Sunday, March 1, 2015

Time Alignment Experiments on the Sound Transformer and The Inspiration

Yesterday, I recorded a series of twelve-second audio tracks of square waves tuned to musical notes in the chromatic scale and played them through the Sound Transformer system while monitoring with a microphone/preamp/oscilloscope placed in front of the horns about three feet away.  The square-wave "test" is regarded as the most discriminating, but perhaps the least applicable generally, because, although it appears to reveal resonance and incoherence in the loudspeaker, the test setup itself, i.e., placing a microphone at a single point in a room at an arbitrary distance from the speaker under test, is encumbered with a load of inconsistencies. (By the way, Japanese investigators have devised a way of testing loudspeaker/room systems that involves placing a matrix of many microphones.)

With the bass and midrange/treble horn mouth ends in the same plane, I could not really see or hear any advantage to adding delay to the midrange/treble signal to achieve "time-alignment".  Therefore, it appears as if nothing is lost by mounting the whole huge speaker flush with an interior or exterior wall in a building, for example.

Last night I processed several music tracks by mixing from stereo to mono, and remixing a stereo track with the bass horn right channel low-pass filtered and the midrange/treble left channel high-pass filtered.  I had processed the square-wave tracks in the same way in order to "split" the bass and midrange/treble information so as to be able to introduce a relative delay in the midrange/treble. This processing simulates a "bi-amped" setup with active crossover.

I hope to listen to these tracks sometime today.

Here is the inspiration for the Sound Transformer Project.  Below is a photo of the horn-loaded Theater Loudspeaker designed by Albert L Thuras and Edward C Wente and reported in the March, 1934, edition of the Bell Labs Record Vol 12, No 7.

Theater Loudspeaker by Albert L Thuras and Edward C Wente
Theater Loudspeaker by Albert L Thuras and Edward C Wente
In 1971 or 1972, I got a look at these speakers in the basement of the Seeley W. Mudd building at Columbia University.  By the way, the metal diaphragm pictured in the lower right is 20 inches in diameter.  The attached voice coil is 8 inches in diameter.  As a result of this experience, I have always thought it would be interesting to build a horn-loaded loudspeaker to hear how it might sound.  I will post more regarding the history of my own interest in sound reproduction later.

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