Massive volume-build-up at 130 Hz (C3 note) - not a room mode

[Les Paul Fan]

Amroc is a good tool to understand room modes, though in my experience the theoretical does not always match the real-world. I believe Amroc assumes the walls are 100% reflective at low frequencies. Unless you have concrete or block walls that is not the case. Also, it does not account for SBIR, which can have a significant impact. REW has a similar mode calculator, but it also allows you to place your speakers and the listening position in the virtual room. I have no affiliation with REW btw. It's just a cool, free tool.

Speaker placement has no effect whatsoever in my room which leads me to belief that my issue is not related to room resonance

 

[jakel]

Speaker placement has no effect whatsoever in my room which leads me to belief that my issue is not related to room resonance

I think that rules out SBIR issues, but does not rule out room modes (or resonances as you call them).

 

[Les Paul Fan]

Amroc is a good tool to understand room modes, though in my experience the theoretical does not always match the real-world. I believe Amroc assumes the walls are 100% reflective at low frequencies. Unless you have concrete or block walls that is not the case. Also, it does not account for SBIR, which can have a significant impact. REW has a similar mode calculator, but it also allows you to place your speakers and the listening position in the virtual room. I have no affiliation with REW btw. It's just a cool, free tool.

Speaker placement has no effect whatsoever which leads me to believe that my issue might not be related to the room (resonance oder room modes)

 

[Rex]

Speaker placement has no effect whatsoever which leads me to believe that my issue might not be related to the room (resonance oder room modes)

When you change speaker placement to put the speakers in a different room — then maybe you can start to rule out room resonance. Until then... same room, same resonances.

 

[chris]

i gig at a place all the time. sometimes they have us on the north side, sometimes on the south side. but it always has the same resonances because it's the same room

 

[strabes]

Speaker placement has no effect whatsoever in my room which leads me to belief that my issue is not related to room resonance

Speaker placement has no effect whatsoever which leads me to believe that my issue might not be related to the room (resonance oder room modes)

You should come to the opposite conclusion. Room modes are based on the dimensions of your room and cannot be fixed by moving speakers around.

 

[Les Paul Fan]

You should come to the opposite conclusion. Room modes are based on the dimensions of your room and cannot be fixed by moving speakers around.

OK. But still, I am a bit lost. If this is the case why then all this business with REW testing to find the optimal speaker location (see Jakel quote: REW has a similar mode calculator, but it also allows you to place your speakers and the listening position in the virtual room). Or did I got something wrong here?

I`m all confused now. I thought it´s all about reflection and absorption. All one has to do is to place the speaker in such a way that the reflecting sound wave does not come right back to the speaker and cannot enhance the signal. Or you treat the room so the wavelength gets absorbed or deflected.

What exactly happens when a note C3 (130HZ) tone leaves the speaker? It gets reflected on the walls and causes resonance of the original 130Hz sound which gets louder and louder? It increases vibration of my string? Or does 130Hz excite standig room waves that finally by itself make the "loudness" of my C3 note go way up? Appreciate if someone with patience could explain that in dummie fashion. I searched the internet and there are lots of articles but I still dont really understand what exactly is going on, who does what to whom in what fashion. Thanks a lot for bearing with me.

 

[GlennO]

The room mode is solely a function of the room dimensions. The location of nodes however, can vary. An analysis with REW won't "find the optimal speaker location". It will help you measure the amount of correction you need for a specific location.

You have a room with two walls 9 or 17 feet apart. You can either treat the room to reduce reflection or you can use room correction software to counterbalance the effect.

 

[jakel]

The room mode is solely a function of the room dimensions. The location of nodes however, can vary. An analysis with REW won't "find the optimal speaker location". It will help you measure the amount of correction you need for a specific location.

You have a room with two walls 9 or 17 feet apart. You can either treat the room to reduce reflection or you can use room correction software to counterbalance the effect.

To clarify, all I said was that the room simulator in REW was a fun tool (or something like that). By "place your speakers and the listening position in the virtual room" I just meant you can do that in the room sim module in REW and see what affect it has on what you hear at the listening position. I did not mean that's the best way, or even a good way, to actually figure out where to place your speakers. Sorry for the confusion!

REW does multiple things, and the room sim is not the primary thing. I think the primary thing is helping you understand the system response for a given listening position (as GlennO said). I used REW to help me find the optimal listening position for my room, to help me find the optimal position for my speakers, subwoofer and sound absorption, and when those were known and in place, to generate an EQ correction file that I could import (or manually enter) into Peace (an EQ software that runs on my PC). I did all of this through trial and error, moving this or that and taking another measurement. How sound works in a room is INCREDIBLY complex.

