Acoustic Measurements: Understanding Time and Frequency

[bblue]

Not just the position. Everything changes in the other room. Further, the entire science of it changes if the room is sufficiently big such as a concert hall. We will then get a diffused soundfield and the bass becomes far better as we get a dense set of modes. The problems we are talking about here are perils of small room acoustics which unfortunately make our life difficult. But yes, if you are talking about a small room, same science will apply. I will not only hear variations due to where I sit, I also hear variations in the identical spot because the room rooms are guaranteed to have different frequency responses due to so many variable contributing to that from speakers to room. For this reason, we need to divorce ourselves from thinking we can ever replicate what the talent heard. Unless both presentations are in anechoic chamber with identical speakers, we are out of luck there.

Ok.

Which is exactly what I said and repeated above. Not sure why you quoted me and reply as if I disagreed.

I'm not either at this point!

No, they don't degrade the *experience.* How do we know that? We test people. We test them with or without that reflection and with the exception of recording engineers, the rest of the world tends to like side reflections better than not having them. Not all reflections. But strong side reflections. Yes, the sound is "distorted." I cover this in my other thread/article: http://www.whatsbestforum.com/showthread.php?8226-Psychoacoustics-of-Room-Reflections.

I can cite research paper and listening test after listening tests to demonstrate the above. For brevity and an interesting data point from someone who should think like you say, here is George Augspurger (designer of PRO listening rooms) in his AES paper, LOUDSPEAKERS IN CONTROL ROOMS AND LIVING ROOMS:

”Third, I did a lot of listening with various amounts of absorptive treatment in the comers behind the speakers. When first-order reflections were largely absorbed I noted that locations of individual sound sources were more precise, that the timbre of individual instruments was more natural, and that the overall stereo picture was more tightly focused. These observations agree well with other reported listening tests. Nonetheless, after extensive listening to classical and pop recordings I went back to the hard, untreated wall surfaces. To my ears the more spacious stereo image more than offset the negative side effects. Other listeners, including many recording engineers, would have preferred the flatter, more tightly focused sound picture.”

I would think (perhaps with too high expectations) that seasoned audiophiles, those who have been around for awhile and have had a lot of time and opportunity to educate themselves and their ears, would not prefer the artificial enhancement of listening room reflections. Instead, realizing that the source is the recording and it should be able to stand on its own if well done and played on an objective and neutral system/room. I wasn't considering neophyte audiophiles.

So if the goal is to have a pretty graph, you are absolutely right. We should create an anechoic chamber and get rid of all reflections. But if the goal is to *enjoy* music, listening tests and research into how we hear, which includes why we don't hear distortions that show up on graphs, tells us that some reflections are beneficial. Others are not. Floor reflections for example bring no benefit since both ears hear it equally. So you would want to absorb them. Likewise front and back walls do not have benefits so while they are not damaging per-se, they can be attenuated if you need absorption. On that point, mid to late reflections can be a negative so all rooms need to have fair amount of absorption in them. The surfaces I just listed make good candidates.

Actually, the (my) goal isn't a pretty graph. It's a fairly critical environment with which you are able to hear what is recorded without significant augmentation. Only then can you appreciate the difference between well recorded and average.

I have explained "why tolerate" them above. I also want to be clear as to which part of the room acoustics we are talking about. In this thread we are all about the bass frequencies/sub. In this region, psychoacoustics does not play a role and the soundwaves act differently. It is important to not mix topics and say that I am talking about EQing everything. Not even close. I am talking about bringing down resonant peaks in bass frequencies. I am not telling you to go and apply EQ to 8 Khz and expect to necessarily do good. You could be making the sound worse if you don't know the cause of that. As to doing things acoustically, let's see the low frequency of a room without EQ. Show me how none of the variations in its frequency response matter.

I have also been referring entirely to the low frequencies below the room transition point. Above that point (major room reflections aside) you are mostly correcting speaker and crossover behavior, which IMO should be done only with convolution applications which can control level, phase and timing.

We always start with minimizing the problem. That solution calls for optimal placement of subs to cancel some of the modes, and use of more than one to cancel some of the others. Once you do this you still have some peaks. Those peaks are then brought down to give us near flat response. I will never have flat response as I mentioned to Terry. It simply is not possible to get lab type results. And you don't get there with acoustic products either. You will absolutely still have peaks and valleys and those sure will have a character.

True enough, though significant peaks and valleys remaining would usually indicate insufficient or improper trapping.

