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分贝仪 article Full article /magsdirx/ax/addenda/media/koya2811.pdfThis article investigates the characteristics of the Radio Shack Sound Level Meter in regard to its accuracy.The Radio Shack Sound Level Meter is a versatile and economical device, used for calibrating home theater speaker levels, subwoofer level matching to main speakers, low-frequency room-mode analysis, and measuring loudspeakers. Conveniently, an output jack is provided on these sound level meters, which allows connections to personal computer sound cards, and permits usage of these devices via PC-based acoustic measurement programs. But how accurate is this device and is it the appropriate tool to be used in the aforementioned activities?The Radio Shack Analog-Display Sound Level Meter (referred to hereafter as the RS SPL meter) has seen a model change. The new version (Photo 1, model number: 33-4050) is more compact than its predecessor. I assume that many users still own the older, now-discontinued version (Photo 2, model number: 33-2050), so I include it here. The Radio Shack Digital-Display Sound Level Meter (Photo 3, model number: 33-2055) features a digital readout of the sound level instead of a moving needle.I had access to two of the older-model RS SPL meters, one new model, and a digital version. I used these four units for this study.FREQUENCY RESPONSEDETERMINATION METHODI devised a method to determine the frequency response of the four RS SPL meters. First, I took a reference measurement using a calibrated Mitey Mike II microphone and a custom-made microphone preamplifier. From its specification sheet, the Mitey Mike II has a flat response in the low-frequency range, to 20Hz. The custom-made microphone preamplifier was designed to have a flat frequency response down to DC. I used LMS, which uses stepped sine signal acquisition1, to perform all acoustic measurements.One speaker I used in making this reference measurement was a 6½˝ two-way bookshelf speaker with a relatively flat frequency response (88Hz ~ 20kHz, ±2dB). Because the bookshelf speaker’s frequency response extends down to only 88Hz (-3dB), I used a subwoofer with an approximately flat frequency response down to 16Hz (16Hz ~ 200Hz, ±2dB) to ascertain the low frequency region. I measured the bookshelf speaker using the ground-plane method2 and the pseudo-anechoic (gated3) method.I performed both measurement methods for the bookshelf speaker because when the RS SPL meter is placed flat on the ground, its microphone element edge is about 1cm off the ground. I surmised that this height differential in the ground-plane measurement may limit high-frequency accuracy4. Therefore, for high frequencies (above 500Hz), I used pseudo-anechoic measurements, which place the RS SPL meter microphoneTube, Solid State,Loudspeaker TechnologyPHOTO 1: RS Sound level meter (new model).
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