## Designing a dome faceted loudspeaker

jeno
Posts: 599
Joined: Sun Jun 29, 2014 10:41 am

### Re: Designing a dome faceted loudspeaker

DeepSOIC wrote:
Sun Mar 17, 2019 4:33 pm
jeno wrote:
Sun Mar 17, 2019 3:41 pm
is the speaker on the picture a 5" Focal?
Nope. It's R2R Audio's one, made here in saint-petersburg, by a few guys in a small lab (almost a garage).
Interesting. Similar design idea like Focal. No dust cap and a fixed phase correction plug.
Remembers me on a long time ago. I also did some experiments with speakers and this an that, but there is a ugly ugly con .. the women acceptance factor
Anyway.. I hope I could help a bit. Maybe you change the dome a bit and use only 4 speakers. Could bring resonace frequency remarkably down.
Keep us updated

cheers
jeno
DeepSOIC
Posts: 7145
Joined: Fri Aug 29, 2014 12:45 am
Location: Saint-Petersburg, Russia

### Re: Designing a dome faceted loudspeaker

I think I am on to something.

I've opened a book "introduction to acoustics" by Rienstra and Hirschberg. There is a section for pulsating-sphere emitter. And man it's difficult to follow. Lots of math, with heavy reliance on symbols defined in earlier sections. And no pictures or practical examples to quickly relate to.
introduction to acoustics wrote:For a compact sphere the first term is dominating. We find exactly the result which we could anticipate from (2.59), the second derivative to time of the volume of the sphere is the source of sound. ... The second term of (6.9) is dominating for large sphere radii. In such a case the action of the wall movement is that of a piston which generates plane waves.
Nontheless, I think I managed to pick and decipher the two key pieces of math, for to derive frequency response of a sphere densely covered by point loudspeaker-like sources. (and after deriving it myself, and rereading the chapter, I realized the result was actually provided in the book, I just didn't recognize it).

For how I derived it, see attached .docx file. I just state here, that I got that for a sphere densely covered by small drivers with heavy cones, the frequency response of the sphere is not going to be flat, even if the drivers are small enough to be considered point-sources, and have ideal lack of directionality.

Specifically, the response is flat up to a corner frequency fc = c/(2*pi*R), where c is speed of sound and R is the radius of the sphere. (It is exactly when the circumference of the sphere equals one wavelength.) Below that frequency, the amplitude should fall like 1/f.

For my sphere, the corner frequency is about 1100 Hz. Go figure. So I multiplied the frequency response of the driver by this sphere-factor, and here's what I got:
theory testing.png (93.63 KiB) Viewed 940 times
Apart from a small hiccup between 1.2-1.5k, the calculated response fits well with the measured one. From 2.8k, they deviate strongly, with real thing having much more output than expected. I would guess, this is because my drivers are too far apart, and above 2.8k they just stop collaborating to make that piston-like radiator, and behave more like a scattered set of point sources. Then probably going into beamy mode at circa 9k.

So I feel like I have a satisfactory explanation to that "BUT WHY" question.
Attachments
pulsating sphere frequency response (en).docx
DeepSOIC
Posts: 7145
Joined: Fri Aug 29, 2014 12:45 am
Location: Saint-Petersburg, Russia

### Re: Designing a dome faceted loudspeaker

This one has a surprisingly similar geometry to mine.
https://www.bksv.com/en/products/transd ... light-4292
OmniPowerSoundSource_Type4292 L_600x600.png (404.43 KiB) Viewed 771 times
DeepSOIC
Posts: 7145
Joined: Fri Aug 29, 2014 12:45 am
Location: Saint-Petersburg, Russia

### Re: Designing a dome faceted loudspeaker

New version is in the works.
46-dome v00.png (119.22 KiB) Viewed 595 times
46-dome v00.FCStd

n of drivers = 46
surface coverage = 68%
DeepSOIC
Posts: 7145
Joined: Fri Aug 29, 2014 12:45 am
Location: Saint-Petersburg, Russia

### Re: Designing a dome faceted loudspeaker

Let me explain, why am I building a new one.

I made a correction filter for the old one. With the filter, the speaker sounds awesome. I also rewired it to become stereo (3 drivers per channel), and surprisingly, it worked much better than I was expecting.
Now I bump into a serious shortage of loudness. The extremely small air volume makes it very difficult for the amplifier to give the drivers some flex, which is really needed for lows. 30W amp is not enough for this puppy, but at a larger power, drivers can overheat.

I got quite attached to the sound of this thing, but I really want it go louder. So, I'm up to building a larger one
DeepSOIC
Posts: 7145
Joined: Fri Aug 29, 2014 12:45 am
Location: Saint-Petersburg, Russia

### Re: Designing a dome faceted loudspeaker

Sound test:
https://youtu.be/JjvcGxaviUU?t=1071
(video is in russian. see timecodes in video description for more music tests.)

Building the new dome is stuck in search for a good driver.
Jee-Bee
Posts: 1966
Joined: Tue Jun 16, 2015 10:32 am
Location: Netherlands

### Re: Designing a dome faceted loudspeaker

Nice project!! can't understand what you say but i think i can follow what said thanks to auto translate...