In search of the power alley By José (Joe) Brusi Engineering, DAS Audio

1 2 3

The following illustrations show modelling for 80, 63 and 50 Hz, respectively, on figures 7-9. Note how the number and location of the lobes varies with frequency.

Fig. 7. Left and right subwoofers. 80 H. Click image for larger view.

Fig.8. Left and right subwoofers. 63 H. Click image for larger size.

Fig. 9. Left and right subwoofers. 50 H. Click image for larger view.

So, where is that alley?

For each frequency we have a map with fingers. If we sum several frequencies and plot them on a single map, the alley emerges, since it represents the only locations where bass sums coherently at all frequencies.

Fig. 10. Powerhouse alley with two subwoofers spaced 14 m (47 ft). Click image for larger view.

The width of the alley depends on the spacing of the subwoofers: as the two sides of subwoofers increase in spacing between each other, the alleyway will narrow.

The next illustration shows the alley for two single subwoofer boxes spaced 14 meters (47 ft). It is not as obvious graphically as it is audibly, but it can still be distinguished as an approximately 4 meter (13 ft) band with more intense color.

Conclusion

The only real way to avoid the powerhouse alley is to place all sub-woofers in a single location. With smaller scale applications, placing all the subwoofers in the centre is normally not a possibility, but the larger shows may benefit from flying subwoofers in the centre.

It is important, however, to understand what is happening, so as not to blame the speakers. And being aware of the zones that get created before taking mixing decisions.

For instance, if the FOH console is located in the centre, we could chose a mix that is somewhat bass-heavy when listened to along the centre line. Off to the sides destructive interference will kick in and establish some frequency balance between the low frequencies and the rest of the system.

José (Joe) Brusi holds a degree in Engineering Acoustics and Vibration from the Institute of Sound and Vibration Research at the University of Southampton (UK). He has worked at different positions with professional speaker manufacturers such as JBL Professional and DAS Audio. He currently manages DAS Audio's Applications and Systems Group as well as the company's AUTOPOL measurement platform. Our thanks to Joe for supplying this article to PSW, and be sure to check out the latest from DAS at www.dasaudio.com.

 

Email this story to a friend.