written by: anoor•edited by: Lamar Stonecypher•updated: 5/18/2009
The primary function of an acoustic system is to amplify an acoustic signal and present it intelligibility to the occupants in the venue. Sometimes this task becomes very difficult due to the room acoustics. This article will address some of the issues that occur as well as some possible solutions.
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Sound System Design
Various venues have different acoustic requirements and each has its own unique solution. Sound systems are designed to accommodate all different types of venues ranging from retail big box open ceiling stores, to churches, to acoustically dampened auditoriums. Each of these venues presents a different design challenge. Assuming that speech intelligibility is an important factor of the venue, a few guidelines can be followed in order to help make the sound system more intelligible.
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Basics of Sound
Before getting into solutions for improving sound system intelligibility, let's review a few basic principles of sound. Sound is the propagation of pressure waves through a medium (in our case it is air). Sound Pressure Level (SPL) has been established with a logarithmic scale called the decibel. This pretty much describes how loud the sound is. Also since sound propagates in pressure waves, a frequency is also specified with the sound in the units of Hertz, Hz. Low frequency is bass and high frequency is treble. The established audible spectrum for the human ear is approximately 20 Hz (bass) to 20 kHz (highs). Average speech has a frequency range of approximately 200 Hz to about 10 kHz. Also the average conversation has a sound pressure level (SPL) of about 70 decibels (dB). A jet engine is approximately 120 dB while a quiet office is around 40 dB. Speech Transmission Index (STI) is a measure of intelligibility of speech. This is primarily dependent on a few factors which include how reverberant the room is and also how much background noise there is in the venue. Articulation loss of Consonants (ALCons) is another metric used to measure intelligibility. ALCons focuses on how much loss there is of the consonants in the spoken language. The reason for focusing on the consonants is due to the fact that the higher frequencies in speech are emitted in the consonants. Vowel sounds usually emit bass frequencies from the throat while consonants produce the high frequencies.
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What Caused that Bad Sound?
Now that we've reviewed some of the basics of sound, let's discuss the primary causes of bad sound. There are a few factors that contribute to a horrible sounding venue. Imagine sitting in the stands at a high school gymnasium during commencement when your little brother, John, who is graduating, has his name called by the announcer. All you hear is a jumble of J's and don't even hear the last name called as the venue echoes and eventually stops. The inherent problems in this type of venue are the hard surfaces that reflect and reverberate the sound. The hard wood floor, the painted shiny cinder block walls, the hard wood bleachers, and even the steel ceiling, all help echo the sound around. Another factor that is adding to the problem is that the sound pressure level from the speaker is generally set very high. At an event like this in a gymnasium with a large crowd, the speakers would most likely be set to 100 dB or higher. Venues like this usually use a cluster of speakers located far away from the audience. This is also a factor that is adding to the reverberation. A combination of all of these items will result in "bad" sound regardless of the venue. Reverberation time is the amount of time it takes for a sound inside of a venue to come to a stop after it has bounced back and forth off the surfaces. A simple test is to stand in the room and clap your hands or pop a balloon and measure how long it takes before the room is silent again. Some of the worst venues in the world have reverberation times of 6 seconds or more. These venues generally have smooth tile or stone floors with high walls and ceilings that are also very smooth such as in cathedrals.
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Good Acoustic Design Guidelines
Now that we understand what causes the issues in the venue, let’s take a look at what can be done in order to make even the worst reverberating venue into an acceptable acoustic venue.
First of all, see if it is possible to add some sound absorption in the room. Whether it is carpet or acoustical panels on the ceiling, the additional absorption will reduce the reverberation significantly and will definitely help the room's acoustics. This is the first and most important step. It also may be the easiest and simplest solution. In the example above, if the gymnasium had several carpets rolled out on the open hard wood areas, the room’s acoustics would be significantly improved. Even some curtains added to the walls would have an effect. The most basic absorption that is commonly overlooked is people. If the venue is nearly empty vs. full of people sitting in the bleachers, there will be a significant difference. Spreading the people out over the bleachers will help reduce reverberation from the hardwood bleachers.
Acoustical treatments such as ceiling panels or acoustical spray on the ceiling are generally very expensive. Another factor is the architectural design constraints. Spaces such as a glass conference room or marble walled auditorium were specifically chosen for their aesthetic effects. Covering them up with acoustical panels or spray is not an option. In this kind of case the next best thing that can be done is to focus on the design of the system. Determine where the audience is going to be and locate the speakers so that they are pointing directly at the audience. Also reduce the volume level of the sound system so that it is at the minimum possible for the application. The more sound there is in the venue, the more the venue will reverberate. The less sound the speaker introduces, the less reverberation and the better the intelligibility.
Moving the speaker is another option that will help improve the acoustical system. Sound propagates as it passes through air. This means that it loses energy as you increase distance from the speaker. If the speaker is emitting 94 dB at 20 feet away, then at 10 feet away from the speaker the SPL is 100 dB. The general rule of thumb is that you lose 6 dB for every doubling of distance. Reduce the distance from the speaker and the audience and also decrease the volume level in order to improve intelligibility. Some very reverberant spaces such as churches or cathedrals use built-in-pew speakers. These speakers are located right in the pew and are directed at the audience at a very close range at a low volume level. This helps reduce the immense reverberation that would be present if the speakers were shooting sound out into the entire venue. This example of multiple speakers is called a distributed audio system. This is preferable to a cluster speaker system in a reverberant venue.
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Each venue has its one unique acoustic characteristics. A sound system designer needs to have a basic understanding of how sound behaves and the also needs to utilize the tools in order to control this acoustic energy in a productive way inside the venue. There really is no single method of solving all of the acoustic issues a venue may have. A precise and engineered solution using a combination of the guidelines listed above will result in dramatic improvement of the acoustics in the problematic venue. Acoustical consultants do this task and generally work with the architect during the design phase of the venue. Unfortunately, sometimes the acoustical consultant is not involved at an early enough stage which may result in difficult acoustic spaces that may require creative solutions engineered for that application. Other times the space is renovated and no acoustical considerations were taken which pose acoustical issues when it is time to put in a sound system. Whatever the case, it is critical to study and review each venue carefully and design and engineer a custom solution.