When sound is emitted from a source, the wave propagates spherically in space. The direct sound from the source reaches the listener first. Depending on the geometry of the room, the first reflections reach him shortly thereafter.
When we consider a short bang as a test signal, it is reflected at the boundary surfaces of the room. The listener first receives the direct sound, then the first reflections and gradually further reflections. The reflection pattern quickly becomes denser and denser until no individual reflections can be distinguished. Then we speak of reverberation.
The position of each reflection depends on the geometry of the room. The amplitude of each reflection depends on the absorption coefficient of the reflecting surface.
<aside> 🎛️ Frequency dependency
Therefore the graph shows the behavior of the room at exactly one position of the source and one position of the listener at exactly one frequency!
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Why is it that behind a speaker, bass is louder, but in front, treble is louder (my current guess is that the vibrations of bass are powerful in a wider radius than the treble)?
The wavelength of the bass sound is really large, so the speaker cabinet itself is too small to effectively absorb much of the sound energy. High frequency (treble) sound has a small wavelength, so it is very directional. It comes out of the front of the speaker, and propagates forward out of the speaker. It doesn’t really go around to the back of the speaker much at all.
In more detail: Take an A-440 note - it’s the A above middle-C. That has a frequency of 440 Hertz, or cycles per second. The speed of sound in air is 1100 ft/sec, so the wavelength is 1100 / 440 = 2.5 feet. That is about the size of your speaker cabinet, more or less. Take an A two octaves up from that. (Note that moving up an octave means doubling the frequency and halving the wavelength.) The frequency is 1760 Hz, and the wavelength is 7.5 inches. Very small, very directional. Take an A that is two A’s below middle-C. It’s frequency is 110 Hz and its wavelength is 10 feet. Much bigger than your speaker cabinet. That cabinet produces the sound, but has no ability to contain it or direct it. It just radiates out in all directions. The reason the high notes cannot be heard behind the speaker is that the cabinet is stuffed full of fiberglass (or something similar) that absorbs all the sound going backward. The only energy that comes out of the cabinet is going forward. For small wavelengths, this works and the sound goes forward. For the really big waves, it just emerges in all directions.
The higher the frequency, the more directional the source. Tweeters are essentially inaudible from behind the speaker cabinet in traditional box designs. They even vary quite a bit as you go from side to side or from near the ground to high up in most designs. They’re meant to be listened to from in front of the speaker, and usually with the tweeter at ear height. The woofer is much more audible from almost anywhere in the room, and subwoofers can be placed in a much wider area simply because our ears can’t detect the sound source’s location below maybe 80Hz.
<aside> 📈 Big waves often have a relatively long wavelength and have greater energy. On the other hand, short wavelength waves carry less energy and dissipate faster
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Reverb time is the time that passes after switching off a sound source, until the level in the room has reduced by 60dB.
<aside> 🎛️ Standing waves reverb
When there are standing waves in the room, the reverb can be much longer at their frequencies.
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Sabin formula
$$ R_{t60} = 0,0163 \frac {V} {a} $$
Where: