The Science[fiction] of Speaker Placement

Jared Taylor —  June 25, 2013 — 8 Comments

Ian_rocks_out

The best part of my job is visiting all thirteen of our sites. I get to meet great people, make observations, comparisons and recommendations. Then, because of my fancy nametag, people usually do what I suggest. But in the hustle of setting up portable church (twelve of our sites are portable) I don’t always have the time to thoroughly explain the reasons behind my recommendations. And when I’m not at the Meeting House, my rule is “say nothing unless you’re asked”. Words to live by, but it can lead to intense experiences of cringing followed by iPhone photos that make their way onto twitter after a few days (to protect the identities of the perpetrators).

One such issue is speaker placement. “Where do the speakers go?” It ought to be a straightforward question – it’s one of the more purely scientific parts of live production – but I’ve seen some interesting answers. Science-fiction answers. It’s time to set the record straight on a few things.

If you’re doing a permanent installation I strongly recommend hiring a professional. A qualified installer offers safety for the speaker hang and valuable computer modeling tools to maximize the design. But if you’re slugging it out in a portable church, like we are at The Meeting House, these three tips are a solid starting point.

1. Find the pattern

A speaker is built to spray out sound in a particular direction called its “coverage pattern”. Sound generally leaves a point-source speaker in the shape of a cone. This means that as sound moves further from a speaker, it covers a wider area. Coverage patterns are measured in degrees, both horizontally and vertically. A typical speaker at one of our sites has a coverage pattern 90° wide and 50° tall. While patterns vary, loudspeakers are almost always wider than they are tall. And it’s worth mentioning that this pattern assumes the speaker is right side up. If you lay a speaker on its side, the horizontal coverage pattern is now the vertical pattern and vice versa. Our typical speaker laying on its side would have coverage pattern of 50° wide and 90° tall. Find out the pattern of your speakers and identify which parts of the room are covered, and which parts aren’t.

2. Point the speakers at the people

Sometimes the easy concepts are the easiest ones to miss. Whether they’re ground stacked, on stands or flown from the ceiling, speakers should be pointed at the audience. Direct sound travels straight from a speaker to the audience – it is clear, and easy to understand. Reflected sound bounces off of walls, floors and ceilings before reaching the audience – in the process it becomes muddled and unintelligible. Your speaker placement should maximize direct sound and minimize reflected sound. Knowing your speaker’s coverage pattern will help you do this.

Just to be clear, pointing the speakers at the walls to “take advantage of the reflections for better coverage” is a bad idea.

… and yes, I’ve seen it done.

3. Even things out (if you can)

Have you heard of the inverse distance law? More than just a fantastic conversation starter at parties, the inverse distance law tells us how loudness decreases as you move away from a speaker. It turns out that when you double your distance from a sound source, you cut the perceived volume in half, or a 6 dB reduction. (The exact figures are debatable, but the point is the same so stay with me). This volume difference is known to affect the migratory patterns of a few types of churchgoers. It’s also about the best reason to put speakers up high and point them down if you can.

Here’s a typical “throw up the speakers” setup. Notice the drastic volume change from the front to the back of the room.

The quick "throw them up on stands"

Fig. 1: The quick “throw them up on stands”

 

Now watch what happens when the speaker is raised up well above people’s heads and angled down (if possible). The volume difference from front to back is much smaller.

Raised up, angled down

Fig. 2: Raised up, angled down

 

Although not likely in portable situations, the addition of a second row of speakers can significantly decrease the volume difference from front to back. This row of speakers is called a “delay” because the audio needs to be time delayed to match what’s coming from the front speakers.

Adding a delay row

Fig. 3: Adding a delay row

 

And finally, since I know you’ll ask, here’s the basic concept behind a line array.

A line array

Fig. 4: A line array

Line arrays are complex systems with a lot more than two speakers per side, so please understand that this diagram is laughably oversimplified! Like a delay system, a line array has different speakers to cover different distances. But, unlike a delay system, the speakers are all grouped together. The elements of a line array are engineered to act together like one speaker – a normal “trap” (trapezoidal) speaker does not do this! We recently chose another line array for the expansion of our Oakville, Ontario location. Stay tuned for a more detailed post on that.

What you can do depends on the equipment you have, the room you’re in and the time you have to set it up. In a portable church situation, ±6 dB from front to back is not bad at all! In a permanent installation, you should really push for ±3 dB. If you’re not sure how your setup is performing, grab your SPL meter and take some measurements across the front and all the way to the back. See where you’re at! Do you have hot spots? Dead spots? Is there anything you can do to improve it?

