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How to calibrate REW or Omnimic's SPL with a smoke detector.

HighImpactAV

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I recently purchased some more smoke detectors and found that they emit an 85 dB signal (at 10 ft) at 3200 Hz. You can read more in The Audibility of Smoke Alarms in Residential Homes. You can use the smoke detector to set REW or Omnimic with a fair bit of accuracy. This is because smoke detectors must conform to ANSI, UL, and NFPA standards.

Open REW's SPL Meter, click "Calibrate," and select "Use an external signal." Make sure you are set to A Weighting (all weightings are actually about the same at 3200 Hz) and Slow. Hold the microphone 1 ft from the smoke detector and press the test button. The signal will go in short bursts so you need to keep an eye on the SPL meter in REW and adjust until the peaks read 105 dB (calculated using this calculator). If you are 10 ft away it will read 85 dB. Click "Finished" in REW when you are done setting the SPL Meter.

In Omnimic, click the SPL/Spectrum tab and set response weighting to "A" Curve. Set the damping to Slow. Press the test button on the smoke alarm and adjust the microphone sensitivity slider to make sure you aren't overloading the input. Hold the microphone 1 ft from the smoke detector. Go to Config > Adjust and set the microphone trim until it reads 105 dB.
 
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DVDdoug

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The link didn't work for me but I found it.
3.1

Piezoelectric Horns Used in Residential Smoke Alarms Smoke alarms (both the ionization and photoelectric types) typically use a piezoelectric horn to indicate to an occupant that the smoke alarm has activated. Piezoelectric horns are popular in smoke alarms because they can output significant sound levels without using much power. Depending on the piezoelectric horn design, the maximum sound pressure level can vary but is typically rated at a minimum of 85 dBA at 10 feet away as specified in the Underwriters Laboratories’ voluntary standard for smoke alarms, UL 217 Single and Multiple Station Smoke Alarms.

There will always be a tolerance and as you might expect, 85dB appears to be a minimum.

It might be "better than nothing" but I'd trust an SPL meter or SPL Calibrator more. I have an "antique" analog Radio Shack SPL meter, and about a year ago I bought a calibrator because I've read that electret condenser mics lose their sensitivity over time (it does have an electret mic) and I thought I could probably re-calibrate it. But my antique meter was "perfect" as accurately as I could read the analog meter!

...Usually with REW, we're more interested in frequency response and SPL level is less important.
 

Speedskater

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I too have a analog Radio Shack SPL meter, they had a pro SPL meter and calibrator at work for a short period of time. My Radio Shack read the same as the pro meter.
 

Blumlein 88

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My old analog RS SPL meter matches my Umik 1 within about 1 db for sound level. It is at least 30 years old.
 
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Keith_W

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That's a brilliant idea! I just checked the SPL of my smoke alarm with my SPL meter. I got 84dB +/- 2dB. That's good enough.

BTW in my state in Australia, all houses need to have smoke alarms installed. I have a vague memory of my smoke alarm installer using an SPL meter to verify the loudness of the smoke alarm. It never occurred to me to use it to calibrate REW's mic.
 

kemmler3D

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Even though the standard only specifies a minimum, this is a pretty clever idea. If people measured the SPL of a few smoke alarms, we could find one on the market that has a tight tolerance, then people could get that smoke alarm and use it for calibrating a mic.

On the other hand, a smoke alarm costs about the same as an entry-level SPL meter...
 
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