EXTECH 407750

User's Guide
Digital Sound Level Meter
Model 407750
EXTECH INSTRUMENTS CORPORATION warrants this instrument to be free of defects in parts and
workmanship for one year from date of shipment (a six month limited warranty applies on sensors and
cables). If it should become necessary to return the instrument for service during or beyond the warranty
period, contact the Customer Service Department at (781) 890-7440 ext. 210 for authorization or visit our
website at www.extech.com (click on ‘Contact Extech’ and go to ‘Service Department’ to request an RA
number). A Return Authorization (RA) number must be issued before any product is returned to Extech. The
sender is responsible for shipping charges, freight, insurance and proper packaging to prevent damage in
transit. This warranty does not apply to defects resulting from action of the user such as misuse, improper
wiring, operation outside of specification, improper maintenance or repair, or unauthorized modification.
Extech specifically disclaims any implied warranties or merchantability or fitness for a specific purpose and
will not be liable for any direct, indirect, incidental or consequential damages. Extech's total liability is limited
to repair or replacement of the product. The warranty set forth above is inclusive and no other warranty,
whether written or oral, is expressed or implied.
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.
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
Electret condensor (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
Meets ANSI and IEC Type 2
External Calibrator
Extech models 407766 or 407744
9V Battery; 20 hour battery life (typical) with low battery
3.2 x 10.1 x 1.5” (80 x 256 x 38mm) / 8.5 oz. (240g)
Model 407750 Version 2.1 February 2004
Meter Description
LCD Display
ON/OFF key
REC (Record) key
MAXHLD (Max Hold) key
C/A Weighting Select key
BA (Background Absorber) key
F/S Fast / Slow Response select key
10. Backlit (LCD backlighting) key
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)
Model 407750 Version 2.1 February 2004
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.
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.
Model 407750 Version 2.1 February 2004
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
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
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
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 MIN/MAX keys
2. When ‘n’ appears on the display, release the MIN/MAX and then the ON/OFF key.
3. The Auto Power Off feature is now disabled. Note that the Auto Power Off feature is reactivated the next time the meter is powered down.
Model 407750 Version 2.1 February 2004
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 software
for data acquisition are sold separately. Detailed instructions are provided with the
Model 407750 Version 2.1 February 2004
Reference Information
Frequency Weighting Characteristics
Frequency (Hz)
A Weighting
C Weighting
Tolerance (IEC 651 Type 2)
Typical A-Weighted Sound Levels
50HP Siren (100')
Jet takeoff (200')
Riveting machine
Subway (20')
Pneumatic drill (50')
Vacuum cleaner (10')
Large store
Small office
Night residential area
Whisper (5'0)
Chain saw
Boiler room
Freight train (100')
Speech (1')
Large Office
Sound studio
Threshold of hearing
Model 407750 Version 2.1 February 2004
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
Calibration and Repair
The meter includes a Calibration adjustment on the bottom of the meter. A sound level
calibrator must be used to complete a calibration. Extech can supply a calibrator for this
purpose. Once the Calibrator is affixed on the microphone, adjust the calibration screw so
that the LCD sound level display matches the calibrator’s output signal.
Extech offers repair and calibration services for the products we sell. Extech also provides
NIST certification for most products. Call the Customer Service Department for information
on calibration services available for this product. Extech recommends that annual
calibrations be performed to verify meter performance and accuracy.
Support Hotline (781) 890-7440
Tech support: Ext. 200; Email: [email protected]
Repair/Returns: Ext. 210; Email: [email protected]
Website: www.extech.com
Copyright © 2004 Extech Instruments Corporation
All rights reserved including the right of reproduction in whole or in part in any form.
Model 407750 Version 2.1 February 2004