Making sense of modes, standing waves and bass traps

[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 and different boundary conditions. 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 might 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

[Syncamorea]

If you combine pressure and velocity absorbers near a corner, wall midpoint, etc., is there any synergy? Are they additive in a linear sense? Or does a velocity absorber diminish the effectiveness of a pressure absorber that is behind it? I'd like to believe that there is some physical arrangement where you get your money for nothin' and modal eliminations for free but am not finding any evidence that this fantasy is reality. I've been reading all of the small room acoustics info I can and since there appear to be no magic bullets available, I feel that my ongoing project is somewhat futile. Sort of a cut-your-losses-and-live-with-it game. It is a 3.8 x 3.4 x 2.4m room that I want to set up as a mixing space. The truth is, even with just minimal treatments in the room, like covering first reflection points to the sides and behind the monitors, the room sounds surprisingly good and the mixes translate pretty well. It just feels overly busy in the ~300 to 3KHz range and that makes it a bit fatiguing to work in for a long mix session.

[gullfo]

like most things, the initial basic steps solve a lot of the issues commonly encountered and then subsequent refine refine that, and usually in smaller increments. so the "magick" is really in taking the time and make the effort to design, measure, plan, and treat to get probably as close as you'll get in most cases.

[Syncamorea]

Thanks for the response, Glenn. I guess part of what I'm getting at is about expectations. I've been using a sort of typical approach to bedroom CR acoustics, such as taking care of early reflections, using general purpose bass traps and a little diffusion, but now I'm scrapping it all and starting over. So I'm in that design, measure, plan stage and don't really know what to expect.

I plan to flush mount the monitors, splay the walls, build a cloud and am removing carpet and replacing with hardwood. For bass trapping, I'm going to try a limited amount of everything - well, not everything. I've done DIY Helmholtz in the past and had pretty good success with it but am thinking this room is too small for that. So I'm thinking about how to best spend my money and more importantly - time.

I like to do DIY and although I can afford to buy pro traps, I'm wanting to do a modular approach so that the positions of traps and diffusers can varied. I do science in my day job and am expert at instrumental methods, so for sure I will measure along the way but am also somewhat suspicious of those results, meaning I think they only tell part of the story.

So, back to the expectations bit - I can rationalize any outcome from having velocity and pressure based traps targeted at the same mode. I could envision a synergy where the bang per buck is awesome - but since I don't see people here focusing on that, I wonder if a pair of traps like this turn out to be less efficient? I'm psyched up to try limp mass absorbers and metal panel absorbers but am a bit leery that I could do a bunch of work and not get the return. So I'm tossing out a set of questions that probably don't have clear cut answers.

[gullfo]

sometimes it's hard to imagine but a large chunk of room volume (when really small room) needs to be in LF trapping - so leveraging the corners (everywhere) and the ceiling (since you need space on the floor) become important aspects of the design. with the soffit mounting and clouds etc you can be surprised by how much absorption can be folded into the space while keeping a lot of life. keep this in mind when creating modular treatments so you don't short change yourself. while many of my designs in the past have alignment with northwards' FTB design approach, having met Thomas and discussed it at length, i will now spend even more time in my designs to pay attention to the details which previously i may have done more intuitively than through calculation.

on the limp mass - i would suggest their use tends to be small percentages rather than large wow! type changes and combining them with porous absorption and other treatments doesn't seem to affect them much if the membrane is tensioned properly, damped properly, and free of contact from other absorption treatments which can impact their efficiency. VPR panels seem to be OK with direct contact with porous absorption.

[avare]

Originally Posted by gullfo

more time in my designs to pay attention to the details which previously i may have done more intuitively than through calculation.

+1. We tend to dismiss the calcualitons in design when trying to keep things simple when describing acoustic design. In the run this just hurts the people coming here for advice.

As a perfect example, how many people have written about disappopintment when building membrane absorbers? How many of those used the Mass-Gap equation? How many built their membrane absorbers NOT against a solid wall like concrete? How many of those people DID NOT use the Mass-Gap-Mass equation?

Calculatingly,
Andre

[gullfo]

actually i typically do a lot of calculations on walls, ceilings, floor, and panel traps (like limp membrane) - i was thinking more calculations of the rear wall reflections and other room cues where i probably tend towards more queuing than less... but fully agree on using calculations to ensure the basics are incorporated into the design.

[Syncamorea]

Originally Posted by avare

How many of those used the Mass-Gap equation? How many built their membrane absorbers NOT against a solid wall like concrete? How many of those people DID NOT use the Mass-Gap-Mass equation?

More homework...

Equations and math are fine with me. My room surfaces are drywall. Do I need to add a layer to make it more massive/stiff?

OP: I apologize if this looks like a hijack. Not intended.

[gullfo]

do you have a room design thread?

[Syncamorea]

Originally Posted by gullfo

do you have a room design thread?

No, I don't yet have one. I've been thinking about it while emptying the room of its current burden. The zero state of the room will be after I get the carpet out, hardwood floor in and have framed/drywalled a door in the 7' wide archway that currently connects it to an adjacent room.