Interior acoustics is vitally important to the functionality of a given space. Interior space has a multitude of uses and applications. The results of good acoustical design can be very rewarding. For example, some concert halls, churches and theaters are admired for their marvelous acoustics. Even small projects without world recognition can function much more effectively with the proper acoustical care. On the other hand, overlooking important acoustical design considerations can cause many problems. Users can become so dissatisfied with a space; it can be classified as unusable. For example, when the German Government moved to the new parliament building in Bonn, officials found themselves in the middle of an embarrassing and costly mistake. The acoustics were so poor and the announcements so garbled, they were forced to move out the next day. If a space is not effective for its intended uses, the consequences are not only annoying, but very costly.
The following is a guide to help understand and enhance proper acoustical design, thereby eliminating expensive mistakes. This is addressed in the following three sections: Acoustical Principles, Acoustical Design Considerations and Acoustical Checklist by Project Type.
ACOUSTICAL PRINCIPLES
SOUND
Sound is produced by a vibration through a medium such as air. Sound travels in all directions from the source as a pressure wave, the same way waves travel through water when a pebble is dropped into a lake.
Frequency (Hertz-Hz) the number of times per second that a sound wave repeats its cycle. A low frequency (large sound wave) repeats its cycle less frequently than a high frequency (small sound wave).
The hearing range of the human ear is from 20 to 20,000 Hz, with the upper range decreasing with age. The frequency range of speech is 125 to 8000 Hz. The frequency of a sound wave is dependent on its length. The lower or deeper frequency of 125 Hz has a wavelength of 9 feet. Conversely, a higher frequency of 8000 Hz has a wavelength of 1 3/4 inches.
Lower frequency sounds are more difficult to control because of their longer wavelength. This is why, when loud music is played in an adjacent room, the high frequencies are blocked, and you only hear the thumping of the bass. Although controlling low frequencies is more challenging, it cannot be overlooked.
Decibel (dB) A unit for measuring relative loudness of a sound. The theoretical threshold of human hearing is zero decibels. The human ear, an amazingly sensitive instrument, is able to hear from 0 to 130 decibels (the average pain threshold). This range of sensitivity is comparable to a bathroom scale capable of measuring from 1 ounce to 300 million tons. This helps demonstrate the challenge involved in effective noise control and acoustics.
NOISE
Noise is unwanted sound that interferes with the function of a given space. Noise is subjective because what is disturbing and unacceptable to one room application may be acceptable to another.
DIRECT SOUND
Direct Sound is sound traveling from the source to the receiver without striking any surface. Direct sound diminishes in intensity as the distance between source and receiver increases. Acoustical planning would be an easier process if all sound was direct, but it’s not.
REFLECTIONS
Reflection is the occurrence of sound striking one or more surfaces before reaching its receiver. These reflections can have unwanted or even disastrous consequences. Reflections are attributed to the shape of the space, as well as the material on the surfaces within the space.
Domes and concave surfaces cause reflections to be focused rather than dispersed. This causes annoying sound reflections, reduced speech intelligibility and adverse effects on music.
Reflective parallel surfaces, such as conference and board rooms, lend themselves to a unique acoustical problem called flutter echo. This is when sound ricochets back and forth between the same spots on parallel surfaces. This problem should be avoided, and can be eliminated, by using absorption surface treatments.
REVERBERATION
Reverberation is the prolongation of a sound in a room, and is caused by continued multiple reflections. In an enclosed space, when a sound source stops emitting sound, it takes some time for the sound to become inaudible. The length of the reverberation needs to be addressed to ensure proper acoustics.
Reverberation Time (RT60) is the time required, in seconds, for the average sound pressure level in a room to decrease 60 decibels after a source stops generating sound. More simply, this is the time it takes for the reflections to go away.
Reverberation time plays a crucial role on the quality of music, and the ability to understand speech in a given space. When room surfaces are highly reflective, sound continues to reflect or reverberate. The effect of this condition is described as a live space with a long reverberation time. For example, a long reverberation time in a conference or board room causes syllables, if not words, to overlap. This in turn degrades the ability to understand the presentation.
Reverberant Field the multiple reflections in reverberant (live) spaces build upon themselves, causing the noise level to increase. The more absorption in a room, the more unwanted reverberant noise levels can be controlled. A project that has a significant noise source may require other noise control procedures.
ABSORPTION
Absorption is a quality in materials that combats unwanted reflections. When sound waves strike a material, part of the sound energy is absorbed and part is reflected. All materials absorb some amount of sound energy. This amount is expressed in terms of a sound absorption coefficient.
Sound Absorption Coefficient The percentage of sound absorbed by a material at a particular frequency. If a material has a sound absorption coefficient of 0.90 at 1000 Hz, it means that 90 percent of the 1000 Hz sound striking that material is absorbed, and only 10 percent is reflected.
Noise Reduction Coefficient (NRC) The average of Sound Absorption Coefficients (at 250, 500, 1000, and 2000 Hz rounded to the nearest 5%) for a given material. The NRC is a single number index of sound-absorbing efficiency.
SOUND TRANSMISSION
Reflection and absorption are properties that directly affect the interior of a space. Oftentimes, it is equally important to address the amount of sound passing through a partition into adjacent spaces.
Transmission Loss (TL) a measurement expressed in decibels, indicating the ability of a material or system to block or attenuate the transmission of sound, at a particular frequency, from one area to another.
Sound Transmission Class (STC) a single-number rating of a construction’s airborne sound transmission performance at a range of frequencies from 125 Hz to 4000 Hz. Higher STC values are more efficient for reducing sound transmission. For example, loud speech can be understood fairly well enough through a wall with an STC of 30. On the other hand, loud speech should not be audible through a wall with an STC of 60.
Tony Sola.
