Question: What is the lowest frequency wave detectable?
About the article "Gravitational wave", section "Introduction".

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Auroral chorus can be detected primarily around the magnetic equator, specifically in two distinct frequency bands, one above the equatorial half gyro-frequency and one below it.
The gyro-frequency ranges from 0.6 kHz to about 1.6 kHz.
Distinguishable on high resolution wideband spectrographs, the wave amplitude grows linearly then switches to non-linear.
Demonstrating a peak distribution near dawn, the auroral chorus is most detectable via ELF/VLF Radio receivers in the middle latitude around 30-60 degrees N. The most numerous recordings of the auroral chorus has been by the Iowa Plasma Wave Group.
[http://www-pw.physics.uiowa.edu/plasma-wave/istp/polar/home.html] They have released many audio interpretations of chorus recordings online along with spectrograph measurements.  
A propagation mode in a waveguide is one solution of the wave equations, or, in other words, the form of the wave.
Due to the constraints of the boundary conditions, there are only limited frequencies and forms for the wave function which can propagate in the waveguide.
The lowest frequency in which a certain mode can propagate is the cutoff frequency of that mode.
The mode with the lowest cutoff frequency is the basic mode of the waveguide, and its cutoff frequency is the waveguide cutoff frequency. 
To be simple, consider a clear medium firstly.
The medium contains small optical absorbers moving with velocity vector formula_1.
The absorbers are irradiated by a laser with intensity modulated at frequency formula_2.
Thus, the intensity of the laser could be described by:formula_3When formula_1 is zero, an acoustic wave with the same frequency formula_2 as the light intensity wave is induced.
Otherwise, there is a frequency shift in the induced acoustic wave.
The magnitude of the frequency shift depends on the relative velocity formula_1, the angle formula_7 between the velocity and the photon density wave propagation direction, and the angle formula_8 between the velocity and the ultrasonic wave propagation direction.
The frequency shift is given by:formula_9Where formula_10 is the speed of light in the medium and formula_11 is the speed of sound.
The first term on the right side of the expression represents the frequency shift in the photon density wave observed by the absorber acting as a moving receiver.
The second term represents the frequency shift in the photoacoustic wave due to the motion of the absorbers observed by the ultrasonic transducer.
In practice, since formula_12 and formula_13, only the second term is detectable.
Therefore, the above equation reduces to:formula_14In this approximation, the frequency shift is not affected by the direction of the optical radiation.
It is only affected by the magnitude of velocity and the angle between the velocity and the acoustic wave propagation direction.
This equation also holds for a scattering medium.
In this case, the photon density wave becomes diffusive due to light scattering.
Although the diffusive photon density wave has a slower phase velocity than the speed of light, its wavelength is still much longer than the acoustic wave. 
Visible light is an electromagnetic wave, consisting of oscillating electric and magnetic fields traveling through space.
The frequency of the wave determines its color: is red light, is violet light, and between these (in the range 4-) are all the other colors of the rainbow.
An electromagnetic wave can have a frequency less than , but it will be invisible to the human eye; such waves are called infrared (IR) radiation.
At even lower frequency, the wave is called a microwave, and at still lower frequencies it is called a radio wave.
Likewise, an electromagnetic wave can have a frequency higher than , but it will be invisible to the human eye; such waves are called ultraviolet (UV) radiation.
Even higher-frequency waves are called X-rays, and higher still are gamma rays.
All of these waves, from the lowest-frequency radio waves to the highest-frequency gamma rays, are fundamentally the same, and they are all called electromagnetic radiation.
They all travel through a vacuum at the same speed (the speed of light), giving them wavelengths inversely proportional to their frequencies.
In dispersive media, such as glass, the speed depends somewhat on frequency, so the wavelength is not quite inversely proportional to frequency. 
Short wave radio waves (in the HF range) are absorbed by the increased particles in the low altitude ionosphere causing a complete blackout of radio communications.
This is called a short wave fading.
These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead.
The ionospheric disturbance enhances long wave (VLF) radio propagation.
SIDs are observed and recorded by monitoring the signal strength of a distant VLF transmitter.
A whole array of sub-classes of SIDs exist, detectable by different techniques at various wavelengths: the SPA (Sudden Phase Anomaly), SFD (Sudden Frequency Deviation), SCNA (Sudden Cosmic Noise Absorption), SEA (Sudden Enhancement of Atmospherics), etc.