Absorbing frequency at RFZ

[nixoblivion]

As far as i know, one can hear direction until around 100Hz...
So does that mean that all absorbant panels at RFZ should absorb up to 100Hz??

If so, i would need a minimum of 10cm + gap of rockwool on all points of the RFZ...

[SAC]

jklp

[nixoblivion]

Originally Posted by SAC

I think you will find that the model for an RFZ is not done with a frequency response plot.

It utilizes the ETC and analyzes acoustic energy in terms of its arrival times.

Acoustical energy below the Schroeder critical frequency, fc, acts in a modal manner, whereas energy above that frequency behaves as specular reflections.

Thus, below the critical frequency, fc, you employ bass trapping, and above fc you employ absorption and diffusion (as appropriate).

Yes i have used ETC's to determine the RFZ. As you said, it measures the energy that arrives at the sweet spot, thus i always thought that lower frequencies were considered as well.

Can you please be more specific about the Schroeder critical Frequency, how do i find this fc? im guessing it depends on the room...

[SAC]

klpjklk

[nixoblivion]

Ok. Thanks, great info.


So lets take my room as example
ca. 8m long - 42,5hz
ca 3,8m wide - ca. 90Hz
ca 2,4m high - ca. 140Hz

So that would mean my room is specular down to 42,5Hz (worst case) and 140Hz (best case)
Am i thinking wrong here?

[tINY]

First reflection absorbers won't help modal behaviors enough to worry about.

What you need to think about is path-length difference. If you can absorb the frequencies down to the response dip (where the reflected path is 1/2 wavelength longer than the direct path) you are in good shape.




-tINY

[avare]

Originally Posted by tINY

First reflection absorbers won't help modal behaviors enough to worry about.

What you need to think about is path-length difference. If you can absorb the frequencies down to the response dip (where the reflected path is 1/2 wavelength longer than the direct path) you are in good shape.

Thank you! I never thought of working with the Δd* to calculate the lowest effective frequency of absorption for the initial sound absorbers.

As far as these absorbers not being effective, they are still effective at low frequencies, but not as much as in a corner. IIRC from a BBC RD report, the effectivenes was approximately one third, compared to being in a corner. Not as effective as a corner, but in a forum where the mantra is "as much bass trapping as possible." getting more low end absoption without consuming floor area, provides another option in the acoustic treatment toolkit.

Andre

*Δd: DanDan got me a roll using characters not present on the normal keyboard layout. Δd is the technical short form for difference (Δ) in distance (d). The Δ is the Greek upper case letter delta, which is used for difference.

[tINY]

Yeah, below that, you have all the other walls to worry about and how they affect the phase difference between your ears. You may not fix the localization cues in the bass, but it's the best you can do for that wall.

I did high speed signal simulation for a while. That's all done in the time domain, so it changes how you think about things a bit.



-tINY

[nixoblivion]

Originally Posted by tINY

First reflection absorbers won't help modal behaviors enough to worry about.

What you need to think about is path-length difference. If you can absorb the frequencies down to the response dip (where the reflected path is 1/2 wavelength longer than the direct path) you are in good shape.

-tINY

Hey tINY.
I dont really get what your saying, but i thinks its a language thing. Maybe you can explain what you mean in just a little more detail?

[tINY]

Measure the distance from the speaker to the ear. Measure the distance from the speaker, to the point of reflection and then to the ear.

You now have the two distances that sound travels to get to your ear - direct and reflected. Subtract the direct from the reflected distance and you have the pathlength difference. The lowest cancelation you get from this reflection point is a tone with twice the pathlength distance as it's wavelength.

So, if you have 6 feet from your monitor to your ear and a wall about 6 feet to your side:

1) The direct path is 6 feet

2) The reflected path (speaker to wall to ear) is about 13 1/2 feet

3) The pathlength difference is about 7 1/2 feet

4) Twice the pathlength distance is 15 feet - which is the wavelength of about 75Hz (speed of sound / distance = frequency)

So, in this case, you should target down to about 75 Hz for absorbtion.





-tINY

[AnthonyRochester]

this is interesting and useful, thanks.

[nixoblivion]

Originally Posted by tINY

Measure the distance from the speaker to the ear. Measure the distance from the speaker, to the point of reflection and then to the ear.

You now have the two distances that sound travels to get to your ear - direct and reflected. Subtract the direct from the reflected distance and you have the pathlength difference. The lowest cancelation you get from this reflection point is a tone with twice the pathlength distance as it's wavelength.

So, if you have 6 feet from your monitor to your ear and a wall about 6 feet to your side:

1) The direct path is 6 feet

2) The reflected path (speaker to wall to ear) is about 13 1/2 feet

3) The pathlength difference is about 7 1/2 feet

4) Twice the pathlength distance is 15 feet - which is the wavelength of about 75Hz (speed of sound / distance = frequency)

So, in this case, you should target down to about 75 Hz for absorbtion.


-tINY

Thanks for this Info. But it confuses me a little.
First of all, it doesnt quite fit together with SAC's explanation!
Which is right? Do they work together?

What also confuses me is that SAC said that you only have to treat the RFZ with 300Hz above in smaller rooms.
But when i calculate for my room, or your example, im WAY lower... So do i really have to treat that low for RFZ??

[SAC]

klpkjl

[tINY]

Originally Posted by nixoblivion

Thanks for this Info. But it confuses me a little.
First of all, it doesnt quite fit together with SAC's explanation!
Which is right? Do they work together?

What also confuses me is that SAC said that you only have to treat the RFZ with 300Hz above in smaller rooms.
But when i calculate for my room, or your example, im WAY lower... So do i really have to treat that low for RFZ??

Your ability to localize sound is dimminished at lower frequencies. If you can get everything under control down to about 3kHz - it will make a noticeable improvement. Down to about 1kHz will conquer most of the perception issues. This is all related to psychoacoustics - your head, ears, and brain.

You can still localize sound at lower frequencies, but the mechanism is different and less effective. In the theoretical examples, there are a couple of different approaches: SAC uses a statistical method for determining when the sound bouncing around the room is sufficiently random that you won't be able to localize the lower frequencies. My more simple approach aims to eliminate the contribution of the wall you are mounting the absorber to.

In the end, you will use 2 or 4" material spaced 2 or 4" from the wall. Both seem to work well enough in a small room...



-tINY

[SAC]

klklzation, and tonality.

[DanDan]

tiNY, thanks for that innovative way of looking at it.
nix, many of the classical concepts don't quite fit well with very small rooms. e.g. there is no Reverb Time RT60. In such small rooms, I reckon it is reasonable to presume that modes will strongly dominate the room decay. They are so strong, causing such pressure peaks and nulls, that they do show up somewhat in the frequency response also.
I would just assume the modal region to be 30-300.
For your interest-
The mode calculator at bobgolds.com and ModeWiz calculate the Schroeder frequency and illustrate the bands of frequencies where modal, ray, etc. behaviour can be expected.

Apart from RFZ and Cloud, there are a couple of other issues at play also. LF ones. The Width mode. The LF reflections of the speaker, there are several.
For these reasons I recommend that RFZ and Cloud panels, be say 10cm thick, with a 10cm airgap, plus FRK or other membrane bonded to the back. (as in nix' OP! ) I could make a case for extending these all the way to the front wall in an attempt to deal with the LF aspects.
DD