standing waves..whats the cause?

[montigue]

Well I am wondering if I am a victim of standing waves. Can anybody tell me what causes them and how can you tell if you have been infected with them?

Monty

[boa12]

monty, to be over simplistic (as usual cause I don't know the techs well ), they come from from interaction between your electronics (crossovers, EQ), speakers/sub(s), the ROOM, & your ears.

you know u have 'em when the bass quality sounds like crap (muddy) or not as good as possible. md & others, had some good tech info on that thread 'bout 2 subs in this section.

this is why i think adjustable crossover(s) are about as important a feature for sound quality w/ a seperate sub as DD or dts decoders. (i detailed my views on that under seb's large vs small thread under HT). now i'll shut my pie hole on it

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[JohnA]

Standing waves are caused by the dimensions of your room. They are a resonance that occurs at some frequency (or frequencies) and every multiple of that frequency until the multiple gets high enough the room absorbs the frequency. An example might be 40, 80, 120, 160, 200, ..... Essentially, your room is resonating like an organ pipe. Other than changing the size of the room, thatr's nothing you can do about standing waves.

The effect of standing waves is that at some frequencies and at certian places in the room, the bass will be strong (peak). A few feet away, or at a different frequency you'll have no bass to speak of (null).

I can assure you, your room has standing waves. Hopefully, they are not too prominent. You can adjust the position of your speakers so that your listening spot is not located in a null or a peak. Another trick is to use 2 or more subs in different locations. Since they are in different locations, each one will excite standing waves in different places. With any luck at all the peaks from one will hit near the nulls from the other and you'll have fairly even bass response throughout the room.

John

[mike stehr]

What would be 'Acoustic coupling'?

Or bass coupling?

[montigue]

just as I suspected. With the addition of my sub I noticed different levels of bass in different locations in my living room. Epecially near the walls and even behind the entertainment center. I also noticed the bass wass boomy at at certain areas in the room and deeper in other areas.

Im thinking your right about getting two subs John. It might even out the bass a little better.

would getting one of those meters from radio shack help in positioning the speakers to limit some of those standing waves from the listining position?

did that make scense?

Monty R

[boa12]

this site's pretty cool & informative. may even talk about coupling somewhere mike.

http://www.deicon.com/listen_rm.html

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[boa12]

isn't bass coupling the combined effect of 2 dif bass sources? like large fronts & a sub covering the same freq ranges? & wouldn't that affect the intensity of the/those standing wave(s)?

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[mike stehr]

Remember now what Al K. said about two subs

phasing each either out, Monty.

Something mentioned of keeping the subs

together, or near the mains, if I recall.

Boa,

I thought 'Acoustic coupling' was when

the amount of bass spl gets to the point

where it starts drowning out other drivers,

I guess phasing them out.

I'm not sure, my knowledge on this subject is

pretty light.

I'll check that site out, I've been lurking

here way to much today, anyway.

THANX!

[boa12]

here's another that talks more on coupling. all i know is i have a huge notch, peak at about 20-38hz & i love it!

bass is clean & tight & there's a lot of it. i can always EQ it down one way or the other for the more balanced listening.

http://www.customaudio.freeserve.co.uk/articles/rpgpaper.htm

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This message has been edited by boa12 on 01-28-2002 at 12:48 AM

[WMcD]

This is a quick and simplified explanation of a standing wave. Err, perhaps I failed in both.

It is most simply understood if in a "thought experiment", you are standing in a tube with a speaker behind you by some distance. The tube is closed at the end you're facing with a wall.

We have a constant frequency or wavelength from the speaker, a constant tone. It travels by your ear and the waves go up and down and you hear the frequency.

You hear a constant volume, at least unless there is a reflection from the wall end. And, without a reflection you could walk back and forth along the tube and the volume "SPL" you hear would be the same at all locations. The waves just go past your ear. The wave is traveling. It does go up and down in pressure.

Actually, the wave is a combination of pressure (particles close together, pushing each other apart) and particle velocity (they are moving 'cause they're being pressured). You have to think about this for a minute. It is how sound pressure, and particle velocity, work hand in hand to propagate sound.

However, the wave hits the wall, and creats an echo. Essentially, the particle velocity at the "standing still" wall goes to zero. The pressure increases just at the wall.

Ha. Just at the surface of the wall there is an increase in pressure above the normal in the room, or tube. We have an echo. The wall has become a "speaker". That is what an echo is. (If the tube was very long and you turned off the driving speaker, you'd hear the echo as a replica of the driving signal. In a valley we could shout and hear the reflection off a mountain wall.)

But consider the tube story. Standing in the tube, you've got a "speaker" on each end, and you're in the middle. Just consider this being in the tube with a speaker with an identical signal on either end.

You can walk along the tube. At some location, the signals from the two speakers are going to be "out of phase" (assuming a constant frequency or wavelength). The pressure cancels and there is no sound at your ear. The waves cancel each other out.

At another location they are in phase, and twice as loud.

In between those locations, the sound pressure increases and decreases, as you walk forward and back along the tube. This is to say the total perceived volume is changing as you walk.

Again, you have to appreciate the extent to which anything is "standing" still. If there was no reflection, the perceive volume would be constant no matter where you walk along the tube. Yet with the reflection, there are peaks and node.

