Ringing Out The Room – Channel or Overall PEQ?

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This topic contains 170 replies, has 21 voices, and was last updated by Profile photo of zzzzzzris zzzzzzris 6 years, 6 months ago.

Viewing 15 posts - 46 through 60 (of 171 total)
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  • #47176
    Profile photo of Dick Rees
    Dick Rees
    Participant

    @DocG…

    I’m not going to get into an endless refutation of your misconceptions regarding audio. Those of us who do this for a living certainly see through your misinformation. I did a point by point reply to clarify the misleading conceptions in your first reply not to cavil with you but to point out for the benefit of the OP and other beginners the fallacies inherent in what you wrote.

    Either your wording was errant or you operate with only a partial knowledge of the basics physics of audio. Your characterizations of GEQ /PEQ are demonstrably fallacious and misleading to those trying to learn. Additionally, your statements about the causes and characteristics of feedback fall way short of the mark.

    I’m not going to continue this as a debate. I come here to help those who are starting out, not to disabuse folks who insist on clinging to what I’ve heard called ” faith-based physics.”

    To the OP and anyone else interested: GEQ, though often used for the task, is not the best tool for dealing with standing waves and/or room resonances resulting from room/system interactions. PEQ is the better tool and should be applied to the mains bus, not the individual channels.

    The bulk of problems in this area lie below 300hz and 3 or 4 notch filters commonly suffice. GCumbee spoke of this in his post above. If you happen to have a DSP unit or DSP built into your power amps or powered speakers, you should take the time and trouble to optimize your system in free space (outdoors) so you’re not dealing with both system AND room anomalies at the same time. It gets tricky fast.

    DR

    #47177
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Consider this: You can stop feedback by turning down the system gain. That says that the feedback at any frequency is dependent on the level of that frequency in the room. If the room tends to feedback at, say 10 kHz, a female vocalist is more likely to generate sound at that frequency than a baritone.

    The first sentence is true.

    The second sentence does not follow from the first as implied and is misleading in that it is the sound at the microphone, not the sound in the room that is the determinant. For a good part of the sound spectrum the propensity for feedback at a specific frequency can be reduced simply by moving the mic a few inches one way or the other toward an anti-node.

    The third sentence is just a poor example for a variety of reasons, primarily that 10kHz is far above the point at which room resonances occur and any sonic anomalies in this region are due to sound reflected directly into the microphone from a mains or monitor speaker or an extremely unfortunate microphone placement.

    #47178
    Profile photo of DoctorG
    DoctorG
    Participant

    DR,

    I too am tiring of this. You simply can’t follow logic. You say “anomalies in this region are due to sound reflected directly into the microphone from a mains or monitor speaker..” My question is, what generates that sound?

    #47182
    Profile photo of mervaka
    mervaka
    Participant

    Resonance, as already stated. Yes it needs stimulus, but the more unstable the system, the less of a stimulus it needs. This can include thermal noise.

    #47184
    Profile photo of coffee_king
    coffee_king
    Participant

    Not sure if reducing the overall volume is an option for your band, but in smaller rooms you should think about that as well.

    I always try to keep the volume of the gig as low as possible (Playing to the volume of the drummer who isnt a massive basher)

    #47194
    Profile photo of Dick Rees
    Dick Rees
    Participant

    My question is, what generates that sound?

    Anything at all. There is no voice that I know of that can produce the 10kHz in your example as anything other than a harmonic. You also cite room resonance:

    .” If the room tends to feedback at, say 10 kHz, a female vocalist is more likely to generate sound at that frequency than a baritone”

    Mervakas post above gets to thhe crux of the matter. First of all, “the room” does not feed back. The sound system feeds back. The only way the room can be excited and resonate (not feed back) occurs as a standing wave directly related to room dimensions. This does not happen at wavelengths as short as 10kHz. There will be many, many other (lower) frequencies which become problematic before 10kHz becomes a problem.

    In fact, you can reduce the propensity of 10kHz to be a part of the problem not just by notching the frequency itself, but by cuts at 1/2, 1/4, 1/8 the value: 5kHz, 2.5kHz, 1.25kHz and so on. Yes, these are octaves…

    For those interested in understanding these phenomena, here’ a link to a calculator:

    http://www.mcsquared.com/metricmodes.htm

    I think you’ve thrown out a random figure in choosing your 10kHz example and linking it to room behavior only applies if the sound is directly reflected into the mic. The wavelength @ 10kHz is 1.35 inches. Such short wavelengths tend to get cancelled out to a great degree by the complex interactions of the reverberative sound field of the room.

    Again, for those who wish to understand the physics involved, here’s another calculator link:

    http://www.sengpielaudio.com/calculator-wavelength.htm

    As I stated before, you are either mistaken in your basic understanding of the physics involved or you are having difficulty expressing yourself in clear terminology which does not render discussion moot. I refer once again to your blanket, unqualified statement of GEQ filters being “narrower” and your confusing of room resonances and feedback. We know what you SHOULD mean, but your choice of terms is so inappropriate as to confuse logical discussion. The end result is semantic quibbling rather than discussion of the topic. I endeavor to provide hard information for those interested in learning the realities of the physics of audio. That way SOMEONE reading this will understand what you seem not to.

    This is getting to be pretty Socratic…if you know what I mean.

