Small center around 90 watts - AVS Forum
DIY Speakers and Subs > Small center around 90 watts
BirdRider's Avatar BirdRider 08:12 PM 03-05-2012
I'm looking into building a center channel for my home theater setup. I've got my system started and can be seen in my signature. I have some basic questions such as what the main goals of a center channel should be? I hear it is the most important piece of the system.

I'd like to design around the HiVi drivers if possible. I was thinking of just doing a Overnight sensations MTM but the 4" drivers are on back order. I've been looking at their other drivers but I've noticed they have a couple different categories.

What type of arrangement should I be looking at that would put me around 90 watts?

I'm thinking either an MTM or a WTMW. I have no idea what that really even means really. It would be nice to go with something that has a proven design or design one myself (with a bit of guidance).

One of the constraints is size. I'd like to make it about, 7-8" deep, less than 9" tall. The width doesn't really matter.

Cost probably sub 300.

I'm also not dead set on the HiVi drivers.

tuxedocivic's Avatar tuxedocivic 10:49 PM 03-05-2012
I think Paul's swope center might do the trick.
BirdRider's Avatar BirdRider 08:02 AM 03-06-2012
Don't you think that would be under powered. I'd have no problem doing it if I thought I could utilize the whole thing, but I fear it is too large for this application.
fbov's Avatar fbov 11:14 AM 03-06-2012
Power has nothing to do with your question as long as you're not power limited, and with 90W, you're not. You don't need matched power handling near as much as timbre matching with your mains, and most especially, clear dialog from a center channel.

It seems obvious that a 3rd ONS TM is the leading choice for a matching center... if ONS TM's meet you needs across the front. That's the bigger question I have from this and the thread you referenced - where are you going? What's the goal?

What I'd like to suggest, especially if the engineering aspects appeal to you, is a simple, basic set of speakers that will keep you entertained, but which will also reveal some degree of shortcomings. The idea is to make you aware of the shortcomings at the same time you're learning more about audio equipment, loudspeakers and rooms. Then you can make a conscious tradeoff among price, performance and capability.

If you want to learn to design speakers, get either Alden's or Dickason's books on loudspeaker design. If you want to understand audio, with a focus on speaker performance requirements rather speaker design, I highly recommend Floyd Toole's book, "Sound reproduction, loudspeakers and rooms." You can build anyone's design, but you will be designing your listening space yourself.

I suggest this as a starting point because you have a fully untreated room, and some geometric constraints that pose challenges. Some approaches to those challenges work better than others, and as an engineer, you want to know why one's better than another. Toole tells you why. Plus, no speaker can sound good if the room sounds bad...

There are two things you can do differently today that will help the sound.

You said: "They put out some good bass too which I wasn't expecting."
That may be a result of placement against a wall. Per Paul...
"...but since they have very full bass (or as some might say "full BSC"), they sounded best put up on a little stand...."

BSC is baffle step compensation, and it's an engineering term you'll want to learn. There are several on-line tutorials. The idea is that the spatial distribution of sonic energy depends on the wavelength compared with the size of the baffle containing the speaker. An "infinite baffle" is an infinite plane with a speaker in the middle - front and back waves never meet. Each (front and back wave) radiates into a hemisphere, a 2-pi steradian solid angle.

As baffles become practical sized (and back waves trapped in boxes), at wavelengths long compared with the size of the baffle, the baffle has no effect and the driver radiates into the entire sphere, a 4-pi solid angle. As the frequency rises, wavelength shortens until it's small compared with the baffle width, and we return to 2-pi radiation.

The transition from 4-pi radiation to 2-pi radiation is called "baffle step" and BSC the compensation for it. It's easy to see that you halve the radiated power density when you double the solid angle. Energy conservation is easy to see. However, diffraction provides a complete description, including intermediate wavelength behavior and appropriate frequency limits for a given baffle size.

Key message: you can greatly change a speaker's bass/low-midrange response in the entire room by changing the speaker's distance to adjacent walls, especially the back wall, and this is an XO design variable. An in-wall speaker design will sound very thin if used free-standing, while a free-standing design will be boomy if it's against the wall. Since Paul thinks ONS sound best away from walls on stands, try different placements/wall distances and listen for differences. I'm sensitive to tonal balance so this effect jumps right out at me.

The second is to get listener's ears away from the walls - your couch is too close to the wall.

Walls limit the motion of air molecules, making them displacement minima, and so pressure maxima, and we hear pressure. Try it yourself; move your ear closer to the wall when playing fairly loud music and see if the lows aren't reinforced the closer you get to the wall. Again, the issue is tonal balance, but this time, for each listener.

That's it for today's "experiments in home theater." Next, we should talk about room treatments; those broad flat walls will enhance some aspects of your listening experience, especially in stereo. However, reducing front wall reflections is a consensus winner for all applications. Not every build has to be at this level, either.

have fun,
BirdRider's Avatar BirdRider 12:53 PM 03-06-2012
I guess I'll try and answer your questions about my goals. I just want a sound system that can compete with much more expensive systems, for a better price. Plus, I like the building asspect that goes into it. I like that I can make them exactly how I want them.

Are you saying that I should maybe think about using a single Overnight sensation as my center? My real goal with the Overnight sensation was just to get me off the ground. I wanted something better that the 10W speaker in my TV. MISSION ACCOPLISHED. Next, I want to start putting together a more complete system, while still utilizing the reciver I just purchased.

