Direct sound

The sound from the speakers which reaches directly from the speaker to the listener’s ears.

Reflected sound

The sound that reaches the listener’s ears, after being reflected on various surfaces (walls, ceiling, floor, etc.).

Reflections reach our ears later than direct sound and thus give us the pleasurable sense of space dimensions.
When reflections are balanced at no lower level than direct sound, they give a larger sense of space and music sounds more alive, natural and pleasant in your room.

Conventional speakers due to high directivity do not emit smooth sound to all directions and so the reflections reaching our ears do not sound like the direct sound. Because of their directivity, conventional speakers are seriously disadvantaged outside the narrow sweet spot right in front of the speakers.



Early Reflections

The early reflections curve is an estimate of all single-bounce, first-reflections, in a typical listening room.

Vertical Reflections

The following two spatial averages are defined as Vertical reflections:

  1. Floor reflection is the spatial average of the measurements at 30 degrees below the main-axis +/- 10 degrees.
  2. Ceiling reflection is the spatial average of the measurements at 50 degrees above the main-axis +/-10 degrees.
Horizontal Reflections

The following three spatial averages are defined as Horizontal reflections:

  1. Front is the average of the measurements at 0 degrees +/- 30 degrees.
  2. Side is the average of the measurements at 60 degrees +/- 20 degrees to either side.
  3. Rear is the average of the measurements at 180 degrees off the main-axis +/- 90 degrees (i.e.: the horizontal part of the rear hemisphere).
On Axis response

The measurement of a speaker’s Direct Sound, just in front of the speaker’s radiation axis, in an anechoic chamber.
This is the most common measurement of speaker frequency response. It represents how the speaker sounds when we are just in front of and close to it, so that the direct sound is stronger than the reflections.

Sound Power response

The average of all the frequency response measurements, spherical in all directions around the speaker. It shows the total radiated acoustic energy at all frequencies around the speaker. Smooth Sound Power similar to Direct Sound is essential for natural and pleasant sound reflections from the walls. Thus it is necessary for rich feeling of space.Sound power represents all of the sounds arriving at the listening position after any number of reflections from any direction.

Sweet spot

Sweet spot is the narrow region in which we must be sited during audio playback to have the best sound quality. According to the international standard ITU-R, in stereo, the sweet spot is the vertex of an equilateral triangle with the other two vertexes being the right and left speaker.

Listening Window

The listening window is a spatial average of the amplitude responses in the ±10º vertical and ±30º horizontal angular range. This encompasses those listeners who sit within a typical home theater audience, as well as those who disregard the normal rules when listening alone.

Feeling of Space

It is the feeling of being in a large space with live music. It is measured by the level difference of the Reflected Sound to the Direct Sound at the listening position. When the Reflected Sound is balanced at no lower level than the Direct Sound, we have the illusion of a bigger space of live music, instead of the sound coming out of the speaker box in front of us. The large value of the factor R indicates that a speaker is able to create the Feeling of Space.

The R-factor is exactly opposite to the directivity index DI. R = Sound Power - Direct Sound.

Apparent Sound Source Size

The sense of size of the sound image in front of us, of which the position is determined by direct sound. The illusion that in our room, just before us, is a live orchestra.

Sound Power Directivity Index

Until November 2013 Sound Power Directivity Index (SPDI) was defined as the difference in level between the On-Axis curve and the Sound-Power curve, expressed in dB, (SPDI = Direct Sound - Sound Power).

But as of November 2013, in ANSI/CEA-2034 Standard the SPDI is defined as the difference between the Listening Window curve and the Sound-Power curve, because in highly-directional systems (e.g., large panel and horn) the listening window curve can be significantly different from the on-axis curve.

SPDI shows the uniformity of sound dispersion in space. An SPDI of 0 dB indicates omnidirectional radiation. The larger the SPDI, the more directional the loudspeaker in the direction of the reference axis.

High directivity means narrow listening window, poor reflections and significant loss of aliveness and naturalness outside the narrow sweet spot. Conventional speakers have high directivity index, with losses between 5db at midrange and 10 db at high frequencies.

Early Reflections Directivity index

ERDI is defined as the difference between the listening window curve and the early reflections curve. In small rooms, early reflections figure prominently in what is measured and heard in the room so this curve may provide insights into potential sound quality.


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