The foundation of this forum is have a mix of solid science and open discussion of audio. All opinions are respected though there is no obligation to accept . What I am describing here is backed by decades of acoustic research. You are hearing 10% of it. Over time, I will post more. The science is all targeted toward bettering the sound in the room. So the goal is the same as what we have here. When Bradley researchers speech intelligibility and finds that room reflections contribute up to 9 db there, we want to know about that even though it goes against one's guts that reflections hurt rather than help. The science tells you that if you take out reflections you take out energy from the room and that has its negative effects. So yes, get ready to hear the best that the acoustic science has to present us. That is what this forum is about. Again, no obligation to accept any of it.

I look for more of a studio control room/mastering room presentation of playback. I don't want to hear how the room modifies it because you lose your accurate point of reference when listening. I guess many listeners don't care that much for that aspect and just want warm and fuzzy when they play something. The problem with that is that 'warm and fuzzy' isn't the same in a different environment. And worse, especially with reviewers that include the room in their listening formula, reviews can often be skewed by the ability of the system (and the reviewed item) to integrate well with their room. Or not.

Well, you should repeat my exercise. Put a single sub in a corner and measure. Take out the first massive peak and tell me if made your room sound less natural. I assure you it will not. There is reason most high-end subs come with built-in DSP filters and software that programs them. Why would they do that if it makes the sound worse?

It won't sound worse, but it still won't be correct.

Reading your posts makes me think that you are thinking that it is being said that EQ should be used as the only tool to fix anything that is wrong in the room. That is not remotely what I am saying. Again, we are talking about narrow but important area in small room acoustics which is bass resonances. We know these will be there. You are right that you can't take out floor reflections using EQ. You better have a thick carpet there. Likewise if your room is too live, no EQ will save it although getting rid of bass resonances is an improvement.

Agreed on that. I see listening rooms with hardwood floors and just shudder at the thought.

Yes, I'm referring to the LF resonances caused by the room. And yes, I did get the impression from your initial posts in this thread that you would be perfectly fine with EQ only resolution of LF problems because it 'was good enough'.

I plan to write a simple tutorial on the top level picture as I think that is most of our problems. Here is a quick version of it. To get good sound, you need to do the following:

1. Start with good speakers. This is what makes sound in the room. I am amazed that entire texts are written on room acoustics but ignore speakers. What makes a good speaker is a subject of another thread .

2. Make sure you have sufficient absorption for mid to late reflections. Too much here causes intelligibility problems and makes for bad sound in general. Too little of it and the room becomes uncomfortable (think anechoic chamber).

3. Deal with problematic reflections (e.g. floor).

4. Optimize your bass frequencies. This is what I am talking about in this thread. Studies show that we contribute 30% of sound fidelity to how good the bass is. So if we fix this, we are one third of the way there.

Ok, I agree with all this, except the method of optimizing bass frequencies. IMO to do it right you need many traps, and then if you must, further it with EQ. Me, I never use inline EQ to the monitoring, unless it can be integrated into an active, already existing path with convolution.

And let's not forget that it's not just the speakers and the room controlling how things sound, but is inclusive of power amps, preamps, and cabling (etc). The electronics can make a profound difference to the interpretation of music by the listener and can have an significant effect on how problematic acoustic areas are perceived in the room.

--Bill

 

[microstrip]

(...) I would think (perhaps with too high expectations) that seasoned audiophiles, those who have been around for awhile and have had a lot of time and opportunity to educate themselves and their ears, would not prefer the artificial enhancement of listening room reflections. Instead, realizing that the source is the recording and it should be able to stand on its own if well done and played on an objective and neutral system/room. I wasn't considering neophyte audiophiles.
(...)

Bill,

Curious. I would consider that neophyte audiophiles, that did not have the time to educate themselves, would prefer just killing the reflections to get immediate granting of the full appealing characteristics of the recording, that surpass the real in many aspects. Only after education they will realize the full potential and limitations of the recording and at this time they would take an educated decision - either stay with large absorption because it sweets their preference, or enter a different way of listening, exploring the enjoyment that spaciousness and envelopment can bring to sound reproduction, with all its problems and vagueness. IMHO, both are respectable ways of listening and as far as I understand, most of the time we debate the second one at WBF.

BTW, I would love be a neophyte audiophile - it would mean I would be in my youth again, something that ended some decades ago. Do you really think that we mature age audiophiles have partial reflexive listening rooms just because we were not able to find the Fountain of Youth?

 

[amirm]

Ok, I agree with all this, except the method of optimizing bass frequencies. IMO to do it right you need many traps, and then if you must, further it with EQ.

The point is that you don't need to have so many bass traps as to make your room look like an anechoic chamber to get smooth response. If you can isolate the bass to a sub, then you can put an EQ in its path and use multiple to arrive at superlative response that will outperform any reasonable effort at doing the same using acoustic material. There is no way you can detect any harmful effects from a parametric EQ in your subs alone.

