Proving SPL meter roll-off

Discussion in 'Speakers & Subwoofers' started by ChrisBee, Apr 25, 2004.

  1. ChrisBee

    ChrisBee Stunt Coordinator

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    Hi

    I've been using my RadioShack/Realistic analogue SPL meter for about 8 years in several different listening rooms and with several different sub/speaker systems.

    But it was only recently that I discovered the "well-known" bass roll-off and "necessary" correction figures.

    Before that I was always rather pleased with my flat frequency responses of my subwoofer & satellite systems.

    But by adding in the correction figures my nice flat response curves are trashed! :b

    So why would I want to spoil my graphs by using the correction figures? [​IMG]

    The question is: Is there a cheap & fool-proof way for an amateur enthusiast to check if his SPL meter has the typical bass roll-off response? [​IMG] Or not? [​IMG]

    Or is it simply that ALL of them bass shy? [​IMG]

    Thanks
    ChrisBee
     
  2. Edward J M

    Edward J M Cinematographer

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    The meter is C-weighted. Without going into great detail, it will read progressively lower than the actual SPL as you go deeper in frequency.

    The meter also has some inherent inaccuracies that cause it to read even lower than the C-weighted curve would suggest. Provided below is a table with the frequency, the theoretical correction factors for the C-Weighted curve, and the commonly used RS Correction Factors. As you can see, there is a difference between the two and it grows larger as the frequency drops.

    Whether each RS meter will display the exact same offsets is up for discussion, but my hunch would be no. If you really want to know what the meter is reading at any particular frequency, get it professionally calibrated.

    Of course, professional calibration will cost you as much as the meter itself....so most people just use the RS correction factors as "good enough". Unless you are into serious audio measurements, I tend to agree.

    Frequency / C-Weighted CF's / Common RS CF's
    10.0 / 14.3 / 20
    12.5 / 11.3 / 16.5
    16.0 / 8.4 / 11.5
    20.0 / 6.2 / 7.5
    25.0 / 4.4 / 5.0
    31.5 / 3.0 / 3
    40.0 / 2.0 / 2.5
    50.0 / 1.3 / 1.5
    63.0 / 0.8 / 1.5
    80.0 / 0.5 / 1.5
    100.0 / 0.3 / 2
     
  3. Mark Seaton

    Mark Seaton Supporting Actor

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    While the meters are reasonably accurate for broad band pink noise measurement, the response varies quite a bit unit to unit so far as frequency response. This is not a hunch, but what many in the pro community have measured. With suitable microphones coming down to less than $100 (Superlux is one of my prefered units) I hope we continue to see better measurement setups available for reasonable money. Considering many spend more on a set of video cables, I don't consider the investment out of line for a hobbyist who wants to actively work to improve their system.
     
  4. Edward J M

    Edward J M Cinematographer

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    Agreed, Mark. I've tested a few RS meters myself and they are all different, especially in the deep bass regions.

    The factory calibrated B&K meter I own for basic SPL monitoring is far better, and follows the c-weighted curve almost exactly down to 10 Hz.

    With burned test tones on a CD, the RS correction factors, an Excel spread sheet, and the RS meter, I think the enthusiast can get a "rough" idea of the in-room FR, and where large peaks and nulls are located. I wouldn't rely on that type of method for precise measurements, because (as you said), each RS meter will react a little (or maybe a lot) differently.

    My test rig didn't really cost me that much. Mic, phantom preamp, cables, adapters, mic stand, a decent multi-meter, a factory sound calibrator, software, professional calibration of the mic/preamp, and an audio-grade sound card set me back around $800. Well worth it for the serious tweaker wanting to get the most out of his system, or for equipment reviews and comparisons where the data will be more closely scrutinized.

    Speaking of mics, I was amazed the ECM8000 measurement mic tested so darn flat right out of the box when I had it professionally calibrated (along with the phantom pre-amp as one unit). The correction factors for the combined mic and premap were never larger than around 2 dB (and often much less than that) from 10-20,000 Hz.

    I also noticed you recently recommended WinMLS (or something to that effect). I saw it can measure things like group delay and impulse response (nice). For the hobbyist who already has RTA software, which version of the MLS software would you recommend for some basic extras like GD and impulse? TIA.

    Regards,

    Ed
     
  5. Brian L

    Brian L Cinematographer

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    As you may or may not know, there is a new version of the RS analog out (there was a thread here somewhere that brought that to light).

    I did take a look at there web site, and it is similar, but does have an updated appearance.

