Extech Instruments 407750 Manual de usuario

User's Guide

Digital Sound Level Meter

Model 407750

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Introduction

Congratulations on your purchase of the Extech 407750. This device measures sound level in dB

and the measurement range can be set automatically or manually. The 407750 offers selectable

frequency weighting (‘A’ and ‘C’) and Time Response (Fast and Slow). The built-in RS-232 PC

interface allows the user to record readings onto a PC in real-time. Careful use of this meter will

provide years of reliable service.

Meter Description

1. LCD Display

2. Microphone

3. ON/OFF key

4. REC (Record) key

5. MAXHLD (Max Hold) key

6. C/A Weighting Select key

7. BA (Background Absorber) key

8. F/S Fast / Slow Response select key

9. DOWN

10. Backlit (LCD backlighting) key

11. UPPER

12. AC adaptor jack

13. Calibration screw adjust

14. AC analog output jack

15. DC analog output jack

16. RS-232 output jack

Note: The Battery Compartment and the Threaded Tripod mount access are located on the back

of the instrument (not pictured)

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Operation

Quick Start

1. Power the meter by pressing the ON/OFF key.

2. The meter’s LCD will count down to zero (99.9, 88.8, 77.7, etc.) and then begin measuring

sound levels. If the LCD does not switch on after pressing the ON/KEY check the 9V battery.

3. Point the microphone toward the source of the sound level to be measured and view the reading

on the meter’s LCD.

‘A’ and ‘C’ Frequency Weighting

Select ‘A’ or ‘C’ weighting via the C/A key. The LCD will reflect the currently selected frequency

weighting. Use ‘A’ weighting to have the meter respond as the human ear would with regard to

frequency response (the human ear boosts and cuts amplitude over the frequency spectrum). ‘A’

weighting is used for environmental measurements, OSHA regulatory testing, law enforcement, and

workplace design. Select ‘C’ weighting for flat responding measurements (less amplitude boost or

cut across the frequency spectrum). ‘C’ weighting is used in applications where hearing

conservation is not an issue; for example, in the diagnosis of malfunctions in electrical, electronic

and mechanical devices.

FAST/SLOW Response Time

Select either FAST (125ms response) or SLOW (1 second response) measurements by pressing

the F/S key. The LCD will reflect the currently selected mode. Selection of ‘Fast’ or ‘Slow’ is

determined by the application and any directives or standards related to that application. For

example, most hearing conservation or OSHA related testing is done using SLOW and A weighting.

MAX HOLD

The meter is capable of taking continuous measurements and only updating the LCD when a higher

reading (than the one presently on the display) is detected. The bargraph display continues to

change while the main LCD waits for a higher reading. Press the MAXHLD key to activate the MAX

HOLD mode. The LCD will reflect the MAX HOLD function. Press the MAXHLD key again to return

to normal operation.

Record (REC) Function

To Record the Maximum and Minimum sound level measurements over a programmable period of

time, press the REC key. The REC indicator will appear on the LCD. Once the REC key is pressed,

the meter begins tracking the highest (MAX) and lowest (MIN) readings. Press the REC again and

the MIN indicator will appear on the LCD along with the lowest sound level reading since the REC

key was pressed. Press the REC again and the MAX indicator will appear along with the highest

reading the meter has encountered since the REC key was first pressed. Press and hold the REC

until the REC indicator extinguishes to exit the RECORD mode.

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BA (Background Noise Absorber) Mode

The Background Noise Absorber allows the user to accurately measure equipment noise by

“eliminating” background noise. The Sound Level Meter first stores the background noise as a

reference level. From there, when a sound is measured, the display will show the sound level

measurement minus the background noise. To operate the meter in BA mode, follow these steps:

1. Power the meter.

2. Press the MAXHLD key (the MAX HOLD icon will appear on the LCD).

3. Press the BA key (‘F” will appear to the left of the SPL display icon).

4. Press the MAX HOLD key again (the MAX HOLD icon will reappear on the LCD).

5. The meter is now displaying the background, reference noise.

6. Power the device under test and note the new sound level meter reading.

7. If the reading changes, the new reading is the sound level of the device. If the reading does not

change, the noise produced from the device is either equal to or less than the background

noise.

8. Press the BA key again to return to the normal mode of operation.

Auto and Manual Ranging

The meter powers up in the Automatic Range mode. In automatic mode the meter automatically

finds the correct range in order to produce the best accuracy. However, if it is desired to set the

range manually, follow these steps:

1. Power the meter

2. Notice the two (2) digit number to the immediate left of the analog bargraph. This number is the

low end of the presently selected range (see the specifications for the ranges).

