HIGH END ´98 (Frankfurt) Catalog S.346

Room Reflections

unwelcome or indispensible ?

The controversal and rather sporadic way that our HIFI specialist press is discussing the matter of room acoustics shows the dilemma in dealing with the spatial hearing. The press emphasizes the phase- and time-constant reflection to assess the roomal emitting reaction of a loudspeaker. This is a sound way of approaching to this problem, allthough the relevant question concerning the effects of sound reflections on the perception of spatial listening is not completely answered.

The importance of these reflections is shown by means of the following experiment. Let the loudspeaker play outside. You will see that the stereoscopic effect will be reduced to the level of the loudspeaker and that spatial effect of depth which you receive in your living room, is missing. Now, at the latest, the question of the emergence of the impression of the room on music reproduction by a loudspeaker unit, which we appreciate so much is arising. Surely, the function of our hearing-system is not cleared in detail because it is a psycho-physical impression of the senses. Nevertheless I will now try a simplified description of spatial hearing with reference to the room reflections.

A sensation for the direction occures, when a sound signal first reaches one ear and then, after a certain delay reaches the other ear. Our brain notes the time difference and is then able to determine quite exactly, from which direction the signal comes. All sound signals following shortly after (i.e. reflections up to 50ms) will be investigated from our ear in a very complex process , whether there is a relation to the first signal or if there are new signals.

By that our ear notes, practically in real time the size of the room and the damping of the room by the minimal delayed sound reflections. This enables us to differ right away between the acoustic of a bathroom or a hall or a damped living room. Room reflections are therefore indispensible for the acoustical orientation, whereas our ear can not accept and evaluate reflections coming from the same direction as the direct sound. We therefore need a listening room to transfer the room acoustic information of the sound carriers into our living room. The listening room constitutes the acoustical projection field. One important condition to create the recording room virtually in the listening room is a well-balanced sound field.

When listening to extremely focussing loudspeakers which therefore produces few indirect sound, the listener hears the direct- and indirect sound of the recording in a spot beam from one direction. This is, however, a completely unnatural sound situation, because the indirect sound of the recording will not be distinguished by us through time masking effects. As a result we distinguish this as a flat acoustic pattern. It is known, that small 2-way loudspeakers have got a wide beamwidth and therefore they show a depth stepping. The world-wide success and acceptance of some representatives of the species of dipole (radiators of 1.order) certifies to this system a better realisation. The ideal, however, is a point source (radiator of 0.order, monopole) with a perfect omnidirectional radiating behaviour. This ideal is mostly reached by an omnidirectional radiator. It is neccessary to take care precisely, of a phase- and time constant radiation to avoid that the room information won´t be clear.

From the psycho-acoustic point of view it is therefor not true, that omnidirectional radiators add something to the music signal with the room reflections, because the opposite is true, only if there is a homogeneous ratio between the sound signal and the sound field you can enjoy a natural music reproduction.

Dipl. Ing. M.Duevel
CD Konzertmöbel

E. Zwicker: Psychoakustik (Springer, Berlin, Heidelberg, New York 1982)
J. Blauert: Räumliches Hören (Hirzel, Stuttgart 1974)
J. Blauert: Räumliches Hören, 2. Nachschrift (Hirzel, Stuttgart 1997)
G. Schwamkrug: Lautsprecher, Dichtung und Wahrheit (Elektor, Aachen 1986)