Making sense of modes, standing waves and bass traps

[Max Dread]

Hi all

This post is me thinking aloud and hoping for a little feedback....and some sharing from my part for others who might be similarly confused.....

I thought I had the issue of modes and standing waves under my belt but recently I've been questioning it. I've therefore tried to make sense of the topic afresh and thought I'd share that here to see if my understandings are correct and for anyone else who - like me - finds it all a bit challenging to understand.

***** 1. Porous Bass Traps Question
I read in a SOS article that "…the surface of the foam needs to be spaced around a quarter wavelength or more of the frequency being reflected from the wall to have the optimum effect."

However, I read in another SOS article that: "…a piece of four-inch-thick foam stuck directly onto the wall is only really effective for frequencies above about 200Hz – 300Hz."

The quarter wavelength for 200Hz is 43cm, for 300Hz it's 28cm. So putting these two quotes together, how would 4"/10cm foam help in that frequency range? Is it a case of diminishing returns - i.e. not as effective but still has some effect? Or have I misunderstood something?

***** 2. Low Frequency Sound Waves, the Incident and the Reflected
I was getting confused by different pictures of how sound waves behave when modal issues are caused. Take this picture for example:



I figured that this must be showing the result of the waveforms interfering rather than the waveforms in action. I thought this because this diagram:



shows the opposite behaviour (it's totally reversed). Is that because it is showing how the actual waves interact?

If so, this picture:



confuses things a little more for me. It shows a full/whole wavelength. But according to this picture, the ¼ and ¾ areas will have a peak/re-enforcement, whereas the middle will have a dip/cancellation. But for a full wavelength the opposite is true – the peak should be in the middle and the nulls and ¼ and ¾. Consider if the reflected wave followed the path of the dotted line instead. THEN the diagram would work. The difference would be that the dotted line would show the correct phase relationship between the incident and reflected wave. But because the incident and reflected waves are together, it misrepresents the actual phase relationship that would exist….

Have I lost it? Am I looking at things all wonky?

***** 3. Modal Nulls vs. Gaps Between Modes
When looking at a frequency response graph I find it hard to tell what the "normal" dB level should be. As such - and with no "central line" so to speak - at some frequencies it is tough to know whether to consider something a null or peak.

This got me onto thinking about nulls more generally. Modes can cause nulls at points where the incident and reflected waves are out of phase causing cancellation. But nulls/holes in the FR can also be caused by large gaps between modes. In treating LF issues in a room, are we tackling all of this? Or are we trying to tackle the modes so that the anomalies they cause bring them closer in line to the "hole" left between widely spaced modes? Or are we trying to somehow raise the "hole" up to a decent level? Or am I thinking about this all wrong?!?


***** 4. Frequency Response vs. Modal Ringing
I've read in several places how porous absorbers are likely to reduce LF decay and therefore problems with modal ringing, but less likely to have much effect on the FR graph at LFs. Why/how much is this the case?

***** 5. The Space between Modes
In an ideal scenario, it is said that modes should be spaced no nearer than 5Hz to each other and no further than 20-25Hz from each other. That makes sense. I wondered whether this applies to axial modes only or should other modes be considered here as well?


***** 6. "Spacing"???
I've been using Bob Gold's mode calculator:

http://www.bobgolds.com/Mode/RoomModes.htm

All is good except I cannot figure out what the "Spacing" field is telling me. Does anyone know?


If anyone is reading this with interest and would like me to add further info for reading, please let me know and I'll go for it! I've put together more info to help me understand all this, and I'm happy to share if anyone feels it would be of interest to them.

Cheers

Max

[Jens Eklund]

1: Q 4 Avare


2: The confusion comes from the different behavior of modes depending on if it’s longitudinal or transverse wave motion. This should clear things up for you:

Standing waves acoustic resonance and vibrations on strings string - Standing waves are stationary waves room modes sound pressure level between hard parallel walls node antinode stationary room acoustic frequency - sengpielaudio Sengpiel Berlin


3: A dip can be caused by SBIR, a mode (if source and/or receiver is in a node), or simply by the lack of modal support. The first two are quite easily dealt with but the last one is somewhat trickier. If you treat other modes surrounding this gap, the situation will improve since the Q of the other modes will be lower and hopefully fill the gap.


4: Velocity based vs. pressure based absorbers


5: This is like chasing the holy grail. In the perfect world the spacing of (all) mode(s) should be even (and a fixed distance in Hz is not a good idea since the spacing will undoubtedly be smaller than this the further up you go in frequency . The “Bonello criteria” is what you’re after here.

At the same time, one needs to realize that the perfect spacing of modes might not be the optimum solution for a given position of sources and receiver but if we’re going this route, it’ll be late when we come back and probably still without the holy grail.


6: Spacing is the distance to the pervious mode in % (since Hz doesn’t work as described above).

[jhbrandt]

+1

Also - Remember that we like to visualize things on paper in 2 dimensions, but the fact is that these waves propagate in 4 dimensions (time included)... Oh and by the way.. The Holy Grail does not exist. hahaha.

Cheers,
John

[DanDan]

Max, it may not ultimately conform to common sense at all. The basic principles do, but real world action is so nuanced it can do a 180.

Here's some mind twisters.

Andre has provided links to tests that show absorbers to be only 1/3 as effective as they should be by theory, when they are out and about in the high particle velocity 1/4 wave and such locations.
Should but don't.

Conversely fibre at only 1/10 or less of the wavelength, at boundaries, where particle velocity tends toward zero, has been shown to have 'significant' absorption.
Shouldn't but do.

Two identical waves, a direct and perfectly reflected one, will add perfectly to create a peak of +6dB above 'the line'. However the null will be -infinity dB. In the real world we see peaks of 4-5dB and nulls of 20dB ish.
Should and do!

I have to say I relish the fact that it just ain't so simple and seems to be full of contradiction. Andre takes Aspirin.

DD

[Audioactive]

http://paws.kettering.edu/~drussell/...rposition.html