All
Search
Images
Videos
Maps
News
More
Shopping
Flights
Travel
Notebook
Report an inappropriate content
Please select one of the options below.
Not Relevant
Offensive
Adult
Child Sexual Abuse
Top suggestions for sound
Device
Monitor
Philips
Monitoring
Glucose
Devices
Air Monitoring
Devices
30-Day Heart Monitoring
Device
Pacemaker
Monitoring
Network Device
Monitor
Home Heart Monitoring
Devices
Sleep Monitoring
Devices
Device Performance
& Health
Cardiac Monitor
Device
Device Monitor
Application
Free Network Device
Monitor
Cardiac Monitoring
Device How to Use
Fall Devices
Training
Android Device
Monitor
Remote Monitoring
Devices
Electronic Monitoring
Devices
Wireless Monitoring
Device
What Is a Heart Monitoring
Device
Medical Monitor
Devices
Length
All
Short (less than 5 minutes)
Medium (5-20 minutes)
Long (more than 20 minutes)
Date
All
Past 24 hours
Past week
Past month
Past year
Resolution
All
Lower than 360p
360p or higher
480p or higher
720p or higher
1080p or higher
Source
All
Dailymotion
Vimeo
Metacafe
Hulu
VEVO
Myspace
MTV
CBS
Fox
CNN
MSN
Price
All
Free
Paid
Clear filters
SafeSearch:
Moderate
Strict
Moderate (default)
Off
Filter
Device
Monitor
Philips
Monitoring
Glucose
Devices
Air
Monitoring Devices
30-Day Heart
Monitoring Device
Pacemaker
Monitoring
Network Device
Monitor
Home Heart
Monitoring Devices
Sleep
Monitoring Devices
Device
Performance & Health
Cardiac Monitor
Device
Device
Monitor Application
Free Network
Device Monitor
Cardiac Monitoring Device
How to Use
Fall Devices
Training
Android Device
Monitor
Remote
Monitoring Devices
Electronic
Monitoring Devices
Wireless
Monitoring Device
What Is a Heart
Monitoring Device
Medical Monitor
Devices
Shop Sound Monitoring Device
Sponsored
$329.00
Extech SL130W Sound Level Alert With Alarm
Amazon.com
Free shipping
$329.00
Extech SL130W Sound Level Alert With Alarm
Amazon.com
Free shipping
$16.99
Decibel Meter Sound Level Reader 30-130Db(A), Hand-Held Sound Noise Meter With Backlight Alarm, Digital Noise Meter Decibel Monitoring Tester
Amazon.com
625
$16.99
Decibel Meter Sound Level Reader 30-130Db(A), Hand-Held Sound Noise Meter With Backlight Alarm, Digital Noise Meter Decibel Monitoring Tester
Amazon.com
625
$188.79
Extech SL510 Sound Level Meter
Amazon.com
335
Free shipping
$188.79
Extech SL510 Sound Level Meter
Amazon.com
335
$98.89
Decibel Meter Recorder 13 Inches Sound Level Meter, Wall Mount Noise Meter For Classroom Sound Meter With 16.4ft Sensor, Buzzer Alarm And Unlimited
Amazon.com
11
Free shipping
$98.89
Decibel Meter Recorder 13 Inches Sound Level Meter, Wall Mount Noise Meter For Classroom Sound Meter With 16.4ft Sensor, Buzzer Alarm And Unlimited
Amazon.com
11
$160.65
Extech Environmental-Series Type 2 Sound Level Meter | Cole-Parmer
Cole-Parmer
335
$160.65
Extech Environmental-Series Type 2 Sound Level Meter | Cole-Parmer
Cole-Parmer
335
$329.00
Extech Sound Level Meter With Led Alert And 110V Adapter | Cole-Parmer
Cole-Parmer
7
$329.00
Extech Sound Level Meter With Led Alert And 110V Adapter | Cole-Parmer
Cole-Parmer
7
$388.00
Traceable Sound Level Meter With Calibration | Cole-Parmer
Cole-Parmer
$388.00
Traceable Sound Level Meter With Calibration | Cole-Parmer
Cole-Parmer
$309.00
Extech 407760 USB Sound Level Datalogger
Amazon.com
340
Free shipping
$309.00
Extech 407760 USB Sound Level Datalogger
Amazon.com
340
$259.00
Extech Type-2 Digital Sound Level Meter With Backlit Display, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
300
$259.00
Extech Type-2 Digital Sound Level Meter With Backlit Display, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
300
$114.00
Extech 407730 Digital Sound Level Meter 40-130Db
Amazon.com
316
Free shipping
$114.00
Extech 407730 Digital Sound Level Meter 40-130Db
Amazon.com
316
$96.90
Extech Digital Sound Level Meter, 40 To 130 Db, 9" X 1.75" | Cole-Parmer
Cole-Parmer
316
$96.90
Extech Digital Sound Level Meter, 40 To 130 Db, 9" X 1.75" | Cole-Parmer
Cole-Parmer
316
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
Free shipping
46% off
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
Free shipping
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
Free shipping
50% off
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
Free shipping
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
Free shipping
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
Free shipping
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
Free shipping
50% off
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
Free shipping
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
Free shipping
66% off
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
Free shipping
$309.00
Extech USB Sound Level Datalogger, 1" X 1" | Cole-Parmer
Cole-Parmer
330
$309.00
Extech USB Sound Level Datalogger, 1" X 1" | Cole-Parmer
Cole-Parmer
330
$429.00
Extech Type 2 Digital Sound Level Meter Kit, 2.1" X 1.25" | Cole-Parmer
Cole-Parmer
298
$429.00
Extech Type 2 Digital Sound Level Meter Kit, 2.1" X 1.25" | Cole-Parmer
Cole-Parmer
298
$242.00
TQC Sheen Sound Level Meter
Industrial Physics
$242.00
TQC Sheen Sound Level Meter
Industrial Physics
$769.00
Extech Data Logging Sound Meter With Sd Card, 3" X 2" | Cole-Parmer
Cole-Parmer
330
$769.00
Extech Data Logging Sound Meter With Sd Card, 3" X 2" | Cole-Parmer
Cole-Parmer
330
$452.00
PCE-SLM 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$452.00
PCE-SLM 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$152.00
PCE-MSL 1 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$152.00
PCE-MSL 1 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$568.00
PCE-SLT-TRM PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$568.00
PCE-SLT-TRM PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$22.08
$39.69
GVDA Digital Sound Level Meter Audio Level Meter Sonometro Sound Meter Decibelimetro 30-130Db
Aliexpress
Free shipping
44% off
$22.08
$39.69
GVDA Digital Sound Level Meter Audio Level Meter Sonometro Sound Meter Decibelimetro 30-130Db
Aliexpress
Free shipping
$29.42
$57.68
Sound Level Meter High Precision Noise Sensor Decibel Meter 30-130DB Digital LCD Noise Monitor
Aliexpress
48% off
$29.42
$57.68
Sound Level Meter High Precision Noise Sensor Decibel Meter 30-130DB Digital LCD Noise Monitor
Aliexpress
$1,949.00
Casella Digital Logging Sound Level Meter Type 2 Kit, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
$1,949.00
Casella Digital Logging Sound Level Meter Type 2 Kit, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
$30.78
$60.35
POROMETISTO-SL01 Digital Noise Meter Handheld Sound Level Meter A/C Weighted 30130Db Audio DB Meter
Aliexpress
Free shipping
48% off
$30.78
$60.35
POROMETISTO-SL01 Digital Noise Meter Handheld Sound Level Meter A/C Weighted 30130Db Audio DB Meter
Aliexpress
Free shipping
$189.00
FLIR Extech SL510 TYPE 2 SOUND LEVEL METER
DigiKey
$189.00
FLIR Extech SL510 TYPE 2 SOUND LEVEL METER
DigiKey
$17.50
$32.87
Decibel Tester High-Precision Sound Level Meter Decibel Detector Handheld Noise Tester 30130Db
Aliexpress
Free shipping
46% off
$17.50
$32.87
Decibel Tester High-Precision Sound Level Meter Decibel Detector Handheld Noise Tester 30130Db
Aliexpress
Free shipping
$329.00
REED R8080 Data Logging Sound Level Meter With Bargraph
Globaltestsupply.com
2
$329.00
REED R8080 Data Logging Sound Level Meter With Bargraph
Globaltestsupply.com
2
$454.00
Extech Digital Sound Meter With NIST Calibration Certificate, 8.2" X 1.25" | Cole-Parmer
Cole-Parmer
$454.00
Extech Digital Sound Meter With NIST Calibration Certificate, 8.2" X 1.25" | Cole-Parmer
Cole-Parmer
$1,210.00
PCE-NDL 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$1,210.00
PCE-NDL 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$396.95
Testo 815 Sound Level Meter
Globaltestsupply.com
$396.95
Testo 815 Sound Level Meter
Globaltestsupply.com
$329.00
FLIR Extech SL130W SOUND LEVEL ALERT, WHITE HOUSING
DigiKey
$329.00
FLIR Extech SL130W SOUND LEVEL ALERT, WHITE HOUSING
DigiKey
5:30
YouTube
Learn Bright
What Is Sound? for Kids
How exactly does sound work? If a tree falls in a forest but no one is around to hear it, does it make a sound? In this video, you and your kids will be able to answer both those questions. There are sounds all around us. Sound is made from vibrations, which are a series of small, fast movements that move back and forth or from side to side ...