Rather than me writing out a bunch of stuff to help explain the basics (and probably screw it up), I suggest that you check out Acoustics Insider YouTube videos and web site. I learned a ton from Jesco. He uses a very pragmatic approach, which I personally needed with my overly anal engineer type brain. GIK Acoustics also has some good education materials on their website.

 

[Les Paul Fan]

I have started to watch the recommended videos by Jesco. One pertinent question for me is where the increased fortified loudness comes from. It´s hard to explain what I want to say. Does the standing wave or the room generate the sound or the speaker. The boominess in my case definitely comes right out of the speaker not out of the room.

 

[jakel]

The room facilitates the standing wave. The speaker creates the wave, and then at certain frequencies (e.g. the room mode frequencies) the room alters your perception of the wave. Let's simplify and pretend we only have two parallel walls and that they are X far apart. Now let's say something creates a sound wave with a wavelength that is equal to X. When we put our head (or a measurement mic) exactly midway between the two walls we will hear double the volume of the source (I think - I might have this backwards, but you get the point), and if we put our head 25% from one wall we might hear close to nothing. Again, I might have this backwards, but you get the point. You can literally try this by finding an online sinewave generator and playing 130Hz (or try lower frequencies), and then moving your head around the room. Try it. You'll be amazed.

Our ears/brains are not very good at locating the source of low frequencies, so you might think you hear the sound coming (only) from your speaker, when it is, in fact, coming from the room also. I would definitely be doing these test with a sine wave, not with your guitar.

 

[Les Paul Fan]

The room facilitates the standing wave. The speaker creates the wave, and then at certain frequencies (e.g. the room mode frequencies) the room alters your perception of the wave. Let's simplify and pretend we only have two parallel walls and that they are X far apart. Now let's say something creates a sound wave with a wavelength that is equal to X. When we put our head (or a measurement mic) exactly midway between the two walls we will hear double the volume of the source (I think - I might have this backwards, but you get the point), and if we put our head 25% from one wall we might hear close to nothing. Again, I might have this backwards, but you get the point. You can literally try this by finding an online sinewave generator and playing 130Hz (or try lower frequencies), and then moving your head around the room. Try it. You'll be amazed.

Our ears/brains are not very good at locating the source of low frequencies, so you might think you hear the sound coming (only) from your speaker, when it is, in fact, coming from the room also. I would definitely be doing these test with a sine wave, not with your guitar.

Thanks for your explanations and your time. With a 130Hz sine wave even at very loud volume I do not get the build up I get with playing the C note on the electric guitar through FM3. In fact, with the 130 Hz sine wave I get no build up whatsoever.

 

[mr_fender]

That's because the synth is not an input. There has to be a feedback loop for the increase to occur. The standing waves are making your guitar sympathetically resonate and your pickups are picking up that resonance and reamplifying it through the FM3 and the cycle repeats until you get a runaway oscillation in the system.

It's just like feedback from a microphone. If the mic is able to pick up the audio directly from the mains or monitors, it creates a feedback loop where the audio gets repeatedly amplified until a runaway oscillation occurs (usually ending in a painful high pitched squeal). Your guitar is more easily able to resonate at low frequencies than high ones, so you're getting a boomy low feedback instead of a high pitched squeal in this case.

 

[danwei]

So have you tested if you can hear it with headphones only also?

 

[Les Paul Fan]

So have you tested if you can hear it with headphones only also?

Actually I did. But they are low budget and I get a weird response. There is no build up / resonance / volume increase. But now each note, not just the C have incredible long sustain which I don´t want either. Maybe this is also some kind of feedback or there is a problem with the headphones. One problem solved another one pops up. I am currently debating with myself if I should shell out lots of money for some high end headphones. Generally I do not like to play or practice with headphones on.

 

[Les Paul Fan]

time stamp 3:55 might be an explanation of what I experience; just came across it by searching "unwanted feedback / FM3"

 

[mr_fender]

Are the pickups in your guitar potted? Unpotted pickups can definitely have resonance and feedback issues at high volume.

 

[strabes]

place the speaker in such a way that the reflecting sound wave does not come right back to the speaker and cannot enhance the signal.

This is SBIR, which can be constructive if the reflections are in phase with the original wave, or destructive if the reflections are out of phase with the original signal.

why then all this business with REW testing to find the optimal speaker location

REW is a good tool to discover problems in your room (peaks, nulls, long decay times, distortion, etc), so you can address them with the right type of acoustic treatment. An untreated room's waterfall graph is going to be a train wreck but you can start by measuring a few different speaker locations to find the best location in that particular room. Then you begin attacking problem frequencies with acoustic treatment.

