Your Project

Good Acoustics

If good acoustics is a prime goal of your building project, then you must ask yourself two questions;

         1 - How good do I need the acoustics to be?
         2 - How important is it to achieve good acoustics?

 

Unfortunately, acoustics is given a low priority in many building projects.  It usually doesn't start off that way, but its high status can be slowly chipped away by cost-cutting measures here, nonprofessional opinions there, and surrender by weary building committee members who just want the project to be completed.  We have seen changes made to structural, lighting, electrical, heating, ventilation and air conditioning, plumbing and room finishing systems cause unacceptable background noise levels, create standing echoes, produce annoying peak room resonances and create various other acoustic anomalies.  Most of these acoustic problems are built into the building, and cannot be repaired without great expense.  Wishful thinking and off-the-cuff remarks often run counter to the laws of physics.

 

Cost is a common excuse for not following recommendations. Certainly the initial construction costs are lowered, for example, by replacing a quiet HVAC system with self contained roof top units placed directly over the spaces they serve, but what is the real cost of having a 65 decibel background noise in your building?

 

Determine Your Acoustic Goals Early and Stick to Them

Each space has an acoustic function, and therefore, each should be given acoustic goals.  These goals should be established even before the architect lays down the first line on the concept drawings since they affect the site orientation and room programming.  Make the acoustic goals a part of your design specifications.

There are over eighty different acoustic measures of a space, but fortunately only three are needed to give a solid foundation for excellent acoustics:

1 - background noise

2 - speech intelligibility

3 - reverberation/liveliness.

1.  Background Noise

The scientific measure of silence is the NC rating (noise criteria). This rating is a measurement of the humanly perceived background noise over the entire audible frequency range.  Standard goals for NC ratings have been established for various types of spaces.  The following is a list of recommended NC ratings.

· Sanctuary/Nave	NC25
· Gathering Space	NC35
· Choir Rehearsal	NC25
· Corridors/Lounges	NC40
· Office	NC35
· Classroom	NC25
· Library	NC35
· Cafeteria/Kitchen	NC40
· Restroom	NC40

2.  Speech Intelligibility

Speech Intelligibility is simply a measure of how much of what is said is understood by those in attendance. Initially, this measurement was accomplished by reading a list of words and having all in attendance write down what they thought they heard.  Now, speech intelligibility can be quickly measured in existing rooms and estimated for planned spaces using computerization.  The most common measurement system is the RASTI method (Rapid Speech Transmission Index) which was introduced as an international standard in 1987.  The RASTI values for the range of conditions are given below.

· Bad	0.0	-	0.3
· Poor	0.3	-	0.45
· Fair	0.45	-	0.6
· Good	0.6	-	0.75
· Excellent	0.75	-	1.0

Although there are no published standards, we recommend that all spaces achieve a speech intelligibility of RASTI 0.65 or above.

3.  Reverberation/Liveliness

Reverberation time is a measurement of the length of time that it takes sound to die down in a room after its source has been shut off.  For smaller rooms, sound does not fade away smoothly and so instead of reverberation, we talk about the liveliness of the room.  Over the past 100 years, surveys have been conducted to determine the preferred reverberation in various types of spaces.  This effort has resulted in a set of curves which show the preferred range of reverberation as it varies with 1) type of space, 2) size of space and 3) frequency of sound.

The data are too complicated to reproduce here; however we believe that it is sufficient to specify that all spaces be designed and shown to fall within the historically accepted limits for reverberation time for their size and use.

 

Specifying the goals for background noise, speech intelligibility and reverberation/liveliness for each space before the design begins is a simple way to build good acoustics into your project.  Costs are controlled by not having to correct acoustic defects caused by early design decisions made without regard to their acoustic consequences.

 

We have analyzed past projects to determine the costs and benefits of achieving specific acoustic goals.  Choosing optimal goals results in optimal payback in user satisfaction, membership growth, worker output, community reputation, attendance and ticket sales.

 

We will be glad to help you determine your project's optimal acoustic goals.

ARCHITECTURAL ACOUSTICS AND NOISE CONTROL CONSULTING

623 26^th Avenue

Rock Island, Illinois 61201

309.788.0673 | Fax: 309.786.5967

 www.kjww.com

By KJWW Engineering Consultants