Speaker design

Eric Best · Post by **Eric Best** » Thu Mar 20, 2003 3:11 am

I want to build some speakers to soffit mount and supplement my nearfields. What kind of considerations do I need to take and are there any available designs that anyone knows about.

Eric

barefoot · Post by **barefoot** » Fri Mar 21, 2003 4:18 am

Eric,

I don't know of any designs available for soffit mounting. All the kits I'm aware of are designed for free standing cabinets, so they include "baffle step" compensation. This is a -6dB shelf in the upper frequencies to compensate for the difference in radiation patterns between the highs and lows. A soffit mounted speaker doesn't require this compensation and will sound too bass heavy if it's there.

One route is to build a regular kit and put an active -6dB low shelf filter in front of it. The advantage here is that you could have your choice of several outstanding kits with proven performance. The disadvantage is you need that very specific active filter which you would probably have to build yourself as well.

The 2nd way is to have a crossover designed specifically for your application. There are a few speaker supply companies that offer this service. This is much more straightforward, but the downside is that these blindly computer generated crossovers typically have mediocre performance. There is actually a fairly large parameter space that can result in a flat on axis frequency response. Finding the one design that provides the best tradeoffs between frequency response, phase response, power response, and dispersion characteristics requires a lot of experimentation and human consideration. The computer designs just get you in the ballpark.

Btw, what's your budget?

Building your own is not a way to get good cheap speakers. The mass produced monitors can almost always win at budget prices. However, building your own can get you an outstanding set of speakers at a relative bargain. Around $600 or $700 is the break point. Below that price and you're simply doing it for the fun. If price versus performance is your main consideration, then you're probably better off buying a commercial speaker. Above that price range is where you really begin to reach high performance. If you choose the right kit you can actually meet or exceed the performance commercial monitors costing two or three times as much.

Thomas

Eric Best · Post by **Eric Best** » Fri Mar 21, 2003 5:32 am

Barefoot, the $700 range is not an issue, neither are building the crossovers (designing them is another story). I wouldn't mind spending up to $1000 to do this, that way I could also afford to buy (or build) quality nearfields. My budget for monitors is about $2000, and I would prefer to build my own.

I have built speakers before but nothing this critical, but I am confident that armed with the correct information I won't screw them up. I have read Vance D's loudspeaker design cook book (about 7 years ago) and understand 90% of it (my B.S. is in Mathematics).

Other questions

Does the type of room design factor into speaker design?

What are the most critical elements for studio monitors, compared to a common Hi Fi design?

Crossover design? Component selection? Box design?

Thanks

Eric

barefoot · Post by **barefoot** » Fri Mar 21, 2003 6:42 am

Does the type of room design factor into speaker design?
The size of the room is probably the biggest consideration in speaker design. Small rooms can have significantly enhanced bass response and may require smaller monitors with less bass to compensate (or a hi pass filter). Large rooms and long listening distances obviously require speaker that can generate more acoustic power.

What are the most critical elements for studio monitors, compared to a common Hi Fi design?
Personally I draw no distinction between monitors and hi-fi speakers. Both should be as accurate as possible.

Crossover design? Component selection? Box design?
What? You want to me write a book?

Well, here are at least a few words:

Well designed crossovers balance on axis frequency response, power response, off axis response, group delay, phase distortion, transient response and power handling.

Drivers should be designed for high linearity and low dynamic compression. Crossover components, wire, and connectors, should be high quality, but "premium" or "signature" components are usually just expensive placebos.

Cabinets should be big, heavy, solid and well damped. The internal shape and damping effects the interval cavity resonances. The resonances that occur in close proximity to the speaker cones are the most critical. For example, avoid deep cylindrical woofer openings in very thick front baffles. Flare them internally instead.

etc., etc., etc., etc., etc.,.........

Eric Best · Post by **Eric Best** » Fri Mar 21, 2003 7:17 am

Barefoot,

Thanks, you have given me the information that I need, it sound like there is no extra "voodo" involved than I was already aware of. I appreciate all of the time from your posts.

Eric

barefoot · Post by **barefoot** » Fri Mar 21, 2003 9:05 am

I would still strongly suggest that you work from an established plan.

