Tikalon Blog by Dev Gualtieri

Spiderweb Microphone

September 9, 2024

One of my brothers worked for many years at Bell Labs, and he took an offered buyout package soon after the breakup of the Bell System to start his own business. His company designed and manufactured vacuum tube audio equipment for musicians and recording studios who believed that vacuum tube sound was preferable to that of transistor circuitry. The supposed reason for this is that vacuum tube amplifiers are more linear, and they have soft signal clipping. Because of his business, he attended many foreign and domestic trade shows each year and had many contacts in the music and recording industries. On a tour of Capitol Studios he was shown a box that was simply labelled, "Frank." The box contained the Telefunken U47 microphone used in Frank Sinatra's vocals during his recordings for Capitol Records from 1953–1960.[1]

Frank Sinatra's Telefunken U47 microphone at Capitol Studios in Hollywood.[2] .

My parents owned nearly every one of Sinatra's phonograph records from his Capitol Records era from 1953–1960.

These records were the 33-1/3 RPM vinyl disks, the audio medium of that time.

(Screenshot from a YouTube video by FirstCom Music.[2])

The U47, only manufactured by Telefunken between 1946 and 1965, is considered to be the best vocal microphone by most recording engineers.[1] It was a condenser microphone with its own internal vacuum tube amplifier. This microphone sells for about $10,000 on the vintage market, and my brother owned one that he received in exchange for restoring two others. The condenser microphone is just one type in a long list of microphones, as given in the table.

**Microphone Types**
Carbon	This microphone consists of granules of carbon separated by metal plates, one of which is a thin plate used as a diaphragm that moves in response to sound. Sound compresses the granular carbon, causing a modulation of its resistance. The frequency response of a carbon microphone is not very good, but it was good enough to be used in telephones until the 1980s, A technique to rejuvenate early carbon microphones was to tap them on a tabletop to loosen the carbon granules.
Condenser	Condenser is an older name for a capacitor. It was given that name since the device condensed electric charge on its plates. Condenser microphones such as the Telefunken U47 are built as a capacitor consisting of a thin diaphragm spaced above a metal plate. A DC voltage is applied through a high value resistor between the diaphragm and the plate. Movement of the diaphragm by sound modulates the voltage.
Electret	Electret microphones are a type of condenser microphone that incorporates a dielectric with a permanent charge that eliminates the need for an applied DC voltage.
Dynamic	A dynamic microphone is based on the same principle as a loudspeaker. Sound moves an inductance coil attached to a diaphragm in a magnetic field, and this produces a voltage by Faraday's law of induction,
Ribbon	Ribbon microphones operate on the same electromagnetic induction principle as a dynamic microphone. A thin corrugated aluminum ribbon is placed under low tension in a magnetic field, and movement of the ribbon generates an axial current in it. This type of microphone has very good frequency response, since the ribbon resonance is below the audible range. Since the ribbon length is much shorter than the length of wire in the inductance coil of a dynamic microphone, its signal is much smaller.
Crystal	These microphones are based on the piezoelectric effect of Rochelle salt (Potassium sodium tartrate tetrahydrate), and they served as the principal type of microphone in consumer electronics in the late 1940s and the 1950s. This was my first microphone, and I discovered its fragility after dropping it and found that it no longer functioned.
MEMS	Micro-electromechanical microphones are a miniature version of a condenser microphone. A flexible diaphragm is created with a small gap above a silicon wafer. Sound modulates the capacitance between the diaphragm and the silicon wafer.

Top-40, News, Weather and Sports.

The author using a dynamic microphone during his top-40 radio disk jockey days.

There was a news announcer during the day who would gather local news, but we would do "rip-and-read news" in the late night hours. Our news service would send a news summary each hour, we would rip this raw text from our teletype, lightly edit during a record play, and read it on the hour and half hour.

(colorized black and white photograph, circa 1970.

Some microphones are designed to accept sound only from particular directions. This allowed recording engineers to properly balance vocals and background music when the singer and orchestra were in the same studio. One microphone in particular, the shotgun microphone, was exceptionally directional. The acoustic transducer was at the bottom of a long, slotted tube. Axial sound waves pass directly to the transducer, while off-axis sounds enter through the slots and mostly cancel because of phase interference. Shotgun microphones do not have a uniform frequency response; so, they are only useful in settings such as press conferences to voice audience members above background noise.

An Audio-Technica model AT815a shotgun microphone, circa 1990s. (Wikimedia Commons image by Pj and Piko. Click for larger image.)

All the microphones listed in the table operate as transducers that convert sound pressure into an electrical signal. Modern microphones have extreme sensitivity, but they are also sensitive to noise caused by air molecules bouncing against their diaphragms. In an effort to solve this thermal noise problem, a team of mechanical engineers from Binghamton University (Binghamton, New York) and the State University of New York at New Paltz (New Paltz, New York) have investigated a sound sensing approach that uses the viscous air flow rather than sound pressure.[3-4] Viscous flow is what vibrates spiderwebs in gentle breezes.[4] Air flow passing a thread of a spiderweb drags the thread.

Calculations showed that a spider silk thread will oscillate at sound frequencies up to 50 kHz.[4] The research team described their device at the 186th meeting of the Acoustical Society of America (Ottawa, Canada, May 13-17, 2024).[4] As a simulated spiderweb, the researchers used an array of thin cantilever beams as their microphone (see figure).[3-4] The beams were 0.5 micrometer thick silicon nitride placed over a hole in a silicon wafer.[4] They used a laser to measure the displacement of the microbeams, first in response to thermal noise, then in response to sound waves from 100 to 1000 Hz.[4] The cantilever velocity matched that of the sound wave, irrespective of the length or width of the beam.[4]

Sound-detecting cantilever beams fabricated over a central hole in a silicon wafer.

Sound-detecting cantilever beams fabricated over a hole in a silicon wafer.

(Adapted from figure in ref 4 using Inkscape.[4] Click for larger image.)

Small microphones based on sound pressure have proportionally more thermal noise than larger ones, but the cantilever response to thermal noise was independent of its size.[3] As the authors explain, small objects have a small Reynolds number, and viscous forces dominate the random thermal forces.[4] The demonstrated cantilever microphone is about 50 dBa less sensitive than the best pressure-based microphones; but, pressure microphones have been perfected over a span of 150 years.[4] As lead author, Ronald Miles of the University of Binghamton comments, "Detecting air flow as a way to sense sound has largely been ignored by researchers, but the principles show that it's worth considering."[4]

Spiderweb Microphone

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