How Much Acoustical Material is Required?

How Much Acoustical Material is Required?

The answer depends on specific measurements involving acoustical principles.

Measuring sound to identify acoustical product needs involves some basic principles.

Understanding these concepts can help evaluate how much acoustical material is required in a given space:

The loudness of direct sound

The loudness of direct sound waves to a listener is determined by the following:

1. The loudness of the original source
2. The listener’s distance from the source
   
   

Reflected sound

When a sound wave strikes a floor, wall, or ceiling, the direction of travel is changed by reflection. The reflection of sound waves follows the same physical law as light reflection: the angle of the incidence equals the angle of reflection.

Multiple reflections

Sound travels about 761 miles per hour in all directions. An original sound is quickly and repeatedly reflected from all surfaces. At any given moment, a listener will hear the current direct sound and also portions of earlier sounds that are reflected. Then, sounds are traveling in every possible direction. The multiple reflections of sound waves have two effects on acoustics: 

1. Loudness increases.
2. It causes reverberation.
   
   

The loudness of combined direct and reflected sound

The loudness of reflected sound is always less than direct sound for two reasons: 

1. Reflected sound travels further, and loudness diminishes with distance
2. Reflected sound loses some energy by absorption at each reflection 

The combination of direct and reflected sound results in loudness greater than direct sound alone. The loudness of reflected sound depends on room absorption, while direct sound depends only on the distance from the source. The overall effect on loudness is determined by absorption present within the room.

Reverberation

Reflected waves will continue ricocheting between room surfaces, losing only a fraction of power by absorption at each reflection. This prolongation of sound is called reverberation, which will gradually diminish.

Reverberation time

Reverberation time is measured in seconds when a sound’s average loudness can be heard before it becomes completely inaudible under quiet conditions. The time may vary from a half second in a very “dead” room to five to ten seconds in an excessively “live” reverberant room.

Speech and communication

The maximum reverberation time for clear speech is about two seconds. As reverberation time exceeds two seconds, speech becomes more difficult to understand. At reverberation times of four to ten seconds, words eventually become unintelligible. 

Music

Optimal reverberation time for:

• Orchestral, choral, and church music – one and a half to two seconds 
• Large organs – two seconds or more
• Chamber music – one to one and a half seconds
  
  

Echo

The distinct repetition of a direct sound is an echo, which gets lost in a highly reverberant room.

An echo is easily distinguished if the interval is greater than half a second in a room with low reverberation time. Sounds reflected from flat surfaces will be less intense than the original direct sound. Sound reflected from concave surfaces has a focusing action that produces very annoying echoes.

Sound absorption and coefficients

Hard, reflective, nonporous interior building surfaces such as glass, wood, plaster, brick, and concrete absorb two to five percent of the sounds striking the surface to reflect 95% or more of the sound. Absorption coefficients are expressed as a percentage of the sound absorbed.

A perfect sound absorber is an open window since it permits 100% of the sound to escape and not return.

Interested in diving deeper into sound measurement and absorption? Download Room Acoustics, an informative resource from MBI Products that includes: 

• Helpful charts
• An explanation of the Noise Reduction Coefficient 
• Reference information about the Sabin formula for measuring sound reverberation, absorption, and the amount of acoustical material needed

Note: There is no substitute for actual sound level measurements done by a competent acoustician with the proper equipment. A thorough professional analysis and evaluation often have better results and a lower overall cost when treating a space with acoustic material.

If you have questions about how much acoustical material you need, please contact us at MBI Products. We’re here to help!