Bass trap placement - high or low? Does it matter? - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com
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post #31 of 110 Old 02-19-14, 03:10 PM
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Babak
 
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sdurani wrote: View Post
Good info, but doesn't apply to the low frequencies being discussed in a thread about bass traps.
The same for post #24.

Also your post could be understood in terms of comb filters:
Quote:
sdurani wrote: View Post
It is reflections from the walls that are causing the cancellation (drop in level) in the middle of the room where you sit. Placing absorption/traps on the walls reduces those reflections, thereby reducing the cancellation at your listening position. To make more efficient use of absorption, you can place the traps where walls meet (corners).
This is a description of comb filters due to reflections canceling out certain frequencies i.e. causing dips.

Also dips in low frequencies are not heard very well by listeners. Lo

The main effect of low frequencies in small rooms are standing waves leading to room modes.
Dips through comb filtering is not such an issue.

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

Regarding modes and bass traps...
The link that I posted (the White Papers from Harman) leads to papers regarding good low frequency performance, including the optimal number and placement of subwoofers.

The paper " Part Three: Getting the Bass Right"
( http://www.harman.com/EN-US/OurCompa...ndRoomsPt3.pdf)
summarises everything in a very good way.

How to avoid exciting room modes is an important point.
You don't need bass traps for room modes that are not excited.

Cheers
Babak
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post #32 of 110 Old 02-19-14, 03:45 PM
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Re: Bass trap placement - high or low? Does it matter?

Quote:
Babak wrote: View Post
Also your post could be understood in terms of comb filters:
Not when taken in context (i.e., in a discussion about bass traps, where liteglow was asking about a drop in the 20Hz to 50Hz region).

I don't want to dissuade you from introducing other concepts (comb filter effects, how our hearing works, spaciousness, perceived soundstage, Precedence Effect, first reflections, diffusors, etc.) into this discussion, but I hope you understand that they aren't related to this particular discussion (using bass traps to address a drop in the 20-50Hz region).

Like I said, it is good information but is applicable to frequencies outside the bass range.

Sanjay
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post #33 of 110 Old 02-19-14, 03:49 PM
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Re: Bass trap placement - high or low? Does it matter?

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Babak wrote: View Post
How to avoid exciting room modes is an important point.
You don't need bass traps for room modes that are not excited.
That's why I asked liteglow if he could move his sub to the location of the null (would prevent that particular room mode from resonating).

Sanjay
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post #34 of 110 Old 02-19-14, 04:28 PM
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Quote:
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Not when taken in context (i.e., in a discussion about bass traps, where liteglow was asking about a drop in the 20Hz to 50Hz region).

I don't want to dissuade you from introducing other concepts (comb filter effects, how our hearing works, spaciousness, perceived soundstage, Precedence Effect, first reflections, diffusors, etc.) into this discussion, but I hope you understand that they aren't related to this particular discussion (using bass traps to address a drop in the 20-50Hz region).

Like I said, it is good information but is applicable to frequencies outside the bass range.
I appreciate your input.

Again, the topic of reflections was not introduced by me. I just responded to an earlier post.

Also taken in the context of low frequencies, your sentence
Quote:
sdurani wrote: View Post
It is reflections from the walls that are causing the cancellation (drop in level) in the middle of the room where you sit.
describes reflections that cause cancellations leading to a decrease in sound pressure level.

That's the same thing that leads to comb filtering and something different than standing waves leading to room modes and nulls.

It doesn't matter where the dips in the frequency response come from - comb filter effects, cancellations through reflections, narrow band filters or EQ's.

Listeners don't hear dips in the frequency response very well - if at all.
That is the case no matter whether it is in the high or in the low frequency region.

The reason for this lies in the way the hearing works.
Microphones work in a different way, and that's why these dips can be measured but hardly be heard.

I hope you understand that the topic whether dips can be heard is relevant for this discussion, as liteglow wrote about dips.

However, as long as we don't know how liteglow made his measurements and how the results look like, all of this discussion is only speculation.


