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SPL meters, Purchase decision
Benliq
post Mar 22 2014, 04:51
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So I got this radio shack spl meter to estimate the level coming out of my headphones.

http://www.amazon.com/Scosche-SPL1000F-135...mp;keywords=spl

I'm not sure if it is accurate enough for the task. Being off by something like 10db can be dangerous for hearing.

some liked it
http://voices.yahoo.com/product-review-sco...ter-693115.html

Others didn't

http://www.diyaudio.com/forums/everything-...cheap-junk.html

Thanks.
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DVDdoug
post Mar 23 2014, 23:46
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Measuring headphones is particularly difficult without an artificial head or artificial ear.

If have the sensitivity specs for your headphones and a multimeter/voltmeter, you might be able "calibrate" the SPL meter by measuring what the level should be with a known-voltage test-tone, then adding/subtracting the difference from the SPL meter's reading.
You'd have to clamp everything down so that nothing moves. (If your headphone's sensitivity rating is given in milliwatts, the conversion is Power (Watts) = Voltage squared/impedance.)

Note that you can't use a multimeter with music (especially a digital multimeter) because they are for constant voltages and you can't count on how the meter is going to respond to constantly-changing music.... The readings are going to jump-around a lot, and it's hard to know what you are measuring.
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Benliq
post Mar 24 2014, 02:21
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Sorry I don't get it, why do I need a multimeter to measure the sound output from a headphone?
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saratoga
post Mar 24 2014, 02:54
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QUOTE (Benliq @ Mar 23 2014, 20:21) *
Sorry I don't get it, why do I need a multimeter to measure the sound output from a headphone?



If you know the sensitivity, you can measure the voltage and then compute the SPL.
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DVDdoug
post Mar 25 2014, 00:03
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QUOTE (Benliq @ Mar 23 2014, 18:21) *
Sorry I don't get it, why do I need a multimeter to measure the sound output from a headphone?
Since you may not be able to trust the SPL measurement, the idea is to find the amount of error, or to find-out how to position the SPL meter & headphones for an accurate measurement.

You only have to calibrate your set-up once (as long as you can get the mechanical setup the same every time you make an SPL measurement).

Here's an example:

I looked-up the specs for the Koss PortaPro. It says 60 Ohms and 101 dBSPL@ 1 mW (0.001W). We can't measure mW directly, so we need to convert mW to to volts... The square root of (1mW x 60 Ohms) = 245mV (0.245 Volts).

If your headphone specs are given in voltage, you can skip that step.

You'll need to generate a constant test-tone with your audio editor, or download a test-tone. (A sine wave at about 1kHz should be good.)

You'll also need a Y-adapter (and probably an RCA adapter or male-to-male 3.5mm cable) so that you can connect the multimeter and the headphone at the same time. (The voltage may change when you connect the headphones, so you should have the headphones connected while adjusting/measuring the volume/voltage.)

If we play the test tone and adjust the volume for a voltage reading of 0.245V RMS, we should have 101dB.... If your SPL meter reads 101dB, or if you can position the meter so it reads 101dB, you know you can trust your SPL meter... If it reads 99dB, you know it's reading 2dB low.

With the SPL meter and headphones in the same position, you can play music and measure the SPL
(without using the multimeter).

Since dB is a ratio, if you are off 2dB at 101dB you'll be 2dB off at any other loudness (assuming the meter is accurate and everything is linear.)

This post has been edited by DVDdoug: Mar 25 2014, 00:12
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antz
post Mar 25 2014, 02:10
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QUOTE (DVDdoug @ Mar 24 2014, 23:03) *
You'll need to generate a constant test-tone with your audio editor, or download a test-tone. (A sine wave at about 1kHz should be good.)

Just to point out that many (most?) test meters are not designed for audio frequency AC measurements. I can't find the spec for my current one but an old one I have quotes 45-450Hz and it has a lower accuracy for AC to boot. It'd be worth looking up the potential errors in voltage measurement to see how that affects the dB figure calculated.
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Arnold B. Kruege...
post Mar 27 2014, 21:28
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QUOTE (antz @ Mar 24 2014, 21:10) *
QUOTE (DVDdoug @ Mar 24 2014, 23:03) *
You'll need to generate a constant test-tone with your audio editor, or download a test-tone. (A sine wave at about 1kHz should be good.)

Just to point out that many (most?) test meters are not designed for audio frequency AC measurements. I can't find the spec for my current one but an old one I have quotes 45-450Hz and it has a lower accuracy for AC to boot. It'd be worth looking up the potential errors in voltage measurement to see how that affects the dB figure calculated.



This one is very flat 20 Hz - 20 KHz in my tests and also somewhat affordable:

http://www.ebay.com/itm/UNI-T-UT61C-AC-DC-...r-/300681515168
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