As far as room treatments go, I don't really plan on staying in this house for more than 2 years. After that, I'll probably move. I want something that is semi universal, in that it isn't designed just to work in one space, IE a complete home theater.

I'm just looking for another starting point. The two Overnights will eventually probably get moved to the rear of the room. Or even a new room all together if need be. So if you are asking if I want a center to complement my Overnight's as left and rights, no, I plan on building left and rights further down the line.

I also plan on adding a subwoofer at one point. I have an Acendant Audio Atlas 15" in my car that I might pull out for this purpose. Seems their pricing has increased since the 7 years that have passed since I bought mine.
BirdRider's Avatar BirdRider 01:20 PM 03-06-2012
Also, can you explain how a larger speaker wouldn't be underpowered?
BirdRider's Avatar BirdRider 01:11 PM 03-07-2012
So I actually found a kit that paul designed that was pretty much exactly what I was looking for.

Already bought it! Lol

fbov's Avatar fbov 01:53 PM 03-07-2012
Speaker/amplifier power requirements are as simple as average listening level and headroom.

The average level is simply how loud you like it. Most folks find 85dB quite loud (85dB is the threshold for long-exposure hearing loss), and if you'll allow me 4 speakers in a 4 meter room, a speaker with 85dB/W@1M sensitivity will need 1W/speaker to achieve that level at the listening position. Average listening level has nothing to do with amplifier power requirements.

Headroom is how much louder the program can get before something hits it's performance limit, as in a solid state amp driven into clipping. The rule of thumb is about 20dB in a good classical program.
- +10dB is perceived as 2x increase in perceived loudness
- +10dB requires a 10x increase amplifier power.
- ROT headroom accomodates a 4x increase in loudness, but requires an amplifier capable of 100x the average power level. Note that this is a transient response situation, so RMS power is very conservative.

I was only saying that with 90W available RMS, you should have enough headroom for normal listening levels in a normal size room. The speaker's power rating isn't significant unless you want a high average level out of a low sensitivity design. The amp's power rating only matters if it clips on transients.

Regarding goals
I suggested 3 of the same across the front because it's cheap and it works. If you've got a progressive build plan, and the ONS are destined to become surrounds, that changes the CC recommendation.

Find a family of speaker designs based on common drivers that includes:
- L/R and CC specific designs.
- both free-standing and on-wall/in-wall XO designs are available.

Then build that CC, in anticipation of building the L/R down the road.

You may want to consider power handling, but moreso sensitivity and your room size and loudness preferences. 3dB of sensitivity is 2x amp power. A 95dB speaker at 10W is the same loudness as an 85dB speaker at 100W.

Sadly, I've got to run... I'll check back later.

Have fun,
BirdRider's Avatar BirdRider 08:41 PM 03-08-2012
I did some quick Inventor mock-ups of the enclosure design. Which one do you guys like? I'm leaning towards the last one.

Would the design change if I made the enclosure less wide but held the internal area the same? I want to shorten the width and make it a little deeper.
BirdRider's Avatar BirdRider 09:39 PM 03-09-2012
Does anyone know if changing the box shape will affect the sound if the internal volume is the same?
maxmercy's Avatar maxmercy 03:44 AM 03-10-2012
Yes, it can. I would run the change past Paul C, the designer.

mayhem13's Avatar mayhem13 05:42 AM 03-10-2012
Hey Bird. I read one of your replies where the OS speakers are a starting point, eventually being moved to surrounds. If this is truly the case, I'd say start with the integration of the center. While Paul's OSMTM center is nice, the Swope center is a MUCH more capable speaker. The only thing remaining will be to build the Swope mains and repurpose the OS for surrounds......sets you up for another fun filled project.
BirdRider's Avatar BirdRider 04:09 PM 03-11-2012
Originally Posted by mayhem13 View Post

Hey Bird. I read one of your replies where the OS speakers are a starting point, eventually being moved to surrounds. If this is truly the case, I'd say start with the integration of the center. While Paul's OSMTM center is nice, the Swope center is a MUCH more capable speaker. The only thing remaining will be to build the Swope mains and repurpose the OS for surrounds......sets you up for another fun filled project.

I think it'll be fine for now. I don't think I could pass the whole swope system by the big boss, aka my wife. I love the look of the OS's drivers too. I think I'll just make some MTM's as towers and call it a day. Honestly the two overnight sensations sound as about a good as I could ever ask for. Getting roughly 5 of them in this room will be nuts.

Anyone know if 1/2 oak ply wood will be good for enclosure building? I got a sheet from Lowes but the top laminates look really thin. I'm not even sure if the middle laminates are oak as well. They had huge 4' x 8' sheets but I had no way of getting that home. I should probably take a picture to explain.
BirdRider's Avatar BirdRider 12:29 PM 03-20-2012
So I have made some progress building this thing. I decided to use rabbit joints on all the corners because it makes it look nice for staining. All my tools are from harbor frieght. They actuall sell a 3/8" rabbit bit in a kit for like $30 and you get like 10 bits. Crazy how much they cost every where else.

BirdRider's Avatar BirdRider 06:10 PM 03-28-2012
Well, I finished this project tonight after about a solid 2 weeks. The paint and stain was the most frustrating part. Overall it looks "good enough" for now. Sounds amazing with the overnights.

maxmercy's Avatar maxmercy 06:41 PM 03-28-2012
I like it.

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