    #47195
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Here’s a link to a better room mode calculator than that linked above:

    http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html

    #47198
    Profile photo of Andreas
    Andreas
    Moderator

    10kHz #3: I also assume that the 10kHz is a badly picked example and would be difficult create with digital desks at all. Not because they’re digital but because they introduce some sort of latency, about a mSec on the Qu. Maximum feedback frequency (at the fundamental) is slightly below 1kHz, getting feedback on harmonics first is pretty unlikely…

    #47206
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Thanks, Andreas.

    For those of you interested in the different types of GEQ filtering, here’s another good link:

    http://www.rane.com/hal/MobileHelp/Advanced/Content/Reference_Block_Topics/BlockReference_Filters_GraphicEQ.htm

    I’m not a big fan of computer screens (eye problems make them hard for me to read), especially if they draw the attention of a sound person to the visual realm instead of the aural. But the EQ trace shown on the Qu screen can be very informative…should you actually own or use a Qu console.

    The common Q value of GEQ filters has been 1.4. Filters with this Q value act over a one octave range, the 1/3 octave nomenclature denoting the center frequencies of the filters, not the width. If you wish a visual representation of such a GEQ filter, set one of your parametric filters to have a Q of 1.4 and have at it. I believe this is 1/3 on the Qu, but I’ll have to check when I get the console out of the vehicle in a couple of days.

    Here’s another link, this one dealing with filter Q and bandwidth:

    http://www.sengpielaudio.com/calculator-bandwidth.htm

    DR

    #47211
    Profile photo of DoctorG
    DoctorG
    Participant

    DR and Andreas,

    Now that I have you focused on the correct issue – the frequency of the sound feeding back — maybe we can communicate.

    Part of the “room” equation are the speakers and the mic. For feedback to occur at a given frequency, all that has to happen is for that frequency to be generated from the mic, transmitted through the mixer, come out of the speaker and hit the mic again at a level sufficient to start a feedback loop. The room itself doesn’t have to contribute to that loop, only the transducers and electronics. You stated that in other terms.

    I said that I was assuming that the operator knew how to take into account the factors having to do with proper speaker placement vs microphone position and the directionality characteristics of each device. I wasn’t addressing those issues; my focus was on the frequency response of the system.

    My choice of 10 kHz and the female voice as my example was perhaps unfortunate but also deliberate; I was just trying to make a point, get you on the subject and at the same time give you something to argue about. A high-pitched female voice can reach 3000 Hz and the 3rd harmonic would be at 9000 Hz. If the microphone can reproduce that frequency strongly, and/or if the mic or speaker has a response peak in that range, feedback can occur at that frequency without room resonance being involved. At any rate, a female voice would be more likely to generate high frequencies than a male. That’s all I was trying to say. Pick another frequency – the argument remains the same.

    If one then reduces the system gain at that frequency with a GEQ or PEQ below the feedback threshold, the feedback can be eliminated. Now that you are talking about the Q values of GEQ and PEQ, you are on the subject I was trying to address; I just did not want to completely ignore all of the factors that contribute to feedback overall, but I wasn’t trying to address them in detail.

    Once all the other factors have been cared for, you are left with the frequency response of the system. If you have smoothed room/speaker response with the appropriate GEQ, eliminated the primary feedback frequencies with HPF. LPF, and the PEQ on the channel in question, and some unwanted feedback is still being generated at the volume level you wish to use, you must resort to the PEQ of the group and main controls, then finally the GEQs on those outputs. With it’s fairly narrow Q values, a GEQ can be used to remove particular frequencies without adversely affecting the overall sound, if used judiciously. If the EQ devices are not sufficient to eliminate the feedback, one must then reduce overall system volume. The Qu has these capabilities.

    I spoke about using an RTA to determine where feedback is occurring; the Qu screen can display this – it is very informative.

    Lord! I hope we have arrived at the answer the operator was asking originally!

    #47213
    Profile photo of Dick Rees
    Dick Rees
    Participant

    Hey Doc…

    Coming back to parrot the corrections to your spurious posts as if you had stated them correctly in the first place or hadn’t gotten into all the ramifications in depth because of this or that is classic trolldom. Once and for all, you lack a basic understanding of the physics of audio…or the practical application thereof.

    Basically, you’re nuts. The PM’s I’m getting are 100% in agreement. To paraphrase W. C. Fields:

    Some of the participants here are habitues. Some are just sons of habitues…

    Go away and stop posting nonsense.

    #47214
    Profile photo of DoctorG
    DoctorG
    Participant

    It hurts when you’ve been shown to be a fool, doesn’t it Dick? I was wondering when you would get to the vitriolic level. Now we all know just the kind of person you are.

    #47216
    Profile photo of Andreas
    Andreas
    Moderator

    Indeed, Doc, we’ve learned more about one particular person, but that’s not Dick…

    #47217
    Profile photo of DoctorG
    DoctorG
    Participant

    I hope that’s a good thing, Andreas, but did you learn anything more about audio technology, or do you feel there is nothing more you can learn?

    #47218
    Profile photo of Dick Rees
    Dick Rees
    Participant

    It hurts when you’ve been shown to be a fool, doesn’t it Dick? I was wondering when you would get to the vitriolic level. Now we all know just the kind of person you are.

    Sorry you’re in such pain, Doc. It appears pretty close to terminal. It also appears your primary intent is to start arguments in this and the other threads in which you’ve posted.

    As to the person I am, I am an audio professional with enough verifiable experience and credibility to earn my living at it. I also am not afraid to post under my real name rather than hide behind a pseudonym.

    You wanted an argument, you got an argument. You should be happy now although you’re still wrong on 90% of the statements you’ve made here.

    You appear to be a classic example of the “majority of one”…

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