How would you calibrate a sound level meter

[Norm Dresner]

Any microphone with a suitable amplifier and an AC voltmeter makes a sound
level meter. Suppose I wanted to make one to set up a stereo system where
the input is single tone at a time and the goal is to adjust a multichannel
equalizer to get approximately overall flat response from voltage/power in
to sound level out. I don't need OSHA or any particular "weighting" scheme.
Can someone give me a rough idea of the sequence necessary to calibrate
something like this?

TIA
Norm

 

[James Meyer]

Any microphone with a suitable amplifier and an AC voltmeter makes a sound
level meter. Suppose I wanted to make one to set up a stereo system where
the input is single tone at a time and the goal is to adjust a multichannel
equalizer to get approximately overall flat response from voltage/power in
to sound level out. I don't need OSHA or any particular "weighting" scheme.
Can someone give me a rough idea of the sequence necessary to calibrate
something like this?

TIA
Norm

I don't think it is practical or necessary to calibrate such a device.

If I wanted a warm fuzzy feeling about the measurements, I would connect
the amplified audio from the microphone to a set of good headphones and make an
A-B comparison using some live audio source(s). If I couldn't hear a difference
between the source directly or through the 'phones, I would assume the sound
level meter is "flat" or "calibrated".

Jim

 

[BobGardner]

the input is single tone at a time and the goal is to adjust a multichannel

equalizer to get approximately overall flat response

============================
If you try to use tones to eq a room, there are nulls all over because of the
standing waves. You move the mic a couple inches and the level is off by 10dB.
Room analyzers put nice flat pink noise out in the speakers, and then all the
leds on the analyzer should be about flat. If you use a tone, you could warble
it back and forth half an octave, and then the spl meter would read the avg
level in that octave.

 

[Pooh Bear]

Any microphone with a suitable amplifier and an AC voltmeter makes a sound
level meter. Suppose I wanted to make one to set up a stereo system where
the input is single tone at a time and the goal is to adjust a multichannel
equalizer to get approximately overall flat response from voltage/power in
to sound level out. I don't need OSHA or any particular "weighting" scheme.
Can someone give me a rough idea of the sequence necessary to calibrate
something like this?

If you're only making 'relative' adjustments at various frequencies then the
meter doesn't need to be calibrated.

3 problems.

1. Only 'measurement grade' mics have a suitably flat frequency response.

2. Room acoustics result in all manner of reflections and 'interference'
that make the type of measurement you suggest impractical.

3. EQing on a graphic equaliser to get a flat response rarely gives a great
sounding result in any case. The result will only be valid for a single
measurment point anyway.


You might like to google for 'smaart live' a program that's used to eq live
sound systems that'll run on a PC. There's a free eval version.

In the end - use as much EQ as you like so it 'sounds ok' !


Graham

 

[Norm Dresner]

I originally wrote:

Any microphone with a suitable amplifier and an AC voltmeter makes a sound
level meter. Suppose I wanted to make one to set up a stereo system where
the input is single tone at a time and the goal is to adjust a multichannel
equalizer to get approximately overall flat response from voltage/power in
to sound level out. I don't need OSHA or any particular "weighting" scheme.
Can someone give me a rough idea of the sequence necessary to calibrate
something like this?

and several responders cuationed me that room acoustics would make the
measurement process at best questionable.

But perhaps I simply gave a bad application of a perfectly good instrument
so let me try again. I have several speakers of differing
acoustic-electrical efficiency by which I mean that in a free-field
environment the acoustic energy output per unit of electrical energy input
differs -- the usual measurements come from the factory in the form of dB
SPL re 1 meter per 1 watt input but those details are unimportant. What is
important is that I have several speakers, each with a restricted frequency
range, which I want to use to create an array with flat response, i.e. the
acoustic energy output per unit electrical energy input is approximately
constant across the relevant bandwidth. This is usually done with
cross-over networks which are in reality nothing but electical bandpass
filters. But in terms of matching the acoustic energy from the various
speakers I also need to attenuate the response of some of them relative to
the others. In general, tweeters are much more energy efficient than
woofers, sometimes as much as a decade in power. To do the matching I want
to be able to measure the acoustic energy out of each speaker as a function
of its electrical input. SO ...
How would one go about calibrating a sound-level meter?

Norm

 

[Walter Harley]

[...]
But perhaps I simply gave a bad application of a perfectly good instrument
so let me try again. I have several speakers of differing
acoustic-electrical efficiency by which I mean that in a free-field
environment the acoustic energy output per unit of electrical energy input
differs -- the usual measurements come from the factory in the form of dB
SPL re 1 meter per 1 watt input but those details are unimportant. What
is
important is that I have several speakers, each with a restricted
frequency
range, which I want to use to create an array with flat response, i.e. the
acoustic energy output per unit electrical energy input is approximately
constant across the relevant bandwidth. This is usually done with
cross-over networks which are in reality nothing but electical bandpass
filters. But in terms of matching the acoustic energy from the various
speakers I also need to attenuate the response of some of them relative to
the others. In general, tweeters are much more energy efficient than
woofers, sometimes as much as a decade in power. To do the matching I
want
to be able to measure the acoustic energy out of each speaker as a
function
of its electrical input. SO ...
How would one go about calibrating a sound-level meter?