329.8K views
May 9, 2023
Sound Level Meter Uses
1:38
SOUND LEVEL METER | Definition and Operation | SVANTEK Academy
YouTube
SVANTEK Sp. z o.o.
23.4K views
Apr 27, 2023
2:36
How to Use the Sound Level Meter PCE-322A
YouTube
PCE Instruments UK
36.8K views
Mar 17, 2017
8:13
Sound Level Meter Explained !
YouTube
Online Acoustics
7.7K views
May 18, 2020
Top videos
11:36
Propagation of Sound
YouTube
Manocha Academy
998.6K views
Nov 21, 2018
1:45
Understanding Sound Waves | MED-EL
YouTube
MED-EL
272.8K views
Aug 16, 2013
9:41
What is Sound? The Fundamental Science Behind Sound
YouTube
Branch Education
614.1K views
Sep 25, 2018
Sound Level Meter Calibration
1:18
How To: Calibrate your Sound Level Meter
YouTube
REED Instruments
18.3K views
Feb 8, 2022
3:01
How to Calibrate the Pro Sound Level Meter With Calibrator (IC-QM1598)
YouTube
Instrument Choice
3.8K views
Jul 26, 2021
1:04
Sound Level Meter Calibration – SVANTEK Academy
YouTube
SVANTEK Sp. z o.o.
2.4K views
Mar 28, 2022
11:36
Propagation of Sound
998.6K views
Nov 21, 2018
YouTube
Manocha Academy
1:45
Understanding Sound Waves | MED-EL
272.8K views
Aug 16, 2013
YouTube
MED-EL
9:41
What is Sound? The Fundamental Science Behind Sound
614.1K views
Sep 25, 2018
YouTube
Branch Education
6:03
Sound Wave Experiments | Waves | Physics | FuseSchool
503.8K views
May 17, 2020
YouTube
FuseSchool - Global Education
1:00
20 FREE CINEMATIC WHOOSH SOUND EFFECTS | High quality 4
…
276.9K views
Aug 10, 2023
YouTube
Sound Effects 4K
5:08
GCSE Physics - Sound Waves and Hearing
451.2K views
Feb 26, 2020
YouTube
Cognito
3:46
Production of sound | Mechanical waves and sound | Physics | Kha
…
329.7K views
Dec 29, 2013
YouTube
khanacademymedicine
16:13
How Sound Works - The Physics of Sound Waves
195.6K views
May 3, 2015
YouTube
Mr. McMath
6:35
What is Sound Intensity? | Physics
119.7K views
Aug 19, 2020
YouTube
Najam Academy
See more videos
More like this
Shop Sound Monitoring Device
Sponsored
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
Free shipping
46% off
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
Free shipping
50% off
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
Free shipping
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
$309.00
Extech USB Sound Level Datalogger, 1" X 1" | Cole-Parmer
Cole-Parmer
330
$309.00
Extech USB Sound Level Datalogger, 1" X 1" | Cole-Parmer
Cole-Parmer
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
Free shipping
50% off
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
$429.00
Extech Type 2 Digital Sound Level Meter Kit, 2.1" X 1.25" | Cole-Parmer
Cole-Parmer
298
$429.00
Extech Type 2 Digital Sound Level Meter Kit, 2.1" X 1.25" | Cole-Parmer
Cole-Parmer
$769.00
Extech Data Logging Sound Meter With Sd Card, 3" X 2" | Cole-Parmer
Cole-Parmer
330
$769.00
Extech Data Logging Sound Meter With Sd Card, 3" X 2" | Cole-Parmer
Cole-Parmer
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
Free shipping
66% off
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
$1,949.00
Casella Digital Logging Sound Level Meter Type 2 Kit, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
$1,949.00
Casella Digital Logging Sound Level Meter Type 2 Kit, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
$242.00
TQC Sheen Sound Level Meter
Industrial Physics
$242.00
TQC Sheen Sound Level Meter
Industrial Physics
$454.00
Extech Digital Sound Meter With NIST Calibration Certificate, 8.2" X 1.25" | Cole-Parmer
Cole-Parmer
$454.00
Extech Digital Sound Meter With NIST Calibration Certificate, 8.2" X 1.25" | Cole-Parmer
Cole-Parmer
$452.00
PCE-SLM 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$452.00
PCE-SLM 10 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$152.00
PCE-MSL 1 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$152.00
PCE-MSL 1 PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$568.00
PCE-SLT-TRM PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$568.00
PCE-SLT-TRM PCE INSTRUMENTS - Test Equipment & Sound Level Meter Supplies
Radwell.com
$22.08
$39.69
