Room modes

[ernestcarl]

EQ doesn't address the harmonics, doesn't work great in all of the time domain, and only works in a small area in the room, and is seldom good to use for cancellations. So it will always be a compromise.

Most of what we do is a compromise.

I’m certainly not in the EQ only camp — I think you would be hard pressed to find anyone that is… folks that make DRC tools say room acoustics matter — even if marketing likes to gloss over that fact.

 

[Justdafactsmaam]

The decay looks the same on those pictures??

The exact same? No. But I clearly see the ringing after the EQ. The frequencies of the room modes clearly decay more slowly than the other adjacent frequencies. Pretty much the biggest difference is the amplitude which is obviously what the EQ should do.

 

[dominikz]

Well, I haven't followed this thread until now, but it's pretty much clear that 20 milliseconds (the period of a 50Hz tone) is longer than the minimum time that the ear can distinguish. Additionally, interaural delay, which has been claimed to be not interfered with at 50Hz, can most certainly have timing issues. Like many people have reported, non-phase-locked sound at 40 Hz and up does not have a direction component, but it most certainly has a spatial sensation component.

That's what I know. I am not eager to get into he-said, she-said arguments.
As to minimum phase, I have measured a lot of things at a lot of frequencies in a room that are not minimum phase. That, however, is totally room dependent.

@j_j Thanks, that sounds interesting and I for one would love to know more. IMHO well-argued counterpoints are a chance to learn something new.
Could you perhaps point me to some reading material on spatial perception of low frequency sounds that explores these points?

Hope you won't consider it argumentative (honestly not my intention), but just to avoid ambiguity: I suppose you agree that flattening a minimum phase (MP) resonance peak in the frequency domain also results in removing the related temporal ringing?

The discussion on whether or not room LF phenomena are generally MP or not is IMHO reasonable and very welcome in this context.
To my understanding non-MP LF behavior in rooms is commonly seen with deep cancellation (e.g. severe SBIR-induced nulls) or with peaks in specific cases where the reflection is louder than the direct sound (link to source). Apart from that, at least in my understanding, a lot of the common room- or placement-induced LF response peaks should qualify as MP (though I fully appreciate that identifying if something is MP or not may not be trivial for most people).
Is this a wrong understanding on my part?

I don’t think this is universally agreed upon and so far every waterfall plot I have seen suggests otherwise.

@Justdafactsmaam As far as I can tell, the plot shared by @JohnPM shows that ringing is completely removed by cutting the 50Hz resonance with PEQ (look at the 50Hz line at the frequency axis in both plots). The ringing that is still visible in the second plot is at 60Hz and therefore a different resonance.

Individual room resonances are often not sufficiently isolated to make it easy to see the effect of an individual EQ filter, but here is an example where that is the case. The after plot is the result of a single EQ filter, Fc 50.00 Hz, Gain -15.00 dB, Q 11.881 targeting a modal resonance with a 60 dB decay time of 1239 ms. When the zeroes of the filter correspond exactly to the poles of the resonance the resonance disappears, all that remains is the decay of the filter.

View attachment 342795


View attachment 342800

Please also note that even a theoretically perfect band-passed system response will always have some ringing (this should explain the decay that is left after removing a resonance), as nicely illustrated by @NTK here:

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The above plots show what the frequency and impulse responses of e.g. a perfect subwoofer with a frequency range of 30-60Hz in perfect anechoic conditions would look like; i.e. we can only get something worse than this with a real 30-60Hz band-limited subwoofer placed in a real room.

 

[ZolaIII]

For illustration purposes only as as much as I remember he has a way to nice room to make it illustrative. On the other hand I have a bad one. Focus is on 43 Hz mode in mine on those photos, so please disregard the rest and I know I could have done some things better all together but it's a small room and even this much is achieved with a lot effort.

 

[j_j]

Hope you won't consider it argumentative (honestly not my intention), but just to avoid ambiguity: I suppose you agree that flattening a minimum phase (MP) resonance peak in the frequency domain also results in removing the related temporal ringing?

Of course, that should work, if you get the right transfer function, and have a very good ARMA model.

This is not as easy as it's thought to be, of course.

More to the point, I have to say that for a standard Redmond Washington office of 10 years ago, a good ARMA model did not suffice, in fact, it seemed pretty clear that that office was not 100% minimum phase at low frequencies. It was, of course, nothing close to that at high frequencies.

As far as the LF spatial sensation, if you have a way to generate a sine tone at 50Hz, and also phase shift it, try just slowly changing the phase at 50Hz in headphones (you also get effects in loudspeakers, but it's hard to be sure you're hearing the time issues there.

Note, it's not a directional cue, it is, to me, at least, a width cue. Dead-on phase is "narrow". A bit of phase shift, and it's wider. Getting past pi/2 radians is "weird".

 

[LIΟN]

Interesting stories. Contributor discussions are always helpful.
One thing I'm curious about is that the story being discussed now is the one on the side that room mode can be removed through EQ? Or does it mean that it even covers the time domain?
It's a little confusing. (Of course, it seems to be a little different from the content of the text thread.)

 

[j_j]

Interesting stories. Contributor discussions are always helpful.
One thing I'm curious about is that the story being discussed now is the one on the side that room mode can be removed through EQ? Or does it mean that it even covers the time domain?
It's a little confusing. (Of course, it seems to be a little different from the content of the text thread.)

Only in one spot in the room.