Better coverage makes a better experience for everybody. And you can mix with confidence knowing the audience is hearing the same thing you are … plus or minus a few dB!

 

[update June 27, 2013]

Here are some diagrams of sloped seating. If you can picture tilting the whole room at the angle of the seats, you’ll see the concepts are no different than the flat seating diagrams.

Figure 5 demonstrates one potential problem with tilting speakers up. I say “potential” because this is just a random diagram, not a real room. In this case either the speakers or the front row would need to move back for better coverage. Or you would choose a speaker with a wider vertical coverage pattern. Still, you’ll have something like a 12 dB volume change from the front to the back of the room.

Fig. 5: mounted low tilted up

Fig. 5: mounted low tilted up

 

Figure 6 is typical of a number of Meeting House sites. When the speaker stands are tall enough and the speaker offers a tilted mount, we set it up this way.

Fig. 6: Middle height

Fig. 6: Middle height

 

Figure 7 shows what the volume difference would look like if you could get the speakers up above the height of the highest row. We’re not able to make this happen at any of our portable sites.

Theatre_slope_3

Fig. 7: Speakers up high

Jared Taylor

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8 responses to The Science[fiction] of Speaker Placement

  1. Jared, a good, simplified as you said, posting and diagrams. I love the line array diagram 🙂 It does get one of the basic principles across.

    I have experience portable sound at two separate Meeting House venues. Both are theatres with seating that is lower at the front of the room that it is at the rear. One uses stand mounted speakers that are tilted upward in order to better cover the rear, upper level seat at the rear. The other has stand mounted speakers that are not tilted upward. While both function reasonably well for the first few rows, the venue that does not have the speakers tilted has very poor coverage much past the first few rows, particularly in the higher frequencies. How may venues tilt the speakers upwards? Why do any of the theatre venues not tilt them upward?

    Lorne

    • Tilting speakers upwards can give you better coverage, depending on the geometry of your room. If you’re trying to mount your speakers low, it’s essential. But there are two challenges:
      1. an upwards slant requires the speakers to be very low in order to cover the lowest rows. The closer those front rows are, the lower the speakers have to be.
      2. tilting upward doesn’t address the volume difference from front to back. If you tilt your speakers at the same angle as the slope of your seats, you have essentially reproduced the layout in figure 1, except the whole room is now at an angle!

      I’ll throw some more diagrams at the bottom of the post, so check back. Portable church sound setups always involve some sort of compromise, but I still think the best possible approach is to get the speakers up high and angle them down. That’s the only way to even out the volume from front to back without a delay line or line arrays. Many of the speakers we use have a double pole bracket that lets the speaker sit straight or at an angle, but many don’t.

    • And Lorne, I think your speakers are 70×70, not 90×50 like these diagrams.

  2. Thanks Jared for an excellent article. At our Richmond Hill site, we initially had the speakers angled down. Months ago we changed to align speakers horizontal. I did not know why we changed. I guess it is because our posts cannot reach the height of the back rows. Any insights on this?

    • Could be – too steep an angle won’t cover the back rows well. I’d be interested to have a look next time I’m up there.

  3. I am finding this to be an interesting discussion.

    Last Sunday I did some measurements of the room (dimensions).
    Screen to rear wall 70′
    Ceiling height 35′
    Height of the rear row 20′
    Speaker height (horn) 7′
    This places the speaker height at approximately the height of the 3rd row
    Only about 50% of the 70 deg. vertical dispersion of our JBL PRX512M speakers can hit peoples ears from that position.
    Tilting the speakers up simply shifts the centre line from the speakers to the audience further up and to the rear. (approximately 1/2 to 2/3 up the way up and to the rear. This is a minor thing, but does put more people closer to the centre line of the horn.

    A 2 box mini line array such as the JBL VRX series with 15 deg per box, on the same stands with the lower box horizontal and the upper box tilted up would direct the vast majority of the audio to the people. This would also allow us to power the top box about 6 Db louder that the lower box, providing a much better front to rear volume level ratio. I would love to try that some time 🙂 Unfortuntely The VRX speakers are much more expensive that the PRX 🙁

    • It’s something we’re keeping our eyes on. A small powered ground-stacked line array is a very enticing option for portable church in movie theatres and I’d love to see where the size/weight/price moves in the next couple years. JBL already has a powered version of the 12″ VRX cabinet. If they had a powered version of the 8″ we’d probably be testing it!

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