Room-to-Room Privacy and Acoustical Design Criteria

Sound and Vibration, Feb 2004 by Weissenburger, J T

With the recent and continuing interest in loft-type condominiums and apartments, privacy between units is a significant issue.1 Unfortunately, architects and designers have little more than Sound Transmission Class (STC) to guide them in choosing wall constructions with an eye to acoustical insulation. Virtually no distinction is made between Sound Transmission Class, Field Sound Transmission Class (FSTC) and Noise Isolation Class (NIC).2 It is well known throughout the architectural acoustics community that even with the use of good construction practice, FSTC and NIC can be significantly lower than STC^sup 3^ - 5 to 10 points. In poor quality constructions, the difference can be much greater. This article examines STC as it relates to speech privacy and offers some guidelines for specifying wall construction based on STC ratings.

Prior to World War II buildings were massive, interior walls and ceilings were lath and plaster. Room-to-room privacy was not much of a consideration. After WWII the structure of buildings changed rapidly. Walls became light weight, ceilings became suspended lay-in and acoustical privacy declined. Now, faced with this in new as well as older multifamily buildings, it is important that privacy bo considered as a design parameter.

Sound Transmission

In describing room-to-room sound transmission, it is convenient but unfortunate that Sound Transmission Class4 (STC) has become the primary descriptor for room-to-room sound isolation. STC is a laboratory5 measurement procedure for transmission of sound through a sample of a wall construction. When the same wall construction is built in the field, it has not only the basic construction as used in the STC test but also various holes, cracks and other paths by which sound can got from one side of the wall to the other. These other paths are referred to as 'flanking' paths. Flanking paths and other differences between the STC lab sample and the "real life" wall in terms of sound transmission can be significant. Unfortunately, there is not a one-to-one link between STC and privacy.

Expectations

Expectations for an acoustical environment in a building, particularly an apartment or condominium, are subjective. That is, different people have different expectations. It is incumbent on the designer to provide an acoustical environment consistent with the expectations of the occupants of the building. A quantifiable set of privacy criteria is needed for design purposes. Most disturbing is the ability to hear and understand speech from the opposite side of a wall. Psychoacoustically, we believe that if we can understand conversation coming our way, then our conversation can be understood going the other way. We want to keep our conversations private from our neighbors. We turn to the method of Articulation Index6 (AI) as a means of quantifying privacy in terms of the percent of unfamiliar sentences (first presentation to listeners) that can be understood correctly.

Privacy

There is no such thing as 'soundproof,' and privacy is a subjective matter. Acoustical privacy could mean either speech privacy or freedom from non-spoken sounds intruding into the acoustical environment, or both. Speech privacy can be quantified to a reasonably precise degree. This is done in terms of the ability to hear and understand a percentage of words, sentences, or other intelligible spoken sounds. Articulation Index is a convenient and proven method for this purpose.

Non-speech acoustical privacy does not lend itself to convenient quantification. To provide non-speech acoustical privacy it is necessary to define the potentially offending sound (loud TV, Hi Fi, music practice, etc.) and how much of this is acceptable in the receiving space. If the criterion is "no sound," the costs are significant. Recording studios, broadcast studios, voice-over booths, hearing testing booths and music practice rooms are examples of spaces that might require such extreme isolation. But it all comes at a price. One might well concede that these "high sound" uses are not compatible with adjacent residential occupancy.

Speech Privacy

Articulation Index was developed to provide a measure of the ability to understand words and sentences under various conditions. The ability to understand sentences, for example, can be equated to speech privacy. For purposes here, privacy is based on the ability not necessarily to hear sound from an adjoining room, but to understand spoken words or unfamiliar sentences. Computation of Articulation Index is a number crunching exercise that considers many factors that influence the ability to understand spoken sentences that may not be familiar to the listener - (1) the level and frequency spectrum of the speaker's voice (the sound source) as received by the listener (receiver), and (2) the level and frequency spectrum of the background sound surrounding the listener. The calculations include weighting the sound reaching the listener by relative importance in each of the sixteen 1/3 octave bands used in the analysis. The relative weightings have been determined by "jury tests" in which many people have listened to test sounds and rated them according to their subjective judgement.