 

[Justdafactsmaam]

Only in one spot in the room.

How big is the spot? How much would head movement affect it?

 

[LIΟN]

Only in one spot in the room.

Yes. But I was wondering if this was actually a medicine in the direction of eliminating resonance, or a story in the direction of mitigating it.

 

[j_j]

How big is the spot? How much would head movement affect it?

Depends totally on the room, I fear.

 

[j_j]

Yes. But I was wondering if this was actually a medicine in the direction of eliminating resonance, or a story in the direction of mitigating it.

It mitigates the PRESSURE component at one point.

Remember there are 4 variables in a soundfield at any one point, the pressure and 3 volume velocities.

 

[Kvalsvoll]

How big is the spot? How much would head movement affect it?

EQ (min-phase) does work in the bass range, and it works so well, also is the only thing that does work when limited to practically viable solutions, so why anyone runs a system without using dsp now in the year of 2024, makes sense only for the incompetent or totally ignorant.

BUT is does not completely eliminate the problem, because the properties of the cause is not a simple minimum phase problem. As @j_j points out above, sound waves have a pressure scalar component and a volume velocity vector, and those are not necessarily in phase. In a standing wave they are out of phase so that where you have pressure cancellation, there is a velocity maximum.

It works well enough to even out freq response and reduce resonances down to levels that are very acceptable, for the practical use of listening to music. It works better at lowest freqs, and eventually above say 200Hz no longer works. For very low f, it works for a some larger area, say you can move a seat or 2, further up in f, you may still move your head, but if you move your listening position your minute adjustments and effort to reduce ringing in the 100-200Hz range no longer works.

It is possible to use multiple sound sources to get better control of the soundfield, and this can give a much more even performance across the whole room, but this is not so straight forward as to just eq for a flat frequency response.

 

[j_j]

EQ (min-phase) does work in the bass range, and it works so well, also is the only thing that does work when limited to practically viable solutions, so why anyone runs a system without using dsp now in the year of 2024, makes sense only for the incompetent or totally ignorant.

I'll have to partially object to that. That may be true for some rooms, and maybe many rooms. It is not a universal statement, however.

BUT is does not completely eliminate the problem, because the properties of the cause is not a simple minimum phase problem. As @j_j points out above, sound waves have a pressure scalar component and a volume velocity vector, and those are not necessarily in phase. In a standing wave they are out of phase so that where you have pressure cancellation, there is a velocity maximum.

It works well enough to even out freq response and reduce resonances down to levels that are very acceptable, for the practical use of listening to music. It works better at lowest freqs, and eventually above say 200Hz no longer works. For very low f, it works for a some larger area, say you can move a seat or 2, further up in f, you may still move your head, but if you move your listening position your minute adjustments and effort to reduce ringing in the 100-200Hz range no longer works.

It is possible to use multiple sound sources to get better control of the soundfield, and this can give a much more even performance across the whole room, but this is not so straight forward as to just eq for a flat frequency response.

And, bear in mind, you mean a "flat pressure response at one point", not "flat frequency response".

 

[Kvalsvoll]

And, bear in mind, you mean a "flat pressure response at one point", not "flat frequency response".

Yes. That is what people measure, what people can measure, so this is what can be adjusted for. Assuming you mean: flat pressure frequency response.

Now, of course, that can not easily be achieved either, in most cases, since cancellations (in p) can not be filled completely.

 

[Kvalsvoll]

I'll have to partially object to that. That may be true for some rooms, and maybe many rooms. It is not a universal statement, however.

How well it works, depends on the properties of the room, yes. But even the worst will improve.

Properties, in bass range, means room dimensions, openings, windows (and their construction), wall construction. Lossy walls, typical wood, are better, concrete walls reflect almost all low freq energy, a true pain to deal with.

Working on this for many years, I have acquired data from many rooms.

 

[j_j]

How well it works, depends on the properties of the room, yes. But even the worst will improve.

Properties, in bass range, means room dimensions, openings, windows (and their construction), wall construction. Lossy walls, typical wood, are better, concrete walls reflect almost all low freq energy, a true pain to deal with.

Working on this for many years, I have acquired data from many rooms.

Pulling down peaks is a very good thing. Filling in near-zeros, not necessarily. Things start to rattle.

 

[Kvalsvoll]

Pulling down peaks is a very good thing. Filling in near-zeros, not necessarily. Things start to rattle.

You get way too much energy at those frequencies you try to boost. It doesn't work well. And creates horrible peaks at other locations in the room.

 

[j_j]

You get way too much energy at those frequencies you try to boost. It doesn't work well. And creates horrible peaks at other locations in the room.

No kidding.

 

[Justdafactsmaam]

All of which is why I plan to bass trap the #%^* out of my room when I do the rebuild. Trap as much as I can then go from there. To the best of my understanding there is no performance downside to bass trapping.

On another note. Is anyone using the Trinnov Waveforming DSP and subwoofer arrays? Looks very interesting. But I wonder what effect it has on the spatial cues being a mono system? Would the crossover have to be below 50 hz? For spatial cues what is the point where stereo separation doesn’t matter?

 

[j_j]

All of which is why I plan to bass trap the #%^* out of my room when I do the rebuild. Trap as much as I can then go from there. To the best of my understanding there is no performance downside to bass trapping.

Fixing the room is always a better win, if you can do it.

As to the particular product you mention, I can't comment.

For spatial cues like "width" below 40Hz. For directional cues, below 90Hz.