The BMF-1 Loudspeaker
We've had this idea. Build a loudspeaker capable of full-range sound, with low distortion and high sensitivity. Amps wouldn't have to work hard, dynamics would be conveyed with authority, but resolution and imaging must be absolutely first-rate.
My friend Mike Galusha had been using a very similar design for the last five years or so. They were somewhat of a legend in the DIY high end audio circles. They were fast, dynamic, very resolving with no fatigue factor. This was the basis for the BMF-1 loudspeaker.
I asked Mike to design a 2 box solution similar to his. He accepted the challenge. We chose the same 9" Air Motion Transformer he incorporated in his design for the mid and high frequencies. This is similar to a ribbon in that it uses, well, a ribbon as the transducer. It's different in that the ribbon (around 5 feet of surface area) is folded like an accordian and instead of radiating by vibrating back and forth, the ribbon is "squeezed" together and apart by the magnetic forces applied to it. The result is a dipole driver with an extraordinarily rapid response rate, enabled by the extremely low mass of the driver and the far smaller distance it travels on each “swing” compared to a dynamic driver. In this technical respect, it shares characteristics with the electrostatic driver.
The discernible motion of each diaphragm flexure is very small, but because of the folded structure, more air is moved than would be by a conventional cone or electrostatic driver of the same plotted surface area.
The folded driver design, combined with the small motion range, means the AMT acts like a point source version of a larger driver, inherently resulting in lower sound reproduction distortion. As a result of its motion pattern, the AMT "spits" the air out in a way similar to the action of shooting a watermelon seed from your hand by squeezing it between thumb and forefinger. The speed of the air as it leaves the diaphragm is approximately five times faster than the speed of the actual driver structure, hence the name, Air Motion Transformer.
We also chose an Italian designed 12" front firing midbass driver in a sealed enclosure. Again this is a high sensitivity, low distortion driver with a large radiating surface area. You can't defy physics if you want to move a lot of air at lower frequencies. This driver coasts along at high sound pressure levels, and presents the full dynamic range in recordings. It's response was very flat across the midband, and it it goes down to about 80Hz in this design.
We chose 2-12" powered subwoofers (also in a sealed enclosure) per speaker to fill in the low end. The low frequency drivers use XBL motors for low distortion. We use 4 of them per stereo pair to ensure that they never have to work so hard as to distort the output. Each subwoofer has its own dedicated 400 watt amplifier.
We like sealed boxes. Placement of a sealed box speaker is easier, and there is no port noise to contend with. The sound just seems tighter, faster and more controlled.
The design controls vertical and horizontal dispersion of the mid and high frequencies. This keeps the mid and high freqency 2nd reflections from arriving at your ears before the primary sound wave. This is what helps the BMF-1's create their wide and deep soundstage. If the width and depth are on the recording, you will hear it, and in a big way.
We start with the speakers 1/3 of the way in to the listening room (measuring from the front baffle), with each speaker 1/5 of the way from each side wall to the center of each speaker. In wider rectangular rooms, we prefer placement along the narrow wall as well. Once the speakers are set in those positions, we try to establish a listening distance 1.2 times further from the speakers than the speakers are from one another. For example, if the speakers are 8 feet apart, we like to sit 10 feet from them. These calculations have worked very well in placing the speakers in rectangular rooms along the narrow wall. The BMF-1's benefit from a lot of toe-in, which reduces the amount of sidewall reflections further.
Specs (so far)
Frequency Response: 20hz-23kHz
43" H x 16" W x 20" D
Weight: 200lbs each
Photos of our BMF project on Pinterest