Page 147 - Master Handbook of Acoustics

Perception of Spaciousness, Images, and Echoes

Assume a reflection delay of 10 msec with the reflection coming from the side. As the level of the

reflection is increased from a very low level, the reflection is completely inaudible. As the level of
the reflection is increased, it finally becomes audible as its level reaches about 15 dB below the
direct signal. As the reflection level is increased beyond this point, the room takes on a sense of
spaciousness; the anechoic room in which the tests were made sounds more like a normal room. The
listener is not aware of the reflection as a discrete event, or of any directional effect, only this sense
of spaciousness.

As the level of the reflection is increased further, other effects become audible. At about 10 dB
above the threshold of audibility of the reflection, changes in the apparent size and location of the
front auditory image become apparent. At greater delays, the image becomes smeared toward the
reflection.
Reviewing what happens in the 10- to 20-msec delay range, as the reflection level is increased

above the threshold of audibility, spatial effects dominate. As the reflection level is increased roughly
10 dB above the audibility threshold, image effects begin to enter, including image size and shifting of
position of the image.
Reflections having a level another 10 dB above the image-shift threshold introduce another

perceptual threshold. The reflections are now discrete echoes superimposed on the central image.
Such discrete echoes are damaging to sound quality. For this reason, reflection level/delay
combinations that result in such echoes must be minimized in practical designs.
Lateral reflections provide important perceptual cues in a sound field. Lateral reflections can
affect spaciousness and the size and position of the auditory image. Olive and Toole investigated a
two-loudspeaker installation and found that the effects obtained from a single loudspeaker are

correlated to the stereo case. This suggests that single-loudspeaker data can be applied to stereo
playback.
Those interested in the reproduction of high-fidelity audio will see the practicality of the results of
these reflection studies. The spaciousness of a listening room as well as the stereo image definition
can be adjusted by careful manipulation of lateral reflections. However, lateral reflections can be

used only after interfering early reflections are reduced. This suggests practical room design
techniques that will be explored in later chapters.

Effect of Angle of Incidence, Signal Type, and Spectrum on

Audibility of Reflection

Researchers have shown that the direction from which a reflection arrives has practically no effect on
the perception of the reflection, with one important exception. When the reflection arrives from the

same direction as the direct signal, it can be up to 5 to 10 dB louder than the direct sound before it is
detected. This is due to masking of the reflection by the direct signal. If the reflection is recorded
along with the direct signal and reproduced over a loudspeaker, it will be masked by this 5- to 10-dB
amount.
The type of signal has a major effect on the audibility of reflections. Consider the difference
between continuous and noncontinuous sounds. Impulses, in the form of 2 clicks/sec, are of the

noncontinuous type. Pink noise is an example of the continuous type. Speech and music lie between