Glossary of Aircraft Acoustic Insulation Terms
- Absorber:
- One of the two treatments for SOUND. It treats noise once it is already inside the aircraft or if inside a wall, absorbs noise before it enters the cabin. It absorbs ambient noise and is like a sponge soaking up water, however in this case it absorbs noise and turns it into low grade heat. Fibrous materials and soft foam are generally good absorbers. They are also generally very light weight and good thermal insulators.
- Acoustic "Short Circuit":
- Slang for one of two things. The first may be a gap that allows noise to enter the aircraft directly like a gap in a canopy. Although more of a vibration "Short Circuit", it might also mean a vibrating piece of equipment or engine that directly contacts the structure and either rattling (a new noise) or transmitting vibration from elsewhere.
- Acoustic treatments:
- For all of the discussion, there are only 4 passive treatments, two are for vibration and two are for noise. They are:
- Isolators: Keep the vibration from ever getting into the structure.
- Dampers: Once the vibration is in the structure, try and reduce the amount of vibration with dampers.
- Barriers: These block the noise from entering the cabin. Like a boat, it is better to keep the water out than have to deal with it once it gets in.
- Absorbers: The last line of defense. Once noise gets inside the aircraft, absorbers soak it up. Placed on the inside as upholstery, they also reduce reflected noise, (echoes).
- ANR (Active Noise Reduction):
- This generates waves of sound to be broadcast against other waves to cancel each other out. A microphone in a headset picks up the noise entering, calculates the frequency and amplitude then broadcasts the response180 degrees out of phase.
- Barrier:
- The second of two treatments for SOUND. It attempts to block noise from entering directly. For example, if the floorboards are vibrating and that in turn is being converted to noise, a barrier can be used to stop it before it enters into the cabin.
- Composite blanket:
- A blanket made of two or more materials that treat noise separately. For example, a felt blanket with barrier uses the felt to absorb noise and decouple vibration from the barrier. The barrier is there to block noise.
- Dampening:
- Standing in the rain. Acoustically, there is no such thing. SEE DAMPER.
- Damper:
- One of two VIBRATION treatments. Once vibration is in the aircraft, dampers reduce the magnitude.
- Decibel, (dB):
- Logarithmic scale invented by Alexander Graham Bell. Named after him, it is why the abbreviation is small "d" and capitalized "B".
- Decouple:
- To isolate from vibration. For example, engine mounts attempt to decouple the vibration of the engine from the airframe.
- Echo:
- Noise that bounces off of walls and other objects. Hard surfaces reflect noise, soft surfaces reduce echoes and may absorb some sound.
- Frequency:
- Cycles in a given length of time. Hertz (Hz) is the same as cycles per second so 100 Hz vibrates one cycle a hundred times in one second.
- Harmonic:
- A multiple of a primary frequency. If an engine is vibrating at 120 Hz. The harmonic frequencies are 240, 360, 480, 600 Hz and so on.
- Isolation mounts:
- The second of the two vibration treatments. It is a way of decoupling vibration thereby reducing vibration energy in the aircraft.
- Law of Conservation of Energy:
- The Law of Conservation of Energy states that energy can neither be created nor destroyed, only converted to another form of energy. This may help explain a lot of what we are doing in terms of controlling noise and making it become another form of energy. Stated another way:
- Isolation mounts (decoupling) reduces the energy that gets into an airframe.
- Dampers dissipate vibration energy into low grade heat by adding shear stress into a gummy foam core.
- Barriers block acoustic energy and therefore reduce the sound inside the cabin. Barriers require a tight fit in order to work!
- Absorbers take sound energy and convert it to low grade heat.
- Massive Barrier:
- The same as a barrier, i.e. it blocks noise the same as a concrete wall does along the freeway.
- Mass Law:
- More of guideline than a law, it states that for every doubling of the mass of a barrier, the sound drops 6 dB. It does not apply to absorbers.
- Octave:
- A doubling of a frequency (it may be any frequency). So octaves of our primary frequency of 120 Hz Are 240, 480, 960, etc.
- Pegasus Aeromarine Inc.:
- Your one source for aircraft noise control systems! We do not sell any single product, we work with a variety of manufacturers to bring you the most effective noise treatment systems then give you options that allow you to make the best INFORMED decision as to what fits your needs. Whether weight, noise or a combination of the two is your priority, we have the right products for you. We show you where each COMBINATION of products fit.
- Tones:
- Narrow frequency band noise. For example, a two blade propeller operating at 2400 RPM (40 Hz), generates two pulses per revolution (80 Hz). Therefore you would see an 80 Hz tone.
- Trim Liner:
- The decorative inside of an aircraft. This can be the single most effective treat in an aircraft since it can be made to treat sounds so many different ways. For example,
- If made from a rigid, non-porous material like Kydex, fiberglass or carbon fiber sheet and if decoupled from the vibrating fuselage, it can become a barrier. Note the fit must be tight with minimal gaps to allow noise to escape.
- If lined with damper, any residual vibrations from the fuselage OR secondary vibrations caused by noise impinging on it will be reduced.
- If covered with an absorbent layer over the top like an absorbing foam, it will soak up ambient noise and reduce echoes.
- Turtledeck:
- Overhead in the cabin, generally aft of the seats.
