Sound Level Meter

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This nifty sound level meter is a perfect one chip replacement for the standard analog meters. It is completely solid state and will never wear out. The whole circuit is based on the LM3915 audio level IC and uses only a few external components. This circuit can also be integrated into audio amp projects.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
C1	1	2.2uF 25V Electrolytic Capacitor
C2, C3	1	0.1uF Ceramic Disc Capacitor
R1, R3	2	1K 1/4W Resistor
R2	1	10K 1/4W Resistor
R4	1	100K 1/4W Resistor
R5	1	1M 1/4W Resistor
D1	1	1N914 Silicon Diode
Q1	1	2N3906 PNP Transistor
LED1-LED10	10	Standard LED or LED Array
U1	1	LM3915 Audio Level IC
MISC	1	Board, Wire, Socket For U1

Notes

V+ can be anywhere from 3V to 20V.

The input is designed for standard audio line voltage (1V P-P) and has a maximum input voltage of 1.3V.

Pin 9 can be disconnected from +V to make the circuit use a moving dot display instead of a bar graph display.

Thanks to help from the forum, this circuit has been improved from the original version to include a peak detector for a more stable and viewable output. This page has been updated with the new circuit.

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Comments

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Alan Sound Level Meter Sunday, October 12, 2014 4:58:58 AM

I want to make a sound level circuit for detecting noise source under a car bonnet (or wherever it might be). Can I use electret microphones with a switched input from mic to mic and thus see which is nearest the noise source?

anonymous Sound Level Meter Sunday, February 24, 2013 3:33:30 PM

I am having the problem "of about 0.6V to pin 5". Regarding "The solution is quite simple though.", please share. I am using a microphone into an LM386. Without the Peak Detector it works as expected. If at all possible I would like to stick the the LM386. The only purpose for my circuit is to make the bar graph respond when someone speaks into the mic. I am not using the sound in any way (not even playing it into a speaker). Thanks!

peter west Sound Level Meter Monday, July 09, 2012 3:30:55 AM

I agree that the peak circuit is shown in the data sheet. I also agree it does not work. There is a good reason why not - the Vbe for the transistor is about 680mv, while the voltage drop across the diode is only 120mv [because there isn't any current with no input] The resulting 500mv+ turns all the leds on, as a number of people has remarked. The solution is quite simple though.

anonymous Sound Level Meter Friday, July 06, 2012 3:20:18 PM

This circuit WILL NOT WORK for line level signals; there will be a constant voltage of about 0.6V to pin 5, which will keep the lowest LEDs on constantly. Don't bother building this - use an opamp detector instead.

Pio Sound Level Meter Tuesday, April 03, 2012 8:16:44 AM

could any1 help me make a sound level meter that goes up to eleven? 11 LEDs that is. can this b altered to do so? also is it possible to add a Pezio elctric microphone to this circuit diagram??

anonymous Sound Level Meter Thursday, December 01, 2011 2:46:05 PM

Does it matter if you use Carbon or Metal film resistors?
(Editor's notes: Nope.)

Dimitris Sound Level Meter Sunday, August 08, 2010 8:46:35 AM

I think that u connect 5 with 3. There is no connection between them. 9 is directly connected to 3. (" All LED switches on when power is connected")

anonymous Sound Level Meter Thursday, June 03, 2010 10:48:17 AM

Built this circuit but it does not work properly. All LED switches on when power is connected. Any workaround for this problem?

Eric Sound Level Meter Sunday, April 11, 2010 9:12:47 PM

I built this exact circuit using a 15V power supply and my LM3915 is overheating very badly. I am using a 1N4148 instead of the 1N914 but otherwise it is all the same. Does anyone have any ideas as to why this is happening? Also, before it gets too hot all the LEDs are lit when no audio signal is present. This would seem to indicate incorrect wiring but both I and another individual have checked it thoroughly. A photo of circuit with all wiring can be provided if needed.

jcs Sound Level Meter Wednesday, January 27, 2010 12:16:39 PM

Just received some RGB LED's from someone made by this company: http://www.lumiron.com/SPECTRO%20300RGB%20-%20T.html I would like to try to drive them with this Sound Level Meter circuit. The supply voltage for the LED's is 12 volts, so it should work fine. Any ideas or suggestions would be much appreciated. I figure 16.7 million colors is enough color to experiment with!!! LOL!!!

