Constant Beamwidth Transducer (CBT) Speakers

[somebodyelse]

https://www.keele-omholt-technologies.com/papers/Keele-2003-10-AES-Preprint-CBT-Paper4.pdf covers Keele's approach to approximating the shading function with banks having 3dB steps that could be done with relatively simple passive arrangements.

 

[TimVG]

Many thanks!

 

[gnarly]

@TimVG, here's paper by Jim Griffon you might find helpful. https://audioroundtable.com/misc/nflawp.pdf
I have a spreadsheet shading calculator somewhere, on some damn hard drive. If I can find it, will post

 

[Cbdb2]

Very nice project there, @gnarly. I’m a bit envious.

My understanding of the tech is that your use of 6 banks of 4 drivers would be more than sufficient to get the full acoustic performance. More of an issue would be the driver spacing, which is too great to create the CBT effect above 4-5 kHz. Plus, assuming the drive units are typical of the breed, they might start to individually beam above 6-8 kHz, which is also something Keele was trying to avoid. Nevertheless, great job.

cheers

His version has 72! Tweeters per side to deal with that.

 

[TiFi Speakers]

It seems that some aspects of living room sound reflections have not yet been fully explained. As a general rule, I have always considered that floor and ceiling reflections are unnecessary or even detrimental to achieving perfect sound at the listening spot. However, there are numerous examples that challenge this belief.

One obvious example is CBT, where part of the sound is directed upwards. Another example, not yet fully explained to me, is the Linkwitz LXMini DIY, which was measured here: https://www.erinsaudiocorner.com/loudspeakers/linkwitz_lx_mini/ (with Erin comments). The same applies to the TiFi Elf, measured here: https://tifi.ee/en/content/10-elf-spinorama. (In TiFi's assortment, there are other speakers that have a much more ideal sound profile than the Elf. Still, the Elf is preferred in listening sessions with a ratio of 95% vs. 5%. There may be other reasons, but it could also be a difference in the vertical HF reflection profile. But can not prove...) The final aspect to consider is Dolby Atmos. I suspect that there was some non-public research behind the selection of the height dimension. Of course, these are all speculations.

The unexplained part is what makes the audio field so interesting, expect as much the "Everything affects everything" when designing speakers.

 

[markus]

In TiFi's assortment, there are other speakers that have a much more ideal sound profile than the Elf. Still, the Elf is preferred in listening sessions with a ratio of 95% vs. 5%. There may be other reasons, but it could also be a difference in the vertical HF reflection profile. But can not prove...

"Easy" to test: Do controlled listening tests with and without a ceiling absorber or reflector.

 

[TiFi Speakers]

Do controlled listening tests with and without a ceiling absorber or reflector.

Thank you for the proposal. You are correct. One of the hints to propose that this is about vertical is the difference in sound in rooms with lower ceiling. There TiFi Elf shines more. But regarding proper double blind test with ceiling absorber I can make a promise. If TiFi Speakers will be as large as Harman - we will do it!

 

[gnarly]

"Easy" to test: Do controlled listening tests with and without a ceiling absorber or reflector.

I thought CBT's are more about mitigating floor reflections, than ones from the ceiling...

 

[Bjorn]

Surprised hos much nonsense is being shared here. The CBT greatly minimizes ceiling reflections. One of its strengths of course.

 

[markus]

Thank you for the proposal. You are correct. One of the hints to propose that this is about vertical is the difference in sound in rooms with lower ceiling. There TiFi Elf shines more. But regarding proper double blind test with ceiling absorber I can make a promise. If TiFi Speakers will be as large as Harman - we will do it!

You could mount a reflective panel at the ceiling that can be tilted with two strings. Should be simple to build and effective for a first evaluation.

 

[markus]

Surprised hos much nonsense is being shared here. The CBT greatly minimizes ceiling reflections. One of its strengths of course.

Yes it does. As far as I can see, no one has claimed the opposite?

 

[TiFi Speakers]

The CBT greatly minimizes ceiling reflections.

It is best to look what has been discussed and illustrated in Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, Third Edition, by Floyd E. Toole, page 278 (picture one of seven referred) There is comparison of 7 different types of sound sources compared in vertical axis. So everyone can estimate there the differences, and also think of profiles of some higher frequencies and different loudspeakers. I suppose better characterization is homogenized ceiling reflections (and sound field over frequencies). As for "reduced" it has to be specific comparison.

 

[Newman]

Surprised hos much nonsense is being shared here. The CBT greatly minimizes ceiling reflections. One of its strengths of course.

Ceiling reflection energy is about 5-7 dB below direct energy, and is full bandwidth.

Compare red vs red responses. Calling that "greatly minimized" is a bit of a leap. See how I refrain from terms like "much nonsense".

Source: Keele, D. B., B&W 801 Matrix Series 3 vs. Audio Artistry CBT36 Ground‐Plane Measurements, 2011​

 

[markus]

Ceiling reflection energy is about 5-7 dB below direct energy, and is full bandwidth.
View attachment 365939View attachment 365938

Compare red vs red responses. Calling that "greatly minimized" is a bit of a leap. See how I refrain from terms like "much nonsense".

Source: Keele, D. B., B&W 801 Matrix Series 3 vs. Audio Artistry CBT36 Ground‐Plane Measurements, 2011​

Psychoacoustically those 5-7dB can be considered "greatly minimized" when it pushes audibility below detection threshold. But yes, Bjorn seems to be a bit frustrated about how others talk about CBT

 

[TiFi Speakers]

Read my post again, yes it was not correctly formulated. Nothing to argue about facts presented. For general interest there is also different way to control bass directivity, referred here in other thread - Thesis of Olli Kantamaa: https://aaltodoc.aalto.fi/server/api/core/bitstreams/53118e62-e0d0-4a6d-949b-b74049a069e4/content.

 

[markus]

Read my post again, yes it was not correctly formulated. Nothing to argue about facts presented. For general interest there is also different way to control bass directivity, referred here in other thread - Thesis of Olli Kantamaa: https://aaltodoc.aalto.fi/server/api/core/bitstreams/53118e62-e0d0-4a6d-949b-b74049a069e4/content.

Nice find! Includes a short and good summary of what science currently knows.

 

[Bjorn]

Ceiling reflection energy is about 5-7 dB below direct energy, and is full bandwidth.
View attachment 365939View attachment 365938

Compare red vs red responses. Calling that "greatly minimized" is a bit of a leap. See how I refrain from terms like "much nonsense".

Source: Keele, D. B., B&W 801 Matrix Series 3 vs. Audio Artistry CBT36 Ground‐Plane Measurements, 2011​

With a normal ceiling it's below 20 dB vs the direct signal with the CBT36. One needs to understand what measurement to look at.

 

[markus]

With a normal ceiling it's below 20 dB vs the direct signal with the CBT36. One needs to understand what measurement to look at.

Can you share those measurements that show a wideband 20dB attenuation? Hope you're not referring to anything IR related as this would be highly misleading.

 

[Bjorn]

Can you share those measurements that show a wideband 20dB attenuation? Hope you're not referring to anything IR related as this would be highly misleading.

Do you understand what specular reflections in a small room are?

If so, explain why an IR would be misleading when the vertical directivity is constant to 250-300 Hz.

 

[markus]

Do you understand what specular reflections in a small room are?

If so, explain why an IR would be misleading when the vertical directivity is constant to 250-300 Hz.

No need to get snappy. IR peaks are dominated by HF. So again, can you please share those measurements?