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Acoustic Data Transmission

June 14, 2021

The once wired world has become wireless. My office desk and floor in the early days of computing were cluttered with a multitude of cables that connected my desktop computer to printers, data storage devices and our building's local area networking. Some of those cables were thick, since they contained the many wires needed for the popular and somewhat speedy parallel data port common on computers of that era. Some of the equipment in my laboratory was interconnected by the even thicker cables of the IEEE-488 General Purpose Interface Bus (GPIB).

Centronics 36pin female connector

A monster from the past. Looking very much like the multi-toothed mouth of some movie monster, this is a Centronics 36-pin female connector. The Centronics parallel port was used first to connect printers to computers in the 1970s. Parallel data connection was later simplified and implemented using a DB-25 connector for high-speed connection to other computer peripherals, such as Zip Drives. (Wikimedia Commons image.)

Today's connection to the Internet is often wireless, enabled by our smartphones. Early home connection to the Internet was made using POTS, the plain old telephone service. The earliest means for this was through an acoustic coupler, a crude device that coupled a telephone handset to a loudspeaker and microphone to allow a tone representation of data to be sent and received. Since POTS had limited frequency response, the data rate was first limited to 110 baud. Later acoustic couplers had rates up to 1200 baud. In one implementation that I used to record data on cassette tape, a 1200 Hz tone represented a "1," and a 2200 Hz tone represented a "0."

Acoustic couplers were needed, since the Bell System would not allow direct connection to the telephone network wiring. The floodgates of telephone connection were opened by the 1968 Carterfone legal decision that allowed direct connection to the telephone wires if the device did not compromise the safety and integrity of the telephone network. Bell's position had been that they couldn't allow connection of devices other than their own, since these foreign devices could generate voltages that would endanger their equipment and linemen. At that point, modems became a common computer device, and the Internet was associated with their signal tones.

Acoustic signaling existed long before the Internet, and one example we have in our technological era is underwater signaling that even included an underwater telephone. Historically, in western civilization, we have church bells and alpine horns. These are mostly alarm sounders, but drums were used for long distance communication of actual messages in Africa. The West Africa talking drum is an hourglass-shaped drum that's designed to allow for changes in pitch that imitates speech. Messages are transmitted through the use of low and high tones along with phrasing and pauses. This type of messaging will soon become extinct, since cellphone adoption in Africa has been rapid.[1]

Ashanti drums (UK National Archive CO1069)

Ashanti drums, along with a drummer and an attentive audience.

The skulls are those of defeated tribal kings, and they are used to adorn the larger drum at the rear.

(Portion of an image from the United Kingdom National Archive, document record CO1069, via Wikimedia Commons.)

In a world in which everyone is trying to steal each other's secrets, it's no wonder that methods were developed to eavesdrop on information leaking from computers, printers, fax machines, and other devices by means of unintended electromagnetic emission and emitted sound. A countermeasure for this was TEMPEST (Telecommunications Electronics Materials Protected from Emanating Spurious Transmissions).

Acoustic cryptanalysis attempts to gain information through unintended acoustical signals that can be obtained from a hacked cellphone or a secretly placed microphone. These derive information from such devices as computer keyboards, impact printers, and inkjet printers. In a more unconventional method, electronic components such as inductance coils and capacitors will emit a faint sound when excited by normal operating currents. In the early days of electronics, some vacuum tubes would produce signals when acoustically excited through movement of their internal electrodes. A converse operation, using ultrasonic signals to take control of a voice-assisted device, is also possible.[2]

Acoustic signaling can be used for good as well as evil. While current television remote controls use infrared for signaling, early versions used ultrasonic waves. The first of these employed mechanically actuated metal tines to generate ultrasonic waves at different frequencies for channel-up, channel-down, sound-on/off, and power-on/off. Later versions sent digitally-coded signals from a transducer operating at 40 kHz. This concept was used for remote control of devices other than televisions, often with just a relabeling of the buttons on a television remote.

A 40 kHz ultrasonic transducer.

A 40 kHz ultrasonic transducer.

Similar transducers were used in early television remote controls. They were also used as the rangefinder for the autofocus feature in the Polaroid Sonar OneStep camera, introduced in 1978.

(Wikimedia Commons image by the author.)

Acoustic data transmission is not a fashionable topic, but James Nesfield published a summary article of its uses at Electronic Design two years ago.[3] One advantage that he cited is that many devices already contain the necessary loudspeaker and microphone for this data transmission mode, so a little software can add value to existing hardware.[3] One particular advantage which would be useful in classroom and conference settings is its pairing-free, one-to-many connectivity that has an advantage over Bluetooth and Wi-Fi, since no connection setup is required.[3]

Nesfield lists the following advantages of acoustic data transfer:[3]
• It Works Offline: The peer-to-peer nature of acoustic data transmission means that Internet connectivity is not required. It will work when no network access is available, which might be an important factor in developing countries.

• It Utilizes Existing Hardware.

• Secure, Private Data Transfer: No internet connection means less of a chance that your data will be stolen. Additionally, acoustic data transmission can still use cryptography.

• It Supports Many Platforms: Acoustic data transmission can work in devices ranging from smartphones, tablet computers, toys, and legacy systems.

• Scalable: Analog data transfer is scalable, and it can be used in simple Internet of things (IoT) devices.

• It Works in Extreme Environments: Proper signal-processing software allows the use of acoustic data transfer in most extreme environments.


  1. Laura Silver and Courtney Johnson, "Internet Connectivity Seen as Having Positive Impact on Life in Sub-Saharan Africa," Pew Research, October 9, 2018.
  2. Sophie Bushwick, "Ultrasonic Attack Device Hacks Phones through Solid Objects," Scientific American, March 18, 2020.
  3. James Nesfield, "Sending Data Over Sound: How and Why?," Electronic Design, June 24, 2019.

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