The human auditory system is designed to detect rapid local variations in air pressure and convert them into what we perceive as sound. These air pressure variations, caused by the repeated compression and rarefaction of air molecules, can be induced by many sources but the source most relevant to us as audiophiles is the motion of the drivers (or panels) in our loudspeakers. The pressure wave caused by such motion propagates through the air of our listening rooms and causes tiny membranes in our ears to vibrate, which in turn allow us to hear the pressure wave as sound. The perceived loudness of the sound we hear is proportional to the pressure of the sound wave.
Having the ability to measure the pressure level of a sound wave in a listening room is extremely useful when attempting to optimize the room's loudspeaker and listener positions. In addition, monitoring sound pressure levels in your listening room can help prevent damage to your hearing from long term exposure to excessively high sound levels.
The Radio Shack sound pressure level meter is a device for measuring sound pressure levels in an acoustic environment. This article will describe the operation of the meter and discuss its use as an aid for listening room setup and monitoring listening room sound pressure levels.
The Radio Shack sound pressure level meter is a small hand-held device for measuring the pressure level of a sound wave in an acoustic environment such as your listening room. The meter can be used to measure sound pressures in the range 50-126dB. This is accomplished through the use of an analogue display which ranges from -10dB to +6dB and a rotary dial that allows you to select one of seven ranges centered at 60dB, 70dB, 80dB, 90dB, 100dB, 110dB and 120dB. For example, at the low end of the scale with the dial set to 60dB, the display will show readings in the range 60-10 = 50dB through 60+6 = 66dB. At the high end of the scale with the dial set to 120dB, the display will show readings in the range 120-10 = 110dB through 120+6 = 126dB. The dial also has a setting called 'BATT' that can be used for testing the meter's internal 9V battery.
Aside from a dial for range selection and an analogue display, the meter consists of a front-mounted microphone and two switches for changing the meter's frequency response or "weighting", and the speed of its response to sound pressure changes. The weighting switch allows for switching between the standard 'A' and 'C' weightings. Choosing the 'C' weighting will make the meter respond more-or-less uniformly over the frequency range from 32 - 10,000 Hz, and the 'A' weighting will make the meter more sensitive to frequencies in the range 500-10,000 Hz. The response switch allows for changing the speed of the meter's response from 'SLOW' to 'FAST'. A slow response setting will make the meter less sensitive to rapid changes in sound level and can be used for measuring average pressure levels. The 'fast' setting is more useful when peak sound levels are being measured since in this mode, the meter will respond to very rapid changes in pressure level.
Although Radio Shack claims that under normal use, the meter shouldn't require recalibration, a hole labeled 'CAL.' is provided which will allow a technician to gain access to the meter's innards and recalibrate it if necessary. Finally, the meter is equipped with a phono-type output jack for connection to other measuring equipment, and a standard threaded socket for tripod mounting. Mounting the meter atop a tripod is recommended in order to avoid reflections off of your body which can alter measurements by more than 2dB.
When setting up loudspeakers in a listening room, the ultimate goal is to choose loudspeaker and listener positions such that sound pressure levels are fairly constant over the audible frequency range. While this may sound relatively simple to accomplish, doing so can be very difficult in practice as the sound waves reflecting about your listening room sum and cancel at different spatial locations, resulting in either increased or decreased sound pressure level at certain frequencies. If your listening position happens to be located at a position of increased or decreased sound pressure at a particular frequency, a boost or suckout at that frequency may be audible, resulting in a decrease in your system's accuracy and musicality.
The Radio Shack sound pressure level meter allows you to measure the sound pressure levels at your listening position given a particular loudspeaker position, and can help you avoid those positions that result in large increases or decreases in sound pressure level. To effectively use the sound pressure level meter for this purpose, a test CD containing a pink noise track and tracks containing test tones over a wide frequency range is required. Such CD's are available from several sources, but I typically use either Stereophile's Test CD 2 or Test CD 3 each containing tracks which fit the bill nicely.
The first step in using the meter for loudspeaker placement is to pick a starting location for your loudspeakers and listening seat. Don't worry about the initial placement too much since you'll most likely be changing it radically once you start measuring. Pick initial locations such that your speakers are equidistant from the side walls, and your listening position is centered between the loudspeakers. Now set the meter's dial to the 80dB setting, the weighting to 'C', and the response to 'SLOW', and locate the meter where your head would be if you were sitting in your listening seat. If you're not using a tripod, try to stand to the side of the meter to reduce reflections from your body that might affect your measurements. Play the pink noise track on your test CD (track 15 on Stereophile's Test CD 2) and adjust the volume of your system so that the meter reads 0dB (which means that the sound pressure level at the meter's location is 80dB since the meter's dial is set to 80dB). Be sure to leave your system's volume control at this level so that subsequent measurements can be meaningfully compared to the pressure at this volume level. Ensure that the meter is at the same location as when making the pink noise measurement, and play the test tone tracks from your test CD (tracks 16-18 on Stereophile's Test CD 2). For each test tone, jot down the tone's frequency and the meter's reading at that frequency. When you're done measuring the sound pressure level for each test tone, you will have a set of measurements of frequency vs. sound pressure level. In other words, you will have the data for a frequency response graph of your room, given the current loudspeaker and listener locations.
Unless you got extremely lucky, your first set of measurements will indicate fairly gross deviations from the 0dB measurement obtained with the pink noise track. In that case, systematically move your loudspeaker and listener positions in small increments, of say two inches or less, and repeat the measurements. Try and resist the urge to make large spatial changes between measurements. Remember, moving a loudspeaker one or two inches can have a radical effect on measured frequency response (after each change in loudspeaker and/or listener position, don't forget to readjust your system's volume so that the pink noise track measures 0dB on the meter with the dial at the 80dB setting).
Don't drive yourself crazy trying to get ruler flat in-room response because in most cases it simply isn't possible - a variation of a few dB across the range of measured frequencies is perfectly acceptable. If, on the other hand, you are measuring gross response increases or decreases of, say, 10dB or more at certain frequencies, then invest the time to try to eliminate them as they will not allow your system to reach its full musical potential.
While simulating the volume level of a large rock band or full symphony orchestra in your listening room can be
a lot of fun, remember to keep an eye on those SPL's to ensure that they are within
safe levels. The following table from the U.S. Department of Labor (also included in
the excellent documentation accompanying the Radio Shack sound pressure level meter)
gives data on permissible sound level exposure.
|Hours per day||Sound Level (dB), A-weighting, SLOW response|
|.25 or less||115|
To use the Radio Shack sound pressure level meter to ensure that you're listening to your system at a safe level, first set the meter's weighting and response switches to the 'A' and 'SLOW' positions respectively. Play some music on your system at your normal listening level (no cheating now!) and adjust the setting on the meter's rotary dial such that the average reading is around 0dB. Note that a setting which is too low will tend to "peg" the needle to the right, possibly damaging the meter. Given an average reading of 0dB, your measured listening level is given by the setting of the meter's dial. For example, if the meter has an average reading of about 0dB and the dial is set to about 90dB, then your listening level is about 90dB. According to the table above, you can safely listen at this level for about eight hours per day, although I would use this table merely as a guide and let common sense prevail. Making a habit of using the sound pressure level meter to measure the average sound level of your system at each listening session is a simple way to ensure that no harm will come to your hearing through regular use of your audio system.
The Radio Shack sound pressure level meter is an inexpensive and invaluable tool for measuring sound levels in your listening room. It is an indispensable aid when optimizing loudspeaker and listening positions and it allows audiophiles to quickly measure sound levels to ensure they are within safe levels. It is truly an essential audiophile accessory.