Bad Bass Node

Billblake · « **on:** 17 Jan 2009, 07:26 am »

Hi, I'm new so please bear with me. I have a question. What exactly is a bass node & how can I get rid of it! It actualy hurts my ears.(puts a lot of pressure on the ear drums).

My room is 17 1/2'x15'x8'. I have tube traps in the corners & sonex covering the front & back walls. The speakers I did have were the Wilson Sophias & now I have Avalon Eidolons. This bass problem seems to be on the back wall. I have to go to nearfield listening to even listen to my music. Could bass traps stop this problem or would something else have to be done.(I don't want to build a new room) & What exactly does a Diffuser do? (I said I'm a little green here). Thanks, Bill

Big Red Machine · « **Reply #1 on:** 17 Jan 2009, 12:18 pm »

IN the room, where are the speakers and where do you sit within those dimensions? YOu need to be off the back wall for sure and bass trapping sounds in order. Six inches thick or thicker of OC703, but we need to know more about your layout first.

Big Red Machine · « **Reply #2 on:** 17 Jan 2009, 12:40 pm »

http://www.audiocircle.com/index.php?topic=63837.msg579172;topicseen#new

youngho · « **Reply #3 on:** 18 Jan 2009, 05:11 am »

Quote from: Billblake on 17 Jan 2009, 07:26 am

Hi, I'm new so please bear with me. I have a question. What exactly is a bass node & how can I get rid of it! It actualy hurts my ears.(puts a lot of pressure on the ear drums). My room is 17 1/2'x15'x8'. I have tube traps in the corners & sonex covering the front & back walls. The speakers I did have were the Wilson Sophias & now I have Avalon Eidolons. This bass problem seems to be on the back wall. I have to go to nearfield listening to even listen to my music. Could bass traps stop this problem or would something else have to be done.(I don't want to build a new room) & What exactly does a Diffuser do? (I said I'm a little green here). Thanks, Bill

I think you are asking the wrong question. Bass modes are standing waves that form within the confines of an enclosed space, like a room. For a rectangular room, the frequencies of the most prominent modes or standing waves are determined primarily by room dimensions and boundary characteristics (i.e. drywall is slightly flexible compared to concrete). A standing wave of a certain frequency has at least one area where the amplitude (loudness) is very soft, due to cancellations (negative interference). This is a node, and there can be more than one, depending on the order of the mode (i.e. first-order mode frequency is ~565 ft/sec divided by the distance between two parallel boundaries in a rectangular room, second-order mode frequency is 2x that, third is 3x, etc). A mode will have at least two antinodes, where the amplitude (loudness) is very loud, due to reinforcement (positive reinforcement), especially near boundaries, but there may be more than two antinodes, depending on the order of the mode.

Bass traps will help, but not fix, the problem you're having with modes. Besides panels like GiK or RealTraps or DIY ones, there are some interesting products from RPG like Modex Plate that cost a lot more. Other approaches to bass management use equalization, which is most effective for one listening position, or multiple subwoofers in various configurations.

There is an amazing website called Google. You can type phrases like "sound diffusion" into it, and a whole bunch of websites that might give you more information about your query pop right up. In short, though, diffusers change the reflection of sound waves from specular (i.e. bouncing off the wall like a light reflected in a mirror), which happens increasingly at higher frequencies, to hemicylindrical for a 2-D diffuser or hemispheric for a 3-D diffuser. Unfortunately, for diffusers to be truly effective at lower frequencies, they need to be deeper, and the more effective ones tend to be a little expensive.

Ethan Winer · « **Reply #4 on:** 18 Jan 2009, 05:21 pm »

Quote from: youngho on 18 Jan 2009, 05:11 am

This is a node, and there can be more than one, depending on the order of the mode

Just to clarify, a node is a null point where that frequency is much softer than other frequencies. But not all nulls are related to modes. You can have nulls at any frequency, depending on how far back the wall behind you is.

--Ethan

youngho · « **Reply #5 on:** 18 Jan 2009, 08:53 pm »

Right, nodes, by definition, are related to standing waves. That's why I wrote "A standing wave of a certain frequency has at least one area where the amplitude (loudness) is very soft, due to cancellations (negative interference). This is a node..."

Nulls or cancellations can occur from boundary reflections, interactions between multiple sources (i.e. in the subwoofer crossover region), or even from acoustical crosstalk (~2 KHz) related to the binaural hearing apparatus. But the original poster asked about a "bad bass node," which is why I thought he or she was asking the wrong question.