I think your angst is in inserting an EQ into the full signal range and only then correct the bass with it. I hear you. That is why I am suggesting for now, to go for active crossovers and send the low energy to the bass where we can use all the tools in our arsenal to arrive at both objectively and subjectively correct reproduction. The fears of another electronic element being inserted in a signal path is really a worry about nothing when we are talking about it correcting for massive distortions in frequency response which are completely audible otherwise.

And let's not forget that it's not just the speakers and the room controlling how things sound, but is inclusive of power amps, preamps, and cabling (etc). The electronics can make a profound difference to the interpretation of music by the listener and can have an significant effect on how problematic acoustic areas are perceived in the room.

--Bill

None of this is material when it comes to how a sub energizes your room. I guarantee that you can't tell the difference between any upstream product when just listening to the sub. Test it. Turn off everything but a sub and then tell me that based on what you are hearing there, you will be able to pick out the nuances you mention above, let alone profound differences. These are orthogonal factors in this context. You should optimize the up stream products for what they do, and the room for what it does.

 

[microstrip]

The point is that you don't need to have so many bass traps as to make your room look like an anechoic chamber to get smooth response. If you can isolate the bass to a sub, then you can put an EQ in its path and use multiple to arrive at superlative response that will outperform any reasonable effort at doing the same using acoustic material. There is no way you can detect any harmful effects from a parametric EQ in your subs alone.

I think your angst is in inserting an EQ into the full signal range and only then correct the bass with it. I hear you. That is why I am suggesting for now, to go for active crossovers and send the low energy to the bass where we can use all the tools in our arsenal to arrive at both objectively and subjectively correct reproduction. The fears of another electronic element being inserted in a signal path is really a worry about nothing when we are talking about it correcting for massive distortions in frequency response which are completely audible otherwise.

Amir,

Can I ask you your opinion about the capabilities of the Behringer DCX2496? It is good enough for this application or do we need higher boost and Q factors than those available in this unit? I have been playing it, programming it using the available PC software and it is very friendly.

Am I correct assuming that you are considering multi subs, each with its separate delay/equalization? I ask because I think that some studies from Harman for subwoofer positioning only assumed a single equalization/amplifier channel.

 

[bblue]

Bill,

Curious. I would consider that neophyte audiophiles, that did not have the time to educate themselves, would prefer just killing the reflections to get immediate granting of the full appealing characteristics of the recording, that surpass the real in many aspects. Only after education they will realize the full potential and limitations of the recording and at this time they would take an educated decision - either stay with large absorption because it sweets their preference, or enter a different way of listening, exploring the enjoyment that spaciousness and envelopment can bring to sound reproduction, with all its problems and vagueness. IMHO, both are respectable ways of listening and as far as I understand, most of the time we debate the second one at WBF.

True enough. I think a lot of the difference in preferences to side reflections has a lot to do with ones ears -- how the sound is directed into the ear canal, and for that matter, the angle of the canal. You can achieve much the same effect by just slightly cupping your ears, or pushing the external ear slightly forward from behind. But it's very frequency selective, and it becomes more significant as one ages and HF (upper mid, actually) response changes.

BTW, I would love be a neophyte audiophile - it would mean I would be in my youth again, something that ended some decades ago. Do you really think that we mature age audiophiles have partial reflexive listening rooms just because we were not able to find the Fountain of Youth?

Ha! Well in my early listening days (early to mid 60's) one of my systems was 12" speakers run full range, plus another 10 or 12" connected via an arbitrary capacitor for a 'tweeter'. I'm sure it sounded awful, so I wouldn't want to revisit that time -- at least not for that reason! What I listen to now bears no resemblance at all. I think I'd rather not listen than listen to something like that.

--Bill

 

[bblue]

The point is that you don't need to have so many bass traps as to make your room look like an anechoic chamber to get smooth response. If you can isolate the bass to a sub, then you can put an EQ in its path and use multiple to arrive at superlative response that will outperform any reasonable effort at doing the same using acoustic material. There is no way you can detect any harmful effects from a parametric EQ in your subs alone.

Assuming you don't have any serious active room modes, that's probably true. My room has quite a buildup of LF 100Hz and below, down to about 25Hz, so I have to use traps to tame that down. Using and positioning subs to try and tame that would result in a lot of standing waves and irregular response depending on just where you are in the room. The room was actually designed with the entire rear wall a bass trap, and the ceiling into an attic space as well. I found out only recently that the rear trap is doing almost nothing because it was designed originally for soffit mounted speakers aimed downward like in a control room. Because of the room layout the only place I can place speakers in a limited area on the floor. The speakers are aimed incorrectly for the soffit trap to work. Some rear corner traps helped, but are not a total solution.

