Sound Level Meters

["Michael S. Gordon" <mgordon@xxxxxxxxxxxxxxx>]

Dear list,
        Some time ago I posted a query about sound level meters (SLMs). Since then I have gotten responses from the list and several of my colleagues here at University of Toronto on that topic. The following discussion collates these responses and summarizes what seem to be the key points for understanding and purchasing SLMs.

First, everyone agrees that when purchasing a SLM, if you can afford it get a Bruel & Kjaer. It is expensive and worth it.

Frequency weighting: Most SLMs have options for frequency weighting, i.e. weighting the intensity to different psychophysical scales. The dB SPL or 'flat' scale has no weighting, dB C is essentially unweighted in the range of human hearing (and very similar to dB SPL). The dB A scale is weighted to the Fletcher-Munson equal loudness contours. dB B is rarely used and probably unnecessary for most users.  The dB A and dB SPL scales will be most noticeably different for tones less than 1000 Hz.
        Other frequency options include band filters. To test a specific frequency range a 1/3-octave band is very useful or for general testing a 1-octave band or greater may be employed.

MIcrophone size: Different microphones will be useful for measuring different frequency ranges and intensities. For most human testing a mic measuring 1/2-inch is most popular, and will probably be most useful. Larger mics are more sensitive to low pressure (1-inch mic is sensitive to as little as 10 dB vs. 1/2-inch mic sensitive to about 22 dB vs. 1/4-inch sensitive to 36 dB). Mic size, however, also influences the frequency range of sensitivity (1-inch mic will respond to about 2.5 to 8000 Hz vs. 1/2-inch responses to 4 to 40,000 Hz vs. 1/4-inch responses to 4 to 100,000Hz). Note that the 1/2-inch mic best covers the human frequency range of sensitivity.

Classification: SLMs are classed at 0 (laboratory), 1 (precision) or 2 (general purpose). Class 0 SLMs will be precise to 0.7 dB, Precision to 1 dB and GP to 1.5 dB. Most human researchers use Class 1 SLMs, but if you want to go cheap you can get a Class 2.

Time constants: SLMs have some options about the time course of their measurements. Slow settings average the intensity of sound over about 1 sec, Fast settings will average over 125 ms and Impulse settings will average over about 35 ms. Fast and Slow options are fairly standard, but some models do allow for impulse and even faster settings if those are required. Impulse is very useful to have if need to measure reverberation time.

Measurement considerations: When measuring it is important to hold the SLM away from one's body. Body reflections of low frequencies (less than 100 Hz) can add up to 6 dB of error. For high frequency measurements reflections are less of a concern. SLMs may be purchased with tripods in order to avoid the measurement error caused body reflections.  For use with headphones, SLM's tend to require a special ear coupler. Also for a headphone measurements a pressure field mic should be used, while for measuring outdoor or room acoustics a free-field mic should be used.
        Because most mics are directional, the SLM must be aimed at the sound source. Errors caused by a badly aimed SLM will become more pronounced as one moves toward higher frequencies (a problem for 1000 Hz or higher).
        Also important for reducing measurement error is constant re-calibration of the equipment. A very responsible scientist might check the calibration before every use of the SLM, and most of us should do it at least every 6 months.

Companies: As stated earlier B & K make the gold standard for SLM's. However the cost will probably be between $15-25,000 with the accessories.

A few people recommended Quest Technologies as a more modestly priced alternative.

Other companies to consider are:
Larson-Davis
Rion
Metronics
Extech instruments
Norsonic

SLMs from these makers run between $800 and $5000, averaging about $2500 depending on the accessories one needs.

Strange as it may be Radio Shack makes a very consistent Class 2 SLM for less than $100. University of Toronto buys hundreds of these for their engineering students and has found them to be quite reliable and accurate to within 1 dB.

The major difference in cost between the models seems to be due to the calibration standard (i.e. class) and resiliency of the equipment. B & K equipment might be expected to last well into the next century (though the mics will need to be changed every 20 years), other companies expect a good 8-20 years from their products, while the cheap ones may last 2-5 years.  


Acknowledgements:
Prof. Hans Kunov, in the engineering department at U of Toronto deserves a great deal of thanks for all his help informing me on this topic. Others deserving acknowledgment for their assistance are Akiko Kusumoto (National Center for Rehabilitative Auditory Research), Ole Juhl Pedersen (Telia), Bruce Schneider (University of Toronto) & Mitch Sommers (Washington University).


Original Post:

Dear list,
        I would like to buy a sound level meter that I can use to calibrate psychoacoustic experiments. I expect that the acoustic presentations will sometimes be made in the free-field and sometimes over headphones, so I need a meter that can be adapted to either circumstance. 
        I would welcome any comments about the sort of capabilities I should look for in a sound level meter, what companies might be good to purchase from, and what price ranges they might have.

--

Michael S. Gordon, Postdoctoral Fellow
CIHR Research Group on Sensory and Cognitive Aging
& Department of Psychology
University of Toronto at Mississauga
Mississauga, Ontario
CANADA L5L 1C6
mgordon@utm.utoronto.ca
http://www.erin.utoronto.ca/~w3gordon/