J & R Sound Level Meter Tuesday, November 17, 2009 8:41:26 PM

This circuit is basically the reference circuit and the reference peak detector from the LM3915 Datasheet. For those who claim it will not work, I was wondering, why then would the same circuit be contained in the LM3915 datasheet, from the manufacurer of the I.C.? The answer has to be that the circuit works. Actually, I've been building these for awhile (blame Mr.MIMs III) and they work fine. For the person who asked, the 1N4148 is a fine replacement for the 1N914 in this circuit (even the datasheet says so). Have a good day :-)

anonymous Sound Level Meter Sunday, August 23, 2009 4:02:39 AM

Yet another circuit on this site which doesn't work. Nice one..
(Editor's notes: Works fine. See multiple posts in the forum. )

anonymous Sound Level Meter Thursday, July 23, 2009 7:10:17 AM

can i connect it to a mike to check the audio level of a person singing into it(mike)?

Bradley Glen Sound Level Meter Saturday, July 04, 2009 11:49:05 AM

I tried this and the problem is that the diode is forward biased through the Diode from the supply . If any DC leaks onto pin 5 of the IC then you are not going to get the proper reading .

simone Sound Level Meter Friday, June 19, 2009 9:28:18 AM

The Q1's bias will not work if the Vin is less than about 0,6V... try an auto bias circuit using the V+ insead of Vin... Then remember that the D1 introduce a voltage drop of about 0.7 V

anonymous Sound Level Meter Saturday, March 21, 2009 5:00:25 PM

Putting an audio signal in makes no difference. Also when I connect the power all LEDs turn bright on too how can I fix this? Any help would be appriciated,

John Sound Level Meter Sunday, December 28, 2008 10:28:02 AM

Hi, i'm new in this kind of things, but have always loved electronics, actually thats what im going to study when i graduate, but i have two questions, all of those triangles pointing down are ground conections, right? And the second is what kind of input can i use and how do i connect it? Can i use a normal rca jack microphone, an electret microphone or what?

darkshadow6921 Sound Level Meter Tuesday, December 09, 2008 5:09:45 PM

Built several of these, Works great. Thanks!

joan_ece_student Sound Level Meter Thursday, October 09, 2008 12:03:06 PM

1N491 diode is not available here. can you give me another as a substitute because the circuit doesn't work. i just used 4148 diode. is it ok? pls response.. tnx!

anonymous Sound Level Meter Tuesday, August 26, 2008 8:23:39 PM

I have just built this project and i can't seem to get it to work. The problem is there are some many errors I could make here. I have a feeling it is either the PnP transistor, or the Silicon Diode. If u can help me out, plus email me.

anonymous Sound Level Meter Saturday, August 02, 2008 11:06:29 AM

looks nice but when i turn the power on all LEDs glow bright too, anyone knows how to fix it? thanks

Christiaan Sound Level Meter Tuesday, June 03, 2008 11:33:30 AM

Thanks for the circuit works well

anonymous Sound Level Meter Friday, May 23, 2008 1:36:23 AM

It works. Used 100k pot for R4.

anonymous Sound Level Meter Tuesday, May 20, 2008 9:11:28 AM

Thanks for the site, though im having a few problems with this circuit. I have tried making a stripboard of it and now just tried a PCB. WIth each, all of the LEDs light brightly when the voltage is supplied. Putting an audio signal in makes no difference. Any help would be appriciated, cheers.

Audioguru Sound Level Meter Friday, March 21, 2008 1:19:03 PM

The circuit will work with any little PNP transistor. The old version of the project doesn't work.

specky Sound Level Meter Sunday, January 13, 2008 3:26:11 PM

i'm finding it difficult to get the transistor so pls send me the circiut diagram of the older version

anonymous Sound Level Meter Tuesday, November 13, 2007 4:28:57 AM

PLEASE help me to get dB METER for 0 - 150 dB, Thanks.

