15.7.2018

The Sweeter Spot

With stereo comes the concept of the sweet spot. Wikipedia defines it as “a term used by audiophiles and recording engineers to describe the focal point between two speakers, where an individual is fully capable of hearing the stereo audio mix the way it was intended to be heard by the mixer.” A very good, albeit not clear enough definition.

You see, stereophonic, multichannel and immersive reproduction systems can create virtual sound images. Some people think of this when they say sweet spot. Others think of the area where you have good and stable rendition of sound colour.

The further away you move from the acoustical axis, the more the sound colour tends to change. The acoustical axis is typically directly in front of the loudspeaker and offers the best, most neutral and flat frequency response. How the sound changes when you move off-axis depends on the individual product design. Genelec is very proud of its product designs which offer minimal, easy to understand, systematic sound colour change. In this sense, Genelec designs offer a wide sweet spot.

The locations of virtual sound images are created precisely only at a certain point in space in relation to loudspeakers. For the most familiar case, the two-channel stereophonic system, this point is defined as being at equal distance from the loudspeakers while the loudspeakers are located at 30-degree angles to the left and right relative to the direction you are facing. Only in this exact position can you experience the studio-designed virtual sound sources at their correct locations. This sweet spot is very narrow with little dependency on loudspeaker design. The effect is created with the combination of two loudspeakers plus the properties of your own hearing system, fooling you to feel that sound appears in thin air. This is a psycho-acoustical effect, and in this scenario, if the listener moves even slightly to the side then all of the virtual sound images also move. A slight movement towards the speakers or away from the speakers starts widening or narrowing the area occupied by the virtual sound images. The existence and locations of the virtual sound images depend on the precise relative timings and levels of the sound coming from the loudspeakers. For this case, there cannot be a ”wide sweet spot”.

For this second case, the quality of the loudspeaker design of course matters. Bad loudspeakers will not create well defined virtual sound images. If the loudspeaker is well designed, audio is less affected by the room or the positioning of the loudspeakers within the room. When well-designed loudspeakers are systematically very similar in production, they will be better able to create more precise and well-defined virtual sound images. Genelec designs have excellent performance characteristics and can be said in this sense to offer the widest and most accurate sweet spot available.

As the correct and accurate reproduction of the virtual sound images depends on the relative timing and level of the audio arriving to the listener from the loudspeakers, it is possible to fine tune this to increase monitoring precision in scenarios where the listener is not located at the standard sweet spot.

Genelec GLM AutoCal performs this function automatically for SAM enabled monitors. After acoustical measurements are taken, the levels and times of flight (delays) of audio for all loudspeakers are aligned at the listening location. This focuses the sweet spot at the location of the measurement microphone. On top of that, the acoustical influences of the room are decreased by way of small adjustments to the frequency responses to reduce the acoustical effects of the walls, floor and ceiling, improving the sound colour at the chosen listening location. As a net effect, the sweet spot becomes maximally accurate at the location of the SAM measurement microphone. This improves the quality of the sweet spot.

How about surround audio and 3D immersive formats? Aren’t these said to “increase the sweet spot”? After all, these have been designed to enable more than one person to sit in the optimal listening area.

The fundamental idea behind surround and immersive reproduction systems is to bring in more real sources of audio. The reason for this is that no matter where one is seated, one can always correctly locate these real sources. This is not true for the virtual sources. When seated off the ideal location, the virtual sound images move. Typically two or three loudspeakers participate in creating these virtual sound images. When we introduce more loudspeakers with less space between the real sources, this offset becomes relatively smaller. However, the basic acoustic and psycho-acoustic principles do not change. If you are not at the ideal location, the virtual image locations will change while the real ones are located correctly. The more real sources are used, the more stable the immersive presentation system can become for the listeners not located at the ideal location. By adding more real sources, this difference can be made smaller.

These three aspects together – controlled directivity with minimal change in sound colour for off-axis position, systematically similar flat and neutral frequency response on the acoustical axis, and the capability of the SAM-enabled loudspeakers to refocus the location of the most precise reproduction – all combine to widen the sweet spot of Genelec monitors to make it the widest in the industry.

But, please pay attention to what people mean when they talk about a wide sweet spot. For stereophonic reproduction systems, only the correct location gives the correct rendition of the sound stage and virtual sound image locations.

 

Aki Mäkivirta

R&D Director