Vertically Stacked Planar Magnetic Speakers

This loudspeaker system continues a previous build based on linear arrays of small wide-range drivers. This time, I tried planar magnetic speakers.

I used LinearTeam WinISD speaker designing software, John Mulcahy's REW room acoustics analysis software and Phonic PAA3 31-band real time spectrum analyzer.

Front speakers

Two Magnepan MMG W per channel are stacked to form tall dipolar line sources.
Inspired by Design Guidelines for Practical NearField Line Arrays, James R. Griffin, 2003:
A near field line array provides a different listening experience versus point source speakers. Among the distinctions that characterize line arrays are:
· Near constant sound levels throughout the listening room
· A wider soundstage
· An image ‘sweet area’ and not a ‘sweet spot’
· Recreates live event sound dynamics

The bass extension is provided by Tymphany Linear Array Transducers (with a tubular form factor) LAT 500-4 in 80 liters vented enclosures filled in the bottom third with a polyester fleece. Bitumen boards damp the inner surfaces. The baffles are mirror oriented at 30 degrees. The length of the external 7 cm diameter vent can be adjusted for room response, actually ~15 cm (Fb ~26 Hz).
I added the pair of TBI Magellan VIII xu subwoofers of my 2007 project.

       

Operating

The speakers are powered by two Rotel RMB-1565 5 channels Class D amplifiers, one for each stereo channel.
Loudspeaker filtering and equalization are done by a Behringer UltraDrive Pro DCX2496 digital system prepared by Ward Maas at Pilgham Audio, following Jan Didden Linear Audio modifications: a linear power supply replaces the smps and a 6-channel volume control gives the DCX pre-amp capability.
The 24-bit / 96 kHz digital signal is provided by a Behringer UltraMatch Pro SRC2496 A/D D/A & sample rate converter. Up to three digital and one analog sources can be connected.

The table below shows the applied filters with the compensations for room effects, phase and loudspeakers linear distortion.

Section Sub Bass Wide range
Level -2 dB –2 dB   inverted polarity 0 dB
111 Hz low-pass 48 dB Butterworth low-pass 48 dB Butterworth
114 Hz high-pass 48 dB Butterworth
Parametric Equalization
    41 Hz  
  168 Hz
  249 Hz
  706 Hz
  11 kHz 		   -3 dB     Q 10  
   -3 dB     Q 10
  +6 dB     Q 3.2
   -3 dB     HS12
  +3 dB     HS12

The green grid in the graph shows the ten octaves and a ±12 dB range.
The red and blue lines display the resulting action of the filters on each section.
The black line is the unfiltered frequency response to a log sweep at the listening position, 3.5m away from the speakers, average of two measures, 1/12 octave smoothing. Lack of energy in the midbass, comb filtering and high frequency rolloff are on the menu. Shocked? Measuring planar speakers in-room produces seldom nice results.
The next figure shows the real time spectrum analysis on pink noise at the listening position, filters applied, average of four measures, and is more representative of what is heard.

Listening

This is my first experience with planar speakers and I'm happy with their natural reproduction, especially of the human voice. The room is filled without pressure. The soundstage is tall, wide and deep, with strong vertical directivity, very precise.
Yet I feel that something is missing: impact. Very enjoyable at moderate levels, it looses relief when played loud. Definitely not suited for funk!

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July 2013