anonymous Sound Level Meter Wednesday, October 24, 2007 8:35:49 AM

I have not built circuit but for those who say it doesn't work may need some audio signal info. Pro-Audio vs Consumer Product Audio line levels are different. Consumer product line standard outputs is -10dBv = .316 Vrms = 0.9 Vp-p. ProAudio line outputs is 1.78dBv = 1.23Vrms = 3.47 Vp-p. So this circuit is more appropriate for cheap consumer products to monitor RCA outputs. You should consult your equipment for actual output levels. e.g. Tape monitor output on my stereo is .150 Vrms and VCR output is 1.0 Vp-p and my mixer line outputs is 1.55 Vp-p when controls are at 70%. Looking at the circuit I would think the right diode would be important. Don't regular silicon diodes drop .7v when forward biased.

anonydr mahamad zayedmous Sound Level Meter Saturday, October 13, 2007 11:00:20 PM

dear sirs i would like to know how to measure soud level meters frpm dental turbine? thanks urs dr mahamad

anonymous Sound Level Meter Sunday, September 30, 2007 5:44:48 PM

give correct circuit and stop deceivine us the beginners.it doesn't work

harHar Sound Level Meter Saturday, September 22, 2007 2:02:23 AM

is the diagarm works?somebody said its not.! can pls do me a schematic diagram like that(sound level meter) that can use as switch of an airCondition unit depending on the noise of a room...

anonymous Sound Level Meter Wednesday, September 05, 2007 1:34:44 PM

nonsense circuit

anonymous Sound Level Meter Sunday, August 12, 2007 8:13:27 AM

it does not work

anonymous Sound Level Meter Friday, July 27, 2007 8:45:58 AM

is the sound level meter's readings/findings displayed?

marc Sound Level Meter Tuesday, July 24, 2007 9:58:11 PM

i would like to build an SPL meter that has an output in terms of decibel readings. Say if the noise reading reached 80dB a voice alarm will be activated. how will im able to do this. can you send me a circuit diagram for that kind of design. thanks!!!

jamiro Sound Level Meter Wednesday, July 11, 2007 7:57:26 AM

The sound lever was was quite good and it all worked out well with an amplifier of high wattage.I built the circuit and was nice.

Tim Sound Level Meter Monday, July 02, 2007 10:54:52 AM

I would like to make a "fun" circuit that would sound an alarm when the noise in a room gets to loud. How would you suggest I do this?

Yvon Chartrand Sound Level Meter Sunday, July 01, 2007 11:45:35 AM

I have built it and it does not work, probably because I try to monitor the line level of the amplifier, and maybe because the input impedance should be adjusted. I do not know how to correct the situation. Any one can help me?Thanks for reading. Yvon Chartrand

tambish. Sound Level Meter Monday, June 25, 2007 3:50:37 AM

ok i think this is a nice circuit.. i make this as my project..

Leandro Sound Level Meter Friday, June 15, 2007 8:14:37 AM

it doesn t work...contect me for very circuit

Ali - Bahrain Sound Level Meter Thursday, May 31, 2007 2:15:38 AM

The one thing I need to know is how to adjust this circuit to adabt with my sound source .. Because as you know, the audio which comes from 50W amplifier is too larger than 2W amplifier, so I think we need to a variable resistor to adjust the strength of income signal & then the IC is programed with correct sound level. This chip "LM3915" provides a logarithmic 3 dB/step analog display >> Anyway, I don't construct this circuit at the present; maybe I do it in the future.

aina Sound Level Meter Sunday, May 20, 2007 9:15:10 AM

Could you give me more details of this circuit

GAYAN Sound Level Meter Thursday, May 17, 2007 12:50:18 AM

VERY GOOD CRICUIT

samad Sound Level Meter Thursday, May 03, 2007 7:03:15 AM

Sir plz full description of sound level meter and operation of circuit

paola Sound Level Meter Monday, April 30, 2007 2:53:21 PM

what could be wrong if all the leds are on? how does it soupuse to capture sound ? i think i need more information could you help me?

H.Frank Sound Level Meter Thursday, April 05, 2007 11:42:12 AM

if you are having trouble making this work, one problem may be your LED orientation. diodes work only when orientation is correct, the positive side (longer lead) should be on the rail side and not the IC side. LED orientation is denoted in the same way that normal diode orientation is noted. but people tend to ignore this notation for LEDs.

