Archive for the ‘monitor’ Category

The “Shark Fin” Cabinet Construction

Sunday, September 23rd, 2007

I have finished building the first cabinet prototype of the Shark Fin Portable PA. I used 1/2″ plywood for the shell of the cabinet and 2x stock wood ripped into specific shapes and angles for the frame of the cabinet. Elmer’s Carpenter’s glue was the main glue for the construction. I used GE Silicone II 100% Silicone Window & Door Caulking to seal the cabinet.


This the bottom panel or base of the cabinet. The wood frame was ripped from scrap 2×10’s that I had lying around the basement. I used the Google Sketchup tools to determine the proper angles to cut all of the bracing pieces. All of the bracing was glued to the base and not screwed so there would be no screw heads on the bottom of the shell to cause the cabinet to be off-balance or wobbly.


Each bracing piece was glued and clamped separately. Elmer’s sets quickly (30 mins), so it didn’t take very long to get all the base pieces attached.


I used 90 degree angle clamps to hold the sides of the cabinet in place to make sure that I cut all of the angles correctly. The angle clamps are essential in getting the pieces together plumb and square. The two side pieces are being glued in place and front panel is locked in to make sure that the side pieces are in the right place.


This is the front view of the panels’ alignment being verified.

Now that the side panels have been glued in place, I am gluing in the bracing pieces for the bottom front panel. Notice that the cross bracing has already been installed to hold the side panels apart. After the side panels were glued in I used self drilling pan-head screws to screw the panels onto the frame for additional strength.


This is the view of the previous photo from the inside of the cabinet.


The front panel has been glued and screwed down. The extension clamps were used to pull the sides of the cabinet into square before the panel was screwed down.


The view from the front after the front top panel has been added to the cab.


Another of the same.


Clamping and gluing the top panel to the frame. No screws will be used here so that there will be no screw heads to interfere with the PA’s controls.


This is the caulking I used to seal the inside of the cabinet. This small tube is much easier to work with inside the box. I never would have been able to get a full-size caulk gun in there.


Detail of the sealed seams inside the cabinet.


Detail of the + and – leads attached to one of the Galaxy Audio full-range speakers. Note that I used colored tape to clearly show the polarity of the speaker leads. I used a battery to determine the polarity of the actual speaker because they weren’t clearly marked.


The two audience facing speakers. The bottom larger speaker is the Selenium woofer.


A full view of the cabinet with the leads and the speakers installed.


This is a detail of the panel where the speaker jacks are installed. The top jack feeds the “monitor” speaker and the bottom jack feeds the bottom two “audience” speakers. The top jack is 8 Ω and the bottom with two speakers is 4 Ω. I used on outdoor blank switch cover plate for the jack panel. These cost about a dollar, are very easy to drill and come with a foam rubber gasket.


Full length view from the back before the cabinet is closed.


Front view of the closed cabinet ready for frequency response testing.

The next steps after testing are determining if a tuned port will be needed to extend the low frequency response of the cabinet, adding the amplifier and battery power to the cabinet, and adding mixer inputs and controls.

The “Shark Fin” Portable PA Project: Design Considerations

Friday, September 14th, 2007

Only a few battery powered PA’s are available on the market today and most of them have design flaws or limitations. My goal is to create a design that uses digital amp technology (Class-D) that runs on standard batteries AA, C or D, or on a lead-acid car battery, that uses small speakers to give a full-range sound. The hope is to achieve relatively flat response from around 85 Hz to about 18 Khz. Who knows if it will work, but here’s the story of the first round of prototyping.

DESIGN GOALS

  1. Ergonomics. The PA should be able to be adjusted and moved easily. The design should allow for the most typical uses of a battery powered PA. The first most common use would be for buskers (street performers.) In these situations the performer usually is riding public transportation to and from the gig, so equipment must be easy to carry or move.

    • They are using the PA both as a monitor of themselves and as the “mains” for the audience to hear as well. Most often the amp is placed on the ground slightly behind the performer. I designed the cabinet to be more effective at being both a monitor and a main speaker by angling one speaker up at a 45° angle and angling the 2 mains speakers at a 75° angle.

    • The controls must be easily accessible when the performer is in the middle of a set of songs. If you have a guitar strapped on and you need to bend over to adjust the volume on a typical amp, the guitar slides off your shoulder. Thus the volume controls (at least) need to be at typical counter height of approximately 36 inches. The design solution for this was to make the controls mounted on a telescoping bar which also serves as a hand truck or wheeled luggage handle.
    • The ease of movement and stability of the cabinet was accomplished by adding in-line skate wheels on the bottom rear corner. The heaviest speaker is mounted in the lowest hole in the cabinet and all of the batteries and electronics are packed in the bottom of the unit. All of these items serve to keep the center of gravity very low, as does the sloped back design of the “Shark Fin.”
  2. Electronics. The PA system should be battery powered, either with 8 AA, C or D batteries or a single 12V lead-acid battery. The amplifiers that we found that have the efficiency that we needed were made by Tripath, a company specializing in what they call Class-T amps (which basically are just Class-D with a T instead). The model that sparked our choice was the TA2024C as used in the Sonic Impact Original Class T Amplifier.

    I chose two different speakers for the project: 2 Galaxy Audio S5N-8 5″ Neodymium Full Range Drivers at 8 Ohms and the Selenium 6W4P 6″ Woofer. The Galaxy Audio’s are the speakers used in the new versions of the Hot Spot mini-monitors. The idea behind these speakers is that they are incredibly light (made from Neodymium) and they concentrate on mids and highs without getting bogged down in low frequencies which are much harder to reproduce. This makes them great for vocal and guitar detail. The Selenium drivers are their to provide some low end for the audience especially. I will probably need to use a low pass filter to get this driver to be as efficient as possible. I may also use a high pass filter on the audience facing Galaxy Audio speaker.

The next installment of the blog will be the actual prototyping of the speaker cabinet. We will be using 1/2″ plywood for the shell and 2x stock ripped to size for the internal bracing. In a production version of the cabinet, ABS plastic would be a much better solution, but it’s not easy to manufacture plastics in your basement.

Modeling a Room in 3D with Google SketchUp

Sunday, September 9th, 2007

It is now free to download and use a high quality 3D modeling software application. Just visit Google’s new 3D modeling center: http://sketchup.google.com/

The drawing above is a scale model of the control room at Indecent Music with one monitor, sans equipment and furniture. Measuring the SPLs in a room and using a model like this helps you to interpret the data you get from your SPL. It also helps you to see how sound might be reflecting in your room. The walls and other surfaces are easy to understand in 3D.

A floor plan view or a horizontal slice lets you write in data points to make the a data model.

It’s pretty easy to see how this kind of a tool could make working in your room easy and and more scientific.

The other thing that I love about SketchUp is that it has a built in tool to get models from Google 3D Warehouse and to upload your own models to share with others. This allows for both online collaboration with colleagues and for collabs with people you don’t know. One of the things that makes 3D modeling so painful, is the need to recreate all of the models that you need to use yourself. Most people end up buying a library of components for use with their own industries. For instance, Kitchen Designers use a variety of different CAD applications to design kitchens, but who wants to model 2300 variants of a Kraftmaid cabinet? So you buy the models from the source.

With the 3D Warehouse, you can check to see if anyone has made something similar already that you can reuse or recycle. Extremely useful. As long as everyone shares, this kind of system works very, very well. Did you notice the model of the monitor? That model was downloaded and imported straight from the 3D Warehouse. I probably saved an hour or more!