I think your angst is in inserting an EQ into the full signal range and only then correct the bass with it. I hear you. That is why I am suggesting for now, to go for active crossovers and send the low energy to the bass where we can use all the tools in our arsenal to arrive at both objectively and subjectively correct reproduction. The fears of another electronic element being inserted in a signal path is really a worry about nothing when we are talking about it correcting for massive distortions in frequency response which are completely audible otherwise.

That could be true depending on what the listener is sensitive to. I personally find inline electronics of less than stellar audio quality quite objectionable. Good bass wouldn't make up for it.

None of this is material when it comes to how a sub energizes your room. I guarantee that you can't tell the difference between any upstream product when just listening to the sub. Test it. Turn off everything but a sub and then tell me that based on what you are hearing there, you will be able to pick out the nuances you mention above, let alone profound differences. These are orthogonal factors in this context. You should optimize the up stream products for what they do, and the room for what it does.

Of course. But for instruments with fundamental frequencies in the low frequency ranges of the sub and strong harmonics in the first few octaves up in the main speakers, there would be a difference based on how the sub matches timing and phase of the fundamentals to the harmonics, and how the room fits in to that formula as well.

But I understand what you are saying.

--Bill

 

[amirm]

Amir,

Can I ask you your opinion about the capabilities of the Behringer DCX2496? It is good enough for this application or do we need higher boost and Q factors than those available in this unit? I have been playing it, programming it using the available PC software and it is very friendly.

I have not used the Behringer. From what I have read, it uses IIR filters which are digitized versions of analog filter which is good if enough accuracy is maintained. I don't know how narrow the Q can be set. REW calculates the parameters for it with a decimal place for both Q and Frequency so likely it is fine enough resolution. So yes, the simple answer is that it should work well .

Am I correct assuming that you are considering multi subs, each with its separate delay/equalization? I ask because I think that some studies from Harman for subwoofer positioning only assumed a single equalization/amplifier channel.

Yes, the original research used identical drive for all the subs. Later, Harman developed Soundfield Management (SFM) which uses an iterative process to drive each sub differently to compensate for non-symmetrical room configuration and differing sub models. It is able to generate better results than using identical units. You can get SFM and auto-EQ for bass in JBL BassQ. See Roger's review/measurements here: http://www.avsforum.com/t/1163240/jbl-bassq

Alas, I am not sure if the box is still sold. We still have them on our price list and my designer has a couple he is not using. PM me if interested in that. Outside of that, SFM lives in JBL Synthesis SDEC-4500 EQ which is what we have at work and in my new theater where the correction came about in this thread. This is a 20-channel EQ with lots of other features and the price is way up there. Harman has patented this technology so it is not going to be offered by anyone else.

To show how well SFM works, here is a pre-SFM measurement across multiple seats:

And after with just a single notch filter and delay I think:

We see that there are no more pesky nulls in there that can't be fixed with EQ.

 

[amirm]

Assuming you don't have any serious active room modes, that's probably true. My room has quite a buildup of LF 100Hz and below, down to about 25Hz, so I have to use traps to tame that down. Using and positioning subs to try and tame that would result in a lot of standing waves and irregular response depending on just where you are in the room.

??? The science of multiple subs says the exact opposite. What the secondary subs do is to cancel out some of the modes created by the other sub. Eliminating modes by definition fixes the problem at that frequency. Here is the actual measurements across multiple seats with one sub from my article on bass optimizations: http://www.madronadigital.com/Library/BassOptimization.html

Now with four subs:

We see clear improvements. The reason is that by eliminating some of the room modes we have actually widened the sweet spot, not made it worse.

The room was actually designed with the entire rear wall a bass trap, and the ceiling into an attic space as well. I found out only recently that the rear trap is doing almost nothing because it was designed originally for soffit mounted speakers aimed downward like in a control room.

Bass frequencies radiate in all directions. Not sure what you mean about directional speakers in this regard. How did you find out it was doing nothing?

I personally find inline electronics of less than stellar audio quality quite objectionable. Good bass wouldn't make up for it.

And you are saying that with respect to inline electronics that are only in the path to the sub?

 

[Wayne A. Pflughaupt]

Great article on a complicated topic amirm! It’s surprising how much controversy this subject generates and how many people don’t really understand waterfall graphs or their use. And might I say, needlessly worry about them! I especially like your stance to mainly concentrate on frequency response and don’t worry about waterfall graphs. I know a couple of fellows at AVS (who shall remain nameless, but their initials are D and L) who would have a meltdown at the thought of that! Really, other than FYI or to measure the effects of low-frequency treatments, there’s no good reason to be concerned with waterfalls, IMO.

