Wireless Audio Technologies:
How Did We Get Here?

How It Works

A hearing loop system relies on the basic principle of electromagnetic induction. A copper wire run around a room or a defined area carries the electrical signal from an audio source, and in doing so generates a changing magnetic field around it. The telecoil (T-coil for short) is a small copper coil built into a hearing aid or cochlear implant. It picks up the magnetic field the loop creates and converts it back into an electrical signal — so the speaker's microphone audio reaches the user's ear stripped of background noise entirely.

The clever part is that the receiver is already built into a device the user is already wearing — no extra equipment needed. Anyone inside the loop area gets the same signal quality, whether they're standing at a ticket counter or sitting in a large concert hall.

History

Built on the electromagnetic field research of Franklin, Faraday, and Tesla in the 1700s and 1800s, the technology was patented in 1937 by British engineer Joseph Poliakoff. The original design was meant to pick up the magnetic field leaking from a telephone handset's speaker coil — hence the name "telephone coil," shortened to "telecoil" or "T-coil." In the 1960s and 70s, Scandinavian countries spread the technology into churches and schools. In 1974, the UK's NHS began prescribing telecoil-equipped hearing aids as standard — a step that accelerated the technology's mass adoption.

How It Works

FM ALD systems convert an audio signal into frequency-modulated (FM) radio waves and broadcast it over the air. The system consists of a transmitter (worn by the speaker) and individual receivers (a headset or a neckloop). Modern systems can also use digital modulation (DM). The earliest FM systems used AM modulation at 480 kHz; over time, the 72–76 MHz band in the US and 169–176 MHz in Europe were allocated to these systems.

Because radio waves pass through walls, sound can be distributed across multiple rooms or large open areas — this is the biggest advantage FM has over infrared systems.

How It Works

IR systems transmit audio by converting it into invisible infrared light waves. The audio signal is first FM-modulated onto an RF subcarrier (typically 95 kHz), and that signal is then sent out through infrared LED emitters. The receiving headset or neckloop detects the light, converts it back into an electrical signal, and delivers the sound to the ear. This dual-modulation structure (FM/AM) is what sets it apart from standard infrared communication.

Because light can't pass through walls, the system is effectively contained within the room — which is both its biggest security advantage and its biggest limitation. Graham Bell's 1880 photophone (a telephone call carried on a beam of light) can be seen as the inspiration for this technology; commercial infrared LEDs followed in 1963.

How It Works

NFMI uses a magnetic field much like telecoil does, but here both the transmitter and receiver communicate through small coil/magnet pairs. The human body absorbs a 2.4 GHz Bluetooth® signal heavily, while the 3–15 MHz range NFMI works in passes easily through body tissue and the head. That's made it the preferred method for "ear-to-ear" synchronization between a left and right hearing aid.

In a typical setup: the right hearing aid picks up phone audio over Bluetooth®, then relays it to the left hearing aid over NFMI. This two-part architecture — 2.4 GHz outward, NFMI inward — was the standard design for many manufacturers, right up until LE Audio and its CIS channels came along to handle that need directly.

How It Works

DECT is a wireless standard developed by ETSI (the European Telecommunications Standards Institute) for voice and data. Because it runs exclusively on the 1.9 GHz band, DECT doesn't share spectrum with Wi-Fi, Bluetooth®, or cell phones, so interference risk is extremely low. DECT 6.0 (the North American version) operates across 5 RF channels; devices use a "listen before talk" protocol to share channels without stepping on each other's transmissions.

Audio is transmitted with 64-bit or 128-bit digital encryption, which is why it's the go-to choice for offices and healthcare settings that need HIPAA compliance. Hearing aid makers like Phonak build DECT adapters to connect home devices — landline phones, for instance — directly to a hearing aid.

How It Works

The 2.4 GHz ISM (Industrial, Scientific, Medical) band is open for unlicensed use worldwide. Proprietary wireless audio systems that run here typically use FHSS (Frequency-Hopping Spread Spectrum) or DSSS (Direct-Sequence Spread Spectrum) to hold a stable connection even amid the congestion of Wi-Fi and Bluetooth® traffic. TV audio transmitters, wireless headsets, baby monitors, and some stage audio systems all use this band.

