The sound of flapping refers to the noise made when something with wings, fins, or other appendages moves them rapidly back and forth. This action of quickly moving the limbs up and down pushes air and creates noise and vibration. The specific sound depends on what is flapping and how quickly the flapping occurs.
What makes the flapping sound?
The flapping sound is created by the interaction between the flapping object and the air. When wings or other appendages push downwards, they displace air which rushes back in to fill the gap as they lift upwards. This rapid disturbance and movement of air molecules colliding creates fluctuations in air pressure. Our ears detect these pressure variations as audible sound waves.
Faster flapping disturbs the air more rapidly, creating higher frequency sound waves. Slower flapping creates lower frequencies. The size, shape and stiffness of the flapping object also impacts the sound through differences in airflow. So a hummingbird flapping its wings makes a very different sound than an eagle due to the differences in their wing size, shape and flapping speed.
Types of flapping sounds
There are many diverse sounds of flapping in nature depending on what animal, insect or other object produces the sound through flapping wings or limbs. Some examples include:
Birds
The flapping of bird wings makes soft swishing or swooshing noises. The larger the bird and the faster it flaps generally produces louder swooshing. For example, an eagle flapping its huge wings as it takes flight makes a very loud swooshing compared to a hummingbird.
Insects
Insects like bees and flies make a buzzing sound with their rapid wing flapping. Their wings are much smaller but move very quickly, creating consistent buzzing. Mosquitos have a particularly high-pitched whining buzz.
Flags
Flags flapping in the wind create a muffled whipping or snapping sound as the fabric is blown back and forth. The speed of the wind impacts the volume and frequency of the flapping flag sound. Strong gusts make louder, faster whipping while a light breeze creates soft irregular snapping.
Sails
Boat sails fluttering and snapping in the wind generate sounds ranging from gentle slapping to loud cracking depending on wind speed. Many overlapping sails create layers of billowing, snapping and rustling sounds. The material and construction of the sail also affects the timbre and resonance of the sound.
Fins
Fish with broad fins and tails make swooshing and swirling sounds when swimming rapidly as their fins flap back and forth pushing the water. Examples like dolphins, whales and sharks create recognizable whooshing fin sounds.
Pool covers
Vinyl pool covers lifted by gusting winds create a raucous flapping commotion of randomly overlapping slaps, cracks and fluttering until the gust passes. The large surface area allows pool covers to generate considerable noise when wind whips them around.
What impacts the sound of flapping?
The specific characteristics of the sound depend on a few key factors:
Speed of flapping
The faster something flaps, the higher frequency sound waves are produced, leading to a higher pitch sound like buzzing. Slower flapping creates lower frequencies and a deeper pitch swooshing sound.
Size of flapping object
Larger flapping surfaces displace more air which creates louder sounds. For example, a flag makes much less noise than a sail, even with the same wind speed.
Rigidity and shape
Rigid, flat and broad flapping surfaces push a lot of air at once, creating noisy cracks and swooshes. Flexible surfaces lead to gentler flutters. An eagle’s stiff wings make robust swooshing compared to an owl’s flexible feathers.
Texture
Rougher or feathered surfaces disrupt airflow and create layered, raspy flapping sounds. Smooth surfaces allow clean airflow for sharper snaps and whips.
Resonance
The resonance of the material being flapped impacts the tone of the sound. Taut fabrics like sails readily vibrate to amplify the sound. Stiffer surfaces like metal dissipate vibration as heat for a truncated sound.
Environment
The location of flapping impacts sound reverberation. Flapping in a wide open space dissipates, while flapping by walls or in a cave amplifies the resonance of the sound through echoing.
Notable flapping sounds
Some examples of iconic flapping sounds:
Owl flight
Owls have specialized feather adaptations that muffle sound to enable silent flight while hunting. Their large, soft wings flap gently with a quiet, fluttering whoosh.
Goose honking
Geese make loud, resonant honking sounds with distinctive vibrato through the rapid beating of their wings when taking off or landing. Their large stiff wings displace substantial air to produce the unique trembling honk.
Sail snapping
A sail will suddenly billow taut and violently snap or pop when wind gusts change direction. The drum-like pop when the sail runs out of slack stands out against the background flutters.
Flag whipping
Whipping flags create rhythmic flutters and rapid snapping cracks when the wind catches the end. There is typically a “pop” when the flag initially flips around with each gust.
Hummingbird buzzing
Hummingbirds flap their tiny wings up to 70 times per second to hover. This rapid fluttering creates the distinctive high-pitched buzzing hum we associate with hummingbirds.
Flapping in technology
Engineers draw inspiration from flapping wings and fins to improve aerial and marine technologies.
MAV flight
Micro air vehicles (MAVs) with flapping wings modeled after insects and birds aim to mimic natural flight mechanisms. The flapping wings provide lift, propulsion and maneuverability superior to miniaturized fixed wings. They produce complex sounds with buzzing and chirping.
Robot fish
Robot fish designed for underwater monitoring use fins and tails flapping like real fish to efficiently propel themselves. The fins disturb surrounding water to generate wake sounds similar to natural fish.
Wind turbines
Modern wind turbine designs aim to reduce noise from flapping blades disturbing air. Slower blade speeds, aerodynamic profiles and dampening materials reduce disruptive swooshing and cracking sounds.
Analyzing flapping sounds
Examining the detailed acoustic properties of flapping sounds reveals more insights. Techniques used include:
Frequency analysis
The distribution of sound frequencies making up the flapping sound determine its pitch and tone. Faster repetitive flapping contains higher, narrower frequency bands.
Waveform analysis
The waveform shape over time indicates the variations in air pressure that define the flapping sound characteristics like bursts, transitions and resonance.
Spectral density
The spectral density indicates which frequencies are emphasized and how energy is distributed between high and low frequencies to give a sound its distinctive timbre.
Decibel measurement
The loudness of flapping sounds corresponds to higher sound pressure levels and decibel values. Measuring decibels quantifies how loud a flapping sound is.
Creative flapping sounds
Beyond naturally occurring flapping sounds, human creativity has invented novel flapping applications:
Slap bracelets
These popular toys are flexible steel strips coated in fabric that makes a loud, reverberant slap when flicked against a wrist.
Cards in bicycle spokes
Clipping a card to a bicycle spoke creates a staccato flapping sound as the wheel turns, imitating a motorcycle or horse hooves.
Flipbook animation
Flipping through sequential drawings in a flipbook make the pictures appear to move and flap. The speed of flipping changes the apparent flapping animation rate.
Conclusion
The sound of flapping encompasses a huge diversity of noises found everywhere from nature to human inventions. Analyzing the aerodynamics and acoustics reveals the physics underlying how movement through air creates sound. Observing and listening to flapping sounds provides insights and inspiration for science, engineering and art. The next time you hear a flag flapping or goose honking, listen more closely to appreciate the hidden complexity within.