I made a post at the Shack a few months ago that hopefully helps clarify the issue as well, although probably not as eloquently as yours:

http://www.hometheatershack.com/forums/rew-forum/59988-rew-eq-filters-ringing.html#post537566

If I may be permitted to perhaps add a bit to an excellent article, and in keeping with your advice to "just pay attention to the frequency response graph," I have at least a couple of problems with with utilizing waterfalls for equalizing bass response, as many people seem prone to do.

First, any equalization implemented in order to achieve the best-looking waterfall is only valid at the location where the measurement was taken. Move away from there and your waterfall graph is “ruined.” For instance, here are some baseline measurements I made at a few seating locations in my room. Note that each one is a bit different and would likewise generate waterfall graphs that looks different. [NOTE: The traces have been shifted in order to separate them for greater clarity.] So if my goal is an optimized waterfall graph, which of these response curves should I use to equalize?

Baseline Readings at Five Locations​

Not that equalizing based on one curve vs. the others would especially make an audible difference (as they're not drastically different each other), but listening to your system is what you do, not gaze at the waterfall graphs it generates. If you can’t hear an appreciable difference between the location where the waterfall graph was generated and the seat next to it - why equalize for a pretty waterfall to begin with?

The next issue has to do with what might be termed “system instability.” I first observed this phenomenon more than fifteen years ago when I got my AudioControl real time analyzer/pink noise generator: I would painstakingly set my 1/3-octave equalizers based on the RTA display, only to check it again a few months later and find, to my great dismay, that my response was no longer “perfect,” but off a little here and there. At least that's what the visual display was showing. There was no detectable (read audible) difference.

What causes this? While a measurement platform like REW is stable enough (being built-into the electronics), the transducers involved - the speakers and elements in the measurement microphones - are not. Their physical (and consequently electrical) properties are altered with changes in temperature, humidity, etc. As such, when you take a second reading with a program like REW six months or a year later, you'll find it doesn't look quite like your original one.

For a couple of examples, here is the graph that shows the effects of temperature changes. It was a cold winter day (for Houston at least), and it was 73? inside the house. After taking a measurement, I set the measurement mic on the floor where sunlight was streaming in through a window. The thermometer registered 86? in the sun. After a half hour or so I took another reading. The red trace is the reading from the warmed-up mic. There is no trace separation in this one; note that the red trace is a dB or two "higher" across the board.

Two REW Readings, Mic Element at 73 vs. 86 Degrees ​

Consequently, it should be no surprise that measurements taken months apart will also show inconsistencies from the “original” measurement [NOTE: The traces have been shifted in order to separate them for greater clarity.]:

Three Baseline Readings, Each @ Two Months Apart​

Bottom line, a waterfall graph generated today with last year's filters isn't going to look as good as it did back on the day you fine-tuned the filters for minimal ringing.

In the end, the best any equalizer can do is reduce the ringing from a room mode and bring it back in line with the rest of the frequency spectrum's decay times (and fortunately, that's all that's needed to improve sound quality). It isn't going to reduce the mode's decay time to something less than the rest of the frequency spectrum.

So all things considered, it seems the best tact would be to apply the "modal equalizing" approach to the true room modes, to get their decay times in line with what the rest of what the waterfall graph displays, and apply a more general, smoothing approach to the rest of the response curve.

Regards,
Wayne A. Pflughaupt

 

[Wayne A. Pflughaupt]

Think of it like this. You have a room with a large resonant peak at 70hz, and want to record an upright bass or cello. The room will significantly modify the character of the bass/cello by resonating with it at that resonant frequency. Eq later during mixing will not correct the problem; that note or range will always sound different than the neighboring frequencies. You can lower its level, but the character remains the same. The same thing would be true if you were in the middle of a room null produced by mode cancellations, and were recording an instrument. At certain notes the cancellation itself will have a unique decay which could not be recovered entirely by eq in the problem area.

The same issues apply in a listening room.

No, there is a huge difference between a listening room and the room where the recording of live instruments takes place.

I think you’re overlooking a fundamental difference between the two. Both will naturally be influenced by a room with the 70 Hz mode you mentioned. The difference is “music production” vs. “music reproduction.” Since the sound reproduction is “artificially” generated by an electronic system, it can also be equalized electronically to reduce or even eliminate the effect of the 70 Hz mode.

