Home > 2-way, 8-10, 90+lbs, Electrostatic, Passive, Tower/Floor Standing > FrankenSound by Justin Boldenow

FrankenSound by Justin Boldenow


General Overview:

I was drawn to the openness and clarity of electrostatic drivers and a dipole system. I chose to build the large coffin-like subwoofer enclosures so that the 15” drivers would be capable of reaching mid-range frequencies and the lowest ends of human hearing. The coffin look is both interesting and functional as unusual shaped enclosures diffract frequencies and help disperse stored energy.

Design Goals:

My first and foremost desire was for monitors that had enough bandwidth, clarity and accuracy to be used as mixing monitors. The electrostatic drivers came with clarity and accuracy so I wanted to design enclosures that could match this and supply the rest of the frequency range needed. I wanted to cover the range of human hearing from high to low accurately.

Design objectives:

• Even full-range frequency response

• Low harmonic distortion

• Dipole system

• Completely original design

Key Features:

• 2-way crossover with potential of 3-way

• Passive crossover with potential of adding an active crossover

• dipole sub/mid woofer enclosure for extended low frequency response and “air”

• Electrostatic tweeters

• Coffin shaped design

• Chalkboard paint

Technical Specifications:

• Bandwidth 25 Hz-20kHz, +- 4 dB

• Max SPL 85 dB with 20 dB headroom

• f3 30 Hz

• f10 16 kHz

Cabinet Construction:

My cabinets are made out of 3/4 inch 13 ply baltic birch plywood and 3/4 inch MDF, this was done to keep the structure solid. Plywood was chosen because of its construction with alternating grain. The MDF’s density helps deaden the cabinet.

The Subwoofer cabinets were designed to be large so as to achieve lower end frequencies. The larger the box was, due to its dipole characteristics, the more resonance it had at lower frequencies. This was done to achieve low frequencies with just a 15in driver instead of using larger drivers and sacrificing mid-frequencies. The coffin design is an irregular shape which helps to disperse stagnant energy held within cabinets. It also fits my personality.

Tweeter:

I chose the Final Sound electrostatic speakers and built the rest of my system to fit it. I chose these drivers because they covered high frequencies and high midrange, which would save me costs in midrange drivers. I also chose these drivers because of their dipole nature and incredible transient qualities. Though the driver wasn’t the flattest in response I believe this can be cured with acoustic tuning of the room that they are placed in. Unfortunately, a standard crossover would not work on these because their impedence nature  had a completely different curve in testing. They were left with the natural crossover that they were manufactured with.

Subwoofer:

For the subwoofer I went with the only infinite baffle designed drivers I could find. The Dayton IB’s were specifically engineered for dipole use and their sound is incredible. Unfortunately the drivers themselves were designed to reach 40 Hz which didn’t fit my requirements. This is why I built such large boxes and gained another 15 Hz lower.

Cross-over:

As I stated before, a standard crossover would not function on the electrostatic drivers and were left to function with their natural manufactured crossover. This implies that I only needed to build a crossover for one driver and the crossover was the simplest crossover to create. I used a 1st order Butterworth at 100 Hz. I chose this frequency because it was roughly where the electrostatic ended and where the subwoofer began to slope downward slightly. Though the total frequency response of the system looks a little rough, I plan to correct a lot of it with acoustically treating its environment and experimenting with black-hole fiberglass/foam.

Testing:

Overall Speaker Performance:

Frequency Response

Harmonic distortion

Step Response

Off Axis Response

Frequency response with individual drivers overlayed

Green is the overall response, Yellow is the subwoofer and Red is the tweeter

Tweeter Performance:

Frequency Response

Subwoofer Performance:

Frequency Response

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  1. June 20, 2012 at 5:24 pm

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