Analog filter for room correction

[abbey road d enfer]

I agree, but the extent to which the room mode is excited does depend on the location of the source: If the source is in the null of the mode, it doesn't excite it (or very litte).

What do you cal a "null of the mode"? Room modes are caused by sound reflecting an even number of times, so the position does not matter; when you get closer to a wall, you get as much further from the opposite.
In fact, walls are velocity antinodes (nulls) and pressure nodes.

What you call reflection modes, is it the same as SBIR (speaker boundary interference response)?

It looks like it. I didn't know an acronym was attached to this well-known phenomenon. And indeed, these are position-dependant, as they involve an odd number of reflections.

Coming back to the original topic: Would it be possible to use a completely passive circuit (for example notch filters) for room EQ? Of course, this would only allow for attenuation, but that's also what is needed in most of the cases. Is it possible to design a completely passive peak filter with variable Bandwidth, frequency and attenuation?

Parametric is close to impossible. Although you may use an active EQ to align your system and then work out a passive version. But why? What is so great about passive? It's like "I want a vehicle with no engine"; you know the answer "Yes, but...".

 

[Neutrino]

What do you cal a "null of the mode"?

I mean the pressure node, i.e. the point where the sound pressure of the mode is zero. For a 1st order mode this would be at the midpoint between two walls. If you put your speaker there, it will not excite the mode. If you put it close to the walls, it will excite the mode to a maximum.
Here is a nice animation of the concept:

http://www.acs.psu.edu/drussell/Demos/RoomModes/driving.html

If the excitation of a mode indeed were independent of location of the source,  subwoofer placement would be far less critical (except for SBIR). It can easily be tested in any room that this is not true.

Parametric is close to impossible. Although you may use an active EQ to align your system and then work out a passive version. But why? What is so great about passive? It's like "I want a vehicle with no engine"; you know the answer "Yes, but...".

I asked about a passive circuit because of simplicity. I never designed a  circuit and only know some basics about electronics. I had a network of resistors and capacitors in mind, but apparently, it is not that easy. 

 

[abbey road d enfer]

I mean the pressure node, i.e. the point where the sound pressure of the mode is zero. 

I'm familiar with this phenomenon. I thought you meant there  was a point in the room where the loudspeaker is inefficient; so yes, there is a number of points (in fact a locus) where one frequency (and some of its harmonics) would be strongly attenuated - it would not be nulled because a) the walls are somewhat absorbing and b) the source is never a true point source.
And indeed that's a good reason to place loudspeakers as close as possible to a wall.
Putting a loudspeaker in the middle of a room may happen in home-cinemas and HiFi installs, but not in a decent control-room.

If the excitation of a mode indeed were independent of location of the source,  subwoofer placement would be far less critical (except for SBIR). It can easily be tested in any room that this is not true.

Room-mode issues depend as much on the location of the loudspeaker than that of the listener, and it's been proved that having a single sub is the worst option - although it's the most frequent case.

 

[ricardo]

Before  you spend time & $$$ on fancy analogue or digital EQ to correct your room, treat the room & move your speakers around first.

- Does your room have perfectly parallel surfaces? -> flutter echoes
- Are the room dimensions poor? -> poor distribution of resonances
- Is T60 inappropriate? -> Sound is too bright or dull
- Is bass lacking / too much/lumpy in your favourite listening position?  Move your listening position and/or speakers.  2 subs are definitely better than 1 and gives you more flexibility.

Open bookshelves (with books) are excellent for diffusors and making walls non-parallel.

If you put a carpet down, don't forget it is best to distribute absorption on both facing surfaces rather than have it all on one.

Don't expect miracles from expensive treatment and bass traps.  You can do everything with bookshelves, carpet & maybe a hat collection or a few hanging rugs.

Once you have done this, Digital Room EQ may provide better results.

If all the above is poor, don't expect ANY EQ to help ... unless you put your head/ears in a vice to ensure the listening position doesn't change.

 

[Neutrino]

Before  you spend time & $$$ on fancy analogue or digital EQ to correct your room, treat the room & move your speakers around first.

Dear ricardo,

I honestly appreciate your help, but have you read what I have written before? ;D

I have spend a whole lot of time and money last year doing exactly that. With all this treatment I was able to reduce the bass response to better than +-5dB and a constant decay time of a bit less than 300ms down to almost 30 Hz which is generally considered as a very good result already.  Of course one can always add additional treatment, but we also need some space to work in, don't we? I would like to improve this even further and my tests with EQs so far have shown that +-2dB or +-3dB at the listening position should be feasible without much sacrifice at other locations in the room. Now I'm looking for a solid and reliable solution to implement that.

But I understand your reaction. Some people (although they are becoming fewer) still think that EQ can solve all they acoustic problems. 

 

[nielsk]

The phase shift introduced by analog filters makes improving the monitoring system a treading water scenario.
With a good digital processor, any distortion from conversions is so far outweighed by the lack of phase shift that I have not even considered using analog room EQ since good digital became available.
You can get a very good processor with EQ, crossover and limiting for a few $100 that vastly outperforms any analog EQ ever made and sounds vastly better.
That being said, if you really want passive analog room EQ I know where there are stacks of 24 band Dukane units, with extremely nice torroidal  inductors and in/ out on every band. Make up gain is an API-ish gain circuit, but all it does is restore gain so for real passive use, leave it out...

 

[abbey road d enfer]

The phase shift introduced by analog filters makes improving the monitoring system a treading water scenario.
With a good digital processor, any distortion from conversions is so far outweighed by the lack of phase shift that I have not even considered using analog room EQ since good digital became available.

“Room resonances at low frequencies behave as “minimum phase” phenomena, and so, if the amplitude vs. frequency characteristic is corrected, so also will the phase vs. frequency characteristic. If both amplitude and phase responses are fixed, then it must be true that the transient response must be fixed – i.e. the ringing, or overhang, must be eliminated” (Toole, The Acoustical Design Of Home Theaters, 1999)
Trying to compensate a MP disturbance with a LP EQ is simply not correct.

As I wrote earlier, the response of a monitoring system (including room) can be decomposed in a product of MP terms and non-MP terms; the former can be EQ'd with a MP EQ, the others could be EQ'd with non-MP filters (more easily implemented in digital than analog), but imply a non-negligible latency, that may or may not be an issue.

 

[ruairioflaherty]

The phase shift introduced by analog filters makes improving the monitoring system a treading water scenario.

I hope you never see a phase plot of a typical speaker, you won't sleep for a week.  Abbey is of course correct, you use a minimum phase EQ to correct minimum phase problems (have a look at the REW link I posted above).

Again weighing in with practical experience I have never heard a linear phase EQ that I liked.  Not one.  Aside from the issue of conversion and latency in this case you definitely do not want to apply low frequency corrections with a linear phase EQ.