Extech Instruments 407750 User manual

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
90
80
70
60
50
40
30
20
10
140
0
Jet takeoff (200')
dB
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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