Low frequency suck out, what can'I do?

Nyal Mellor · « **Reply #100 on:** 2 Jun 2011, 05:15 pm »

For the resonance at 25Hz:
You can fix 25Hz with EQ as long as the peak in frequency response is minimum phase. But the highpass idea from McTwins is another suggestion that could work. Personally I'd use a parametric EQ filter though.

For the dip at 35Hz:
The narrow band EQ filter used to deal with the peak at 25Hz is not going to effect the suckout just above 35Hz. To flatten a suck out you need to need to figure out if the dip is SBIR related or to do with the lack of modal reinforcement at that frequency.
- If SBIR related then changing sub positioning will change the dip frequency (since the path length difference to the listening position changes). The other option with SBIR related dips is to place thick absorbers at the boundary that is causing the null
- If lack of modal reinforcement then again changing placement may improve things, since you may be able to position the sub near to or in the center of a node. The other option is to use EQ to boost the frequency but be careful here that you don't run out of headroom with the subwoofers.

Bear in mind that I've tried to keep the above general recommendations rather than specific for the room at hand; I haven't looked at the specific room in question or the placement of subs and speakers...

McTwins · « **Reply #101 on:** 2 Jun 2011, 05:44 pm »

Hi
Nyal Mellor....
How do you distinguish between a resonance and SBIR. I want you to show me how you do this in practice and add some measurements as well. As far as I know, there is no EQ that can fix a modal resonance.
Thanks

McTwins · « **Reply #102 on:** 2 Jun 2011, 05:57 pm »

Hi
Here is a room that is with concrete walls(bank vault) at a dealer with a dimension 5x5x2,5 meters.

http://i1223.photobucket.com/albums/dd505/mctwins1/valvet.jpg

and before

http://i1223.photobucket.com/albums/dd505/mctwins1/Before.jpg

and after

http://i1223.photobucket.com/albums/dd505/mctwins1/After.jpg

and this is a problem around 30Hz and has been fixed by Matts SMT. He can't be the only one that is showing some measurements.

very impressive, regarding that this is a very small room.

Thanks

Nyal Mellor · « **Reply #103 on:** 2 Jun 2011, 11:07 pm »

Quote from: McTwins on 2 Jun 2011, 05:44 pm

Hi
Nyal Mellor....
How do you distinguish between a resonance and SBIR. I want you to show me how you do this in practice and add some measurements as well. As far as I know, there is no EQ that can fix a modal resonance.
Thanks

Hi McTwins - request ackowledged. I have a blog article in my head to put the results of one band of parametric EQ at 25Hz that shows how properly applied EQ reduces ringing. That should be up tonight on my blog.

Distinguishing between a resonance and SBIR is a good question. The simplest way to do so is with an excess group delay measurement. See here for more details: http://www.hometheatershack.com/roomeq/wizardhelpv5/help_en-GB/html/minimumphase.html

audioblazer · « **Reply #104 on:** 3 Jun 2011, 12:39 am »

Nyal,
Let me see whether I get U. I remove all my surround / centre channel interconnect. Use my preamp HT by pass. Switch on my pre/power for HT. Plug the XTZ Y connector to the preamp output and hopefully I can measure my 2 channel + sub. Thks

Nyal Mellor · « **Reply #105 on:** 4 Jun 2011, 05:00 am »

McTwins, and gracious thread readers please cast your eyes on the promised article on room mode equalization / EQ, showing hard proof of the efficacy of equalization in combating both the frequency response peak and time domain ringing associated with room modes. The measurements are taken from a room I calibrated and my good friend and acoustics accomplice Jeff Hedback designed.

McTwins · « **Reply #106 on:** 4 Jun 2011, 09:39 am »

Hi
Nyal mellor...
Interesting reading, but I am not convinced.

I can agree that one can EQ a room if it is treated acoustically as possible. I would like to see how it affects in an untreated room and how much this EQ can fix the problem. If I am using Helmholtz resonators I can always remove ringing and adjust the modal decay in an untreated room.

Decreasing the peaks at 25Hz and 50Hz, as stated in the article, thus removing the ringing one schould have seen an increase in the region between 25Hz-50Hz. There is also a suckout at 56Hz due too EQ that wasen't there before.
When one reduces the ringing one schould be able to see other changes in the freq response at the same time, meaning an increase in level between 25Hz-50Hz. That's why I am not convinced.

The graph showing Impulse response, schoulden't it be Step response?

Regarding the article about group delay and minimum phase there is three graphs that shows lenght, width and hights.

How can one measure this in a room? For me it is impossible to understand how one can measure in a room in one axiall direction, seperately from each other, and try to read out graphs and evaluate minimum phase, group delay, and so on.

