A good-quality sound level meter (SLMs) has the potential to be one of the most useful tools for the church sound system operator. Using an SLM on a regular basis and with the proper technique will ensure that you are providing a safe worship environment with volume levels that are not threatening or a distraction to the congregation.

SLM Defined
An SLM is a sound-pressure-level (SPL) meter that has frequency-weighting filters (equalization) designed to allow measurements that emulate how humans hear. Sound pressure level is the scientific value of the volume level of sound or noise and is measured in decibels (dB). 0-dB SPL, which is equal to 20-uPa (microPascal) pressure level, is the threshold of human hearing and is what one would experience when deep within a subterranean cave or within a large anechoic chamber. Because humans do not hear with a linear frequency response, and our sensitivity to frequencies also changes as the volume level goes up or down, SPL measurements made with a flat-response SLM do not correlate to how we perceive complex sounds. SLMs are therefore provided with frequency-weighting filters that provide reasonably accurate measurements that are representative of how we hear. Almost all SLMs provide A- and C-weighting filters. The frequency response of these filters may be seen in the Graph #1.

Graph 1

A- and C-weighted measurements apply different amounts of equalization to the measured signal. When properly applied, the weighted measurements may offer a close correlation to the actual perceived loudness by the human ear.

SLMs also provide two response settings, Fast or Slow, which vary how quickly the readings are displayed. Without these response settings, the dynamic range of most sounds would result in constantly and rapidly changing readings that would be impossible to visually monitor on the meter. For measurements of program music or spoken word the slow setting provides a more consistent and readable display of sound pressure level. SLMs have four different classes of accuracy from Class-0 (+/- 0.4dB) through Class-3 (+/- 1.5dB), based on ANSI (North American) and IEC (European) standards. For general sound system and architectural noise measurements, Class-2 SLMs are sufficiently accurate. All but a very few SLMs feature range-selecting switches that allow them to measure from very-low to very-high sound levels. One downside to this feature is that the user must ensure the levels being measured are within the selected range. Out-of-range readings are often incorrect.

Guidelines for SLM Measurements
When using SLMs to measure sound, care must be taken in how the device is positioned to ensure accurate measurements. One basic rule is to keep the SLM away from your body, which can introduce reflected sound into the SLM microphone and influence the measurements. You should also hold the SLM at 70°-80° to the sound source(s) to ensure accurate response at high frequencies. In live-sound-system operation, it is common practice to "walk the room" while holding the SLM to obtain multiple readings across the space. When mixing, we more frequently need to position the SLM in, or close to, our line-of-sight (towards the stage or platform) so that occasional SPL readings may be made while watching for cues and making mixing adjustments. Invariably, this means that the SLM is stood on or leaned against the console's meter bridge. Large format (40+ channels) consoles may have enough surface area that they too will influence measurements if the SLM is laid down on the mixer surface. Likewise, when measuring out in the sanctuary, close proximity to walls will also change our measurements. As is the case with any measurement tool (and specifically those with microphones), handle and store your SLM with care and it will continue to work like new. Foreign particles (dust, dirt, magnetic particles) that are allowed into the mic, switches and connectors will eventually cause deviations in accuracy or outright failure.

Relative versus Absolute Measurements
The issue of measurement accuracy brings us to an interesting aspect of conducting sound level measurements. Sound level measurements may be made in two different ways or for two different purposes: absolute and relative SPLs. Absolute measurements are referenced to a known calibration sound source and the readings represent the actual sound pressure level. Non-calibrated or poorly made SLMs provide inaccurate (some would say meaningless) measurements of absolute sound pressure levels. Relative measurements indicate the change in sound pressure levels that we measure, and are not dependent on calibration to a reference level. In fact, relative measurements may be made with any SLM, even one with poor frequency response or that is held improperly-as long as all of the subsequent (relative) measurements are made with the same device, with the same weighting filter, same response time and held in the same manner.

Why Measure SPLs?
The whole issue of appropriate volume levels in sound reinforcement can become a quagmire of scientific evidence versus human subjectivity, intuition and demographics. It is far beyond the scope of this article to discuss many of the details and various facts or issues that are entailed. But suffice it to say that as sound system operators in houses of worship, we are at least partly (if not largely) responsible for how our sound systems are used. The personal health and personal taste of the parishioners we serve mandate that we at least have a basic understanding of how SPLs relate to and impact human hearing. Below are two tables showing common sound sources versus basic sound pressure levels and their perceived impact. We invite you to read further about this complex subject and to this end we provide several informative sources at the end of this article.

Table-1: Examples of sound sources and their relative sound pressure level

Safe exposure levels are outlined and discussed in detail at these websites:
http://www.experiencingworship.com/articles/sound/2001-9-How-Loud-is.html
http://www.osha-slc.gov/OshStd_data/1910_0095_APP_G.html

Technical details of sound-pressure-level measurement and equipment may be found at these websites:
http://www.npl.co.uk/npl/acoustics/publications/soundmeasurements/measuring.html#2
http://www.norsonic.com/web_pages/background_noise.html

Another very good source on SLMs and an excellent handbook for church sound folks is:
Handbook for Sound Engineers, 3d Edition. Ballou, Focal Press (pages 1465-1469)

The editor would like to thank Jim Long of Electro-Voice, Inc. for his help with this article.

Tom Young is principle consultant with Electroacoustic Design Services based in western Connecticut. He has just entered his 30th year professionally involved in live sound and seldom wishes he still played bass or went into lighting instead. He can be reached at dbspl@earthlink.net.