Mini-Line Array Box, But Used As A Center Channel?

[fredygump]

I have had another terrible idea, and I'm looking for someone irresponsible enough to encourage me!

I want to build a center channel to go with my main speakers. My speaker design is a bit unconventional, so I'm looking for the same with the center channel: https://www.diyaudio.com/community/...on-of-integrating-multiple-subwoofers.401343/

My first idea was to use the same coaxial driver I have in my mains--12" B&C 12FHX76. But that would be a really tall center speaker! And I have found that while it plays down to 100hz reasonably well, the mids sound a lot better when I cross it higher.

So the next logical step is to do a MTM style center, but instead it is a WCW...woofer, coax, woofer. I'd use a 5" or 6" coaxial driver, probably B&C. And 2 woofers the same diameter as the coax.

Then I had the worst idea. Why not make a box like a line array boxes used in pro audio? It would be the MTM style, but the drivers are angled so they are closer together than a standard MTM design. I think this would help horizontal directivity? This one could be a 2 way or 3 way. The 3 way version would use a coaxial, and the 2 way would have a horn/ compression driver.

Right now I only need a center channel, but if it works out, I'd eventually build a few more for surrounds as well.

Requirements are frequency response from 100hz to 20k(-ish), 115dB (impulse, peak) across that range, and compact (i.e. thin) enough to work as a center channel. Drivers are 6" diameter or smaller, and use a compression driver/horn for high frequencies. I want the design to control horizontal directivity as much as possible.

This image is a good representation of the general concept. They are common in pro audio, but I've never seen a center channel made like this. It's probably a terrible idea?

 

[fredygump]

I did some research and I found a driver combination that would work for a 3 way version.

For a coaxial I could use the Beyma 5cx200fe for highs and mids. According to VituixCAD it will need to be crossed to a low mid woofer at ~500hz. They recommend crossing it to the horn at 3,500hz. The published response for the horn looks pretty even

Then I can use 2x FaitalPro 6fe100. This will cover 100-500hz.

Now I just need to work out the cabinet dimensions to see if it would be an acceptable size.

 

[mayhem13]

That Beyma Coax has a low Fs for such a small midwoofer…..I would not hesitate to cross it to your woofers in the 350hz range. At that point, there’s no need for tilting anything…..you’re well within center to center distance 1/4 wave area to avoid any destructive combing. My little KEF UniQ 5” center is crossed at 250 to a pair of 6” woofers and the speech intelligibility is fantastic out to +/- 30 degrees.

 

[fredygump]

This is what I was looking at. I guess F3 is 344hz, so yeah, I guess I could cross it a little lower.

 

[fredygump]

Here's a rough drawing that seems to work OK. The simplest design.

These drivers don't really seem to care what size enclosure they are in.

The proportions look like what I would expect from a center channel. 24"x 16"x 8".

This would work, but I still think I want to angle the drivers. Why? Because I want to build it into a TV stand with 19" racks built in. So I'm tempted to squeeze this down to 19" (internal width)

 

[fredygump]

This is probably a smarter way. I'm still thinking of a design to double as a surround speaker. It is starting to look like something to me.

One idea was to try to copy Genelec's "Concealed Woofer" design. I'm seeing that possibility re-emerge. I just need a "wave guide" baffle across the front. I don't know anything about designing a functional wave guide, so I'll try to come up with something that looks the part.

 

[mayhem13]

Here's a rough drawing that seems to work OK. The simplest design.

These drivers don't really seem to care what size enclosure they are in.

The proportions look like what I would expect from a center channel. 24"x 16"x 8".

View attachment 1306845

This would work, but I still think I want to angle the drivers. Why? Because I want to build it into a TV stand with 19” So I'm tempted to squeeze this down to 19" (internal width)

……the use two 5” midwoofers as well……you only need 2 octaves of playback here with extension to 100hz…..another very easy ask.

 

[fredygump]

the use two 5” midwoofers as well……you only need 2 octaves of playback here with extension to 100hz…..another very easy ask

Do you have anything in mind?

I didn't list this as a requirement, but I want to use sealed cabinet.

It appears to me that the combination of sealed cabinet, 5-6" driver, and target of~115dB @100Hz (combined output) is quite limiting. Small subwoofers have low sensitivity. Small pro audio drivers are designed for higher frequencies. The rest don't have enough xmax.

I'm searching through usspeaker.com right now. They have more options than parts express. The B&C 5FG44 looks promising. Also Ciare HWG130-4 might work.

I haven't modeled them yet. I'd rather spend my day doing speaker design, but instead I had to work all day today doing "emergency" service work...

 

[Patrick Bateman]

No offense, but I've always found that speakers that have a dramatic change in the power response sound weird, and the problem isn't fixable with EQ, because EQ can't change polar response.

With line arrays in a theater or the like, the large height of the array can maintain directivity all the way down to 500hz, 250Hz, or even lower.

With line arrays that are eight inches tall, you're going to get a dramatic change in the power response, because the lens on the compression driver yields a narrow vertical directivity but only down to the height of the element. In your case, that's about 2200Hz.

 

[mayhem13]

Do you have anything in mind?

I didn't list this as a requirement, but I want to use sealed cabinet.

It appears to me that the combination of sealed cabinet, 5-6" driver, and target of~115dB @100Hz (combined output) is quite limiting. Small subwoofers have low sensitivity. Small pro audio drivers are designed for higher frequencies. The rest don't have enough xmax.

I'm searching through usspeaker.com right now. They have more options than parts express. The B&C 5FG44 looks promising. Also Ciare HWG130-4 might work.

I haven't modeled them yet. I'd rather spend my day doing speaker design, but instead I had to work all day today doing "emergency" service work...

