The final design consideration in the three part breakdown is the receiver. This is the experience of the person hearing the sound, and within this are two key considerations: volume and clarity. When attempting to block the transmission of sound from one space to another, the hope is that sufficient attenuation has been added to the path so that the receiver will hear zero volume from the source. Many times this is impractical and unnecessary. Many times clients tell us that they would like one room to be “soundproof” from another. To make it fully “soundproof” according to a precise definition of that word would usually be quite costly. What needs to be determined is the acceptable level of sound transmission for the client. In some cases it may be acceptable to hear sound, but it is imperative that all clarity be attenuated. This is often the case in Office areas, in which private conversations are essential, but sound transmission is acceptable. Another key consideration is that sound is often heard but not perceived. This is because most indoor spaces have a certain level of ambient noise that masks minor intrusive noise transmitted into the space. The general objective for a space is “quiet” rather than “silence.” Depending on the type of space, sometimes these ambient levels of sound can be quite high for the persons occupying that space to be reasonably content. In the acoustic design industry the measurement for ambient noise is understood as an NC (Noise Criteria) rating, and there are recommended NC levels based on the usage of a space. For example, a worship auditorium should not exceed NC -25 (this would be a sound decibel level in the area of 38). Noise that is heard from mechanical units, air ducts, and all other ambient sources that fall below NC-25 will generally not be perceived by the persons within that space.  For classrooms the NC can be between 30- 40 (dBA 38-47) and in open Office areas it can be between 40- 50 (dBA 47-56). In fact, in general open Office areas it is desirable to have the ambient level of noise higher in order to mask semi-private conversations. Understanding the fact that there are ambient levels of noise is a key consideration in understanding how much sound attenuation is needed from one space to the next.

The other key design consideration with respect to the receiver is sound clarity. In rather small spaces this is typically not a concern but as a space becomes big enough to hold more than 75 people this issue becomes much more complex, because the sound must not only be adequately loud but also properly distributed. When a space exceeds 53,000 cubic feet in volume, and if a voice must travel more than 50 feet a sound reinforcement system will be required. Through sound amplification it is easy to address the loudness necessary, but the real challenge is to properly distribute that sound. As stated before, sound waves bounce, and as they bounce they arrive at the receiver at different time intervals which causes echo. And if the time interval is sufficiently long this will destroy speech intelligibility. The proper choice and placement of speakers with appropriate delay built into the system is essential and can only be done by a professional. There are however, some design considerations that can mitigate the problems of reverberation (echo). Large, flat, highly reflective surfaces, particularly on the back wall of the worship auditorium or behind the speaker, can be treated with absorptive surfaces such as acoustic panels or curtains. These are surfaces that receive a direct hit from the sound source and create the most danger for long reverberation. Creating diffusion on the side wall surfaces can break up what is known as flutter echo. These are surfaces that usually take an indirect hit from the source, and full absorption is not necessary or even desirable. Reflective but diffuse (sawtooth as an example) surfaces are ideal in these areas. It is important to understand that some reverberation is desirable, especially for music. Inadequate persistence of sound within a space can make music sound muddy and lifeless. The design trick is to have the right amount of reverberation while not destroying the clarity of speech. 

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