Using an SPL meter FAQs

Discussion in 'Subwoofers' started by Nimby, Jan 7, 2005.

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  1. Nimby

    Nimby
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    Don't panic! :D

    It looks daunting. But it's far easier to do the test than to read the instructions:

    The idea is that you download short test tones onto your computer from the BFD (snapbug) set-up site. These tones will sit in your music folder ready to play like tracks on an album. You can use this to your advantage and play the tones autromatically like a long, boring music CD. Some people burn a CD with the tones to have it handy. Choose your own poison. There is no right way. But each method offers advantages and disadvantages.

    Set the SPL meter to C-Slow and the 70dB range. Then place your meter at head-height where you normally sit when listening to music or watching films.

    Either rest the meter on the back of your armchair or perhaps use a tripod. Placing the meter elsewhere will still record the sub's output of course. But not have much to do with your listening position which is what really matters.

    Play the short test tones through the subwoofer one at a time. A phono to phono cable can be run straight from your computer's sound card (line level speaker out) to the sub's low level input socket.

    Those using high level subwoofer connections will need a phono lead to the back of your stereo amp from your computer. A bit more fiddly but the same applies. Leave the volume alone once you have the meter showing 70dB on you first tone (or whatever starting sound level in dB you choose)

    You read the sound level in dB off the SPL meter of course. You can click between ranges of sound level on the meter if the needle goes off-scale or the digital version indicates a change in dB range while testing.

    One set you must not change the volume (or gain) on the subwoofer once you start. Or you have to go right back to the beginning.

    The idea is to read the sub's acoustic output on the meter when the sub is fed the same level of input signal but at different frequencies.

    Run downwards from (say) 150Hz in 1, 2 or 5 Hz steps. The closer the steps the more accurate your curve but the longer it takes. You might think testing is fun. But a partner or near neighbours might not! :mad:

    Scribble the dB figure from the SPL meter against the frequency of the tone legibly onto a bit of paper. Keep doing this until the meter no longer records any output from the subwoofer as the tones get very low.

    DO NOT TURN UP THE VOLUME ON THE SUB TO MAKE VERY LOW TONES AUDIBLE!!

    Remember to leave the gain (volume control) competely alone once you start a test run of tones. Very deep tones below 20Hz are inaudible to most people ayway. Trying to make them audible by turning up the gain might seriously damage your subwoofer!

    You should now have a frequency response curve for your subwoofer. But it's in the form of an inscruitable column of figures.

    You can do a test run with and without your main speakers if you have the patience. But the speakers will need to be calibrated properly for level with the sub first or the results will be meaningless.

    You can now plot your dB figures manually onto graph paper and scan the paper version into your computer as an image (if you have a scanner). Or use Excell to do it for you straight onto your computer.

    Then all you have to do is post the graph on the forum and we all talk it to death. :devil:

    http://www.snapbug.ws/bfd.htm

    Nimby
     
  2. eviljohn2

    eviljohn2
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    With thanks to Nimby for his excellent post I'd like to add a couple of extra points:

    Signal Generators
    If your computer is connected to your sound system it might be easier to use a signal generator such as SigJenny:
    http://www.natch.co.uk/downloads/SigJenny/SigJenny.html
    for generating test tones. Either this or burning tones to a CD will work just fine, I personally find a signal generator to be a little more flexible. There are some around that will even generate pink and white noise for those that are interested.

    Different SPL Meters
    There are loads of different SPL meters available, most of which will do a reasonable job for our purposes. Just bear in mind that at the cheap end of the price bracket (ie. <£100) they're all fairly unreliable at low frequencies. In order to do a proper analysis you'd need some Real Time Analysis Software for your computer, a proper microphone and accompanying mic preamp, all of which can add up to several hundred pounds.

    Correction Values
    There is a lot of talk about correction values for SPL meters. It should be emphasised that as far as I can tell there is only one set of correction values available on the internet and these are only suitable for the old Radio Shack Analogue SPL Meter. You're welcome to try them with the new Radio Shack meter or any other meter for that matter, just bear in mind that nobody's sure if they're right. Personally, I'd rather stick with the straight values from the meter rather than adding corrections which could also be wrong.

    Finally, there's a forum for Room Acoustics and Calibration available further down the main page for anyone that wants to delve further into this subject. It doesn't stop with flattening your subwoofer response, you can treat your room to do the whole audible frequency range if you want. :)
     
  3. Ian J

    Ian J
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    I have copied Nimby's post from a seperate thread which is why it appears to be answering a question that hasn't been asked.

    As this is something that is frequently asked I will leave this as a sticky thread with thanks to Nimby and Eviljohn for their explanation
     
  4. Nimby

    Nimby
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    The Mac is supported by REW but there are a few foibles.

    An SPL (Sound Pressure Level) meter is a microphone which measures sound levels and displays the results on a meter or digital display as dB. (deciBells)

    The most popular inexpensive example of the SPL meter is the analogue Radio Spares SPL meter. Available from BK:

    http://www.bkelec.com/HiFi/Sub_Woofers/rssplmeter.htm

    Room EQ Wizard is a very popular and sophisticated free subwoofer equalisation program.
    REW measures the frequency response of the subwoofer. Then suggests modifications to the input signal using BFD filters to eliminate problems with the bass acoustics of the room. If the subwoofer doesn't play a particular problem frequency so loud (because of a cut filter) then it can avoid exciting that sound in the room.

    Calibration files to correct the RS SPL meter's poor sensitivity to low frequencies are available from The HT Shack.
    This is the same website where Room EQ Wizard is available as a free download to registered members.

    THE BFD is the DSP1124 better known as the Behringer Feedback Destroyer.
    A clever, but affordable, [&#163;85] parametric equaliser which has 10 sets of 12 filters all individually programmable and storable.

    Working in dual mono or stereo the BFD has recently been discontinued so grab an example if you can still find in stock. They can still be found at musical instrument shops supplying groups, DJ & PA systems.

    Avoid US examples on eBay as they will be the wrong voltage for the UK/Europe.

    EDIT: We have moved on from that post involving single frequency test tones to REW. Which uses short sweeps from 2Hz to 20kHz and instantly graphs the results in many different ways. Avoid single frequency test tones if you possibly can.
     
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