Unlike standard Bluetooth®, these systems run manufacturer-specific protocols, so devices from different brands generally aren't interoperable. As a separate, competing space, UHF (470–698 MHz) wireless microphone systems also occupy the professional-certified stage audio market.

How It Works

Bluetooth® is a short-range radio protocol developed by Dr. Jaap Haartsen at Ericsson in 1994, named after the 10th-century Viking king Harald Bluetooth, who united Norway and Denmark. It runs on the 2.4–2.485 GHz ISM band, frequency-hopping (FHSS) across 79 channels. The A2DP (Advanced Audio Distribution Profile) for audio was standardized in 2003, and it set the SBC codec as the mandatory baseline.

Bluetooth®'s physical bandwidth ceiling is theoretically 2–3 Mbps, but the data rate actually usable for audio is usually well under 1 Mbps once protocol overhead, error correction, and other Bluetooth® services sharing the same band are accounted for. That's why every Bluetooth® audio codec is lossy — true lossless transmission is only theoretically possible in aptX Lossless's high-bitrate mode running over Bluetooth® LE.

How It Works

AirPlay started life in 2004 as AirTunes, alongside iTunes 4.6 and the first-generation AirPort Express. It routes audio over Wi-Fi using RTSP (Real Time Streaming Protocol), with device discovery handled by Bonjour (Zeroconf). Audio data travels over the AES-encrypted Apple Lossless (ALAC) codec, which fully preserves CD quality (44.1 kHz / 16-bit).

AirPlay 2 launched in 2018 with iOS 11.4, bringing multiroom support — running multiple speakers in sync. But to make multiroom work, audio quality got capped at 256 kbps AAC — a decision that sparked real controversy in the audiophile community.

How It Works

Sonos was founded in Santa Barbara in 2002 and shipped its first product, the ZP100 ZonePlayer, in 2005. At founding, the company ran into a hard problem: the 802.11 Wi-Fi standard of 2002 wasn't reliable enough for real-time, synchronized multiroom audio. So Sonos built its own proprietary mesh network protocol, SonosNet.

On SonosNet, every active Sonos device acts as a network node and extends the network's reach. When one device is wired to the router, the system routes all the other Sonos devices through that wired connection — cutting down on Wi-Fi traffic. Synchronization is kept under 1 ms, so there's no "echo effect" as you walk from room to room.

How It Works

WiSA is a wireless home-theater audio protocol developed by Summit Semiconductor (later renamed WiSA Technologies) and standardized by an industry association starting in 2012. It runs on the 5 GHz DFS (Dynamic Frequency Selection) band — spectrum historically reserved for weather radar and military systems, and kept clear of the crowded consumer Wi-Fi band.

The system continuously scans 24 frequency channels; if interference shows up on the current channel, audio hops instantly to another one. 8 independent audio channels (7.1, or 5.1.2 with Atmos) are sent uncompressed at 24-bit/96kHz — well beyond CD quality (16-bit/44.1kHz). The WiSA HT variant delivers speaker synchronization within ±2 microseconds.

How It Works

Google Cast works on a different logic than AirPlay: in the "Cast" model, the receiving device (speaker, TV) pulls the music stream directly from an internet service like Spotify or YouTube Music. The phone or computer only acts as a "remote control" — the music keeps playing even if you turn that device off. This architecture removes AirPlay 1's problem of the stream stopping when the source device drops off the network.

Chromecast Audio launched in 2015 as a small USB dongle with a 3.5mm output, supporting 24-bit/96kHz audio. It was discontinued in 2019, but the Google Cast protocol lives on in Nest speakers, TVs, and smart displays.

How It Works

LE Audio is built on the LE Isochronous Channels (CIS and BIS) introduced with Bluetooth® 5.2. It supports two core transmission modes:

• →CIS (Connected Isochronous Stream): Used for left-right ear synchronization in TWS earbuds and point-to-point phone-to-headset connections. Low latency, bidirectional.
• →BIS (Broadcast Isochronous Stream) = Auracast™: One-way audio broadcast from a single transmitter to an unlimited number of receivers. There's no pairing — if an Auracast™ broadcast is nearby, anyone can tune in, the way you'd change a channel.

At the heart of this technology is LC3 (Low Complexity Communications Codec). LC3 delivers the same or better audio quality at roughly 50% lower bitrate than SBC. That efficiency translates into lower power consumption, longer battery life, and the low latency that's critical for hearing aids.