Acoustic signature design

Summary¶

A multirotor's acoustic signature is dominated by propeller tip speed, not motor noise or frame vibration. Tip speed is the product of rotational speed and propeller radius; reducing either reduces tip speed and therefore noise. Ghost's design exploits this directly: large-diameter props driven by low-KV motors produce the same thrust at substantially lower RPM than the small high-KV motors on other family members, reducing tip speed and acoustic emission by approximately 10–15 dB(A) — a reduction the human ear perceives as roughly half as loud. This is the primary design decision of the Ghost variant.

Concept¶

The tip speed mechanism¶

Propeller noise is generated primarily at the blade tips, where the local velocity relative to the surrounding air is highest. This tip velocity has two components: the rotational velocity (propeller RPM × tip radius) and the forward velocity of the aircraft. In hover, only the rotational component is present.

Tip speed (m/s) = RPM × π × diameter / 60

For a standard 6-inch (152 mm diameter) drone at 14 000 RPM: Tip speed = 14 000 × π × 0.152 / 60 = 111 m/s

For Ghost's 12-inch (305 mm diameter) props at 3 200 RPM (sufficient for hover at 1 360 g AUW with MN4108 480KV on 4S): Tip speed = 3 200 × π × 0.305 / 60 = 51 m/s

The 51 m/s vs 111 m/s difference in tip speed corresponds to approximately 10–15 dB(A) reduction in broadband noise emission. The A-weighting (dB(A)) reflects human auditory sensitivity — low-frequency content from large slow props is less audible than high-frequency content from small fast props even at equal acoustic power.

The KV-diameter relationship¶

A motor's KV rating (RPM per volt) determines the no-load RPM at a given voltage. T-Motor MN4108 at 480 KV on a 4S (16.8 V full charge) produces approximately 8 064 RPM at no load. Under the load of a 12-inch prop, actual operating RPM at hover throttle is much lower — typically 3 000–3 500 RPM. This is the fundamental trade: low KV forces low RPM at any given voltage. The same voltage that spins a 3 400 KV P1804 motor at 56 000 RPM no-load spins the MN4108 at 8 000 RPM. Large props at low RPM carry as much or more air per unit time as small props at high RPM — the difference is acoustic efficiency.

Detectability implications¶

Acoustic detection range is the distance at which a sound source can be distinguished from background noise by a human listener. At 65–70 dB(A) source level (typical 6-inch drone), the detection range in quiet outdoor conditions is 80–150 m. At 50–55 dB(A) (Ghost), the same outdoor background noise reduces detection range to 20–50 m. This is not invisibility — it is a meaningful reduction in the warning time available to an observer.

For Ghost's operational context (perimeter watch, security-sensitive survey, night operations), the difference between 150 m and 30 m acoustic detection range is operationally significant. The awareness curriculum measures this directly using Bandit Part B Exercise B2.1.

Reference¶

Note: Absolute dB(A) values are estimates from published data for comparable platforms; exact Ghost values require field measurement after build and maiden flight.

Procedure¶

Measure Ghost acoustic detection range (Bandit Part B methodology)¶

1. Place Ghost in hover at 50 m AGL in Loiter mode. Operator monitors QGroundControl from the launch point.
2. A second operator (the listener) walks away from the hover point on flat terrain in calm wind conditions (< 2 m/s).
3. The listener walks until they can no longer reliably hear the aircraft, then walks back until they can reliably hear it. Mark this distance.
4. Repeat in three directions from the hover point. Average results.
5. Compare against the same measurement on Bandit — the ratio is the acoustic reduction Ghost achieves in this specific environment.

Rationale¶

The 10–15 dB(A) reduction from Ghost's large-prop low-RPM design was considered sufficient to justify the non-printable CF plate arm architecture and the Li-Ion battery pack complexity. A smaller noise reduction — achievable with slightly larger props on a printed arm — would not justify the departure from the family's print-at-home philosophy. The 10 dB threshold was chosen because it represents a perceptual doubling of quietness to the human ear — a reduction that is operationally meaningful, not just measurable on a meter.

Connections¶

yaml requires: - [propellers](<./propellers.md>) - [brushless-motors](<./brushless-motors.md>) - [lift-and-thrust](<./lift-and-thrust.md>) related: - [ghost-variant](<./ghost-variant.md>) - [cf-plate-arms](<./cf-plate-arms.md>) - [operational-security](<./operational-security.md>) - [hover-and-forward-flight](<./hover-and-forward-flight.md>) leads_to: - [ghost-variant](<./ghost-variant.md>)