I still don't understand why you need absolute calibration. Seems to me all
that matters is that you get them all equal; for that all you need is a flat
mic and flat electronics (and a free-field testing environment).

But if you insist: there are devices you can buy, for under $100, that
produce reference sound levels (inside a sealed cylinder sized to fit the
reference mic). Check audio gear suppliers, like Musician's Friend,
Sweetwater, etc.; or, I'm sure I saw one in the latest MCM catalog. They're
called "microphone calibrators".

Frankly I'll be awfully surprised if this approach actually leads to a
speaker system with flat frequency response in your listening environment.
Seems to me it would be much more productive to use the spectrum analyzer
approach already suggested, with pink noise. I don't see how you can
meaningfully test the speaker response out of the enclosure and sans
electronics, because those things interact. But I suppose it wouldn't be
the first time I've been awfully surprised.

 

[mike]

I originally wrote:




and several responders cuationed me that room acoustics would make the
measurement process at best questionable.

But perhaps I simply gave a bad application of a perfectly good instrument
so let me try again. I have several speakers of differing
acoustic-electrical efficiency by which I mean that in a free-field
environment the acoustic energy output per unit of electrical energy input
differs -- the usual measurements come from the factory in the form of dB
SPL re 1 meter per 1 watt input but those details are unimportant. What is
important is that I have several speakers, each with a restricted frequency
range, which I want to use to create an array with flat response, i.e. the
acoustic energy output per unit electrical energy input is approximately
constant across the relevant bandwidth. This is usually done with
cross-over networks which are in reality nothing but electical bandpass
filters. But in terms of matching the acoustic energy from the various
speakers I also need to attenuate the response of some of them relative to
the others. In general, tweeters are much more energy efficient than
woofers, sometimes as much as a decade in power. To do the matching I want
to be able to measure the acoustic energy out of each speaker as a function
of its electrical input. SO ...
How would one go about calibrating a sound-level meter?

Norm

I've spent a LOT of time in the past messing with speaker systems.
1) you don't need absolute calibration. You just need flat frequency
response.
2) for multiple speaker systems, you'll have phasing problems out the
wazoo. Measuring one speaker at a time won't tell you the whole story.
3) Room acoustics make tone measurements USELESS, USELESSS, USELESS.
You'll have a much better chance getting good sounding results with pink
noise.

If I were gonna do this again, I'd use one of the sound programs to make
a pink noise CD. I'd play that CD in my system as the source.

I'd use that same sound program with a "flat" microphone into my sound
card to view the result.

It was 20 years ago and all that was done with dedicated hardware, but
the results were the same...I pitched the equalizer and all the random
speakers. Went out and bought some good speakers.

mike

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[Pooh Bear]

several responders cuationed me that room acoustics would make the
measurement process at best questionable.

Indeed. You would need to measure ina 'free field' environment to make the
results meaningful. Out of doors would be reasonably ok.

I have several speakers, each with a restricted frequency
range, which I want to use to create an array with flat response

This is not a trivial task. Many ppl, have been here before you. Kinda like
looking for the Holy Grail.


Graham

 

[Mook Johnson]

If you want to do it right and not too Xpensive, put that Radio Shack SPL
meter aside and use this.

http://www.speakerworkshop.com/

then go here to learn how to build the jigs and how to take proper
measurements.

http://www.speakerworkshop.com/Links.htm

the RS meter has a response curve that is not linear. There are pages that
show you ho to make it linear but you are likely better off just building a
preamp for a $3 panasonic electro mic that IS linear (enough).

good luck

 

[Leon Heller]

If you want to do it right and not too Xpensive, put that Radio Shack SPL
meter aside and use this.

http://www.speakerworkshop.com/

then go here to learn how to build the jigs and how to take proper
measurements.

http://www.speakerworkshop.com/Links.htm

the RS meter has a response curve that is not linear. There are pages that
show you ho to make it linear but you are likely better off just building a
preamp for a $3 panasonic electro mic that IS linear (enough).

Knowles makes very good electret mics, also, that are suitable for accurate
measurements. Where I used to work our acoustic division often used them in
the lab as they were much cheaper than the ones supplied with their
measuring equipment.

Leon

 

[mike]

I originally wrote:




and several responders cuationed me that room acoustics would make the
measurement process at best questionable.