    I wonder how long before we get a new set of correction factors for the new one?

    BGL
     
  6. Edward J M

    Edward J M Cinematographer

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    I'd bet the guts are the same. It is definitely still c-weighted, so at a minimum apply the c-weighted correction factors I provided above. Anything else you apply beyond that is a bit of a crap shoot, I'm afraid.
     
  7. ChrisBee

    ChrisBee Stunt Coordinator

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    Hi,

    Thanks for the interesting responses.

    I still can't get my head round the basic problem:

    If the RS meter is SO inaccurate, then why does the needle return to EXACTLY the same spot for so many sinewave test tones?
    Yesterday the needle dropped to zero between 1Hz stepping test tones and returned within a hairsbreadth all the way from 43-70Hz @ 84dB. I then re-adjusted the PCi crossover to 60Hz. To remove a trough at the previous 40Hz setting. Resulting in a flat response according to the RS meter from 15Hz to 70Hz(+-2dB)

    But then of course I had to add in the meter correction values and it all went North as I went South! [​IMG]

    I'd post the graph if I could find a way that didn't involve a website URL. I haven't found an image "browse" button yet to upload an image direct from my computer. :b

    ChrisBee
     
  8. Edward J M

    Edward J M Cinematographer

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    There is a difference between accurate and precise.

    You weigh 225 pounds. Your scale reads 217 pounds. Every day you step on the scale (at your weight of 225 pounds), the scale reads 217 pounds.

    You lose 13 pounds and now weigh 212 pounds. Your scale now reads 204 pounds. Every day you step on the scale (at your new weight of 212 pounds), it reads 204 pounds.

    The scale is precise, but it is not accurate. See the difference?

    The RS meter may very well be fairly precise in its ability to repeat data or convey changes in SPL. But it is not accurate in an absolute sense, both due to the c-weighted filter, and compounded by its own inherent additional inaccuracy.
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    And, to add on to what Ed said about accuracy and precision (I was just about to post my own similar version) is that because of this:


    Remember that most of us simply use the Radio Shack meter for balancing levels between channels. That is precision, and the meter is actually very good for that. But if you want to know what the absolute freq response of your sub is, not gonna do that very well.


    But I have my own theory too. For example, I use the Radio Shack meter and ETF software with my PC to do rudimentary measurements for my ht. I can certainly believe that there are differences *between* meters at different frequencies. But I wonder about the scale of the inaccuracies of any one individual meter *within itself*. What I mean by that, is that let's say I measure 20 Hz to 200 Hz. Meters will read differently. But is it more of a trend? Or more of problems with discrete measurement frequencies?

    I.e.:

    Freq Hz....True dB....Meter X.....Meter Y
    ..20............75.........73...........73
    ..40............77.........76...........79
    ..80............78.........76...........77
    ..160...........77.........75...........80

    I believe real meters act more like Meter X than Y. (Look at the "trend" for X vs the "haphazardness" of the errors for Y.) So even though a lot of people put forth the inaccuracy of the Radio Shack meter as an issue, I believe that you can still use one meter by itself for a lot of interesting things in a home theater.
     
  10. MikeTz

    MikeTz Stunt Coordinator

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    Chris:

    If you can truly calibrate the frequency response of your room to within 6dB from 20Hz to 1KHz you are blessed. Enjoy that acoustically flat room! Most people have rooms that vary more than 20dB below 200Hz. Sounds like your room dimensions and speaker placement are spot on.

    Get a drink, fire up the music, and smell the roses.

    MT
     
  11. ChrisBee

    ChrisBee Stunt Coordinator

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    I like your argument on accuracy and precision Edward.

    My argument is that there was no variation. I didn't gain or lose weight (or dBs) over an extended part of the lower frequency response according to my meter scale.

    So, is a rising trend in the bass below 30Hz (after making the RS corrections) a good thing? [​IMG]

    It was dead flat before the corrections from 70Hz down to the cliff edge at 14Hz.[​IMG]

    But now I've got a ski jump ramp 10dB high @ 15Hz with a clear leap into subsonic oblivion! [​IMG]

    ChrisBee
     
  12. Edward J M

    Edward J M Cinematographer

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    I don't know what type of subwoofer you own, but if it has a reasonably flat anechoic response down to 20 Hz, then in a typical room it will display a rising response. This is due to a phenomenon known as room gain.

    How soon the response starts to rise, and to what extent, is largely a function of the room size. A smaller room will start to rise earlier and exhibit more overall gain.