Al-Ameen Sound Level Meter Friday, March 30, 2007 7:27:25 AM

it works make sure you connections are proper and correct diode directions

ArbStar Sound Level Meter Friday, March 30, 2007 7:17:47 AM

VERY Good Great thing I did it for science fair project :)

ArbStar Sound Level Meter Saturday, March 24, 2007 4:30:17 PM

It Doesn't work !!

Davis Sound Level Meter Thursday, March 22, 2007 6:18:42 PM

Doesn't Work!!!!

b.jagadeesh kumar Sound Level Meter Saturday, March 03, 2007 5:55:26 AM

very good 9885740831

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Alan	Sound Level Meter	Sunday, October 12, 2014 4:58:58 AM
I want to make a sound level circuit for detecting noise source under a car bonnet (or wherever it might be). Can I use electret microphones with a switched input from mic to mic and thus see which is nearest the noise source?
anonymous	Sound Level Meter	Sunday, February 24, 2013 3:33:30 PM
I am having the problem "of about 0.6V to pin 5". Regarding "The solution is quite simple though.", please share. I am using a microphone into an LM386. Without the Peak Detector it works as expected. If at all possible I would like to stick the the LM386. The only purpose for my circuit is to make the bar graph respond when someone speaks into the mic. I am not using the sound in any way (not even playing it into a speaker). Thanks!
peter west	Sound Level Meter	Monday, July 09, 2012 3:30:55 AM
I agree that the peak circuit is shown in the data sheet. I also agree it does not work. There is a good reason why not - the Vbe for the transistor is about 680mv, while the voltage drop across the diode is only 120mv [because there isn't any current with no input] The resulting 500mv+ turns all the leds on, as a number of people has remarked. The solution is quite simple though.
anonymous	Sound Level Meter	Friday, July 06, 2012 3:20:18 PM
This circuit WILL NOT WORK for line level signals; there will be a constant voltage of about 0.6V to pin 5, which will keep the lowest LEDs on constantly. Don't bother building this - use an opamp detector instead.
Pio	Sound Level Meter	Tuesday, April 03, 2012 8:16:44 AM
could any1 help me make a sound level meter that goes up to eleven? 11 LEDs that is. can this b altered to do so? also is it possible to add a Pezio elctric microphone to this circuit diagram??
anonymous	Sound Level Meter	Thursday, December 01, 2011 2:46:05 PM
Does it matter if you use Carbon or Metal film resistors? (Editor's notes: Nope.)
Dimitris	Sound Level Meter	Sunday, August 08, 2010 8:46:35 AM
I think that u connect 5 with 3. There is no connection between them. 9 is directly connected to 3. (" All LED switches on when power is connected")
anonymous	Sound Level Meter	Thursday, June 03, 2010 10:48:17 AM
Built this circuit but it does not work properly. All LED switches on when power is connected. Any workaround for this problem?
Eric	Sound Level Meter	Sunday, April 11, 2010 9:12:47 PM
I built this exact circuit using a 15V power supply and my LM3915 is overheating very badly. I am using a 1N4148 instead of the 1N914 but otherwise it is all the same. Does anyone have any ideas as to why this is happening? Also, before it gets too hot all the LEDs are lit when no audio signal is present. This would seem to indicate incorrect wiring but both I and another individual have checked it thoroughly. A photo of circuit with all wiring can be provided if needed.
jcs	Sound Level Meter	Wednesday, January 27, 2010 12:16:39 PM
Just received some RGB LED's from someone made by this company: http://www.lumiron.com/SPECTRO%20300RGB%20-%20T.html I would like to try to drive them with this Sound Level Meter circuit. The supply voltage for the LED's is 12 volts, so it should work fine. Any ideas or suggestions would be much appreciated. I figure 16.7 million colors is enough color to experiment with!!! LOL!!!
J & R	Sound Level Meter	Tuesday, November 17, 2009 8:41:26 PM
This circuit is basically the reference circuit and the reference peak detector from the LM3915 Datasheet. For those who claim it will not work, I was wondering, why then would the same circuit be contained in the LM3915 datasheet, from the manufacurer of the I.C.? The answer has to be that the circuit works. Actually, I've been building these for awhile (blame Mr.MIMs III) and they work fine. For the person who asked, the 1N4148 is a fine replacement for the 1N914 in this circuit (even the datasheet says so). Have a good day :-)