GVDA Digital Sound Level Meter Audio Level Meter Sonometro Sound Meter Decibelimetro 30-130Db
Aliexpress
Free shipping
44% off
$22.08
$39.69
GVDA Digital Sound Level Meter Audio Level Meter Sonometro Sound Meter Decibelimetro 30-130Db
Aliexpress
$29.42
$57.68
Sound Level Meter High Precision Noise Sensor Decibel Meter 30-130DB Digital LCD Noise Monitor
Aliexpress
48% off
$29.42
$57.68
Sound Level Meter High Precision Noise Sensor Decibel Meter 30-130DB Digital LCD Noise Monitor
Aliexpress
Shop Sound Monitoring Device
Sponsored
$329.00
Extech SL130W Sound Level Alert With Alarm
Amazon.com
Free shipping
$329.00
Extech SL130W Sound Level Alert With Alarm
Amazon.com
Free shipping
$16.99
Decibel Meter Sound Level Reader 30-130Db(A), Hand-Held Sound Noise Meter With Backlight Alarm, Digital Noise Meter Decibel Monitoring Tester
Amazon.com
625
$16.99
Decibel Meter Sound Level Reader 30-130Db(A), Hand-Held Sound Noise Meter With Backlight Alarm, Digital Noise Meter Decibel Monitoring Tester
Amazon.com
625
$188.79
Extech SL510 Sound Level Meter
Amazon.com
335
Free shipping
$188.79
Extech SL510 Sound Level Meter
Amazon.com
335
$98.89
Decibel Meter Recorder 13 Inches Sound Level Meter, Wall Mount Noise Meter For Classroom Sound Meter With 16.4ft Sensor, Buzzer Alarm And Unlimited
Amazon.com
11
Free shipping
$98.89
Decibel Meter Recorder 13 Inches Sound Level Meter, Wall Mount Noise Meter For Classroom Sound Meter With 16.4ft Sensor, Buzzer Alarm And Unlimited
Amazon.com
11
$160.65
Extech Environmental-Series Type 2 Sound Level Meter | Cole-Parmer
Cole-Parmer
335
$160.65
Extech Environmental-Series Type 2 Sound Level Meter | Cole-Parmer
Cole-Parmer
335
$329.00
Extech Sound Level Meter With Led Alert And 110V Adapter | Cole-Parmer
Cole-Parmer
7
$329.00
Extech Sound Level Meter With Led Alert And 110V Adapter | Cole-Parmer
Cole-Parmer
7
$388.00
Traceable Sound Level Meter With Calibration | Cole-Parmer
Cole-Parmer
$388.00
Traceable Sound Level Meter With Calibration | Cole-Parmer
Cole-Parmer
$309.00
Extech 407760 USB Sound Level Datalogger
Amazon.com
340
Free shipping
$309.00
Extech 407760 USB Sound Level Datalogger
Amazon.com
340
$259.00
Extech Type-2 Digital Sound Level Meter With Backlit Display, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
300
$259.00
Extech Type-2 Digital Sound Level Meter With Backlit Display, 8.25" X 1.25" | Cole-Parmer
Cole-Parmer
300
$114.00
Extech 407730 Digital Sound Level Meter 40-130Db
Amazon.com
316
Free shipping
$114.00
Extech 407730 Digital Sound Level Meter 40-130Db
Amazon.com
316
$96.90
Extech Digital Sound Level Meter, 40 To 130 Db, 9" X 1.75" | Cole-Parmer
Cole-Parmer
316
$96.90
Extech Digital Sound Level Meter, 40 To 130 Db, 9" X 1.75" | Cole-Parmer
Cole-Parmer
316
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$224.56
Dwyer | SM-100 | Handheld Digital Sound Meter.
Blackhawk Supply
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
Free shipping
46% off
$22.83
$42.86
Handheld Decibel Meter, Digital Sound Level Meter 30-130Dba SPL Decibel Monitoring Tester Sound
Aliexpress
Free shipping
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
Free shipping
50% off
$24.07
$48.98
USB Charging 30-130Db A/C Weighted Digital Sound Level Meter Audio Level Meter Sound Meter
Aliexpress
Free shipping
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
Free shipping
$21.68
Digital Sound Level Meter Audio Level Meter Sound Meter Decibelimetro 30-130Db Decibel Meter
Aliexpress
Free shipping
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
Free shipping
50% off
$21.90
$44.56
SMART SENSOR Digital Sound Level Meter 30-130 DB Decibel Meter LCD Backlight Sonometer Noise Meter
Aliexpress
Free shipping
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$234.66
Bluetooth Sound Meter, NIST Certificate Of Calibration
Certified MTP
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
Free shipping
66% off
$57.96
$171.19
Nobsound AK2515 MIC Audio Spectrum Analyzer VFD Sound Level Meter VU Meter Screen Display Music
Aliexpress
Free shipping
Feedback
Feedback