Save time in the field by presetting your speaker output delay and level offsets.
Here’s a model from a recent show where I estimated the level and delay for a pair of relay speakers and line of front-fils.
If you know the difference in distance between two sources, you can accurately estimate their time offset, and therefore, the delay necessary to bring them into alignment. The challenge is finding the correct crossover point.
Drawing About Sound
Crossover (spatial): An acoustic crossover in a spatial domain, i.e. the location where elements combine at equal level.Bob McCarthy, Sound Systems: Design and Optimization
Finding a spatial acoustic crossover point in the field is pretty simple. Once you have defined the on-axis (ONAX) point for source A and source B, match their levels, then walk in a line between the two points until they sound equal. Place our measurement microphone at this point, then use the audio analyzer for verification.
You can simulate the process in a modeling and prediction software like MAPP XT, but I have been practicing a quick offline method of drawing speaker coverage shapes and estimating their crossover points that you can do on a plane (or even with pencil and paper in 2D).
Here’s a recent show that I worked on using JBL VRX 932 speakers on stands. The distance from the speaker to the end of coverage is 61.66ft. I can divide this distance by the forward aspect ratio of this 100º speaker to find the coverage width.
61.66ft / 1.31 = 47.1ft
For more on forward aspect ratio, please read One Simple Tool to Find the Right Size Speaker for Any Space. These topics are also covered in detail inside of Pro Audio Workshop: Seeing Sound.
This gives me a quick sketch of the coverage shape.
For comparison, here’s a similar speaker’s prediction at 4kHz.
Here’s how you would do this in the field with the audio analyzer:
- Match solo ONAX levels B1 to A1.
- Find XAB (the acoustic crossover between A and B) between ONAXA and ONAXB where A1 and B1 match in level .
- Set delay.
Now let’s do those steps in our model with distance measurements.
B1 level = 20 * Log10((distance from ONAXB to B) / (distance from ONAXA to A))
In our model that would be -2.4dB.
20 * Log(22.59 / 29.78)
You can drop that formula into google to verify it.
Find XAB at the center of their coverage pattern overlap.
B1 delay = (distance to A1 – distance to B1) * 0.9
In our example model that would be 24.17ms.
(48.01 - 21.16) * 0.9
Once we got into the room the coverage shape of B1 changed due to an obstruction. This changed the level offset, but the delay was very close to my estimate.
Much of the front-fill processing changed because we replaced two of the 928 speakers with 932 models and were able to level set and delay them independently.
Will this work for subs?
Yes for delay. If you have followed Merlijn van Veen’s Relative/Absolute Method, then you already have a preset for the relative relationship of the spectral crossover alignment of your main and sub. Use your 3D model for the absolute step.
No for level. Low-frequency energy will enjoy room gain and I’m less confident that a distance measurement will help you estimate the level. If you try it, let me know.
Have you tried presetting your DSP using distance measurements from a model? How did it go?