standing waves and cancellation & exageration ?

lolojr1

Member
2004-10-03 5:20 am
can anyone explain to me about standing waves in a room/car and what causes them / stops them ?

if i can find the wave length how do i know if there is a cancellation ?

if there is a cancellation does it cancell the sound at the point of the standing wave or everywhere in the room ?

is there a way to stop them non matter how redicilous or unorthadox of a soloution ?

i also have read that they cause a cancellation and/ or an exaggeration how do i take adventage of the exaggeration ?
 
aka Room Modes...

You can get rid of them mostly by using Dipole bass, but this requires ALOT of equalization for flat response. It will be pretty even around the room though. If you equalize an omnipole sub it may be flat in only one spot and everywhere else have big peaks and nulls.

To take advantage of Maximum exaggeration place the sub in one corner and sit in the opposite corner...
 

lolojr1

Member
2004-10-03 5:20 am
so if the sub, a ported one, cant use dipole has to stay in the exact location and it makes a standing wave is there a way to stop it or cancell it with out adding subs or changing its location ?

if there is a room mode a 100 is there one at 200 and at 50 hz also ?
 
If the first multiple is 100, there won't be any room modes at 50, and the one at 200 will be the 2nd multiple. But if the room mode at 100 is the 2nd multiple, so yes there's a room mode at 50. 200 will be the 3rd multiple...

You can stop them also with bass traps and things like that, it's quite expensive, depending on the result you want to have.
 
If you completely lined the walls of the room with subwoofers I'm fairly certain you wouldnt have too may problems with room modes. Or you could place an very large number of subwoofers randomly throughout the room.

With a reasonable number of subwoofers you dont want semetricical placement or two subs placed together in the same position
 
To really get a handle on it do this:

Take or make a dish that has the basic room shape that you are interested in. The dimension ratios are the only important thing. Fill it with some water. Take an eye dropper or whatever and drop water in various posistions. Think about the scale of the room and the possible placements of the speaker. THe places where the wavefront started by the drop of water cancel out quickly are the enignodes. In a real room they would be dead spots. The ideal wave pattern is a good clean back and forth reflection until there is no more wave. No criss crosses and such. So play around with it and see where the drop of water makes for the best wavefront and the best decay of the wave. You will have a very good idea what a real room will behave like as long as you were faithfull to the original room ratios.

I had to do this with a difficult client who believed that I had ripped himm off with a dual 15" subwoofer. He thought that it had no bass. ( How I don't know, I don't pick them just work for them ) I found a square baking dish that matched his square living room and demonstrated how there was no way on earth the placement he decided on would produce anything. Then we put the box in the approximate sweet spot shown in the simulation. A smile was evident and a happy customer!

Computers are not allways the best thing for simulations :)

Mark
 
A complication:

Few homes built these days have anything resembing a rectangular room or even a L-shape. I'm taking about the tyical living-dining-central area. Bedrooms are another matter. This makes the standard room mode example as given earlier problematic. There is software where you can make a sort of CAD drawing of your room and let the program calculate the modes.

I tried one a couple of years ago. I have highly irregular rooms with multiple levels, non-orthagonal ceiling-wall relationships, half height walls, and so on. It doesn't actually look that unusual until you try to put it in to a 3D drawing. That took nearly a week of spare time. Then when had the program do it's thing, it just ran, and ran and ran. All I had at the time was a 433Mhz Pentium III. I let it run 24 hours and gave up. An acquantance with a background in microwave took a look and told me the architect have designed a huge coupled-cavity oscilator. Fortunately, that's only at a very low frequency that my sub can produce and I can't hear.

