Bass trap placement - high or low? Does it matter? - Page 9 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #81 of 110 Old 02-28-14, 08:30 AM
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Re: Bass trap placement - high or low? Does it matter?

Hi

before jumping on conclusions too early, I would like to understands, what the "nulls" and "dips are, how they've been measured, where they come from.

Examples
Quote:
phazewolf wrote: View Post
But I ended up with a few large deep nulls that when I remove the traps are not there.
or
Quote:
phazewolf wrote: View Post
The null goes away if the speakers are flat against the wall at least with the old ones. The new Legacy focus se who knows what is going on as I have not run REW as of yet.
What measurement showed the "nulls"?
How was it measured, how does the curve look like?

-------------------

The term "null" refers to room modes. They are nulls of the standing waves that appear due to resonances of the room.
Not the nulls cause problems but the maxima of the standing waves.

At the nulls the standing waves do not cause any increase of sound pressure.
This does not influence the direct sound from the speakers/woofers.
A sound pressure of x from the direct sound plus an additional sound pressure of zero leaves the sound pressure x from the direct sound.

The the maximum points the standing waves cause anin crease of sound pressure at the certain frequency.
This adds to the direct sound pressure.
The sound pressure x from the direct sound plus a sound preyyure of y>0 due to a maximum of the standing wave gives an increase of sound pressure at the frequency of the standing wave.

-------------------

"Dips" in measurements can have a different cause.

They appear if one measures the room response curve.
The room response measurement is the combined sound pressure of direct sound and the room response (reflections, reverberant sound).

If you combine/overlay the direct sound with the reflected sound, you will observe an interference between those two sounds - depending on the frequency and the phase delay of direct vs. reflected sound (more precise: the wavelength and the difference of travel distance between direct an reflected sound).

Interference leads to peaks for certain frequencies (where the direct and reflected sound are in phase and this add to each other) and dips for other frequencies (where the phases are shiftet by 180, e.g. the direct sound is at high point the reflected sound is at a low point, resulting in a cancellation, zero sound pressure).

-------------------

A microphone only measures sound pressure changes.
So it does not distinguish between direct and reflected sound - but only the resulting sound pressure.

The ear distinguishes sound patterns also depending on the arrival time and their coherence.
Tu it in a simple way early arriving patterns with high coherence are identified as direct souund, late arriving patterns with low coherence are identified as reverberant sound.

The ear does not use the sound pressure.


So the microphone shows a result that is perceived in a completely different way.

-------------------

Additionally the ear responds differently to peaks and dips in the direct sound.

Experiments were made where the direct sound was manipulated.
One set of manipulations was intriducing peaks at different frequencies.
The other set had dips in the same frequencies.

The result was that listeners responden to peaks much stronger than to dips.
In many cases they did not respjnd to dips at all. Only strong dips could be detected.

So, another question is how relevant the dip is in terms of perception.

-------------------

The tricky thing is the right interpretation of measurements.

So it is inportant to know what has been measured and how it was measured (signals, microhphone setup, length od measurement widwows etc.)

It can be that dips and peaks are measurement artefacts.

-------------------

I would recommend several different measurements, for example:
  • Room reverberation (RT60 or RT30)
  • Waterfall diagram

The interpretation of those would be the basis for further thoughts.

Cheers
Babak
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post #82 of 110 Old 02-28-14, 09:32 AM
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Re: Bass trap placement - high or low? Does it matter?

Not disagreeing with anything being said here - i kinda think that this problem is being over thought a little....

He already stated that the null went away when he placed the old speakers against the wall.... hence he moved the source to change the wavelength location in the small room, hence room mode.......

If when the new speakers are looked at and the problem comes back, location could then be checked and then the crossover points between the subs and the mains (and phase).

Knowing the problem is half the battle.

Gordo
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post #83 of 110 Old 02-28-14, 09:34 AM
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Re: Bass trap placement - high or low? Does it matter?

Quote:
phazewolf wrote: View Post
Bass traps can do one thing a extra sub can't and that is help with the decay times in the bass region. That said I think in the case of this room there maybe something else going on think about it.
Well, without having to go into a 12 page post, I'll just say no. "Bass traps" are simply absorbers that convert sound energy to heat. Fixing the frequency domain, i.e, making it smoother, will also fix you visual time "problems".
But for "audio" problems in the low bass region, you need no such things. What you can "see" in a measurement and what your 2 ears perceive, are two different things. Too many people approach acoustics "intuitively". That is the wrong approach. Perceptual science/psychoacoustics is far from intuitive.
My advice, worry only about amplitude peaks. Cut them down to avg level. Forget narrow dips. Broad ones, a combination of multiple sources and EQ is perfect for HT.

Quote:
phazewolf wrote: View Post
If he moved the sub all over the room and still has the same issue at the same frequencies then it could be several issues combining to cause the dip. Not using that line of thought maybe just maybe a trap could help more then a sub.
No. Something effective for those frequencies would be enormous and physically change the volume of the room. Plus I consider it foolhardy to throw away energy at those frequencies.
Still curious about how he measured, some of it makes no sense.

Quote:
phazewolf wrote: View Post
My room had a 30db peak at 57hz that could not find a way to fix with moving stuff around. When I started adding traps it helped it a lot but then show other issues that were being masked. The bass traps took care of son of the peak and moving stuff fix a lot of it too. But I ended up with a few large deep nulls that when I remove the traps are not there.
Exactly. "Traps" are big, "dumb" pieces of absorber. They're not smart enough to know exactly what frequencies to surgically remove. Active OTOH, is not...at least in skilled hands.
Why didn't you just EQ the peak? You're not one of those 2ch anti-EQ "purists", are you?

cheers

AJ

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post #84 of 110 Old 02-28-14, 12:27 PM
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Quote:
ajinfla wrote: View Post
"Traps" are big, "dumb" pieces of absorber. They're not smart enough to know exactly what frequencies to surgically remove.
That's right.

