Schwartz Engineering & Design

The following are questions and answers that came out of our in-booth presentations of the PFD Microphone Prototype 3 at the 127^th AES convention in New York City 9 through 12 October 2009.  They are in no particular order of importance.

1.  Do I need a license or permission to build a PFD mic for my own personal or educational use?

No. A license is only required to manufacture PFD mics.

2.  Can I buy a clone of your PFD mic Prototype 4 when its ready?

Yes; $2,500 plus shipping. Power supplies are included.

3.  Is the smoke fluid you use readily available?

4-ounce bottles of the fluid can be purchased from Viking Manufacturing in Texas , or you can buy Lionel smoke fluid from your local hobby shop. We've also used disco fog fluid, which is glycerin-based.

4.  Is the smoke type you used at the 127^th AES conference the best option?

We don’t know. Presently, we are experimenting with a water nebulizer.

5.  Are the 5 mw red trace line laser and Thorlabs PDA100A photo detector the best choices?

No. A better-matched solution is desirable, as is a detector with a greater dynamic range. A photodiode might work, too.

6.  Is smoke or vapor density critical?

Yes, as are the optical properties of the particles or droplets in terms of reflectivity and absorption.

7.  Are the smoke nozzle shape and aperture geometry important?

Yes. Proto 3 has a bullet-shaped nozzle, inside and out, with a 5/16-inch by 3/64-inch aperture. We don’t know if that is optimal. A NACA duct profile might be better.

8.  Is the smoke velocity important?

Yes. Proto 3 runs at about 3 inches per second. We don’t know if that is optimal.

9.  Can you figure out a way to recirculate the smoke or vapor?

Yes.

10. Is a smaller version of the mic possible?

Yes, smaller is better; less noise contribution from the housing and a better sound pressure to smoke column ratio.

11. Why does Proto 3 sound so bad?

The mic sounded bad mainly due to the closed-end plastic tubing construction. That configuration and its material ring.

12. Is the horizontal slice orientation of the laser beam optimal?

Maybe not. One suggestion is to split the laser beam with a prism and intersect the half-beams at the smoke ribbon. Then, particle motion would reflect the phase of the sound.

13. Wouldn’t a CMOS digital imaging chip like in a high-speed video camera be a better choice than analog silicon?

Probably, especially since the output would be digital.

14. What’s the frequency response of Proto 3?

Proto 3 had a response of 100 to about 3,000 Hz. We didn’t measure the flatness.

15. Why bother with this approach, given how good conventional mics are?

It’s all about the potential for exceeding conventional designs. In a PFD mic, the limits of S/N and frequency response become those of the optical system, as opposed to an existing mechanical system that has reached its full potential.

16. Where did the idea come from?

Long story short: David Schwartz noticed the effect of conversation with his wife on the column of smoke from an oil lamp on the table between them during dinner.

17. Does the smoke or vapor stream have to be smooth?

Yes. We used coffee stirring straws packed into the smoke supply tube to create a laminar flow. If the stream coming out of the nozzle becomes turbulent, the mic won’t work. Another approach could be to modulate the smoke or vapor stream at an ultrasonic frequency and then low-pass filter that out of the signal. We haven’t tried that, yet.

18. Shouldn’t you have included a way for the sound pressure to exit the back side of the detection chamber?

Probably. The issue is, how to do that without excessively disturbing the particle stream.

19. When will a studio-quality version of the PFD mic be available in the market?

If we decide to produce a limited number of Schwartz branded mics, they could be ready by the 128^th AES convention. Otherwise, the schedule will be up to the licensees.