International projectionist (Jan-Dec 1947)

THE sound level meter is a device for measuring the intensity of sound calibrated in db. It is a small, compact instrument, weighing approximately 24 pounds and measuring 9^2 x 13^2 x 12 inches. It accepts sounds from all directions, measuring sound received on a direct line from the source as well as any reflected sound, thereby giving an over-all measurement of all sound at a particular point. The total sound is registered upon the meter in db. automatically giving the sound or noise level of that area. The sound level meter finds many practical applications. In determining studio location, for example, some quick and effectual samplings of the noise levels at various points within a structure can be made to determine the point of lowest noise level. Furthermore, the sound level meter also measures any outside sounds or noises which reach it. Thus, the effect of nearby railways, adjacent factories, street noises, etc., upon a given location are readily discernible. In establishing the location of loudspeakers in a noisy area the sound level meter is invaluable in determining the amount of audio power necessary to override the sounds normally present in this room or area. After the normal sound or noise level of a particular area has been determined, a loudspeaker operating from sufficient electrical power and possessing the proper directional characteristics may be selected. Noise and Noise Levels Also, after the point of highest sound intensity has been found within an area, the location and facing of the loudspeakers in relation to the source of this predominant sound may be better determined. Previously the term "noise level" was interchanged with the term "sound level." This is understandable when it is realized that noise or sounds unpleasant to the ear possess levels of intensity or loudness just as musical notes or pleasant sounds possess levels of intensity or loudness. Noise level merely refers to the intensity of noise in terms of db. Noise is defined as any unwanted or undesired sound. It is usually irregular and non-musical in character and of a disagreeable, irritating quality. While noise is usually made up of a wide range of frequencies, one or two particular frequencies may predominate. The presence of predominant frequencies in noise is cited because of the problem which is sometimes created by this condition. Should these predominant noise frequencies be the same as frequencies reproduced by the loudspeakers, the * A chapter in the recently published "The Architects Manual of Engineered Sound Systems," reviewed in IP for Sept., 1947, p. 28. ACOUSTICS II. Sound Level Meter — Noise and Noise Levels — Sound Frequencies — P. A. Requisites — Reverberation Data Copyrighted by Sound Products Section, RCA* sound level will build up and intensify these noise frequencies. The end result is an apparent intensification of the noise frequencies with no appreciable increase in the clarity or loudness of the desired sound. In order to overcome noise normally present in a room or area the loudspeakers must provide sufficient sound power to override the noise. For the reproduction of speech in the average quiet location a sound level between 60 db and 70 db is needed to provide comfortable listening. If the program seriously exceeds this level, it is uncomfortably loud for the listener; if it falls below this average level, it cannot be properly heard. On the other hand, in a noisy location where an average noise level of 70 db or more may already exist, the sound power from the loudspeakers must be increased by an equivalent number, of db to overcome the noise already present. Music Requires Higher Level For the reproduction of music in the average quiet location a slightly higher sound level is required than is needed for speech. The different parts of a musical program differ in loudness and vary over a broader range. A sound level from 80 db to 100 db is usually adequate, but if a high noise level is encountered, this range must be increased to make the program heard above the noise. If a sound system must reproduce both speech and music, meeting the power requirements for music obviously assures ample power for speech. Under most circumstances a high noise level may be overcome by (1) sufficient power, (2) the right loudspeakers and (3) proper location and facing of loudspeakers. If the noise level of a room is especially high and objectionable, however, the value of the acoustical treatment to reduce the over all noise level should not be overlooked. The noise quieting accomplished by these means will often create a quieter, more desirable listening area and at the same time permit better sound system performance. Sound Frequency Range In the installation of any sound equipment the range of sound frequencies that the equipment is required to handle is always an important factor. Sound frequency is defined as the number of cycles completed per second (abbreviated c. p. s.). Again referring to the example on loudspeakers, each complete trip of the diaphragm — starting from its neutral position, moving to one side, and finally back to neutral again — is termed a "cycle." The rapidity with which these cycles occur per second determines a sound's frequency or its pitch. Highfrequency sounds are high-pitch sounds, such as the treble notes on a piano or the shrill tone of a whistle. Lowfrequency sounds are low-pitch sounds, such as the bass notes on a piano or bass drum. The human ear is sensitive to a frequency range from approximately 16 to 20,000 cycles per second. Although expense and size precludes the design of microphones, amplifiers and loudspeakers which can reproduce this entire range of frequencies, sound equipment, whether it be for speech only, music only, or a combination of both, must be designed to transmit uniformly most of this range of frequencies with as little distortion as possible. Microphones used for paging, announcing, or speech only should be designed for this particular application. A slightly wider range and flatter frequency response is needed for choral and orchestral work. More frequently a microphone is used for both music and speech, necessitating the use of an 22 INTERNATIONAL PROJECTIONIST • October 1947