An interesting development has occurred in recent times: folks have become much more aware that the acoustic space plays a critical role with respect to the final sound of the room.
In other words, unless you “fix the room,” changing the sound system may in fact not solve the problem at all. And truth be known, bringing the room acoustics up to an “acceptable level” is often actually easier than setting up a PA system!
Testing One, Two, Clap
Go into the room, clap your hands. If the trailing echo and reverb is longer than 1 second, it will affect the intelligibility of the system. Intelligibility is a term that describes our ability to understand what is being said. Back in the dark ages, Gregorian chant was perfectly suited for the long trailing reverb that was common in houses of worship.
And the spoken word? Only the elite could speak and understand Latin, so for the average person, following the sermon had very little to do with understanding the message.
Today this has changed. Music has become rhythmic; the language in the church, nightclub or conference center is the language of the people. It is all about communicating a message. And for this to be effective, the message must be clear.
Solving The Problem
This article is not about creating a performing arts center out of a gymnasium. It’s about reducing the reverberant field so that the sound system can actually do its job.
The human voice ranges from 200 Hz to about 4 kHz, with a peak around 500 Hz. The louder we speak, the more accentuated our voice will be in the mid-range. Acousticians are specialists that have the expertise to deliver optimized high-end rooms. Here, we’re only going to tackle the basics so that you can turn a problem space into an acceptable one.
Fletcher-Munson curves show us that human hearing is also most sensitive to the mid-range. This provides us with the first key ingredient to improving intelligibility: we need to focus our attention on the mid-range.
The next bit that we need to consider is that low frequencies have much more energy than high frequencies. And further, that bass frequencies below 300 Hz are deemed to be omnidirectional and high frequencies tend to beam like a flashlight. Therefore, bass frequencies are very difficult to manage while mids and highs are relatively easy.
So let’s pick the easy fruit off the acoustic tree first! Draw a plan view of the venue. Position the main loudspeakers and trace the dispersion angle of the horns.
As the sound expands away from the cluster, it will reflect off the walls, causing primary reflections. These reflections combine with the direct sound emanating from the loudspeaker to cause an effect known as comb-filtering, whereby some frequencies will “phase cancel out” while others will be reinforced, depending on the wavelength and distance.
To make matters all the more interesting, the comb-filtering will vary depending on where you’re sitting in the room. So changing seats will have a dramatic effect on how well the output is heard.
By attenuating the reflections, comb-filtering is minimized while voice intelligibility and sound localization are improved. This is typically done by mounting acoustic panels on wall surfaces. The more panels that are used, the more sound will be attenuated, and the higher the quality of the acoustic panels, the more even the absorption will be.
Most rooms work well with about 25 percent wall coverage, but this is usually determined by available budgets. With 20 to 25 percent wall coverage in mind, sketch out a plan as to the best use of the available inventory.
Start by positioning the panels where the most obvious reflections will occur. In long rooms, this is usually along the side walls where the PA clusters will produce the most energy and subsequent primary reflections that will echo into the listening area.
In wider rooms, look at the back or “receive” wall where flutter or slap back echo will occur. Most spaces benefit with some treatment in each of these areas.
Good judgment and common sense go a long way to making a room work well. In other words, you may have to compromise due to windows, doors, traffic areas, and so on.
One of the most common questions: how do we make a room work for all the different types of events it hosts? This problem is common to houses of worship, and other spaces that must work well with both speech and music.
With some types of music, such as orchestral works or chorus, a longer reverberation time will enhance the performance, while with voice, long reverb trails make it difficult to comprehend what is being said.
There are basically two ways to approach the problem: introducing movable acoustics or utilizing artificial reverberation. Heavy theater drapes can be very effective. These can be set up to manually be opened (absorptive) or closed (reflective) depending on the performance.
Electric motors can also be installed to automate the system. To work properly, the drapes must have sufficient mass to trap the sound within the air cavity behind. The downside is that these heavy drapes can be very costly, especially if you employ automation.
The other primary option is treating the room using acoustic panels and then adding artificial reverberation when needed. There are high-end venues around the world that successfully use this approach, and for the “regular venue,” adding digital reverb to the mix can often produce acceptable results.
It’s worth noting that musicians play better when they’re in a comfortable acoustic environment. For instance, a chamber orchestra or chorus will perform better if they’re surrounded by some natural room reverberation. This can be introduced by positioning reflective panels (i.e., clear Plexiglas) above the musicians or by applying some movable drapes to their area so that the walls can be exposed and made reflective when needed.
Understanding The Specs
Gauging the performance of an acoustic panel is just as easy as comparing microphones or loudspeakers. The sound absorption coefficient curve shows how well an acoustic panel will absorb energy at the various frequencies. A factor of 1.00 equals 100 percent absorption at that given frequency.
Be aware that acoustic panel makers will often promote a single digit “average” that has no practical real-world value. This is akin to saying a sound system is 100 percent effective at 1 kHz.
So just as you check the specifications to see how well a loudspeaker produces an even frequency response, also look at the panel data when it comes to absorption. In particular, make sure that the panel is actually able to absorb energy in the voice range. Further, if you’re trying to control bass, evaluate thicker panels that are able to “reach down” and absorb lower frequencies.
A good trick is to introduce an air cavity behind the panel (like a plenum ceiling or the drapes discussed above), which can often lower the effective frequency response by an octave without adding much in the way of cost.
In light of current budget restraints, there has been a shift toward reducing costs. Large panels are expensive to ship because they require special delivery using a flatbed truck, and further, a lift truck is needed to enable installation.
Sound and A/V companies are well equipped to improve the actual sound of rooms, but for years, acoustics have often been ignored or left out of the equation. With modern sophisticated loudspeakers and digital electronics, it has become apparent that simply changing the sound system still leaves plenty of room for improvement. Smaller panels that are grouped in a modular format have gained popularity because they can be shipped using a common carrier, and these can often be installed using a simple ladder. This approach reduces costs while still furthering acoustic performance.
As a result, more attention is being paid to room acoustics, often before a new system is considered.