position than a traditional speaker that does follow the inverse square law. Lastly, these types of speaker are advertised as being "room friendly" and mitigating undesirable early reflections, so the customer base is skewed toward those who care about reducing unwanted early reflections.

It's not the highest frequencies that radiate backwards. It happens lower in frequency even with a narrow CBT like the JBL here. Besides, these drivers actually do beam. It's only CBT36 of the CBTs that's been sold that's is capable of maintaining the directivity to the highest frequencies, but the sound wrapping around the baffle starts more in the midrange.

With the CBT36 you generally have much less early arriving reflections compared to most speakers due to the avoidance of the floor reflections and minimization of ceiling reflections. I've seen that many times in measurements. But obviously there are designs that minimizes early reflections even more, when they are also narrow horizontally. That's the case with a horn design I have which has a directivity of 80°x50°. If one wants the combination of constant and narrow directivity in both planes and very low baffle step, a huge horn can give that without beaming. Or with a different CBT design for that matter.

The CBT we're working on has a wider baffle, so baffle step will happen lower in frequency. If I remember correctly it's 11 cm wider at the widest compared to CBT36.

 

[Rick Sykora]

Just to keep them out harm's way, my ground plane CBTs are placed against a back wall (as far as the arc allows) and sound just as good as if placed further out in the room.

No doubt that above the baffle step, a larger baffle is going to have more forward output, but a minimal baffle design like the B&W 801 is going to have to lower forward output too (just like the CBT). So, for a speaker that has a wider baffle how much less backward radiation is there at higher frequencies?

In any case, if I have to try to place a wide baffle speaker in my room, it better be in the wall or my wife will kicking them to the curb!

 

[BenB]

Just to keep them out harm's way, my ground plane CBTs are placed against a back wall (as far as the arc allows) and sound just as good as if placed further out in the room.

Do you have measurements to show the difference at the listening position?

 

[Rockfella]

Seems like JBL overall has good line up of speakers .. in almost all price ranges.

 

[Rick Sykora]

Do you have measurements to show the difference at the listening position?

I had no reason to measure as IO (and others) were pleased with the result. This was over a year ago and have only moved them once. I wanted to try them as surrounds. While OK, not near the improvement as using them as mains.

It is a family room that is used for multiple purposes (measuring the Direcitva design rn). With the CBT soundstage, there really no longer is a single listening position but the usual sweet spot is about 15 ft from the CBTs. As I mentioned, due to curvature (and a shelf behind them), they are already almost 3 feet into the room. This is way further into the room than the BG towers that preceded them.

 

[yourmando]

Edit: here are two 3D graphs which should help with understanding of how the sound field is shaped to be a horizontal donut as opposed to sphere:

View attachment 118970

View attachment 118971

Hi Amir--did you test with the "broad" or "narrow" vertical coverage setting?

The "broad" setting doesn't look afflicted by the lobing issues from about 8khz and up.

Also, the vertical beam width is smoother, staying level from about 1khz on up, where the "narrow" setting beam width continues to narrow as we go higher.

Also, the vertical off-axis frequency response is much smoother in "broad", and at the more extreme angles like 40 degrees, attenuation is actually greater in broad!

So "narrow" is great for a bit more SPL in longer throw applications, but if the specs are accurate, then "broad" is a good way to go for a smoother vertical pattern w/o the lobing issues.

 

[MerlinGS]

As I mentioned, due to curvature (and a shelf behind them), they are already almost 3 feet into the room. This is way further into the room than the BG towers that preceded them.

Without derailing the thread too far, do you mind if I ask you which BG ribbons and in what configuration? And to bring it back to the thread, sonically how did they differ and resemble the CBTs? Thanks