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post #35 of 110 Old 02-20-14, 12:19 AM
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Re: Bass trap placement - high or low? Does it matter?

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Babak wrote: View Post
I hope you understand that the topic whether dips can be heard is relevant for this discussion, as liteglow wrote about dips.
Again, if you want to obfuscate a discussion about low frequencies and bass traps with concepts (comb filter effects, spaciousness, perceived soundstage, Precedence Effect, first reflections, diffusors) that apply to frequencies outside that range, then that's up to you. However, if you could offer something (anything) that could help liteglow with his 10dB dip between 20-50Hz (besides telling him "listeners don't hear dips"), then that would be even better.

Sanjay
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post #36 of 110 Old 02-20-14, 07:30 AM
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Re: Bass trap placement - high or low? Does it matter?

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Again, if you want to obfuscate a discussion about low frequencies and bass traps with concepts (comb filter effects, spaciousness, perceived soundstage, Precedence Effect, first reflections, diffusors) that apply to frequencies outside that range, then that's up to you. However, if you could offer something (anything) that could help liteglow with his 10dB dip between 20-50Hz (besides telling him "listeners don't hear dips"), then that would be even better.
There's no need to get offensive.
Nobody wants to obfuscate anything, so please take it easy.

Maybe I was not clear.

As I wrote before, the topic about reflections was not introduced by me but I only responded to that.
So you should place your complaint about that somewhee else.

Obviously you have overread my other comments to liteglow's bass dip issue.
I also understand that some points appear obfuscating if the connections are not clear.
So I'll try to explain it in another simpler way.

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

The first question is, whether the dip is a problem at all-
It would help to see the measurement

There are tolerance limits for variations the room response curve.
Is the dip going below the lower limit of that tolerance range?

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

The second question is what causes the dip.
It would be good to know how the measurements were made.

The dip could be a measurement artefact.
Or - if it was a measurement of the room response curve - the dip can be a cancellation between direct and reflected sound.

If it is a cancellation issue it doesn't matter because the ear does not perceive those cancellations that are measured by the microphone.
Ears and microphones work differently and hence response differently to that.

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

The third question is whether the dip can be heard at all.

Experiments were made using dips with a certain center frequency, a certain width and a certain amplitude with peaks that have the same parameters.
Listeners perceived the peaks pretty clearly whereas they did only weakly respond to the dips, in many cases they did not perceive them at all.

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

So I think that this is an issue that could possibly not be relevant.

It can be a measurement artefact or a dip that is not perceivable (but measurable) or both.


As long as we don't have more details about the measurements, nothing can be added to the discussion.
There is no point in making speculations.

And there is also no point in picking words and starting a dispute about them.


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post #37 of 110 Old 02-20-14, 08:20 AM
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Re: Bass trap placement - high or low? Does it matter?

Hi

regarding the question, whether the dip is too big one can refer to the recommendations of the ITU (International Telecommunication Union)

Recommendation ITU-R .1116-1
Methods for the Subjective Assessment of Small Impairments in Audio Systems Including Mulstichannel Sound Systems

http://www.itu.int/dms_pubrec/itu-r/...0-I!!PDF-E.pdf

Especially interesting for this topic:
8.3.4.1 Operational room response curve
It reads as follows:
Quote:
The operational room response curves are defined as the one-third octave frequency responses of the sound pressure levels produced by each monitor loudspeaker at the reference listening position, using pink noise over the frequency range 50 Hz-16 kHz. The measured operational room response curves shall fall within the tolerance limits given in Fig. 2.

The differences between the operational room response curves produced by each of the (stereo or multichannel) front loudspeakers at the reference listening point should not exceed the value of 2 dB within the whole frequency range.
This is why I asked about details, how the measurement was made.
Was it measured with pink noise?
Was is measured in one-third octave bands?
How does it compare with the tolerance limits for operational room response curve on page 13?

It would also be helpful to see the RT60 measurements of the room


Cheers
Babak

Last edited by Babak; 02-20-14 at 08:51 AM.
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post #38 of 110 Old 02-20-14, 09:31 AM
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Re: Bass trap placement - high or low? Does it matter?

