BBE Sound 262 Manual de usuario descargar pdf (Pagina 11)

MED ASSO C IAT E S I NC. ANL- 940 - 1 U L TR ASO NI C M I CRO PH ONE AND AMPL I FIE R

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MED Associates, Inc.

Figure A.1 shows a plot illustrating the correction factor (in dB) that should be subtracted from SPL

measurements taken at distances less than 1 meter to scale them to the equivalent SPL that would be

obtained at 1 meter. Some commonly used measurement distances are indicated. To obtain the

correction factor for any distance less than one meter, simply take the logarithm of the distance and

multiply by 20. Subtract this value from the SPL you measured and this will give you the equivalent SPL

at 1 meter. It is convention to express SPL measurement s at a distance of 1 meter. However, always be

sure to state the distance used in your measurements when publishing papers. This will aid other

readers in interpreting your data, and represents good scientific practice.

Example: Setting up to make measurements that reflect expected pressure levels at one-meter.

1. Determine the distance from the sound source to the microphone, e.g., 0.25 meters.

2. If possible, set the A-to-D converter gain such that the largest expected analog signal provides a

digital output slightly smaller than the maximum converter output count.

3. If possible, adjust the scaling factor of the display software such that the peak value of a 1 volt

50 kHz sine-wave input produces a reading of

94 dB + 20 log(distance)

which for a distance of 0.25 m yields

94 dB + 20 log(0.25 m) = 94 dB – 12 dB = 82 dB

The above scaling may be accomplished by either using the known gain factor for the A -to-D converter

and the display software, or by applying a 1 volt peak (0.707 V rms) 50 kHz signal to the con verter input

using a signal generator. The test generator output level may be determined either by direct

measurement, or, if the generator provides a calibrated output, by using the set value directly. If it is

not possible to scale your A-to-D recording hardware and software, you will need to do post hoc scaling

of your recording signals using some other data processing method (e.g. export data from your recording

system to an Excel spreadsheet).

NOTE: if the signal generator output is set using the generator's output level control, be sure to load the

generator output with the proper load. For example, if the generator output impedance is 50 Ohms, the

output must be loaded with 50 Ohms. In the case where the A-to-D being used has a high input

impedance, the stated generator output voltage must be doubled (i.e., a setting of 0.5 V peak will

produce a 1 V peak signal into a high impedance load). Finally, if a test signal of 1 V peak causes the

output of the A-to-D to saturate, reduce the amplitude of the test signal by a factor of ten to 0.1 V peak

and replace the factor of +94 dB with +74 to compensate for the reduced signal level.

A note on rms values

Expressing signal levels in rms equivalent values assumes an averaging over either time or space, an d is

usually used to express the spread of a measurement. The rms value of a single measurement is the

measured value. In the case of a pure sine-wave, the measurement is assumed to be averaged over a

full cycle and is typically used to calculate the power of the signal. For example, a sine-wave with an

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BBE Sound 262 Manual de usuario Pagina 11