Verifying true acoustical balance with speakers and amp.

Discussion in 'Mastering' started by realdynamix, Jan 24, 2004.

  1. realdynamix

    realdynamix Well-Known Member

    :) A member just raised another question by trying to find a problem in his amp.

    The question is, how can you be sure you have

    (1) True electrical balance from the power amp?

    (2) True acoustical balance from the speakers for each channel?

    This question refers to each side of a system with respect to each other.

    --Rick
     
  2. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Fairly easily.

    Go to Radio Shack and purchase their sound level meter and a small DVM (digital volt meter)

    Go home and measure the output of each speaker with pink noise (Pink Noise can be downloaded off the net or you can make your own with programs like Wavelab) Use a MONO pink noise as levels for stereo pink noise can vary from channel to channel.

    Here is how to do it: Turn the volume control on the mixer so that at one foot away the speaker reads 83 dBSPL (speaker A). (C weighting, slow response on the RS meter) Now measure the other speaker (speaker B) to see what its SPL reading is. Do the measurement at the same exact distance from the speaker and without changing the monitor level. It should be the same. Now switch the speaker leads, Left to right and right to left. The speakers should both be putting out the same level.

    If they are not.....

    Then it maybe the speakers or the amplifier. Reset the speaker wires to their original configuration. Apply enough volume control to have the dB meter read 83 dBSPL from the speaker you started with (speaker A) Now measure the AC voltage at speaker terminals(not at the power amp) do the same with the other speaker.(speaker B). the difference should be within a half of volt or less.

    If you need more voltage to get the same reading as the first speaker then there maybe something wrong with the speaker or the amplifier. Redo the test switching the wires. If it is necessary to put out more voltage across the B speaker across the A amplifier channel to get the same SPLs then you may have a bad speaker. If you put the same level into amplifier channel and feed that channel into the "good" speaker you should have the same voltage across the amplifier output no mater which amplifier channel is loaded with the same speaker. If it is not then you may have a bad amplifier or more likely your monitor volume control is not "tracking" correctly.

    I have seen volume controls be off as much as 3 db between channels and be dependent on where the knob it set, NOT GOOD.

    Repeat the SPL measurements with different settings of the volume control (some slightly higher some slightly lower Say 60 dBSPL and 90 dBSPL. Translate everything into SPL levels and keep track of the difference in levels.

    When you get though you will know for sure whether it is the amplifier or the speakers and you will know where on the volume control you have the closest match.

    Hope this helps.
     
  3. Barefoot Sound

    Barefoot Sound Active Member

    Here's one of the simplest ways to check the amplifier balance. Take a single speaker and connect the positive terminal to the positive terminal of one side of the amp. Connect the negative speaker terminal to the positive terminal of the other side of the amp. So, the left and right positive amp terminals are driving the speaker (this won't hurt your amp). Now play an equal amplitude 1kz sine wave, pink noise, white noise or whatever (as long as it's stable) through both channels. Now measure the speaker output with a sound meter level, or just a microphone and a panel meter will due. Adjust the output of one of the amp channels until the speaker output reaches its minimum. The amp and the entire monitoring chain up to the speakers are now balanced.

    If the speaker levels aren't matched when the amp levels are matched, then the frequency response curves probably don't match up very well either. In this case, I would just try to adjust the channels as best you can by ear.

    But if you really insist on a more objective methodology, the simplest way I can think of is to connect a speaker to each channel, with one speaker having and opposite polarity as the other. Stand the speakers facing each other approximately 1" (25mm) apart with an omni microphone between the woofers. Play a 200Hz sine wave through both channels. Adjust one side of the amp until the microphone signal is minimized.

    Thomas
     
  4. Michael Fossenkemper

    Michael Fossenkemper Distinguished past mastering moderator Well-Known Member

    Thomas, that's a very interesting way to balance an amp, i'll have to give it a try. I usually do the other Thomas's way but using specific freq's as well as white noise and pink noise. having been a mixer for many years, i've notices that most volume pots are way out of wack. especially the mini SSL pots. I always patched the large pot to what ever speakers I wanted to mix on. I now use a digitally controller volume knob, it seems to be much more uniform at any level. The only draw back is that it's not passive and the signal runs through some op amps. Anyway, I've found that you should do the 1' or 1 meter distance as to minimalize your room effect. After you are sure that your amp and speakers are uniform, then you can deal with the room. If your room isn't perfect, which most aren't, moving a few inches nearer or further can produce better imaging and left right balance. But that's a whole other topic.
     
  5. Ammitsboel

    Ammitsboel Member

    That sounds like a very good idea Thomas... have to try that, first with my ears :) and then with a mike to see if there's any difference.


    Michael, there's much better ways around than that.
    At the moment I use 2 attenuator tranformers and a 24 positon switch. It pull the socks off anything when speaking of dynamics and clarity in all the different positions.
    But If you just whan't the matched channels then find a good Nobel pot, that's far better than opamps and digital control of volume.

    Best Regards,
     
  6. realdynamix

    realdynamix Well-Known Member

    :) Interesting answers all! I am drawn to the "max phase cancellation" slant in these responses. I have performed Tom's method with the meter, test signals, and DVM, however I used a power meter designed for measuring amp output, it's circuits represent a load to the amp as a speaker would. I would generate a tone, measure it's electrical output and adjust the controls to be equal. Just a slight touch either way can be 3db, a noticeable difference. IOW, the position of the controls is a basic reference at best.

    Even with the sound level meter positioned, I get in the ballpark, but not really sure if there is an influence of surfaces nearby.

    That's what makes the phase difference an interesting approach. It seems that method would get you to home plate. Mine, being midfields, would force me to use either the mid driver, or the LF driver for measurement as the sound would not really mix together for some distance.

    I can adjust each of the mid and high drivers separately, here we have pots again so I know a visual isn't going to be honest with me.

    How about this, after electrical verification, using Thomas' method with the Omni mic, for each set of drivers, using tones well within the parameters of the crossover frequency's?

    --Rick
     
  7. Michael Fossenkemper

    Michael Fossenkemper Distinguished past mastering moderator Well-Known Member

    I just realized that each of my speakers weighs 100lbs. I don't think I'll be trying the phase approach. Henrik, you are totally right about the pot. I've had it for awhile and i'm used to it but looking to change. I've had my eye on a Cello mastering console which sounds amazing with a price tag that goes along with it. anyway, I'm going to try a couple of Thomas's approaches. thanks
     

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