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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 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]
References:
- Ken Theriot, "The Frank Sinatra Microphone," homebrewaudio.com, September 22, 2022.
- Jason Rudd, "Frank Sinatra's Famous 'Telly' Neumann U47 Microphone," YouTube video by FirstCom Music, August 14, 2019.
- Junpeng Lai, Mahdi Farahikia, Morteza Karimi, Zihan Liu, Yingchun Jiang, Changhong Ke, and Ronald Miles, "Effect of size on the thermal noise and acoustic response of viscous-driven microbeams," J. Acoust. Soc. Am., vol. 155 (April 10, 2024), pp. 2561-2576, https://doi.org/10.1121/10.0025546. A PDF file is available here.
- Rachel Berkowitz, "Spider-Inspired Microphone Detects Tiny Gusts of Sound," Physics, Vol. 17, no. 89 (May 30, 2024).
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