North Creek http://www.northcreekmusic.com/ has a couple of fantastic designs. Their CM-7 and especially the Borealis would make great nearfields. The Rhythm would make a great main in a medium size room. The Borealis and Rhythm kits use some of the most linear drivers available and the crossovers and box alignments are top notch. You can even order fully broken in and hard matched driver sets for a little extra. The Revelator tweeter option is incredible, but it also comes with the additional cost of very fancy crossover components which I don't think are necessary. I mean those giant 10 gauge inductors look cool as hell, but they're way overkill.

You could modify the box aspect ratios and move the ports to the front to better suit them as recording monitors, just don’t change the front baffle width if they will be free standing. I would make them shorter and deeper. And of course these would need that -6dB low shelf filter in order to work properly in a soffit mount (I can assist you with this too).

Another consideration is you may or may not want such similar designs for your nearfields and mains. The sameness would give good consistency, but differing designs might give different "perspectives" on your mix. Zalytron also offers some excellent kits as well. http://www.zalytron.com/ Personally, the North Creek systems are my favorite. Just pardon his superstitions about using huge inductors!

Thomas

Eric Best · Post by **Eric Best** » Sat Mar 22, 2003 5:04 am

Thomas,

My control room will be 2400 cubic ft.

How much could I vary the geometry of the rhythm cabinet if they were going to be soffit mounted? Would it make much of a difference if I kept the same volume and went to a more standard ratio? I could make a shorter and wider face?

Would it make a difference if I was to align the woofers vertically or horizontally in the soffits?

It fits within my price range.

Thanks

Eric

barefoot · Post by **barefoot** » Sat Mar 22, 2003 11:10 am

Changing the dimensions is fine as long as you maintain the same volume. Deep cabinets are desirable in my opinion because they help reduce rear reflections that can radiate through the woofer cones. Also try to keep the port at least a few inches away from the woofers.

Personally I would also keep the front baffle the same width as the original design. This way if you ever want to use the speakers in a non soffit mount application the crossover alignment will work properly.

You definitely don’t want to mount the speakers horizontally. They have much more response variability in their vertical plane as opposed to their horizontal plane. If you think about which direction you move around in the most while mixing, it's horizontally. Usually you're planted in your chair and moving side to side. So you want the horizontal speaker response to be the most stable. Vertical driver alignment gives you this.

Have you thought yet about how you would build the low shelf filter?

Thomas

Eric Best · Post by **Eric Best** » Sun Mar 23, 2003 4:49 am

I haven't even looked at it, if you have ideas, I am ready to listen.

barefoot · Post by **barefoot** » Wed Mar 26, 2003 5:23 am

Eric,

The circuit is very simple. It's basically just a passive low shelf filter (two resistors and a capacitor) with a unity gain buffer in front of it. Below I've illustrated two simplified circuit ideas. The first uses three integrated circuits from Burr-Brown. The INA134 balanced line receiver, the OPA604 opamp, and the DRV134 balanced line driver. You can design your own +/- 15V power supply or order a kit online. Marchand Electronics sells the PS10 kit. http://www.marchandelec.com A simple good quality power supply. This circuit would have outstanding performance and you'd need some other rather high end electronics to view it as any sort of weak link.

The second circuit idea is sort of a no holes bared audiophile version. It uses Jensen input and output transformers and a discreet JFET "Super Buffer" amplifier kit designed by Borbely Audio. You can buy the Super Buffer and the +/- 24V power supply at Audiokits. http://www.audiokits.com/ When you price out these components you'll find that this one will be just a slightly more expensive endeavor than the first design.

Thomas

Eric Best · Post by **Eric Best** » Tue Apr 22, 2003 7:52 am

Hey Barefoot,

I've been checking out designs to build. What freqeucy is usually target for the baffle step compensation in a typical speaker, or does it vary from design to design? Does it depend on the width of the baffle?

Thanks,

Eric

barefoot · Post by **barefoot** » Wed Apr 23, 2003 3:51 am

The step frequency depends on the narrowest baffle dimension, which is typically the width. The -3dB frequency is given approximately by:

f3 = 115851 / Width in mm

or

f3 = 4560 / Width in inches

Thomas

Eric Best · Post by **Eric Best** » Wed Apr 23, 2003 4:22 am

Thank you sir!