anonymous	Sound Level Meter	Sunday, August 23, 2009 4:02:39 AM
Yet another circuit on this site which doesn't work. Nice one.. (Editor's notes: Works fine. See multiple posts in the forum. )
anonymous	Sound Level Meter	Thursday, July 23, 2009 7:10:17 AM
can i connect it to a mike to check the audio level of a person singing into it(mike)?
Bradley Glen	Sound Level Meter	Saturday, July 04, 2009 11:49:05 AM
I tried this and the problem is that the diode is forward biased through the Diode from the supply . If any DC leaks onto pin 5 of the IC then you are not going to get the proper reading .
simone	Sound Level Meter	Friday, June 19, 2009 9:28:18 AM
The Q1's bias will not work if the Vin is less than about 0,6V... try an auto bias circuit using the V+ insead of Vin... Then remember that the D1 introduce a voltage drop of about 0.7 V
anonymous	Sound Level Meter	Saturday, March 21, 2009 5:00:25 PM
Putting an audio signal in makes no difference. Also when I connect the power all LEDs turn bright on too how can I fix this? Any help would be appriciated,
John	Sound Level Meter	Sunday, December 28, 2008 10:28:02 AM
Hi, i'm new in this kind of things, but have always loved electronics, actually thats what im going to study when i graduate, but i have two questions, all of those triangles pointing down are ground conections, right? And the second is what kind of input can i use and how do i connect it? Can i use a normal rca jack microphone, an electret microphone or what?
darkshadow6921	Sound Level Meter	Tuesday, December 09, 2008 5:09:45 PM
Built several of these, Works great. Thanks!
joan_ece_student	Sound Level Meter	Thursday, October 09, 2008 12:03:06 PM
1N491 diode is not available here. can you give me another as a substitute because the circuit doesn't work. i just used 4148 diode. is it ok? pls response.. tnx!
anonymous	Sound Level Meter	Tuesday, August 26, 2008 8:23:39 PM
I have just built this project and i can't seem to get it to work. The problem is there are some many errors I could make here. I have a feeling it is either the PnP transistor, or the Silicon Diode. If u can help me out, plus email me.
anonymous	Sound Level Meter	Saturday, August 02, 2008 11:06:29 AM
looks nice but when i turn the power on all LEDs glow bright too, anyone knows how to fix it? thanks
Christiaan	Sound Level Meter	Tuesday, June 03, 2008 11:33:30 AM
Thanks for the circuit works well
anonymous	Sound Level Meter	Friday, May 23, 2008 1:36:23 AM
It works. Used 100k pot for R4.
anonymous	Sound Level Meter	Tuesday, May 20, 2008 9:11:28 AM
Thanks for the site, though im having a few problems with this circuit. I have tried making a stripboard of it and now just tried a PCB. WIth each, all of the LEDs light brightly when the voltage is supplied. Putting an audio signal in makes no difference. Any help would be appriciated, cheers.
Audioguru	Sound Level Meter	Friday, March 21, 2008 1:19:03 PM
The circuit will work with any little PNP transistor. The old version of the project doesn't work.
specky	Sound Level Meter	Sunday, January 13, 2008 3:26:11 PM
i'm finding it difficult to get the transistor so pls send me the circiut diagram of the older version
anonymous	Sound Level Meter	Tuesday, November 13, 2007 4:28:57 AM
PLEASE help me to get dB METER for 0 - 150 dB, Thanks.
anonymous	Sound Level Meter	Wednesday, October 24, 2007 8:35:49 AM
I have not built circuit but for those who say it doesn't work may need some audio signal info. Pro-Audio vs Consumer Product Audio line levels are different. Consumer product line standard outputs is -10dBv = .316 Vrms = 0.9 Vp-p. ProAudio line outputs is 1.78dBv = 1.23Vrms = 3.47 Vp-p. So this circuit is more appropriate for cheap consumer products to monitor RCA outputs. You should consult your equipment for actual output levels. e.g. Tape monitor output on my stereo is .150 Vrms and VCR output is 1.0 Vp-p and my mixer line outputs is 1.55 Vp-p when controls are at 70%. Looking at the circuit I would think the right diode would be important. Don't regular silicon diodes drop .7v when forward biased.