Thanks

McTwins · « **Reply #107 on:** 4 Jun 2011, 11:20 am »

Hi
i forgot to ask...

When I see the freq response, shoulden't it be at a same level as in the first measurement?
In this case , the level schould be raised by 8dB and that requiers extra power from the amplifier, or one would lose the headroom. That's the disadvantage of the EQ in the digital domain.

To be able too see an accurate measurement, the level schould be at 100dB or more, so one is away from the background noisefloor.

And also, that's why you see the differen't in the impulse response graph.

This is my untreated(brown) and treated(green) Step Response in my dedicated room.

Thanks

bpape · « **Reply #108 on:** 4 Jun 2011, 02:09 pm »

EQ can be effective if (big if) the center frequency and the Q are exactly matched to the problem frequency in question. If either is off, even slightly, then it can actually cause more ringing than it is fixing at other frequencies. I would agree that EQ is a last resort only to be used when it's not possible or practical to fix the problems via positioning and treatment - and then, only in the deepest bass, and never for modal nulls.

Bryan

McTwins · « **Reply #109 on:** 5 Jun 2011, 11:03 am »

Hi
Interesting comment on the blog, Ethan. I was thinking of asking similar question.

Bpape...This link supports what you are saying, and I can agree.

http://www.realtraps.com/art_audyssey.htm

Some more interesting acoustic makover we have picked up from Euphonia forum.

While Svanå Matts built his unique worldclass Listening/Tracking room(Level 3) to Marten Design, another company had responsible for a smaller listening room in the same building.

The smaller room dimension was perfect in the Bolt "foot" and had no parallel walls. Playing and talking in that room was a nightmare becouse of the axiell modes was perfectly seperated and it created togheter with the oblique and transversial resonances a amazing bathroom voice and as you see below, a rollercoster in the bass.

Matts now use this room to teach his exclusive Roomtuners that help him around the world.

Final test should be performed like this in a day.

Before

After

After visting this room they also know that all this Golden ratio and Bolt foot is made for much larger rooms.

Thanks

Nyal Mellor · « **Reply #110 on:** 5 Jun 2011, 10:12 pm »

Quote from: McTwins on 4 Jun 2011, 09:39 am

Hi
Nyal mellor...
Interesting reading, but I am not convinced.

I can agree that one can EQ a room if it is treated acoustically as possible. I would like to see how it affects in an untreated room and how much this EQ can fix the problem. If I am using Helmholtz resonators I can always remove ringing and adjust the modal decay in an untreated room.

Decreasing the peaks at 25Hz and 50Hz, as stated in the article, thus removing the ringing one schould have seen an increase in the region between 25Hz-50Hz. There is also a suckout at 56Hz due too EQ that wasen't there before.
When one reduces the ringing one schould be able to see other changes in the freq response at the same time, meaning an increase in level between 25Hz-50Hz. That's why I am not convinced.

The graph showing Impulse response, schoulden't it be Step response?

Regarding the article about group delay and minimum phase there is three graphs that shows lenght, width and hights.

How can one measure this in a room? For me it is impossible to understand how one can measure in a room in one axiall direction, seperately from each other, and try to read out graphs and evaluate minimum phase, group delay, and so on.

Thanks

Couple of things:

- impulse response is more used to show how a room behaves acoustically; I have never come across step response in relation to acoustics

- the graphs in the article are theoretical, to show how axial modes can combine into a response that has non flat excess group delay

- why would the spl level of other parts of the frequency response change if I only applied EQ to the peaks?

- we both agree that EQ is best applied AFTER acoustic treatment.

- I can show some graphs in a future article showing how applying acoustic treatment actually makes it easier to effective apply EQ by flattening the excess group delay measurement.

- In many cases the passive treatment required to deal with resonances at low (<100Hz) frequencies can be large, visually obtrusive and expensive. In these cases EQ is a reasonable alternative if carefully applied. And yes, I have had positive measured AND subjective improvements from application of EQ in a room with NO bass treatments.

Nyal Mellor · « **Reply #111 on:** 5 Jun 2011, 10:13 pm »

Quote from: bpape on 4 Jun 2011, 02:09 pm

EQ can be effective if (big if) the center frequency and the Q are exactly matched to the problem frequency in question. If either is off, even slightly, then it can actually cause more ringing than it is fixing at other frequencies. I would agree that EQ is a last resort only to be used when it's not possible or practical to fix the problems via positioning and treatment - and then, only in the deepest bass, and never for modal nulls.

Bryan

Agreed! Only to be used below about 120Hz...

Nyal Mellor · « **Reply #112 on:** 5 Jun 2011, 10:16 pm »

Quote from: McTwins on 4 Jun 2011, 11:20 am

Hi
i forgot to ask...