Billblake · « **Reply #6 on:** 19 Jan 2009, 04:32 am »

Sorry, I was just going by what someone told me. Maybe I misunderstood him. Yes, I do have some things to learn. The bottom line for me is my ear drums feel swollen or full when I listen to my speakers & it seem to start when I got the Wilsons. These speakers don't work real well at nearfeild listening & this is what I was custom to. I had to take my seat closer to the back wall & this is when the problem started. I do think I have some fluid behind both eardrums. It doesn't bother me to much untill I play my music & then the pressure starts up. I did bring my speakers in a little,away from the side walls & this seemd to help for sure. I know that no matter what, I'm going to get some good bass traps. Sorry again for any confusion. Bill

youngho · « **Reply #7 on:** 19 Jan 2009, 03:41 pm »

I don't think that the Wilson Sophias lend themselves to nearfield listening, given their size. The Stereophile measurements showed an emphasis of bass frequencies around 60-70 Hz or so. If you were set them up so that they were "firing" down the length of the room, especially if you didn't place them closer to 25% of the way down that length (or if you weren't sitting closer to 25% of the distance from the rear wall), you would expect them to energize a standing wave (second-order length mode, or around 64-65 Hz), which would be most noticeable when you got close to the back wall. So this might have been a poor match between speakers and room.

If you're having ongoing problems that have continued even after switching the Sophias for other speakers, then obviously they weren't the issue. Fluid behind the eardrums will typically result in decreased perception of higher frequencies, so when I have an ear effusion, things tend to sound dull and boomy, and I get headaches quickly from listening to music or speech. Listening to anything that is loud for a while will result in gradual adaptation for most people, as a branch of the facial nerve causes the ear drum to tighten up and become less responsive, unless they have hyperacusis.

My suggestion would be to try listening to your music through decent headphones. If you don't experience this problem, then it's probably your room. If you do, then it's a problem with you.

Ethan Winer · « **Reply #8 on:** 19 Jan 2009, 04:12 pm »

Quote from: youngho on 18 Jan 2009, 08:53 pm

Right, nodes, by definition, are related to standing waves.

Yes, and standing waves can occur at both modal and non-modal frequencies. That's all I was trying to clarify. No disagreement.

--Ethan

Billblake · « **Reply #9 on:** 19 Jan 2009, 05:48 pm »

So, Let's say it is the speakers that just don't jive with the room. Now let's say the problem is what you think it is( which by the way, I agree ). Would there not be anyway to take care of this problem without building a new room or getting new speakers? If the problem is in the 90hz on down,wouldn't major bass traps help....maybe help a lot? Bill

Vinyl-Addict · « **Reply #10 on:** 19 Jan 2009, 07:57 pm »

Quote from: Billblake on 19 Jan 2009, 05:48 pm

So, Let's say it is the speakers that just don't jive with the room. Now let's say the problem is what you think it is( which by the way, I agree ). Would there not be anyway to take care of this problem without building a new room or getting new speakers? If the problem is in the 90hz on down,wouldn't major bass traps help....maybe help a lot? Bill

I think, and this is just my opinion because I am no expert in acoustics, that a "good" parametric EQ would be necessary even if you use bass traps. I have a similar problem in my room (who doesn't?) and I have as much bass trapping as I can use. I think you should first maximize speaker placement as well as your listening postion, then implemement effective bass trapping and then & only then think about a parametric EQ if you still have issues.

Billblake · « **Reply #11 on:** 19 Jan 2009, 08:16 pm »

That sounds like a pretty good idea. Really,this is what I had in mind anyway. Now placement of my speakers & listening position will take a little time since I will do it by ear. Experment after experment!

Ethan Winer · « **Reply #12 on:** 19 Jan 2009, 10:21 pm »

Quote from: Billblake on 19 Jan 2009, 05:48 pm

wouldn't major bass traps help....maybe help a lot?

Yes, bass traps are always the best solution. EQ is useful at very low frequencies, say, below 60 Hz. Otherwise, bass traps are the best solution for peaks, and nulls, and ringing. (EQ only helps peaks.)

--Ethan

youngho · « **Reply #13 on:** 21 Jan 2009, 04:15 am »

Quote from: Ethan Winer on 19 Jan 2009, 04:12 pm

Quote from: youngho on 18 Jan 2009, 08:53 pm
Right, nodes, by definition, are related to standing waves.
Yes, and standing waves can occur at both modal and non-modal frequencies. That's all I was trying to clarify. No disagreement.

