Introduction Meter Basics Requirements & Desirable Qualities Test and Observations Available Meters References Appendix	Requirements and Desirable Qualities of Sound Level Meters The characteristics and desirable qualities of sound level meters are: ● Accuracy ● Cost effective ● Challengeability in court ● Wind Screen required ● Calibrator ● Simple to operate and have these essentials: ▪ Fast & slow ▪ Allow calibration in field ▪ Wind Screen ▪ Rugged carrying case ▪ Light weight, small size ▪ Environmental ruggedness ▪ Instructions
Accuracy. This, of course, is the most import attribute of any scientific measuring instrument. It is essential, however, to distinguish between accuracy and precision. Accuracy implies closeness to the true level, whereas precision implies how small a change can be measured. A very highly precise meter, if improperly calibrated, can be completely inaccurate. On the other hand, a meter possessing only Type 2 precision can, and if properly calibrated, maintained and used will produce highly accurate sound level measurements. Cost effective. This is a much more important criterion than it might at first appear. As will be mentioned below, sound level meters must be calibrated at least once a year by a laboratory. Often, after rugged field use, repairs are necessary. The cost of maintaining and calibrating a more expensive meter, which will inevitably have more features, not all of which are useful, will be greatly increased. Also, two less expensive sound level meters will gather twice as much information as the most expensive single instrument. Challengeability in court. This is an issue that is often given more attention than seems to be warranted. There is a general perception that if one party to a law suit or the advocate of one side of an action uses a Type 1 meter, and the other side uses a Type 2, the results from the more expensive Type 1 meter will automatically be accepted and those of the less expensive Type 2 meter rejected. There are, of course, judges and magistrates that are this shortsighted, but, with proper explanation by a true expert, as to the accuracy and precision of the meters, this misperception should be debunked. Nevertheless, with the recent advance in the availability of high-quality micro electronics, the difference in cost between Type 2 meters and Type 1 meters has in the last few years narrowed. So, if one expects his results to be challenged in court, a Type 1 meter is recommended. Windscreen is required. It is absolutely futile to attempt to make outdoor sound measurements without fitting a proper windscreen. (The windscreen is typically a sponge ball that attaches to the end of the microphone.) The microphone responds to changes in air pressure, but the sound level meter can’t distinguish between those changes in air pressure caused by an acoustic signal and those caused by wind. There is almost always wind out of doors. A further benefit is that should the sound level meter be dropped or bumped, the windscreen can provide some protection to the microphone.   Sound meters with and without spongy windscreen attached to the end of the microphone. The windcreen is esential for making accurate oudoor (field) measurments.   Calibrator and calibration. Calibration is the most often challenged and the most often ignored aspect of measuring sound in the field. It is absolutely essential that the sound level meter be calibrated frequently during each measurement day. Further, the meter and field (external) calibrator must be returned to either the manufacturer or a qualified acoustic laboratory at a period specified by the manufacturer, usually once per year, for a full calibration. Only properly calibrated instruments give accurate readings. Meters with out of date calibration simply cannot be trusted. It should be mentioned that the calibration of the meter is greatly affected by the condition of the microphone. It is the nature of most measurement microphones as used in sound level meters that is they are dropped or bumped, they will continue to function, but they will overestimate the measured sound level. Therefore, if the meter is dropped or inadvertently struck, or if the microphone is dropped or inadvertently banged, the entire meter (and calibrator) must be sent to the acoustics laboratory or the manufacturer for calibration before the meter is further depended upon.   Field (external) calibrator inserts into the end of the meter's microphone. It is essential to frequently calibrate the sound level meter when making field measurments.   Simple to operate. Simplicity is essential, but the minimum features that a sound level meter should have are as follows: Fast and slow. Meters should have fast and slow response time. In the author’s opinion it should have only fast and slow response time. "Fast" and "slow" are measures of the time constant that the meter uses to integrate the sound level signal. "Fast" is designed to capture transient maxima, using a time constant of 1/8 second, while "slow" is intended to measure more steady-state sounds, and uses a time constant of 1 second. Since virtually all sounds generated by Forest Service activities are steady state, that is, lasting about a second or more, slow time weighting is recommended. Especially with digital type meters, reading "fast" levels is a near impossibility. Then, why are such meters equipped with "fast"? Because in many scientific applications, the meter is used as a "front end", that is, to drive a recording device, either analog, such as a tape recorder, or digital, such as a data recorder, and "fast" levels may thus be captured. Note: Some sound level meters are equipped with “peak” or “max hold” features. The use of these features inevitably leads to overestimating the sound level present. Some modern sound level meters do not come equipped with slow frequency response, but do have an “equivalent level” response, with a settable time constant. A one second equivalent level is very close to the "slow" response and is acceptable as a substitute for it. Nonetheless, it would certainly be the author’s recommendation that sound level meters be equipped with slow response. Allow calibration in the field. It is essential that the sound level meter be supplied with an external calibrator, as previously discussed. The sound level meter must be calibrated using the external calibrator at the beginning and the end of each measurement day. It is desirable to calibrate every half hour or hour to see if battery drain, environmental conditions, etc. have affected the calibration of the meter. Some sound level meters are equipped with an internal calibrator, that is, a regulated voltage source which calibrates the electronics of the meter. This is fine except that the feature of the sound level meter most likely to go out of calibration is the microphone, and internal calibrators do not assess the calibration of the microphone. Therefore, internal calibrators should not be relied upon. A further "calibratability" feature is a removable microphone. If the microphone can be removed, the electronics of the sound level meter can be much more easily calibrated, and, the microphone itself can likewise be calibrated, by the calibration laboratory. Therefore, a removable microphone is a highly desirable feature, as is a standard ½” or 1” microphone size. Wind screen. As mentioned above, a wind screen designed for use with the sound level meter is an essential part of your equipment. Rugged carrying case. The importance of a rugged case cannot be overestimated. Sound level meters, though quite well designed, are still precision electronic instruments and rough handling will inevitably damage them. This is particularly true of the microphone. Also, it is essential that spare batteries, calibration adjustment screw driver, etc. should fit snugly in the case, or inevitably they will not be available when needed. Light weight and small size. These features are essential to do effective field work. Gathering valid acoustic data in the field is always difficult. The limiting factoring on accuracy and precision is certainly not the characteristics of a good Type 2 meter, but the technique and knowledge of the technician gathering information. Small errors in measurement distance, distance the microphone is held from the ground, microphone orientation, etc., will overwhelm any inaccuracies in the meter itself. Thus, light weight and small size, and thus, convenience of use, are very important. Environmental ruggedness. Regardless of how careful a technician is, the sound level meter will be subject to rough handling. Small size and light weight contribute to environmental ruggedness as does a minimum number of controls, knobs and switches. Instruction. The sound level meter should come with complete instructions not only on how to operate the sound level meter, but on how to make sound level measurements. There is a great deal more to measuring sound than merely taking the meter out of the box and turning it on. Different microphones have different directional responses. Some need to be pointed at the sound source, some need to be held perpendicular to the sound source and some at an angle. Without complete instructions, the technician using the sound level meter does not know whether he is obtaining the accuracy the manufacturer built into the sound level meter or not.   Sound level meter kit: sound meter, windscreen, calibrator, instructions, and carring case.