The Importance of Sound

The perfect room has a depth three times the room’s width, lower ceilings or reflective clouds, stadium seating, thick outer walls, and diffusion on the walls along with absorption. There are always exceptions, but as a guideline, these elements work quite well in unison, and many premier concert halls and performance venues have been built with these concepts in mind.

Below is a more detailed look at what all those phrases actually mean, and why they matter!

Room Structure

Take the 3:1 rule for example – which states that rooms should be three times deeper than they are wide. The concept behind this rule is that the long side walls are closer to most of the audience and provide early lateral reflections.

Envelopment

Envelopment describes the listener’s sense of being surrounded by the music and the presence of the sound and is typified by your sense that the music is being created right in front of you rather than on a stage a hundred feet away.

In performance spaces, envelopment is the attribute that supports participation from the audience and congregational singing. One of the common mistakes in acoustics is to only use absorption on the side walls of an auditorium which deadens all lateral reflections and leads to less participation.

In other words, if you want people to sing and respond to the action on stage then envelopment should be considered when designing or changing a space.

Using Reflections Correctly

Early reflections have a large impact on the subjective pleasing nature of the sound. Lower ceilings and reflective clouds have similar effects to the longer side walls – they allow for faster reflections to the listener. Generally, we perceive any sounds arriving between 35 milliseconds of each other to be the same sound. Reflections arriving less than 20 milliseconds from each other add to the pleasing or intimate nature of a sound while any reflections arriving after 50 milliseconds or greater are destructive to the original sound.

Simply put, large spaces have a more difficult time managing early and late reflections because of the greater distance between the walls or ceiling and the audience. As acoustics designers, we use ceiling clouds and reflective panels to artificially create the sonic signature of the room.

Stadium Seating

Stadium seating allows for greater clarity for each listener as they have an unobstructed listening path to the initial sound. That’s right – it’s about more than just a better view!

Wall Structure

Thicker outer walls allow for greater bass retention in a performance space. The density and total thickness of the wall are what will keep the larger sound waves from passing through the wall and reflect them into the room instead. The lower the note in music the larger the sound wave that is created.

Many auditoriums suffer from problems with an uneven tonal balance in the room because the bass and treble were not appropriately considered when building the room. Designing the right outer walls of the room is only one consideration.

Absorption & Acoustic Panels

The other major consideration is what type of absorption you decide to use in the room. Based on their thickness and values the acoustic panels or insulation in a room are designed to absorb a certain range of frequencies.

Assuming you have purchased acoustic panels from a reputable manufacturer, a standard 2’ fabric panel is designed to absorb frequencies from the high end down to 250-500Hz. So by adding acoustic panels to change your reverb time, you can also change your tonal balance of the room without realizing it.

Diffusion

One solution to having too many acoustic panels is by adding diffusion. It is one of the most key aspects of an acoustics plan and room space design that is often overlooked because of the additional costs. Most performance spaces will have a mixture of diffusion and absorption.
While absorption helps to remove unwanted echoes and control the reverb time, diffusion helps to disperse sound waves evenly throughout the room. You might have seen this accomplished with small panels attached to the walls in a studio space.

For large performance spaces, however, diffusion is not accomplished with panels but by architectural decisions that use materials to repeat specific patterns to create diffusion in the sound waves. One common example is using an acoustically modeled wood slat wall with varying widths of boards and spaces to create large diffusers. The benefit to diffusers is they will break up walls that would create short echoes without changing the spectral balance of the room like an absorptive panel would.

Measuring Acoustic Success with Reverb Time

After taking all of these factors into consideration, the best way to measure their effectiveness is with reverb time. Reverb time is not the only predictor of great acoustics but is the most widely studied and understood.

The reverb time is created and controlled by the cubic volume of the space and absorption that is placed in the room. Reverb Time or RT60 is calculated by measuring the time it takes the reflecting sound to drop 60dB below the original sound. Generally for a mixture of speech and music your target reverb time is between 1.5 to 2.5 seconds.

In generic terms the longer the reverb time the better for music and the shorter the reverb time the better for speech. Longer reverb times give the performance space warmth while shorter times will increase the articulation of the music or speech.

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Historically, much of what we know about acoustics comes from the study and construction of concert halls. Today, the majority of performance spaces now rely on amplification of the sound sources, which changes the source of sound from on the stage to about 20-50 feet above the stage. Even with this change though, the acoustic principles explored above remain the same. Taking them all into consideration toward the purpose of your room will ensure that your space has the best possible acoustic sound.