When I see the freq response, shoulden't it be at a same level as in the first measurement?
In this case , the level schould be raised by 8dB and that requiers extra power from the amplifier, or one would lose the headroom. That's the disadvantage of the EQ in the digital domain.

To be able too see an accurate measurement, the level schould be at 100dB or more, so one is away from the background noisefloor.

And also, that's why you see the differen't in the impulse response graph.

This is my untreated(brown) and treated(green) Step Response in my dedicated room.

Thanks

- This room was very quiet (<30dB), so 75dB was a fine level to take this measurement at.

- Headroom with this system was not an issue. Of course it needs to be taken into account when thinking about EQ so your point is valid.

- Are you sure you are measuring at 110dB SPL? That would be EXTREMELY loud (like nightclub levels). More likely is that your ARTA is not calibrated properly. I have never seen any acoustician measure at 100dB...

McTwins · « **Reply #113 on:** 6 Jun 2011, 05:50 pm »

Hi

- why would the spl level of other parts of the frequency response change if I only applied EQ to the peaks?

Hard evidence for me is to achive the same as proper tuned HH boxes do:

When they lower the ringing, they also even out the lower frequences with out loosing any headroom. They lower the "Q-value" for the hole room.
What i can see from your graphs is that you don't trigger the mode as much as before, downsides is you get 8 dB less efficiency and a suckout just above 50 hz were much of the fun HT impact is.

Whats happening above 60Hz is also interesting to see because you EQ tuning at 50Hz.

Regarding my step response, this is the one you use for the lower freq. Maybe your impulse response is showing only a 25 Hz tone(one single tone).

Step response is obtained as a time intergral of the impulse response according to ARTA. It is valuable for monitoring of low-freq system behavior, but I asume you know this already.

I bealive that Matts from SMT AB have a calibrated mic and also transport a Sonotube 21" bass to get this high SPL.

Audioblazer,

sorry for have been hi-jacked your thread like this but I hope you felt that this was interesting to see, from all of this meausurment, that your room can bee fixed. So now you know who to call in the future to get your problem fixed, it is Matts

.

Thanks

Nyal Mellor · « **Reply #114 on:** 7 Jun 2011, 02:47 am »

Audioblazer, yes I am sorry we have hijacked your thread! McTwins I have nothing further to add and in any event I am not sure we were making any progress

Best to call it a day

audioblazer · « **Reply #115 on:** 7 Jun 2011, 03:19 am »

Nyal, maybe u can help to answer a question I posed earlier. Usually at what height from the floor should a diffuser be place? Does it matter if I place all the diffusers - front wall& side wall near listening position at 24" from floor and diffuser at side wall behind speakers at 30" ( due to subpanel at 24" position). Thks for your contributions to this thread

Jeffrey Hedback · « **Reply #116 on:** 7 Jun 2011, 12:11 pm »

Hello Audioblazer,

I'll jump in and offer that (unfortunately) there is no usual height for diffusor placement other than the mid band (vertically) of the the wall surfaces. But the real answer can only be offered by proper study of your specific space and the type of diffusor (1D, 2D, QRD, PRD, Binary Amplitude...etc...). It might be a good time to reset and post a plan view of your room and description of your desired diffusors (and please excuse me if I missed that information somewhere in this thread).

Nyal Mellor · « **Reply #117 on:** 8 Jun 2011, 12:32 am »

Quote from: audioblazer on 7 Jun 2011, 03:19 am

Nyal, maybe u can help to answer a question I posed earlier. Usually at what height from the floor should a diffuser be place? Does it matter if I place all the diffusers - front wall& side wall near listening position at 24" from floor and diffuser at side wall behind speakers at 30" ( due to subpanel at 24" position). Thks for your contributions to this thread

What I've been taught and read is that the center of the diffusers used on walls should be cover at least vertically a foot below ear height to a foot above ear height. But this is only a rule of thumb since exact placement depends on your space, location of risers, height of seats, number of speakers, etc etc. Hope that helps and thanks for the kind words of appreciation, glad you found the material I posted useful!

audioblazer · « **Reply #118 on:** 8 Jun 2011, 03:33 pm »

Nyal, with my ears position at around 44" ( about tweeter position) and the diffuser size 25" x 46", it would place my diffuser at around 21" above floor. thks

audioblazer · « **Reply #119 on:** 8 Jun 2011, 03:48 pm »

jeff, below is a rough sketch of my current room and the intended diffuser. Placing problem is due to the presence of a subpanel located at 26" from floor and about 30" from back wall. Helmholtz resonator located at 11' fr from wall to handle first reflection