Technology

In stereophonic reproduction it is normal to hear a sound on the left or on the right, close or far away. But what about its height? 

Sometimes we are ill at ease when we hear a trumpet, a chorus or an organ with a flat sound. As if it were glued to the top of the loudspeakers. 

Could it be that stereophony only has two dimensions?

stereolith vs speaker

What is stereophony?
Derived from the Greek word stereos, meaning "solid", stereophony means the reproduction of sound with the impression of acoustic contour. In essence, this means three dimensions: width, depth and - obviously - height.

Any sound we perceive has three dimensions. The flute has a sound of its own, just like the piano. A concert-hall environment has a volume which, in itself, characterizes its acoustics.

Our hearing process
In reality, we perceive these dimensions and the position of sound sources in space. That is where we come to a most important point: the localisation of the source of the sound. Our ears "listen" even while we sleep; they are permanently active. When a sound or noise appears we automatically try to determine its origin. Then we try to recognise what kind of sound or noise it is and, finally, we try to situate ourselves in relation to this noise, i.e. we become aware of the acoustic environment.

However, as long as the sound source is not localised, our hearing is jeopardised and we are incapable of correctly analysing the nature of the sound and its environment. Although hearing provides us with the pleasure of music, above all it allows us to live and even to survive, hence its extreme importance! To cross the street without perceiving the arrival of a 40-ton lorry can have dramatic consequences...! 

Hearing means that a noise or sound attracts our attention, whether we like it or not. Independent of our will, our hearing sense has the function of an alarm (as do our other senses). After having located the source of the sound, we begin to ascertain and to analyze the sound message according to our life span and according to our sound patterns. This will determine our decision to cross or not to cross the road.

Localisation and identification
Having two hears does not only have the advantage of better hearing, but permits - and that is essential - localising precisely the direction of sound sources. As our ears are located on either side of our heads, in most cases they can determine where the sound is coming from, providing the sound intensity is higher than our own threshold of sensitivity and that the frequencies are neither too low nor too high. Indeed, a sound coming from the left reaches our left ear before the right one as the sound will take 10 - 900 microseconds longer to reach the right ear. It is this minute time difference which permits us to perceive the direction of the sound.

This so-called localisation phase of hearing lasts for less than a 1000th of a second. It represents the maximum time which sound needs to travel the distance between our two hears. In addition, our ears will not only perceive a time difference but also a different timbre and sound pressure. Having located the source of the sound, our sense of hearing will then try to recognise and analize the sound. Finally, a more or less important reverberation, as well as the re-echoing - or its absence - inform us about the acoustic environment. 

Hearing does not record a continuous flow of information. It proceeds to a selection of which less than 8% serve to localise and identify the sound. The hearing phases are automatically switched on when a new sound event occurs. It then synchronises the new sound to its own rhythm where music is concerned.

Stereophonic recording
During a stereophonic recording, the sound space is encoded by a two-channel (left and right) matrix. For the actual recording, two microphones are placed approximately 30 cms. distant to each other in order to respect the hearing physiology. Each of the microphones (left and right) provides a signal which is slightly different depending on the location of the sound source.

This slight difference is of great importance, as spaciousness is determined only by this difference. In addition to the actual, purely musical information, the recorded stereophonic signal also contains information regarding space.

Stereophonic reproduction
In order to reproduce a stereophonic recording which faithfully renders not only the sound but also the accompanying accoustic spaciousness, it is of paramount importance to establish very specific hearing conditions. It is obvious that the hardware (record player, tape recorder, tuner, amplifier and loudspeakers) has to correspond to the standard of hi-fidelity and be correctly connected.

Although today's electronics do not pose any major problems, this is not the case with speaker boxes. Their location is critical and their performance dependent on the acoustic characteristics of the room.

How do we perceive stereophony?
When equipped with two speakers, the room in which we listen is not the stage of a stereophonic reproduction; it is only the carrier of two different types of information: the left and the right signals. With the help of these two types of information, the listener can - if certain conditions are respected - mentally reconstitute the stereophonic "image". As paradoxical as it may seem, it is the location of the two speakers which permits us to separately perceive the left and right signals, transmit them to our brain which interprets them by more or less successfully reconstituting the musical spaciousness. This is only true if the listener is placed in a symmetrical axis with regard to the two speaker boxes. If he is placed slightly more to the left or the right, there is a time difference in which the sound reaches the two ears, thus jeopardising the reconstitution of the stereophonic image. To modify the sound level with the balance knob does, unfortunately, not correct defect.

The ideal spot
As described above, it is seldom possible to have an ideal listening condition. It can only be found at the studio where the actual recording was made. We have to jungle with the appearance of our rooms and the space available. Very often we only obtain a two-channel, instead of a stereophonic reproduction.

This is a great pity as we cannot reproduce the true dimension of a flute or a piano and its position in space. Whatever the sound intensity while listening, the fact of reproducing music with two speakers does not guarantee a true stereophonic audition.

What can be done?
All that is needed to guarantee the total symmetry of the two speakers in the entire room and, thereby, ensuring a three dimensional reproduction of sound is simply to integrate the two speakers into...one! It was by completely rethinking the acoustic phenomena, based on the physiology of hearing, that the idea of the Stereolith® box was born. By coupling acoustically and by opposing the left and right speakers in the same box, one brings about the true stereophonic matrix reproduction.

On the appearance of a signal, a lobe of acoustic pressure is generated on either the right or left side to couple with a complementary lobe of sound pressure on the opposite side. Through propagation, this double lobe extends and reconstitutes by its form a wave front, the angle of which determines the original direction of the sound source. The distance between the left and the right speakers determines the wave length at which the coupling was made. To cover an extended spectrum of frequencies, the Stereolith® is equipped on its tapered sides with several couples of speakers in a line and at various distances. Although the speakers are placed on the tapered sides, the Stereolith® box diffuses sound in all directions without reflecting walls or surfaces. As it is independent from the place where it stands, it functions just as well in the open as in a very reverberating room. 

Therefore, it is not necessary to be in a precise spot since the Stereolith® spreads acoustic waves throughout the entire room which are virtually identical to those transmitted by an orchestra in a concert hall. As the stereophonic image does not have to be reconstituted, our hearing sense immediately locates the sources of sound in their own acoustic environment, which places us in a musical world effectively having a third dimension...