For narrow band reduction one can use Helmholtzresonators or plate/membrane resonators.

Or use a couple (4-6) of Vicoustic Vari Bass modules. They can be tuned to specific frequencies.
http://www.vicoustic.com/hifi-home-c...trap/panel/470


I can also recommend the following absorbers :

The Super Bass Extreme work for broader band absorption from 60 to 100 Hz.
http://www.vicoustic.com/hifi-home-c...trap/panel/473


The Wave Wood panels also work as broad band bass traps (125 to 350 Hz) when mounted over the corners, eg between wall and ceiling.
http://www.vicoustic.com/hifi-home-c...trap/panel/626

Cheers
Babak
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post #85 of 110 Old 02-28-14, 02:35 PM
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Re: Bass trap placement - high or low? Does it matter?

Quote:
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Or use a couple (4-6) of Vicoustic Vari Bass modules. They can be tuned to specific frequencies.
http://www.vicoustic.com/hifi-home-c...trap/panel/470
Babak, have you used this product? And if so how well did it perform?

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post #86 of 110 Old 02-28-14, 04:27 PM
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For the measurements I used REW with a calibrated mic. I don't have any of the files anymore they were not recovered when the hard drive crashed so I am only going by my memory which is poor a lot of times.

The hole at 107hz was about 35db deep and if I recall went down to 103 and upto 111hz but I can't be sure.

I have no issue using a eq to fix a peak I just prefer moving everything I can first to make things as smooth as I can first.
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post #87 of 110 Old 02-28-14, 08:52 PM
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Re: Bass trap placement - high or low? Does it matter?

Hi
Quote:
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Babak, have you used this product? And if so how well did it perform?
In my room the Wave Wood panels mounted over the room corners were sufficient.
The speakers are located on 1/4 of the room length and the room width i.e. on nulls of several and the listening position is in the middle of the room, i.e. on nulls of other modes.

So room modes are not really a problem in my room.

A friend of mine distributes Vicoustic products in Austria and I could hear the Vari Bass Pro modules
in several different rooms.
They work very well if enough of them are installed.
4 are sufficient for small rooms, 6 for medium sized rooms and large rooms possibly need 8 of them.

They work best if they are part of a bigger acoustical concept.
That means
  • measure the reverberation times of the room (RT30 or RT60)
  • use the broad band absorbers (Wave Wood over corners and Super Bass Extreme) to reduce the overall reverberation times at low frequencies
  • measure again to identify increase of bass response due to room modes
  • install the Vari Bass modules against specific modes.
It is an iterative process of measuring and improving.

So in my rooms the measurements after installing the Wave Wood panels showed that there is no real need to do anything about the modes.

Of course the Vari Bass modules are not magic but only use physics.
But using them are less effort than calculating, building and tuning Helmholtz resonators by yourself.
Especially the possibility to tune them easily to every frequency makes them very convenient and flexible.


Cheers
Babak
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post #88 of 110 Old 02-28-14, 09:04 PM
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Hi

Quote:
phazewolf wrote: View Post
For the measurements I used REW with a calibrated mic. I don't have any of the files anymore they were not recovered when the hard drive crashed so I am only going by my memory which is poor a lot of times.

The hole at 107hz was about 35db deep and if I recall went down to 103 and upto 111hz but I can't be sure.
Thanks for the details.
Do you remember which kind of measurement you performed using what kind of signal?
Do you remember the position of the mic in the room, that means in relation to all room boundaries (floor, ceiling, walls)?

Those distances could give a hint, whether the hole was caused by interference of the direct sound with the sound reelected by one of the boundaries.

As written before...
In my opinion one gets a better idea of the room response and room modes by measuring the reverberation times (RT60 or RT30) and a waterfall diagram of the room response curve and it's decay.

Cheers
Babak
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post #89 of 110 Old 03-01-14, 03:26 PM
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The mic was at ear height 38" from the ground the location is 8' from the rear wall this had the best sounding bass other locations has large drops in bass and you could hear things missing. From the left wall I am 70"

As I have said the room is split into 2 half's so very limited placement.

When I get time I will measure more but what I knew of of the room was with my old Polk SDA speakers which needed to be placed with in 6 to 12 inches of the front wall to work correctly. The new speakers are placed completely different.

The sweeps were made with REW as I said and it was a normal sweep I can't really recall the rest of the details anymore.
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post #90 of 110 Old 03-05-14, 12:02 PM
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Re: Bass trap placement - high or low? Does it matter?

Quote:
Babak wrote: View Post
That's right.
For narrow band reduction one can use Helmholtzresonators or plate/membrane resonators.

Or use a couple (4-6) of Vicoustic Vari Bass modules. They can be tuned to specific frequencies.
http://www.vicoustic.com/hifi-home-c...trap/panel/470
Hi Babak,,

Is there any data (3rd party preferably) showing the efficacy of those type products?
What is the cost?
That would be interesting to compare, vs the same cost subwoofers. Subwoofers that would increase output, lower distortion and be actively adjustable...and most likely much smaller and less obtrusive, both physically and spatially.
Of course, with subs, usually at most, only 3-4 would be needed.
I've never quite grasped the concept of going through the trouble of creating energy....only to throw it away. Seems wasteful to me.

cheers

AJ

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