Just to clarify, Ethan, could you please give some examples of standing waves at non-modal frequencies in the bass region in typical small room acoustics? By "modal", I mean axial, tangential, oblique, etc. By "standing wave," I'm using the standard definition of "a wave characterized by lack of vibration at certain points, between which areas of maximum vibration occur periodically. Standing waves are produced whenever a wave is confined within boundaries, as in the vibrating string of a musical instrument." This is in opposition to areas of constructive and/or destructive interference from multiple sources or a single reflection.

Thanks. I can't think of any.

Ethan Winer · « **Reply #14 on:** 21 Jan 2009, 04:31 pm »

Non-modal peaks and nulls are caused by comb filtering, and the frequencies are related to the distance from a reflecting boundary. For example, you can have peaks and nulls outdoors caused by reflections off a single wall. The classic case of non-modal peaks and nulls in a room is from reflections off the wall behind you. In that case the peak and null frequencies have nothing to do with the room dimensions. As I see it, acoustic interference is the parent of comb filtering, and room modes are a subset of that when a space is totally enclosed. More here:

Comb Filtering
Bass Waves in the Control Room
Non-Modal Peaks and Nulls

--Ethan

youngho · « **Reply #15 on:** 21 Jan 2009, 05:33 pm »

What you're talking about are not standing waves.

Ethan Winer · « **Reply #16 on:** 21 Jan 2009, 05:39 pm »

Quote from: youngho on 21 Jan 2009, 05:33 pm

What you're talking about are not standing waves.

Yes they are! Standing waves do not require room modes. All that's needed for waves to stand still is constructive interference. This happens at peak pressure points. In the case of reflections off the rear wall, or off a singe boundary outdoors, the waves stand still at distances of half-wavelength multiples.

Maybe you missed it, but the "Internet acoustics guys" went around the block a few times about this a few years ago. Initially some of them insisted that standing waves require resonance, but in the end that was proven not to be true.

--Ethan

youngho · « **Reply #17 on:** 21 Jan 2009, 05:43 pm »

Can you define "standing wave" for me? Maybe something from a physics textbook? What you're describing is simple interference. Can you provide a link to this discusson of the "Internet acoustics guys"?

Ethan Winer · « **Reply #18 on:** 21 Jan 2009, 06:36 pm »

Quote from: youngho on 21 Jan 2009, 05:43 pm

Can you define "standing wave" for me?

This explanation uses ocean waves and electrical waves as an example, which are both unbounded, showing that waves can stand still without resonance:

http://en.wikipedia.org/wiki/Standing_wave

Quote

Can you provide a link to this discusson of the "Internet acoustics guys"?

There were many discussions over a few years before this was resolved. But this is pretty much the last one where it was resolved to my satisfaction:

http://groups.google.com/group/alt.sci.physics.acoustics/browse_thread/thread/4555bfce5a413bd6/

You'll see some haranguing from a few old-timers, but if you read it all you'll see a consensus from the heavy-weights.

--Ethan

youngho · « **Reply #19 on:** 21 Jan 2009, 07:46 pm »

I think the problem is that we are using different definitions of "standing wave," which is why I asked you to define it.

In the discussion you linked, you seem to be defining it as "ANY case where acoustic interference causes a deep null (we can define "deep" separately)." I assume you meant also "in a fixed pattern." By this definition, an open baffle bipole speaker creates standing waves around it towards the sides. A subwoofer with its phase set incorrectly creates standing waves with the main speakers. Diffraction effects should even be considered as standing waves.

When it comes to ocean waves, light waves, and electrical waves, there's a reason why I originally wrote "standing waves at non-modal frequencies in the bass region in typical small room acoustics". I was very precise about this for a reason. Interestingly, when you look at the Wikipedia article you linked, the ocean and electrical standing waves have periodic distribution of nodes and antinodes, and there was a mathematical description of standing waves. Do these apply to comb filter effects?

In essence, as far as I can tell, you're arguing that virtually all room acoustics are standing waves, with the possible exception of the direct signal from the loudspeakers, and if they're not exactly equidistant from the listener, then these are also standing waves, as well. In fact, there's even a standing wave inside the listener's head at around 2 KHz or so from acoustic crosstalk, by the definition that I'm assuming that you're using.