Such is not the case with acoustical instruments like the upright bass or cello you mentioned. Neither has a built-in equalization function. So you’re right - the room acoustics where the recording takes place needs to be fixed in order to accurately record the acoustic instrument. But that has absolutely no relevance to the listening room, as each listening room is different with its own specific set of room modes. As amirm mentioned, electronic de-emphasis of a listening room’s modes will allow for accurate music reproduction from a sound system.

But naturally, that won’t be a help if you invite a musician to play his upright bass or cello in your listening room. Make sense?

Now suppose you EQ a 1/2 octave dip at 91 Hz? That just reduces the speaker's output in that range, but it doesn't do anything to the character of the room in that range.

I don’t think anyone believes or even pretends that equalization changes the room.

It still behaves in the same manner, obscuring detail. It'll be a little less obvious because the triggering level is lower, but it's still there.

Of course, equalization doesn’t eliminate the room mode. No one pretends that it does. And proper parametric equalization does far more than make the mode “a little less obvious.” If the sound reproduction system is robbed of energy (i.e. exaggerated amplitude) at the offending frequency, the room mode is brought back in line with the decay times the rest of the room is exhibiting.

Take a look at these two waterfall graphs that show the effects of parametric equalization applied to a 41.9 Hz room mode. The second graph includes a parametric filter set at 42 Hz, with the overall level of the signal after equalization raised to match the SPL reading the mode was displaying before being equalized. In other words, 41.9 Hz are at the same SPL in both graphs (~90 dB). We can clearly see that the frequency where the mode was located (41.9 Hz) now displays a significant improvement in rate of decay compared to what it did before equalization, and that the rate of decay is now on-par with the room average. As amirm noted, equalization corrected both the amplitude and time domain issues the mode exhibited.

You have to fix it at the source (room modes usually)

Well, since the source of the room mode is typically the room's dimensions – how do you propose we “fix it at the source?”

I get the impression you don’t have any first-hand experience using parametric EQ to tame low frequency response? I’ve never heard anyone who did complain they didn’t get a dramatic improvement in low frequency sound quality.

Regards,
Wayne A. Pflughaupt

 

[microstrip]

(...) Outside of that, SFM lives in JBL Synthesis SDEC-4500 EQ which is what we have at work and in my new theater where the correction came about in this thread. This is a 20-channel EQ with lots of other features and the price is way up there. Harman has patented this technology so it is not going to be offered by anyone else. (...)

OK, it seems the best way to have optimized bass with multiple subwoofers is using a proprietary technique and adequate equipment. Is the setup fully automatic, or does he need some assumptions from the owner and can be custom tailored? Does the unit include an audio analyzer with a calibrated microphone?

 

[terryj]

OK, it seems the best way to have optimized bass with multiple subwoofers is using a proprietary technique and adequate equipment. Is the setup fully automatic, or does he need some assumptions from the owner and can be custom tailored? Does the unit include an audio analyzer with a calibrated microphone?

Not sure he said that.

re the dcx, it has more than enough power to do up to six distributed subs. With a bit of trickery, not that you'd want it, you can easily do more.

It has enough resolution in the parametric parameters, and has full control over delays and such. Dunno if this would ever be a problem, note tho that it can ONLY delay the subs. Ie of no use if for whatever reason it turns out the mains needed any sort of delay (obv that applies to any such unit)

My only personal concern with the dcx is build quality and or reliability, NOT any sound quality.

Especially when dealing with subs. Amir made an important point earlier, one that would be instructional to any person concerned with adverse sound consequences when dealing with subs.

Play the subs on their own. Turn off the mains power amps, disconnect one lead, whatever. Just have the subs alone playing, sit at the LP and listen.

Listen. Listen to the most goddawful garbled a-musical noise you could ever imagine.

THEN come back and tell us whether or not *you* still think you will get worthwhile sound improvements by buying a better sub cable (if that is your bent)!!! Similarly with eq in the bass (or subs). It could very well be something a person has to experience to get, but apart from stuff like hum or whatever, the benefits it brings to the table are immense.

It truly is one of those cases where you don't know what you have been missing (or listening thru) till suddenly it is gone. Man, if there were one DBT everyone could pass it is this one. Turn the eq on and off. Note not only the bass improve, but the .....entire...perceived....audio....spectrum.

THERE is suddenly your new midrange resolution, and we did not touch anything but the subs.

 

[microstrip]

Not sure he said that.

re the dcx, it has more than enough power to do up to six distributed subs. With a bit of trickery, not that you'd want it, you can easily do more.
(...)

Terry,
You are an optimist. Perhaps the DCX has power enough, but I doubt on the user power and knowledge.
Just look at the optimization algorithms described in the Todd Welti and Allan Devantier papers - I think I would spent years just studying the paper and setting the bass, even with only four subs!

