I received this message via email and thought the question and my response would be good for public consumption.
Hi Dave,
Thanks in advance for your time - it is appreciated. I'm considering
buying your kit with completed crossovers to build the floor standing
version. This would be my first project. I'm leaning towards using
two layers of high quality 3/4 inch baltic birch plywood for all panels
(possibly more for the bottom and back) and maybe lumber for the
baffle.
I'm no expert and have virtually no technical aspirations! I just want
to build a high quality speaker, happen to have access to some
relatives who are professional wood workers, and above all would like
to have some fun...
You're probably going to cringe, but mainly for aesthetics I'd like to
modify the basic floor stand design. Something like the Usher CP 6311.
I'm not after absolute accuracy, as I realize that my room acoustics
and listening position may a have greater audible impact than the
changes I'd like to try. In your opinion, how much would the following
affect the overall audible sound (in aggregate)?
1. Retain the baffle width, roundover and driver placement of the
monitors - I wouldn't change the interior width of the cabinet, just
make the side walls twice as thick.
2. Make the overall height slightly taller - either by making a sealed
bottom chamber or using 4 layers of 3/4 inch panels, etc.
3. Slope the entire speaker cabinet approximately 5 to 7 degrees (top
and bottom panels would remain level).
Also, what would be the absolute maximum internal volume for a floor
stand version; at what point would be the acceptable internal volume
affect your design - is there a range?
Thank you again, and I apologize for all of the questions! I'm not
sure if this will get off the ground, but...
Joe
I generally recommend MDF because it is more stable and easier to machine and veneer. MDF moves extremely little with moisture changes, and the edges are also very stable. This is NOT true with normal plywood, but solid core plywood is much better. The edge might hold tight. I have heard that solid core plywood can be very stable over time, but I still have some hesitation. I am only @80% certain the solid core plywood will remain tight. The edge might delaminate. Also, MDF is commonly used as the core material for kitchen cabinets because it is very stable. I believe that stability when changing moisture content is the primary reason for using MDF.
Years ago I learned from a work cohort there are also differences in MDF. He provided me a nice glossy pamphlet produced for a conference for folks engaged in the manufacture of compressed lumber products. This pamphlet addressed the differences in MDF designed for cabinets, floors and general purpose. I was surprised to learn there are changes in the resins and base material used. These changes resulted in desired stability and hardness characteristics intended for purpose specific application.
Following this, I ordered some cabinet-grade MDF through a local cabinet shop. The cabinet grade MDF was slightly softer and weighed slightly less than MDF from the local lumber yard. I used the MDF, but really didn't find any significant difference between the MDF from the lumber yard and the cabinet shop. I couldn't hear any difference, and I the materials were equally stable over summer & winter season in Great Falls, MT. My hunch is that that the greater mass of the lumber yard MDF should be acoustically superior, but I cannot support this with any findings. My summary is... don't fuss with cabinet grade MDF. Use the MDF from the local lumber yard.
I have used no-voids plywood for internal bracing. I have also used it in a 2-ply sandwich on the back panel of the 1801. I use 3/4" plywood glued to 3/4" MDF for the back panel. Commonly this is called Baltic Birch, but there are other varieties too. It's indeed stronger than MDF, and fairly stable. Intuitively, the greater strength of the Plywood should have an impact. Unfortunately I couldn't hear any audible difference between the plywood and when used for internal bracing or the back panel of the 1801 cabinet.
I have not used no-voids plywood for an entire speaker. I don't see a serious problem with this, but have a very slight hesitation. I will first express the potential audible impact, then address the issue of lamination. My memory is somewhat foggy, but I vaguely recall and article in Speaker Builder Magazine that compared 2 speakers having different construction materials. One cabinet was contracted from MDF. The other cabinet was constructed from Baltic Birch plywood. The audible impact was non-existent. Also, many pro-sound speaker cabinets are constructed from plywood. I have also seen a few loudspeaker horn cabinets that were constructed from no voids plywood. Indeed it's stronger and weighs less. Nonetheless, I have not experimented with this and cannot fully ordain the use of plywood for a complete speaker. Plywood is a more resonant material than MDF, but the impact maybe me minimal or nil.
My primary concern with the use of plywood is that it might delaminate. If the edge grain of the plywood delaminates any veneer overlay will also delaminate. If very good no voids plywood is used, the results will very likely be favorable. Indeed TC Cain uses no-voids plywood in their speakers.
http://www.cain-cain.com/BEN/index.htmlOn this subject, I am compelled to convey my sentiment regarding the intentional use of a very resonant cabinet material in a very resonant cabinet design. There are a few folks marketing this. I wholly disagree with this approach. The goal in a speaker cabinet is to remain stationary, not vibrate, and firmly hold the driver(s). Any and all desired waveforms emanating must come from the drivers, not the cabinet. If this cabinet vibrates, it's distortion.
I think... that completes my thoughts on the use of good plywood in loudspeaker cabinets.
Now to the subject of a backward leaning speaker. I believe that having the acoustic center of the tweeter slightly delayed is easier for the crossover designer, and will potentially eliminate parts from the crossover circuit that exist primarily to adjust phase. This is mostly science, and partially artwork. Inductors and capacitors have several impacts, and one of them is shifting the phase of the signal across them. The degree of phase shift across componets is determined by the amount of ac/dc resistance across the component. As the AC component rises, the phase shift will become more significant. Inductors delay the signal. Capacitors advance the signal. As the entire filter circuit takes shape, it's difficult to get the design axis higher. The woofer phase must be advanced or the tweeter phase must be delayed. This phase difficulty often necessitates the use of more componets in the signal path. Or, the baffle can be tilted backwards. I fervently believe that a simplified crossover the primary reason why many loudspeakers lean backwards.
The measured design axis of the 1801 is at the base of the 1801 is at the base of the tweeter flange. However, I really don't hear a significant impact when vertically moving away from this design axis. Perhaps there is some impact when 2-3' vertically off axis. I don't hear anything "magical" when my ears are positioned precisely on the design axis. Hence, I don't see a slight rearward lean problematic - especially if the cabinet is slightly shorter.
I also don't believe changing the width is a problem. I haven't experimented with anything wider than 9", but a narrower cabinet is fine. I recommend reading the discussion string on the 1801F here:
http://www.audiocircle.com/index.php?topic=7577.0With regard to volume I think that @ 20 to 25 liters is ideal, but slightly larger is viable. Certainly the 1801F is slightly larger. One of my very first areas of experimentation in loudspeaker construction was changing cabinet volume. I found that the port had to be adjusted slightly, but changing cabinet volume didn't have a monumental change in bass performance. A smaller cabinet sounded tighter, and a bigger cabinet sounded deeper. This is because the dampening remains constant, but the spring-effect on the driver increases as the cabinet grows larger. However, the driver used is far more important than the size of the cabinet. Regardless of cabinet size, the W18 didn't/doesn't have the bass quality of the SS8545, Accuton C95 or Meniscus 838. The W18 bass dynamics are very good, but not world class. The W18 midrange is... a different story.
Hopefully that answers all the issues presented above. Whew, that became very long.
Dave
