Walls, crowds and 2.4 GHz: why Auracast coverage changes

The band Auracast lives in

Bluetooth LE, and so Auracast, uses the 2.4 GHz band from 2400 to 2483.5 MHz, in 40 channels 2 MHz apart (Bluetooth Core Specification). The SIG's transmitter guide names typical obstructions: "thick walls, metal structures, water, etc." People belong on that list too, as the next sections show.

Signal levels in one minute

Signal strength at a seat is given in dBm. The scale is logarithmic: every 10 dB is a factor of ten in power. So -60 dBm is ten times stronger than -70 dBm, and a hundred times stronger than -80 dBm. Every wall, body and meter of distance between the transmitter and the listener takes some away.

Walls

Typical losses at 2.4 GHz from Huawei's Wi-Fi planning table:

Treat these as starting points, not measurements of your building. Real walls vary in thickness, build and moisture, and the signal also reaches seats by reflection. A 15 dB brick wall cuts the power to about a thirtieth; 25 dB of concrete to well under a hundredth.

What it means:

• Treat a solid wall as a new zone. Rather than pushing a transmitter's power to reach a side chapel through a solid wall, give the chapel its own transmitter. (Bettear's rule of thumb says the same: a solid wall means a new zone.)
• Hidden transmitters underperform. The SIG guide asks installers to avoid "enclosures such as cupboards, or behind metal partitions or counters." Auri: "Avoid placing transmitters behind solid walls or large metalwork."
• The neighbors may still hear it. A wall reduces the signal; it rarely stops it. See the note on privacy in our comparison with loops, FM and infrared.

People

The human body absorbs 2.4 GHz signal. A 2018 study in the journal Sensors measured about 10 dB of extra path loss when one person stood in the path, and about 15 dB with two people, in a building with concrete walls. Its simulations found about 10 dB of extra loss with the receiver 1 m from a person's body, about 6 dB at 4 m, and much more when the receiver was right next to the body.

A full room puts many bodies in the path. The effect depends on where the transmitter is:

• Low transmitter, seated audience, listener at the back: the signal passes through rows of people. Expect a big drop from empty to full.
• High transmitter: the direct path passes over heads. The SIG's guide: "Placing the Auracast transmitter above head level will minimize this effect."
• Hearing aids and earbuds: they sit against the wearer's head, so the wearer's own body is in some of the paths. Auri's manual notes that range drops when receivers are covered or obstructed, and that a single earbud or hearing aid has much greater range with line of sight to the transmitter. Plan margin for them.

Planning for both

1. Design for the full room. The SIG asks installers to "repeat the coverage check when the room is being used." Auri: coverage "should be validated in normal use of the space."
2. Mount high and central. It is the simplest way to take the audience out of the path. See transmitter placement.
3. Count the walls, not just the distance. A short path through brick can be weaker than a long open one.
4. Measure at listening height. The UK code of practice uses 1.2 m for seated listeners and 1.7 m for standing.
5. Keep a margin. A room that only just works empty is unlikely to work full. Our commissioning guide covers testing it occupied.

Common questions

Why does it work in rehearsal but not on the day?

A common reason is that the room is full: people absorb the signal. The radio environment can also change with the event, so test with the room in use.

Can we just turn the power up?

Sometimes, within the transmitter's and the region's limits. But more power also reaches further into neighboring rooms and adds to the busy 2.4 GHz band. The SIG's advice is to use the lowest power that covers the intended area. Moving or adding a transmitter is often the better fix.

Do glass walls block Auracast?

Plain glass costs little (about 4 dB in Huawei's table). Heavy colored glass costs more (8 dB) and armored glass far more (about 25 dB). Test across any glazing that matters.