Some acoustic terminology

Despite the practical nature of this work I use some basic terminology of the acoustics.



Second order reflections that occur when a sound is reflected in a perpendicular corner like underneath the side balconies of a hall.

[Acoustician Henrik Möller's personal description]

In this work also mentioned as a part of the controlling reflections, which then again is author's own term describing the important acoustic feedback.



The scattering or random reflection of a sound wave from a surface. The direction of reflected sound is changed so that the listeners may have the sensation of sound coming from all directions at equal levels. Most rooms are not diffused, and reverberation times can be different in different parts of the room due to the room's modes. The amount of diffusion can be measured by taking reverb measurements in many different locations in the room and then taking the average of the differences in decay times.




The portion of the room impulse response consisting of the combination of the true direct sound, which has traveled directly from the sound source to the listener, and the various reflections within the first 20 milliseconds after it.

[McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc]



Early reflections are sounds that arrive at the listener after being reflected maybe once or twice from parts of listening space, such as walls, ceilings and floor. They arrive later than the direct sound, often in a range from 5 to 100 milliseconds, but can arrive before the onset of full reverberation. The early reflections give your brain the information about the size of a room and for the sense of distance of sounds in a room. They have an important role in determining the general character and sound of the room. Lateral early reflections are especially important for the spatial impression of a concert venue.




The three main parts of the impulse response are the direct sound, early reflections and late reflections (reverberant sound).



A room is said to have acoustical intimacy if sound in it gives the subjective impression of a small room. The degree of intimacy in a space corresponds to how soon after the direct sound the first reflection reaches the listener's ears.


"Intimacy" refers to the feeling that listeners have of being physically close to the performing group. A room is generally judged intimate when the first reverberant sound reaches the listener within about 20 milliseconds of the direct sound.

[Encyclopaedia Britannica]



The overtone resonance is based on the matching overtones of at least two simultaneous fundamentals complementing each other and creating a resonant sensation. Basically, the more consonant the played harmonies are the more overtone resonance is achieved. However, the resonant sensation highly depends on the overtone structure of the sound of each instrument and therefore their combinations.

(Author's personal popular description)



The time it takes in seconds for a sound to decay 60 dB or one-millionth of its original sound level after the source has stopped in an enclosed space. Commonly referred to as RT60.




The property of materials (such as air, walls or acoustic panels) that changes sound wave energy into heat energy. When a sound wave that hits a surface is not reflected it is absorbed.




Also called stationary wave,  combination of two waves moving in opposite directions, each having the same amplitude and frequency. The phenomenon is the result of interference - that is, when waves are superimposed, their energies are either added together or cancelled out. In the case of waves moving in the same direction, interference produces a travelling wave; for oppositely moving waves, interference produces an oscillating wave fixed in space.

[Encyclopaedia Britannica]