    If your meter showed a flat in-room response without RS correction factors applied, that is merely happenstance and coincidence, and I wouldn't read into it any deeper (pun intended).

    Regards,

    Ed
     
  13. ChrisBee

    ChrisBee Stunt Coordinator

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    Thanks Edward and all those who responded. [​IMG]

    The sub is a 16-46PCi in a 30 foot long attic room.

    Needless to say I'm delighted with the performance.

    It was interesting that setting the sub's crossover to 40Hz to exactly match my floorstanders roll-off point did NOT make for a flat response.

    Though it sounded great from the first notes. I had a 10dB sawtooth dip centred precisely on 40Hz. Raising the crossover frequency to 60Hz (quite arbitrarily) flattened the response perfectly and smoothed the upper bass.

    Without the RS meter I would never have known about this dip or a shallower one at 90Hz.

    Thanks again for your time in answering my questions and countering my doubtful assumptions so politely. [​IMG]

    ChrisBee
     
  14. Brian L

    Brian L Cinematographer

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    Sounds like the sub and mains were out of phase at or near 40 hz?

    Come 'on Kevin, you are the undisputed king of those that obsess over subwoofer phase issues...what say you?????[​IMG]

    BGL
     
  15. Kevin C Brown

    Kevin C Brown Producer

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    Brian- Actually, I would guess that the following is the case: [​IMG]

    Remember that the crossover freq is just the beginning of when the low pass and high pass slopes start. But there is also the natural roll off of the speaker too.

    Because of this, a lot of people feel that the minimum crossover freq you should have for a set of speakers is 1/2 or 1 full octave above the -3 dB point.

    So Chris, in your case, if the -3 dB point is 40 Hz, that means that the natural roll off of the speaker's freq response has already begun. So now you are applying the high pass at that same freq, which further rolls off the freqs below 40 Hz. You add those two slopes together, and bingo, you get a dip at 40 Hz (and below), until the sub takes over.

    So, you accidentally did it right [​IMG], in that 1/2 an octave up from 40 Hz is 60 Hz (1 full octave doubles to 80 Hz), and the dip goes away because you have now moved the slope of the high pass to the mains far enough away from their natural roll off, that they aren't "adding" together anymore. If it was a phase issue, then generally speaking, that dip should have simply moved to 60 Hz but it wouldn't have completely disappeared.
     
  16. ChrisBee

    ChrisBee Stunt Coordinator

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    Hi

    I'm using high level connections. The main stereo speakers are fed full range and NOT filtered in any way. I followed this method of connection due to the highest priority being on musical sound quality. Though the sound quality (and bass!) on films is amazing despite being in stereo only.

    The floorstander's -3dB point is just below 38Hz measured from the listening position.

    In retrospect I should really have tried using the phase control to flatten the dip at 40Hz. It amazed me how well lifting the sub's crossover to 60Hz flattened the response.

    Though I cannot admit to hearing any difference despite the removal of this 10dB dip @ 40Hz. [​IMG]

    Thanks again
    ChrisBee
     
  17. Edward J M

    Edward J M Cinematographer

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    Chris - your fine choice in a subwoofer explains your frequency response.

    The 16-46 is tuned to 16 Hz and displays an almost flat response to that frequency in an anechoic environment. Place it in a moderate size room, and you will almost assuredly have a rising response down to the tune point, or even a bit deeper.

    All of my SV subwoofers have exhibited a similar curve - starting at about 30 Hz I start to see room gain. In the 16 Hz tune, my PB2-Ultra stays elevated above the 30-100 Hz level even out to 12 Hz, and that's at Quick Sweep sound pressures of 100-110 dB at the listening position.

    Regards,

    Ed
     
  18. ChrisBee

    ChrisBee Stunt Coordinator

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    Edward

    I bought the 16-46 primarily for classical organ music reproduction in my stereo system.

    Before it arrived I was terrified it would drown everything out with typical HT "bash, crash & boom" and I would probably have to sell it straight away. [​IMG]

    Instead it has been an absolute revelation on every kind of music and film. Never intruding, its effortless ability to go from silence to 110dB and back to silence again in an instant and totally without distortion is utterly amazing. [​IMG]

    If I say it was worth every cent just for its for it's bass drum sound alone you'll know exactly what I mean as a fellow SVS owner.[​IMG]

    Thanks
    ChrisBee

    My little tribute to The SVS 16-46 PCi as a seriously musical, high-level connection, high-end subwoofer:

    http://mysite.freeserve.com/svs_pci_music/index.jhtml
     

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