But perhaps I simply gave a bad application of a perfectly good instrument
so let me try again. I have several speakers of differing
acoustic-electrical efficiency by which I mean that in a free-field
environment the acoustic energy output per unit of electrical energy input
differs -- the usual measurements come from the factory in the form of dB
SPL re 1 meter per 1 watt input but those details are unimportant. What is
important is that I have several speakers, each with a restricted frequency
range, which I want to use to create an array with flat response, i.e. the
acoustic energy output per unit electrical energy input is approximately
constant across the relevant bandwidth. This is usually done with
cross-over networks which are in reality nothing but electical bandpass
filters. But in terms of matching the acoustic energy from the various
speakers I also need to attenuate the response of some of them relative to
the others. In general, tweeters are much more energy efficient than
woofers, sometimes as much as a decade in power. To do the matching I want
to be able to measure the acoustic energy out of each speaker as a function
of its electrical input. SO ...
How would one go about calibrating a sound-level meter?

Norm

You're still focusing on the wrong problem.
How do you intend to attenuate the drive to the speakers? Unless you
plan to have an amplifier for each one, your attenuator is gonna really
mess things up. That's why purists don't like passive crossovers.
Attenuators are much WORSE.

There are phase shifts. In the crossover, in the attenuator, due to the
relative placement of speakers. You need to measure amplitude and phase
from the listening point to all speakers. The more speakers you cobble
together, the smaller the sweet spot will be...well, I guess there's a
limiting case with an infinite number of speakers in an infinite space
that might work well...

Try this experiment. Put pink noise into your speakers. Use the sound
card spectrum analzyer on the microphone. Experiment with the high
frequency response as you translate/rotate the microphone in space.
Also see what happens if you put a small flat plate behind the
microphone. Just the shape of the microphone and any box that it might
be in can have major effects on the measurement.

Many have gone where you're going. Some have never come back ;-)

Having said all that, it appears that you won't be satisfied until you
calibrate your microphone.
If you're in Portland, Oregon, I have a General Radio Type 1562
Sound-Level Calibrator that we could try out. It put out sound, but
I've never felt the need to use it.
mike


--
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Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
Compaq Aero floppy,ram,battery.
MINT HP-41CV, 2-METER AMPS, 200CH SCANNER
http://www.geocities.com/SiliconValley/Monitor/4710/

 

[Norm Dresner]

You're still focusing on the wrong problem.
How do you intend to attenuate the drive to the speakers? Unless you
plan to have an amplifier for each one, your attenuator is gonna really
mess things up. That's why purists don't like passive crossovers.
Attenuators are much WORSE.

Yes, ultimtely I plan to have a bi- or tri-amplified system but wanted a a
way to determine the gain/attenuation in each channel.

...

Having said all that, it appears that you won't be satisfied until you
calibrate your microphone.
If you're in Portland, Oregon, I have a General Radio Type 1562
Sound-Level Calibrator that we could try out. It put out sound, but
I've never felt the need to use it.
mike

Sadly, I'm on the other coast in MD. But I'll see if I can find something
like that.

Thanks for your comments

Norm

 

[legg]

I originally wrote:


scheme.

You are attempting to calibrate the amplifier and loudspeaker in the
described exercise.

To do this, the measuring equipment, signal source and test setup must
be pre-calibrated and standardized to your application, even to the
point-source or physical shape of reception (say - location of ears on
a standard head).

If the measurement is made with simple VU or SPL measurement, then the
source must vary over the frequency range of interest - say a slow
sweep or spot signals.

Formatted signals for this are available from

http://www.marchandelec.com/sweeps.htm

http://62.242.11.12/midiutil/index.htm

Whether what you get will be better than just dumping the equalizer
altogether is moot.

To calibrate metering - the standard technique is to compare it to
reference calibrated standards. The calibration standards are produced
by gnomes in secret laboratories in the Black Forest.

RL

 

[Norm Dresner]

To calibrate metering - the standard technique is to compare it to
reference calibrated standards. The calibration standards are produced
by gnomes in secret laboratories in the Black Forest.

Do you have a URL for the Black Forest ;-))

Norm

 

[John Woodgate]

(in <[email protected]>) about
'How would you calibrate a sound level meter RESTATING THE PROBLEM', on
Wed, 1 Sep 2004:

Do you have a URL for the Black Forest ;-))

http://www.blackforest-airport.com/E/

 

[Jim Thompson]

(in <[email protected]>) about
'How would you calibrate a sound level meter RESTATING THE PROBLEM', on
Wed, 1 Sep 2004:
http://www.blackforest-airport.com/E/

See that smog. You're looking west into France.

...Jim Thompson

 

[mike]

Yes, ultimtely I plan to have a bi- or tri-amplified system but wanted a a
way to determine the gain/attenuation in each channel.

...




Sadly, I'm on the other coast in MD. But I'll see if I can find something
like that.

Thanks for your comments

Norm

There was one on ebay yesterday.
mike


--
Return address is VALID.
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
Compaq Aero floppy,ram,battery.
MINT HP-41CV, 2-METER AMPS, 200CH SCANNER
http://www.geocities.com/SiliconValley/Monitor/4710/