THERE is suddenly your new midrange resolution, and we did not touch anything but the subs.

I also experienced it with the addition of two very large tuned membrane bass traps, even without subs - but the Aida's go down to 18 Hz in my room!

 

[jkeny]

Ahh, I use impulse graphs all the time, but never from that 'far a distance'. Yep, once you scale waaay back then et voila, they are the graphs!

And in raising those questions, I also outlined how I would test my hypothesis..

Ran a few quick ones this morning, want to do a bit more digging still, but for now and quickly I can say without a shadow of a doubt my hypothesis is busted. No question, zero maybes, completely and totally wrong.
.......

So when you turned down the volume you DIDN'T see a decrease in the post-ringing as per Amir's graph?

 

[amirm]

OK, it seems the best way to have optimized bass with multiple subwoofers is using a proprietary technique and adequate equipment. Is the setup fully automatic, or does he need some assumptions from the owner and can be custom tailored? Does the unit include an audio analyzer with a calibrated microphone?

Which unit? BassQ? If so, I have not used it but I believe it is full automatic. The JBL Synthesis version is also fully automatic but then you can manually change the settings it programmed for every sub. The BassQ comes with four microphones so it can optimized across more than one position. The JBL requires a dealer calibration system that has 8 microphones.

 

[amirm]

Terry,
You are an optimist. Perhaps the DCX has power enough, but I doubt on the user power and knowledge.
Just look at the optimization algorithms described in the Todd Welti and Allan Devantier papers - I think I would spent years just studying the paper and setting the bass, even with only four subs!

Indeed. The paper talks about the impossibility of doing this manually. You have four subs. For each, you need to program level, delay and filter with a variable response. Now multiple that by 4. So you have 12 independent variables, across multiple measurement points. There is no way you can arrive at the optimal settings manually. The paper uses a fancy algorithm that is able to predict if it is headed in the right direction or not and keep ittirating until it finds the optimal solution in reasonable amount of time/computing power.

 

[terryj]

Terry,
You are an optimist. Perhaps the DCX has power enough, but I doubt on the user power and knowledge.

You mean your own user power and knowledge? Well maybe, but another way to look at that is 'self fulfilling prophecy'!

In any case, I was trying to answer a question you asked recently about the dcx, even if you have since rejected it. The short answer is 'yes, it can do all that you need'.

Just look at the optimization algorithms described in the Todd Welti and Allan Devantier papers - I think I would spent years just studying the paper and setting the bass, even with only four subs!

The multiple subs paper? IIRC *most* of it was computer simulation, up to a ridiculous number like a thousand? I think they decided that there was no real gain after about four. Been a few years tho mind.

So four subs, set independant delays and measure, eq, voila.

But yes, you are absolutely correct, it is easy to totally stuff it if done wrong, I mentioned this earlier and feel we should not gloss over it. In that case, I can most definitely see the appeal in a quality unit that can do it automatically for you.

I also experienced it with the addition of two very large tuned membrane bass traps, even without subs - but the Aida's go down to 18 Hz in my room!

I don't doubt it for a second. The point is, and it seems you agree with me, that bass is a funny animal and how it is presented to you has ramifications for everything else perception wise. To that end, I feel it most important that we *fight* any lingering audiophile fear of 'inserting components in the signal path'. Those days are looong gone. It seems an absurd notion when a quick simple measurement at your LP will make the usual persons jaw drop...if they understand what they are seeing that is.

I take it the measurements you have been presenting here are of your aidas?? Is that on their own or are there subs too.

In any case, without coming across as a prat I hope, there are significant improvements to be had. A few subs in addition (if not there) will be greatly beneficial.

Amir speaks of four, I only use two, but then again I have 18's in the mains as well so really four distributed bass sources. My 18's in the mains don't go do 18hz as yours do. So you will prob gain even more than I did.

The next quest for you, good luck!

jkeny


.......
So when you turned down the volume you DIDN'T see a decrease in the post-ringing as per Amir's graph?

Nope. Nothing. Not a change anywhere.

Someone can correct me if needed, pretty sure the impulse graph does not show spl per se, but rather a percentage of full scale deflection or something. As it shows 'comparisons' rather than absolute, that explains why simply lowering the volume still gives the same graph as it were.

Any of those 'swings' we saw ONLY came down or changed with the application of a filter, a mere change in volume did nothing to it at all.

Still need to do a bit more to get a better understanding, but *our* (note I am dragging you in with me in this sinking ship haha) thoughts on the matter-or at least this particular one-hold no water.