So what is the "standing wave" talk? It is actually to say that as one walks along the tube, there are wave-like areas of high and low perceived volume. It is the intensity of the sum of the two "speakers" which seems to stand still at a given location. Hence, "standing waves".

As you might imagine, a given room is a three dimensional tube. This makes the set up very complicated.

You have to go to another level of "thought experiment". You have single subwoofer putting out a constant frequency. And you've got six walls to reflect it. And and you walk around the room, all relative distances vary.

The bottom line is that it is the room which creates "standing waves" of response at a given frequency.

What happens when you change the frequency or wavelenght? All the nodes change.

Gil

This message has been edited by William F. Gil McDermott on 01-28-2002 at 12:34 AM

[boa12]

thanks gil. great real life explanation. i edited my post above accordingly

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[T2K]

Good explanation Gil.Although I have no schooling in this area and therefore have no business commenting,I am at risk of totally embarrassing myself by revealing something that I think I have learned by trial and era.So here goes.

Is it possible to avoid or diminish 'standing waves' by way of 'venting a room'? In other words assuming a hypothetical 16'X20' room with three walls and at each corner of the fourth wall adding two additional hypothetical rooms that would have a common wall mid-way of the fourth wall of the 16'X20' room.Sort of a 'Y' shaped cluster of three rooms,in other words.

Have some more thoughts but I'd better keep them to myself.

Thanks for any input from anyone.

Keith

Keith

[Klewless]

Here is another take on standing waves! Why don't they just sit instead?

Seriously though, it is very easy to hear standing waves. Go get an inexpensive audio oscillator, the kind like you once saw in high school science labs. There are two kind of oscillators. 1) Low level which you can plug into your amp. 2) High level you can connect directly to your speaker.

Then run the frequency from high down to low using your oscillator. As the frequency gets lower and lower you will be able to walk around the room and hear spots where the sound is loud at THAT FREQUENCY and other spots where the sound seems to just go away completely. The spots occur at different locations AND at different frequencies. These spots are called standing waves because they seem to be stuck standidng at one place.

At even lower frequencies, the spots will seem to occupy the entire room (or half of it, or one-fourth of it, etc, etc). This thing is going on at all frequencies. You don't hear them at the high end because the wave length of the sounds are very small and are randomly scattered throughout the room so that you cannot dinstinguish individual sounds. The reason for this effect is that the high frequency waves are small RELATIVE to the room walls and furniture in it.

But at the low frequencies however the wave length starts to become as large as the room itself. Now your random distribution of the sounds are much less prominent because there are fewer of them and because of their size. Hence you hear them as individual rather than diffused sounds.

You cannot stop this. Lots of things are done to try and control their location within your listening space, hence the term "sweet spot" comes to mind.

In ten words or less, standing waves are the result of the sound bouncing around within the room in such a way that they appear to be standing in only a few places within the room and either interfere with each other (null) or reinforce each other (loud).

This was a very simplistic explanation of their cause and not terribly scientific. I hope this helps out. It is not a fault of speakers, amps, or any other part of the system. It is the result of your listening in a room smaller than the frequencies your are trying to hear.

Of course, you do have an alternative. Listen outside or go build yourself an acoustic sound absorbing chamber to the tune of several thousand dollars. I don't think you'd really like the resulting sound though.

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John P

St Paul, MN

[boa12]

i've been embarassed to say this, & the sound in the sweet spot, is pdg. but sitting at this computer in a room open to a hall off the main sound (living) room, i swear the overall sound quality is better in here.

in addition to keith's ?, what are the best room dimensions? i think i just need a new house to complement the sys.

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[montigue]

well, after reading William and klewless's explanation of standing waves I definitly have a very good understanding of what causes it. Thank you guys for that information. Thanks to Boa and Keith as well for you inputs.

I think as long as the listining position is free of standing wave it shouldnt matter too much.

one more question, I just bought drapes to enclouse the HT room for more of a theater effect. Having the room encloused by all four corners, will it cause more standing waves?

Monty

[Klewless]

boa12,

I too have experienced the sound in other rooms. In my case I'd almost believe the bass is perfect! Of course you lose the up front stage effect.

I have my own private little theory. When your system needs improving, just sit someplace else in the room. You'd be amazed at how much money you can save doing that!

montigue,

Even your "sweet spot" will be in a standing wave at some frequency. The trick is to move things around (or add expensive baffles, filters, equalizers, etc) to minimize the effect or else to balance it so that things sound clean and musical. All rooms will have standing waves.

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John P

St Paul, MN

[Guest BobG]

Floyd Toole has published a very informative white paper on the topic of standing waves and the loudspeaker + room combination. Find it at http://www.harman.com/wp/pdf/Loudspeakers%20and%20Rooms%20-%20Working%20Together.pdf

It is worth the time to read.

[T2K]

BobG,thanks for the link.I think the standing wave issue is misunderstood.But then again,my room is not a box or rectangle.It consists actually of 2 walls,a 1/2 wall, and then another 1/2 wall.One half wall at a 45 degree angle.Opens on two ends to other rooms.My subs are littered about,perpendicular to the mains 1/4 way down one (the only) side wall,another on rear wall,and another on angled side wall shooting at about a 45 degree angle toward my RPTV.Hard to describe but the only noticeable 'problem' area is a slighly noticeable peak in the kitchen.John Albright's post pretty well describes my situation....

Keith