Sorry Fred…..I can’t suggest anything for hearing damage at home levels. Roll that back to a reasonable 100db, and there’s plenty of options

 

[fredygump]

because the lens on the compression driver yields a narrow vertical directivity

I don't know if I misunderstand, but I'm not going to use a line array wave guide.

On my main speakers I'm using B&C 12fhx76 coaxials, and I chose 12" to control directivity at the crossover. They have 60x40 radiation pattern, so I'm hoping to choose a horn or coaxial driver that is similar.

I am still open to a 2 way (horn + woofers) or 3 way (coaxial + woofers). I'm still open to suggestions,

For example, I found the Ciare PR614 horn--it claims a 60x40 pattern, and it claims to extend down to 500hz. I could use it in a 2 way arrangement. I don't know what compression driver would play down to 500hz, but it is interesting.

 

[fredygump]

I can’t suggest anything for hearing damage

Lol!

I explained this is about dynamic peaks, not average SPL. Because movies have much higher dynamic range than stereo music recordings! Music is boring by comparison.

High end studio monitors will boast high SPL numbers. Why? Because transients. Dynamic range. Genelec's 8351 monitors boast 113dB SPL. And they are typically used as near fields!

THX reference level is 85dBa + 20dB headroom, so 105dB. You don't listen at 105dBa (average)...but the dynamics occasionally reach that level, momentarily. If you account for the distance you are away from the speaker, you may need something close to 115dB @ 1 meter to achieve the 105dB impulse response at the listening position.

 

[Patrick Bateman]

Ah, I thought you were opting for the CA8 AMT, which has a beamwidth of 70x20.

With a single unit, the pattern begins to widen at 2200Hz, because the waveguide is eight inches tall (2200Hz.)

Add another and the pattern would hold until 1100hz, add two more and the pattern will hold to 550Hz.

TBF, the waveguide looks well designed, and they cleverly juggled the parameters so that even if there's only ONE unit, the horizontal and vertical beamwidth are matching at the 1500hz crossover point. (70x70)

https://www.eighteensound.it/media/W1siZiIsIjIwMjIvMDQvMTIvMTlfMDFfNDhfMTAyX0NBOF9BTVQucGRmIl1d

I gotta admit, this might just work with one unit. I hadn't considered the possibility that they'd be clever enough to design it so that the vertical and the horizontal would match, due to the widening of the vertical beamwidth as the wavelengths exceed the height of the waveguide.

 

[mayhem13]

Lol!

I explained this is about dynamic peaks, not average SPL. Because movies have much higher dynamic range than stereo music recordings! Music is boring by comparison.

High end studio monitors will boast high SPL numbers. Why? Because transients. Dynamic range. Genelec's 8351 monitors boast 113dB SPL. And they are typically used as near fields!

THX reference level is 85dBa + 20dB headroom, so 105dB. You don't listen at 105dBa (average)...but the dynamics occasionally reach that level, momentarily. If you account for the distance you are away from the speaker, you may need something close to 115dB @ 1 meter to achieve the 105dB impulse response at the listening position.

Sounds like you’ve got your work cut out for you then. I’m not sure even the 5cx cd isnt going to suffer from thermal and power compression at those levels given it’s small size and lack of any mechanical heat dissipation. Add in the heat from the woofers voice coil and the ferrite magnet as a big heat sink. With such a small enclosure, you’re likely going to have to factor in some method of thermal relief in order for the the system to remain isothermal. And remember, stuffing will increase the temp as the rear wave friction hits the fibrous materials through friction. With this speaker rack mounted, if I were to assume sharing a semi closed cabinet shared with electronic gear?…… one might consider an Al baffle or box with fins. If not, you risk the symptoms of an adiabatic system where half way through a 2 hour movie, your drivers are overwhelmed with thermal compression.

Given your available space, I’m thinking a Unity/Synergy type system would be the better option if there’s more depth within your rack to accommodate.

 

[fredygump]

I’m not sure even the 5cx cd isnt going to suffer from thermal and power compression at those levels given

I don't think you understand the capabilities and power requirements of a pro audio coaxial? And the power consumption is determiend by the average, not the peaks!

At 85dB:
woofer = 0.25 watts
horn = 0.007 watts

At 115dB:
woofer = 200 watts (rated for 200 watts continuous)
horn = 8 watts (rated 20 watts continuous)


But after all of that, I'm not committed to this driver or even to using a coaxial driver.

 

[fredygump]

I hadn't considered the possibility that they'd be clever enough to design it so that the vertical and the horizontal would match

I don't know that it is difficult to do. You just have to understand that a single one of these cabinets is in effect a big coaxial driver. Same is true of a "MTM" design.

The directivity of the woofers is determined by the distance between the outer edge of both of them. If it is 12" from edge to edge of the opening, the speaker will have the directivity of a single 12" driver horizontally. In the same way, the verticle directivity will be determined by how tall the opening is.

Essentially it is an MTM, but the drivers are pushed to gether to fix the directivity issues they often have.

 

[fredygump]

Back on track. This is the 3-way version. The openings for the woofers are 12" apart, so that should match the directivity of my 12" coaxials.

The "box" in the center is for the 5" coaxial, and it has to be removable so I can install the drivers! The real line array does essentially the same thing. I thought about making it a cylinder too.

I thought 8" was still kind of tall for a center channel, but I did some research and found many center channels are around 8".

I was also thinking about hiding four 5" woofers inside to reduce the height. Maybe I'll draw that up tomorrow...

 

[fredygump]

Here is the 2 way horn version. The Faital Tactrix horn claims to work down to 800hz, but most likely I'd still cross it around 1khz.

I know next to nothing about compression drivers, so for this drawing I used Faital HF10AK dimensions. Horn is Faital LTH102. Any expertise on compression drivers is welcome!