The goal is to minimize SBIR with your speaker placement, or use it to your advantage. In most residential rooms there is rarely enough space to do sufficient bass trapping behind the speakers so usually in a typical residential setup the best position for the speakers is centered on the short wall, directly against the wall. This will help ensure the rear reflections are as in phase with the original wave as possible, resulting in a boosted low end, which you can then compensate for by reducing the bass trim on your monitors. Obviously if the reflections are out of phase, you'll get a dip in frequency response, which cannot be fixed with room correction software or by boosting certain frequencies because the out of phase reflections will also be boosted. The same goes for nulls created by room modes but that's a different topic.

Crude math illustration might help: problem frequency original wave amplitude + reflection amplitude = resulting amplitude hitting your ears
In phase: 5 + 5 = 10 (will show up as a peak in REW)
Out of phase: 5 + (-5) = 0 (will show up as a dip in REW)
Out of phase but boosted to compensate: 10 + (-10) = 0 (roughly the same dip)

What exactly happens when a note C3 (130HZ) tone leaves the speaker? It gets reflected on the walls and causes resonance of the original 130Hz sound which gets louder and louder? It increases vibration of my string? Or does 130Hz excite standig room waves that finally by itself make the "loudness" of my C3 note go way up? Appreciate if someone with patience could explain that in dummie fashion. I searched the internet and there are lots of articles but I still dont really understand what exactly is going on, who does what to whom in what fashion. Thanks a lot for bearing with me.

Just do some research on room modes, youtube is helpful. Here is a good article, read all 4 pages. For some reason it displays weird in my browser so make a PDF if you need to. https://arqen.com/acoustics-101/room-setup-speaker-placement/

Forget about the specific feedback issue on the guitar in this instance until you get your room's issues somewhat under control. If you're going to DIY some panels with building insulation don't bother making anything under 6" deep because shallow velocity-based absorbers can only absorb high frequencies due to the longer wavelength of low frequencies, so you'll just end up making your room sound dead and won't address the low end issues which require much deeper absorbers, especially if they're velocity-based i.e. building insulation.

 

[jakel]

Well said, @strabes ! Great article as well. I'll definitely go through that a few times.

 

[Rex]

OK. But still, I am a bit lost. If this is the case why then all this business with REW testing to find the optimal speaker location (see Jakel quote: REW has a similar mode calculator, but it also allows you to place your speakers and the listening position in the virtual room). Or did I got something wrong here?

There are a lot of things at play here. A lot of ingredients in the soup. You have feedback in a resonant room. And there are optimal places to put a speaker in a resonant room, even though the resonant room is not optimal itself. Just like there are optimal places to be in a boat in a storm, even though actually being in the storm is not optimal itself.

I`m all confused now. I thought it´s all about reflection and absorption. All one has to do is to place the speaker in such a way that the reflecting sound wave does not come right back to the speaker and cannot enhance the signal. Or you treat the room so the wavelength gets absorbed or deflected.

At its root, resonances are all about reflection. But it takes a little math to understand how reflections cause resonance. It’s more intuitive to just realize that rooms have different resonances, depending on their size and shape.

What exactly happens when a note C3 (130HZ) tone leaves the speaker? It gets reflected on the walls and causes resonance of the original 130Hz sound which gets louder and louder? It increases vibration of my string? Or does 130Hz excite standig room waves that finally by itself make the "loudness" of my C3 note go way up? Appreciate if someone with patience could explain that in dummie fashion. I searched the internet and there are lots of articles but I still dont really understand what exactly is going on, who does what to whom in what fashion. Thanks a lot for bearing with me.

That “getting louder and louder” thing is all about feedback. You strike the string. It sends a signal through your amplifier. That signal comes out your speaker as sound. That sound hits your string — which is already vibrating at 130 Hz — with its own 130 Hz sound. That makes your string vibrate more strongly, which puts a stronger signal into your amplifier, which makes your speakers louder, which hits your string harder and makes it vibrate even more strongly, which puts an even stronger signal into your amp, which makes your speaker louder still, which hits your string even more strongly... do you see the pattern?

That feedback loop is affected by any losses or gains that occur. At a room resonant frequency, gain is added to the feedback loop, and the feedback gets stronger. If you turn down your volume knob, there is a loss, and the feedback gets weaker. If you add sound-absorbing material, there is a loss, and the feedback gets weaker. With enough losses in the feedback loop, the feedback disappears altogether.