Which is kinda ironic given the sinking ship metaphor

 

[amirm]

For a couple of examples, here is the graph that shows the effects of temperature changes. It was a cold winter day (for Houston at least), and it was 73? inside the house. After taking a measurement, I set the measurement mic on the floor where sunlight was streaming in through a window. The thermometer registered 86? in the sun. After a half hour or so I took another reading. The red trace is the reading from the warmed-up mic. There is no trace separation in this one; note that the red trace is a dB or two "higher" across the board.

Two REW Readings, Mic Element at 73 vs. 86 Degrees ​

Consequently, it should be no surprise that measurements taken months apart will also show inconsistencies from the “original” measurement [NOTE: The traces have been shifted in order to separate them for greater clarity.]:

Hi Wayne. Welcome to the forum! It is great to have you here. And thanks for the kind words. It is good to not feel lonely on this topic .

I was fascinated by the above measurement and observation. Did not realize the variation can be this significant. Thanks for posting it.

 

[amirm]

The multiple subs paper? IIRC *most* of it was computer simulation, up to a ridiculous number like a thousand? I think they decided that there was no real gain after about four. Been a few years tho mind.

You are thinking of the wrong paper Terry. This is a separate paper "Low-Frequency Optimization Using Multiple Subwoofers," TODD WELTI AND ALLAN DEVANTIER

It starts by summarizing Todd's paper that you are thinking about and then gets into SFM which it shows with actual measurements compared to predictions.

So four subs, set independant delays and measure, eq, voila.

As I noted, the key to the success of this solution is to also apply filters to subs independently. That one parameter optimized across four subs presents an infinite case. Add the other two params and the "brute force" solution of trying every alternative goes beyond any computing power we have. So they suggest an optimization algorithm to cut it down to manageable set.

Amir speaks of four, I only use two, but then again I have 18's in the mains as well so really four distributed bass sources. My 18's in the mains don't go do 18hz as yours do. So you will prob gain even more than I did.

Two is actually one of the optimized solutions also. That is what I have in my new theater. Four in the corners however gives you more power since you use the corner to your advantage to get additional gain. With two, the optimal position is in the middle so you lose fair amount of your sub power. In our theater at work we had to double the number of subs to compensate (the two placed next to each other look like one larger sub).

 

[terryj]

You are thinking of the wrong paper Terry. This is a separate paper "Low-Frequency Optimization Using Multiple Subwoofers," TODD WELTI AND ALLAN DEVANTIER

It starts by summarizing Todd's paper that you are thinking about and then gets into SFM which it shows with actual measurements compared to predictions.

Thanks.

As I noted, the key to the success of this solution is to also apply filters to subs independently. That one parameter optimized across four subs presents an infinite case. Add the other two params and the "brute force" solution of trying every alternative goes beyond any computing power we have. So they suggest an optimization algorithm to cut it down to manageable set.

This is something I have looked at (albeit briefly) in my own situation.

Firstly, I think we need to make the distinction between HT and stereo listening. My guess that most here are into 2 ch mainly. Anyway, I most certainly am, movies bore me to death usually.

So seat to seat variation demands are very different. In my case all I need is at my seat, who cares about the rest (well, there is only one seat but you get the idea). And in any case, movies as such and music are two very different animals. A lot of the time, movies in the bass is not music so much but also bangs and crashes. Even on those occasions where there is genuine music, or the need for smooth bass FR, still the visual sense is far far more important, ie we can forgive a lot when we are watching (due to the primacy of the visual sense)

So trying to get the perfect response in many different locations (impossible, or at least less attainable than one position) is fortunately less needed.

So back to 2 ch, then the demands you just outlined become more possible and are not the huge task this might imply. You CAN quite easily set independent levels, you CAN individually time align each sub, and with the dcx as it has come up you CAN set individual filters on each channel as well as global filters.

The wash up? I have not found too much difference between setting individual filters on each channel (which when interacting that 'perfect' response with a second 'perfect' response you get a non perfect response anyway) which then needs final eq, or just setting levels and time, then correcting the whole.

At least in my mind or situation it is a settled matter.

You may as well set the time properly, your relative levels, THEN correct the interaction.

Two is actually one of the optimized solutions also. That is what I have in my new theater. Four in the corners however gives you more power since you use the corner to your advantage to get additional gain. With two, the optimal position is in the middle so you lose fair amount of your sub power. In our theater at work we had to double the number of subs to compensate (the two placed next to each other look like one larger sub).

This refers to the paper I got mixed up with? Again from a while ago, I think it was four in the corners, or mid walls work too. And that two worked only a little less well than four, after which no real benefit in going. Even tho two wored well, I at least feel less need to go four as I include my other two 18's in the distributed bass mix.

Besides that, four way active plus stereo subs is about as complex as I have any desire to be!