anonydr mahamad zayedmous	Sound Level Meter	Saturday, October 13, 2007 11:00:20 PM
dear sirs i would like to know how to measure soud level meters frpm dental turbine? thanks urs dr mahamad
anonymous	Sound Level Meter	Sunday, September 30, 2007 5:44:48 PM
give correct circuit and stop deceivine us the beginners.it doesn't work
harHar	Sound Level Meter	Saturday, September 22, 2007 2:02:23 AM
is the diagarm works?somebody said its not.! can pls do me a schematic diagram like that(sound level meter) that can use as switch of an airCondition unit depending on the noise of a room...
anonymous	Sound Level Meter	Wednesday, September 05, 2007 1:34:44 PM
nonsense circuit
anonymous	Sound Level Meter	Sunday, August 12, 2007 8:13:27 AM
it does not work
anonymous	Sound Level Meter	Friday, July 27, 2007 8:45:58 AM
is the sound level meter's readings/findings displayed?
marc	Sound Level Meter	Tuesday, July 24, 2007 9:58:11 PM
i would like to build an SPL meter that has an output in terms of decibel readings. Say if the noise reading reached 80dB a voice alarm will be activated. how will im able to do this. can you send me a circuit diagram for that kind of design. thanks!!!
jamiro	Sound Level Meter	Wednesday, July 11, 2007 7:57:26 AM
The sound lever was was quite good and it all worked out well with an amplifier of high wattage.I built the circuit and was nice.
Tim	Sound Level Meter	Monday, July 02, 2007 10:54:52 AM
I would like to make a "fun" circuit that would sound an alarm when the noise in a room gets to loud. How would you suggest I do this?
Yvon Chartrand	Sound Level Meter	Sunday, July 01, 2007 11:45:35 AM
I have built it and it does not work, probably because I try to monitor the line level of the amplifier, and maybe because the input impedance should be adjusted. I do not know how to correct the situation. Any one can help me?Thanks for reading. Yvon Chartrand
tambish.	Sound Level Meter	Monday, June 25, 2007 3:50:37 AM
ok i think this is a nice circuit.. i make this as my project..
Leandro	Sound Level Meter	Friday, June 15, 2007 8:14:37 AM
it doesn t work...contect me for very circuit
Ali - Bahrain	Sound Level Meter	Thursday, May 31, 2007 2:15:38 AM
The one thing I need to know is how to adjust this circuit to adabt with my sound source .. Because as you know, the audio which comes from 50W amplifier is too larger than 2W amplifier, so I think we need to a variable resistor to adjust the strength of income signal & then the IC is programed with correct sound level. This chip "LM3915" provides a logarithmic 3 dB/step analog display >> Anyway, I don't construct this circuit at the present; maybe I do it in the future.
aina	Sound Level Meter	Sunday, May 20, 2007 9:15:10 AM
Could you give me more details of this circuit
GAYAN	Sound Level Meter	Thursday, May 17, 2007 12:50:18 AM
VERY GOOD CRICUIT
samad	Sound Level Meter	Thursday, May 03, 2007 7:03:15 AM
Sir plz full description of sound level meter and operation of circuit
paola	Sound Level Meter	Monday, April 30, 2007 2:53:21 PM
what could be wrong if all the leds are on? how does it soupuse to capture sound ? i think i need more information could you help me?
H.Frank	Sound Level Meter	Thursday, April 05, 2007 11:42:12 AM
if you are having trouble making this work, one problem may be your LED orientation. diodes work only when orientation is correct, the positive side (longer lead) should be on the rail side and not the IC side. LED orientation is denoted in the same way that normal diode orientation is noted. but people tend to ignore this notation for LEDs.
Al-Ameen	Sound Level Meter	Friday, March 30, 2007 7:27:25 AM
it works make sure you connections are proper and correct diode directions
ArbStar	Sound Level Meter	Friday, March 30, 2007 7:17:47 AM
VERY Good Great thing I did it for science fair project :)
ArbStar	Sound Level Meter	Saturday, March 24, 2007 4:30:17 PM
It Doesn't work !!
Davis	Sound Level Meter	Thursday, March 22, 2007 6:18:42 PM
Doesn't Work!!!!
b.jagadeesh kumar	Sound Level Meter	Saturday, March 03, 2007 5:55:26 AM
very good 9885740831