Passive Cardioid - Measurements, Simulations, Math?

[purebordem]

Hi all,

I have been fascinated with passive cardioid designs. While the concept is relatively simple "its a like a cardioid microphone in reverse", reality proves to be a bit thorny. In my experience the literature for these designs is sparse, the patent's somewhat vague, and repeatable proper simulations do not seem to be readily available. There has been quite the banter in the Directiva R2 requirements gathering thread.

At minimum, I thought it good to at least compile the various sources of information in a singular place for digestion/discussion. Below is collection of all the various bits of information I have found while researching the topic. Vaguely categorized by type...

Please by all means, if you have a valuable source of information, post it and I will try and add it here. While I do not hope that the DIY community will crack the full trick to this approach at time soon, I believe we can at least get a bit closer. If we can even get the point of an accurate simulation, that would be a win in my book.

Articles

• General
  • Directivity Control Part I & II
    • https://www.linkedin.com/pulse/directivity-control-part-1-survey-loudspeaker-systems-steve-mowry/?trk=related_artice_Directivity Control Part 1: A Survey of Loudspeaker Systems_article-card_title
    • https://www.linkedin.com/pulse/directivity-control-part-2-diy-unidirectional-system-steve-mowry/
• Fulcrum
  • https://www.fulcrum-acoustic.com/wp...e-Passive-Cardioid-Loudspeaker-Technology.pdf
• Comsol
  • https://www.comsol.com/paper/download/199319/vaucher_paper.pdf
• Geithain
  • https://www.me-geithain.de/en/assets/media/products/studio/RL901K/en/PPThomsen-RL901K-02-05-eng.pdf
• Kimmo
  • https://web.archive.org/web/20210122211214/https://kimmosaunisto.net/CardSub/CARDSUB.html
    • Currently "unavailable" so using Web Archive version

Patents

• Fulcrum
  • https://patents.google.com/patent/US10123111B2/en
  • https://patents.google.com/patent/WO2017210605A1/en?q=cardioid+speaker&oq=cardioid+speaker
• Meyer
  • https://patents.google.com/patent/US8428284B2/en
• Dutch & Dutch
  • https://patents.google.com/patent/NL2013741B1/en
  • https://patents.google.com/patent/EP3018915B1/en?assignee=Dutch+&+Dutch+B+V
• Altec
  • https://patentimages.storage.googleapis.com/85/eb/36/f6607caf2450b5/US3722616.pdf
• Sony
  • https://patentimages.storage.googleapis.com/94/88/75/ba9ff777125fac/US6665412.pdf

Calculators and Charts

• Porous Absorbers
  • http://www.acousticmodelling.com/porous.php
• Absorption Coefficients
  • https://www.bobgolds.com/AbsorptionCoefficients.htm
• Acoustic Impedance
  • https://www.hicalc.com/program/135

Simulation Tools

• AKABAK/ABEC
  • http://www.randteam.de/AKABAK3/
• Comsol
  • https://www.comsol.com/
• ANSYS
  • https://www.ansys.com/

Other

• Coefficient to dB and back
  • https://blog.thermaxxjackets.com/decibel-drop-noise-reduction-coefficients-for-material-combinations
• Dutch & Dutch 8C Development Thread
  • https://www.diyaudio.com/forums/multi-way/192737-2-waveguide-cardioid.html

 

[purebordem]

I personally have dived down the AKABAK rabbit hole hard since I lack the time or resources to do iterative real world designs. After probably a few hundred variations on a theme, I am not 100% convinced AKABAK can do a full simulation of a cardioid speaker. However I also could still be screwing things up

A few things I have noticed so far as related to cardioid sims and AKABAK...

• Smaller enclosures are more touchy about the port and its associated parameters.
  • I seem to have better luck with a smaller squarish hole than the long rectangular profile at smaller volumes
• Using internal blocking around the port is makes adjusting delay between the front of the speaker and the sides much easier
• The internal blocking and the damping material are fairly reliant on one another, which makes sense
  • By adjusting one or the other I could get either smooth even attenuation across the 100-500hz territory, or I could get higher attenuation at a smaller frequency range; trading off for something vaguely more dipole-ish in the rest of the range.
    • Dutch 8C shows something similar albeit higher between 500-1Khz

Fwiw I have been using a mockup of the Purifi 6.5 on the LEM side of things. Obviously without the crinkly bits, but it was at least a starting point.