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As I wrote before, the topic about reflections was not introduced by me but I only responded to that.
Are you able to separate low frequency reflections in the modal range vs reflections above the transition frequency that result in comb filtering, spaciousness, perceived soundstage, precedence effect, first reflections, diffusors? I only ask because the post you responded to was about the former but the information you posted is about the latter, giving the impression you can't separate the two concepts.

Sanjay
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post #39 of 110 Old 02-20-14, 11:08 AM
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Re: Bass trap placement - high or low? Does it matter?

Sanjay,

Quote:
sdurani wrote: View Post
Are you able to separate low frequency reflections in the modal range vs reflections above the transition frequency that result in comb filtering, spaciousness, perceived soundstage, precedence effect, first reflections, diffusors? I only ask because the post you responded to was about the former but the information you posted is about the latter, giving the impression you can't separate the two concepts.
I don't have clue why you try to provoke me by using this offensive style.

In my point of view there is a difference between reflections that lead to cancellations (comb filtering) and room resonances blow the Schroeder Frequency causing standing waves that lead to exaggerated response and booming bass at certain frequencies (modes).

So if you really want to go down the route to be picky on terms and definitions:
When looking at low frequency standing waves (room modes) we are talking about resonances and not about reflections.

What is written here about reduction of sound pressure due to reflections has nothing to do with resonances/standing waves/room modes but is a comb filter effect taking place also at other frequencies.

In my understanding those are the two concepts that need to be seperated but are mixed up here.

--------------------
Regarding spaciousness, reflections precedence effect etc.:

My answer in post #29 was a reply to the posts #24.

Post #24 stated
Quote:
rkeman wrote: View Post
A typical room has many areas of cancellation and reinforcement that vary by frequency.
That's describing comb filtering, similar to other posts that deal with reflections that cause cancellations.

Quote:
rkeman wrote: View Post
The best compromise is finding listening and loudspeaker locations that yield the least variation and yet provide good imaging.
Good imaging is dependent on reflections and includes spaciousness. Precedence Effect plays a major role here.

Quote:
rkeman wrote: View Post
Try moving the seat closer to the rear wall [...]
This would increas early reflections, probably also those earlier than 5 ms. That reduces imaging, soundstage. The Precedence Effect plays a major role here as well.
So I would reccomend not to move the seat too close to any wall.

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

My other posts #31, #34, #36 and #37 did not deal with those topics at all.
Can it be that you did not read them because you are still stickig to my post #29?

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

Room acoustics deals with a whole bunch of topics at the same time.
One cannot really look at optimal positioning of the loudspeakers and the seat in regards to room modes without looking at the consequences in regards to the wall reflections at the same time.
The optimization of one parameter can lead to a worse situation with a different parameter.

Maybe that is also one thing that seems to obfuscate things.
One simply cannot look at one parameter in isolation.

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

So, again my three main points from those posts:

It would be interesting to see the data and the graph from the measurement.
Then we could see what the observed dip really looks like and wheather it is beyond the tolerance levels.

It would also be good to know how the measurements were conducted.
Maybe the dip is more a measurement artefact.

The human ear does not really perceive dips very well. Often it does not perceive them at all.
It is more sensitive to peaks.
So maybe the observed dip is just a detail that only looks ugly in a measurement but is not relevant for the listening experience.

Cheers
Babak

Last edited by Babak; 02-20-14 at 11:30 AM.
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post #40 of 110 Old 02-20-14, 11:58 AM
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Re: Bass trap placement - high or low? Does it matter?

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Babak wrote: View Post
In my point of view there is a difference between reflections that lead to cancellations (comb filtering) and room resonances blow the Schroeder Frequency causing standing waves that lead to exaggerated response and booming bass at certain frequencies (modes).
In that case, you understand that you replied to my post discussing how bass traps could help a room resonance below Schroeder by posting a reply about comb filtering, spaciousness, perceived soundstage, precedence effect, first reflections and diffusors, all of which are discussed in the context of frequencies above Schroeder.

Sanjay
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