So how to get around all this? No one has mentioned the old trick of putting the sub in the listening position and moving around the room with your SPL meter or RTA mic until you find the location the the smoothest response. If it's practical put the sub there. Remeber elevation off the floor matters both for the listening postion and the subs position. Be preparred t settle for second or third best location the the first choice turn out to be a doorway or some other impractical spot.
 

lolojr1

Member
2004-10-03 5:20 am
so ok i have some really god responses thank you
i have read about room modes and even revisited my master handbook of accoustics and found that no modes occour below the lowest note of the longest dimnension of the space between walls.
is that correct i might have it backwards not realy sure at the moment but i get the idea

now this brings me to another question how do room modes happen in a room where the walls are angled in at the top like the upperstairs bedrooms in an older house where the walls are about 3-4" tall and the roof meets there and continues up tp a peak at the ceiling how do i use this shape one of the online room mode calculator ? its not a rectangle not a square and the walls and ceiling are the same thing(well kind of)

thanks for the help and i am sure i will have more questions
 
do room modes happen in a room where the walls are angled in at the top like the upperstairs bedrooms in an older house where the walls are about 3-4" tall and the roof meets there and continues up tp a peak at the ceiling how do i use this shape one of the online room mode calculator ? its not a rectangle not a square and the walls and ceiling are the same thing(well kind of)

I believe id depends on the actual angles. I doubt it can be hand calculated. However, the rule of thumb seems to be that the more non-parrallerl surfaces you have the more even the pattern in the room becomes. If you think of the pattern that occurs in a rectangular room as having the granularity of very large beech balls, adding some non-parrallel surfaces replaces the super large beech balls with regular ones. More odd angles shinks them to basketballs etc.

Rukes of thumbs are not perfect, of course. The comment earlier about dipole subs is significant but may be a source of frustration as there are very few commercial ones available and they are pricey. The ultimate source on the topic is at www.linkwitzlab.com. I think the key observation is "dipoles create nulls orthagonal to the axis of radiation". I have dipole main speakers (not subs, alas) and the statement is indisputable true. The result is to reduce the room mode generating surfaces from six to two.

Another way to attak the problem is bass traps, again were are talking more money although they can be DIY'd.
 
Water in a pan only works for rectangular shaped rooms, and only tells you about two of the three modal directions (and it is room modes, not room nodes, though for any given mode there may well be several nodes). Height is important as well, sometimes particularly so, as it often produces modal irregularities in the upper bass range that are energized by a majority of music and easy to hear.

The method suggested for placing the sub in your listening position (at ear height) and crawling around the room listening and measuring is by far the most practical real-world method for finding decent placement. Simulations require expensive software, lots of knowledge, and are still only as good as your ability to accurately define the problem (and for a problem with lots of people and furniture in the room, not to mention unknowns about the structure itself, your ability to define it accurately is pretty poor). What you can't achieve by good placement you can address with a two-pronged method: room treatments and active equalization. Room treatments are large, expensive, and/or require building in to room structures to work well for low bass frequencies. Active equalization definitely helps but is not a cure-all. Doing what you can on the placement and treatment side makes the equalization side easier and more rewarding.

Try using some punctuation in sentences. Commas are nice. Periods are even better! ;)
 
Simple was the question simple was the answer!

Water in a pan only works for rectangular shaped rooms, and only tells you about two of the three modal directions (and it is room modes, not room nodes, though for any given mode there may well be several nodes). Height is important as well, sometimes particularly so, as it often produces modal irregularities in the upper bass range that are energized by a majority of music and easy to hear.


RHosch

I agree that your assesment is correct. My statement was simplistic.

I have done the complex calculations to simulate room modes ( Thanks. Proper term slipped into the dark recesses of the mind ) And a water pan no matter how faithfull to the shape of the room is only functioning in two dimensions. The wavelengths are very low even if a drip pot is used. I've only ever fooled around up to about 50hz.

The third dimension is indeed an important variable. There are some decent shareware programs that will produce accurate simulations. I think sitting duck software did something called ( close but probably exact ) "modes for abodes."

THe illustration was simple to help explain the basics. And usually seeing is believing. All of us have had to explain things to people untill we were blue in the face. Let them see it for themselves and then there is an understanding oh I see now.

For someone with experience your idea of carefull placement and listening is one of the quickest methods and is the one I actually use. ;)

Mark