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World Overcomers Outreach Ministries Church 2_002_10-08-19_1540w_800h.jpg
A IV6 Modular Vertical Array configuration at World Overcomers Church in Memphis, Tennessee.

Testing and Design Work Key to Figuring Speaker Needs for Space

In this second of two parts, we look at the Critical Distance test, while looking at the importance of intelligibility and clarity, when making the decision on the right speaker for the space.

In the first part of this two-part article, "Making the Best Loudspeaker Choice for Your Church," we began to discuss how to work with a public speaker to calculate the acoustic characteristics of a space, to test for Equivalent Acoustic Distance, or EAD, along with another test for Critical Distance.

Good decisions often become self-evident, when we clearly identify our needs and objectives.

While EAD tests for the natural communication path between a speaker and listener, Critical Distance includes the effects of the loudspeakers in the signal path.

  1. Play a pink noise generator into the sound system. Be sure there’s no signal clipping in the mixer or amplifier.
  2. Place a conventional sound level meter in the front row of seating at ear height. Adjust the sound system pink noise level, such that it’s equivalent to the typical usage for speech: about 60dB to 70dB A-Weighted.
  3. With the pink noise calibrated to the front seat value, move the sound level meter as close to the loudspeaker as possible. If feasible, do it within one to two meters from the speaker. Take note of this level.
  4. Slowly move the meter away from the loudspeaker, taking record of the level which should be dropping, as you move further away.
  5. Stop moving the meter away from the speaker when you notice the level on the meter is no longer changing. The distance from this location to the loudspeaker is approximately the Critical Distance. That is, the distance at which the loudspeaker’s directivity pattern effectively can focus sound adequately into the space.

Try this along a few different paths, through the seating. If you found the level only dropped slightly (a few dB throughout the seating) and there are no complaints about hearing speech clearly, then your system probably offers good coverage uniformity.

However, If the level stops changing near the front of the audience (or perhaps even before the sound reaches the audience), then the system clearly does not fit the acoustics of the space.

Especially if you know that intelligibility is poor, then this likely means that the current loudspeaker has very little directivity and is not providing adequate, direct sound to the listeners.

When a system makes a speaker louder, but not clearer, this is exactly what’s going on. Generally, if you found that the Critical Distance was not near the furthest seats, this more clearly indicates that a higher directivity product is needed. This would point toward either a larger format point source speaker, or a line array. This is certainly true if the Acoustic Distance in the EAD test as described in the first part of the article, also was found to be a very short distance.

As you conduct technical research on loudspeaker designs for your worship and meeting spaces, the design services you employ should also be able to provide predictions about the sound system’s performance and take measurements on-site to more thoroughly evaluate the venue’s acoustics.

Loudspeaker designers should demonstrate at minimum, uniform sound coverage simulations (direct sound). But it’s also important to simulate intelligibility (STI, STIPA), and music clarity (C80).

It may seem odd that we’ve just discussed loudspeaker technology, without a single technical loudspeaker spec reference, or a product mentioned.

While that may seem unusual, I’ve found from consulting with innumerable clients, that starting with a clear understanding of the acoustic environment, frames up all other decisions.

This step cannot be ignored. It’s like prescribing medicine, without first examining the patient.

Good decisions often become self-evident, when we clearly identify our needs and objectives.

Simply stating them goes a long way to shaping a vision for the technical system that fits our needs. Stating what we know about the acoustics of a worship space or meeting room is the first step to good sound system design because the acoustics will dominate all our decisions.

Now that you’ve made a couple basic evaluations of your worship or meeting space, in our next article, we will address questions like “do I need a line array?” giving more specific guidance relating to loudspeaker types, to fit various venue architectures. This will be more like matching the vehicle to our needs, rather than picking a vehicle, because it’s fashionable or comes at a “great price” (because it just might).

Addendum:  Additional Guidance on Acoustic Specifications

If you have retained a sound system designer, there are several more technical assessments that can be made, both in on-site testing, and in computer simulations. Some or all the following should be considered during the design process, as they are all commonly predicted in design software, and most professional audio measurement systems perform these tests.

A key metric for worship spaces is speech intelligibility. There have been a few metrics such as C50 and %Alcons that have developed over many decades, the current is called “STI” (Speech Transmission Index). STI calculates an overall rating, that considers several factors in the signal flow in a system, between a speaker and listener.

A similar test, called STIPA, is used to quickly evaluate the STI through an amplified system. It is affected by reflected noise, ambient noise, and distortion, caused by components in the sound system. STI rates a measurement position on a scale of zero to one. Acceptable scores for lectures, preaching, and inter-personal communication begin at 0.5 STI. Most designers will target for at least 0.6 STI, while scores above 0.7 are very hard to achieve in large spaces.

In a similar way, when taking the “vitals” of an acoustic space, the reverberation time measures how long it takes for sound signals to naturally fade away. If sounds from speech do not fade away (because of acoustic absorption) fast enough, they will echo around the room, becoming a detriment to comprehension.

At that point, what was a meaningful word, phrase, or sentence, may become its own confusing noise, thanks to excessively reflective acoustics. On the other hand, if sound decays too quickly, the space can feel uncomfortably artificial and “dry.”

What’s an appropriate reverberation time for your venue?

For speech, we can certainly identify a range of acceptable use – typically somewhere around one second. The reverberation time, though, is measured at all frequencies, and the effect of all frequencies must be considered. That’s where STI, %Alcons, or C50 are helpful metrics. They are based on the RT, but give us values, related to our experience as listeners.

For music, there’s also an acceptable range of use in a metric called C80 – a measure of how much signal to noise exists 80ms after sound has passed from the sound source to a listener.

The acceptable range of C80 varies, depending on the type of music for the venue. Slow tempo music with sustained notes (symphonic, acoustic, organ) works best with a low C80 (-2 to +2), while higher tempo music (jazz, pop, rock) requires a higher C80 (+4 or +6). These scores follow very closely with reverberation time. As improvements are made to the venue to benefit one, it affects the other.

Some of these factors may be familiar from other reading, and each of these metrics deserves its own article to describe them fully.

As a technical researcher for your worship facility, you can use these as ways to evaluate loudspeaker design proposals, beyond just the basic plotting of direct sound coverage. Direct sound coverage is key to any system design.

Usually, uniform coverage ensures that all listeners receive a similar experience. But, it’s also possible to have uniform coverage, but the wrong type of loudspeaker sound source. This is where we begin evaluating various types of loudspeakers and the directional patterns, that they create to focus sound on key audience areas.

Future Topics on Loudspeaker System Selection:

How to Interpret and Use a Loudspeaker Spec Sheet
How to Listen to a Evaluate Loudspeakers in a Demonstration
Subjective Versus Objective Sound Qualities
What is a Venue Polar, and Why do I Need One?

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