3. To change the range, press the UP key to raise the range or press the DOWN key to lower the

range. The two digit number on the left of the bargraph will change with each key-press.

4. An advantage of Manual mode is that it takes less time for the meter to take a reading. In Auto

Range mode the meter must first locate the correct range before displaying a measurement.

LCD Backlighting

Press the BACKLIT key to illuminate the LCD. The backlight will remain on for 5 seconds and then

automatically switch off to preserve battery life.

Auto Power Off

To preserve battery life, this meter has an automatic power off feature. If the unit is not used for

approximately 20 minutes, the meter shuts off. To override this function, follow these steps:

1. From a power OFF condition, press and hold the ON/OFF and MAX HOLD keys simultaneously.

2. When ‘n’ appears on the display, release the MAX HOLD and then the ON/OFF key.

3. The Auto Power Off feature is now disabled. Note that the Auto Power Off feature is re-activated

the next time the meter is powered down.

Analog Outputs

The meter includes an AC and a DC analog output. These outputs are proportional to the displayed

sound level and are ideal for use with chart recorders and dataloggers. The AC output is 0.707V

rms full scale and the DC output is 10mV per dB. The labeled 3.5mm output mini-plugs are located

on the bottom of the instrument.

RS-232 Output

The meter includes an RS-232 PC interface jack. This PC interface allows the meter to store and

display readings on a PC as they are recorded. The interface cable and 407752 software for data

acquisition are sold separately. Detailed instructions are provided with the software.

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Calibration

To calibrate the meter, an external calibrator such as the Extech Instruments 407744 or 407766 is

required in addition to a small screwdriver.

Turn the meter on and set the parameters of the 407750 to the following before proceeding:

Response: Fast

Function: A weighting

Range: 50 to +100 dB

Place the calibrator gently over the microphone of the meter. Set the calibrator to output 1kHz sine

wave at 94.0dB. Adjust the calibration potentiometer, located at the bottom of the meter, until the

display shows a reading of 94.0 dB.

Battery Replacement

When the low battery message appears on the LCD, the 9V battery has fallen to a critically low

voltage level and should be replaced as soon as possible. The battery compartment cover resides

at the rear of the meter. Remove the rear battery compartment screw and remove the battery

compartment cover, change the battery, and replace the compartment cover.

Safety: Please dispose of batteries responsibly; never dispose of batteries in a fire, batteries may

explode or leak. If the meter is not to be used for 60 days or more, remove the battery and store

separately.

Specifications

Display Backlit 2000 count LCD with analog bargraph

Display update rate Main LCD digits: 0.5 seconds; Bargraph: 50mS

Analog Bargraph 1dB steps with 50dB display range

Microphone Electret condenser (0.5” diameter)

Measurement Bandwidth 31.5Hz to 8KHz

Measurement Range A weighting: 30 to 130dB; C weighting: 35 to 130dB

6 ranges in 10 dB steps: 30 to 80dB, 40 to 90dB, 50 to

100dB, 60 to 110dB, 70 to 120dB, 80 to 130dB

Accuracy / Resolution ± 1.5dB / 0.1dB

Time response selections Fast (125ms) and Slow (1 second)

AC and DC Analog outputs 0.707VAC rms at full scale; 10mVDC / dB;

3.5mm output jacks

Standards Meets ANSI and IEC Type 2

External Calibrator Extech models 407766 or 407744

Power 9V Battery; 20 hour battery life (typical) with low battery

indication

Dimensions/weight 3.2 x 10.1 x 1.5” (80 x 256 x 38mm) / 8.5 oz. (240g)

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Reference Information

Frequency Weighting Characteristics

Frequency (Hz) A Weighting C Weighting Tolerance (IEC 651 Type 2)

31.5 -39.4dB -3dB ±3dB

63 -26.2dB -0.8dB ±2dB

125 -16.1dB -0.2dB ±1.5dB

250 -8.6dB 0dB ±1.5dB

500 -3.2dB 0dB ±1.5dB

1 k 0dB 0dB ±1.5dB

2 k +1.2dB -0.2dB ±2dB

4 k +1dB -0.8dB ±3dB

8 k -1.1dB -3dB ±5dB

Typical A-Weighted Sound Levels

50HP Siren (100')

130

120

110

100

140

Jet takeoff (200')

Riveting machine

Chain saw

Subway (20') Boiler room

Pneumatic drill (50') Freight train (100')

Vacuum cleaner (10') Speech (1')

Large store Large Office

Small office Residence

Night residential area

Whisper (5'0) Sound studio

Threshold of hearing

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