Bass alignment methods and group delay

[Steve F]

I thought my rambling response gave my feelings but I'll try again.

I can't directly answer whether Peter was correct or not regarding 10ms being acceptable and 50ms being unacceptable. I haven't done that specific test. Forced to choose, I would say "no".......

I just don't believe that the typical variation of group delay is a real world factor in speaker preferences......

As others have pointed out, the room effects are likely to overshadow the relatively minor differences between system types and cutoffs.

David

Thanks for your direct opinion. Yes, this is essentially what I gleaned from your previous responses, but I just wanted your direct answer to the direct question.

PM was a very bright guy, and had a thought-provoking way of looking at things. And who among us who remembers could not have been amused when he was banned from all future Bose press conferences after his relentless "emperor has no clothes" questioning at some Bose press event. (I forget which one).

Anyway, thanks again for your opinion.

Steve F.

[kkantor]

Steve,

I keep delaying my reply to your question, because I vacillate between very short answers and very long ones! Here are a few comments:

1- Let's do a thought experiment: First, consider an extremely large system , with an Fc of 1 Hz. I think it is clear that resonance-related issues like Qts, group delay, alignment order, etc, will have almost no impact on the sound. Next, consider a tiny system, with an Fc of 1,000 Hz. Here, a lot of musical energy resides directly in the resonance region, and below. Every real-world system is going to fall somewhere between. As Hegel observed, the human mind is attracted to binary frameworks. We all want to align with an alignment, pick our heros, and be done with gray areas. But, I don't think nature always cooperates.

2- I will say that smaller systems probably do favor lower-order systems, for two reasons. Excursion is better controlled below resonance, and the area under the curve below cutoff is greater.

3- The physics and psychophysics of very deep bass in listening rooms is a surprisingly under-researched area, IMO. (There are notable exceptions, like Fielder's work at Dolby.) Most discussions are based on speculation and theory, not experimental data.

4- In my own casual experiments, I find that even the basic behavior of home listening rooms is complex. For example, many room boundaries eventually become transparent as frequency goes down, and do so at varying rates. So, while the response of a given room location is fairly predictable down to 40 Hz, it is not so down to 20 Hz. Even more disconcerting is the fact that it can take many seconds for the steady state response at very low frequencies to stabilize. At these frequencies, there can be reflection paths that are very long... like all the way to outer walls of the building, or the neighbor's building.

5- I knew Peter Mitchell and have debated this subject with him on several occasions. To me, there is very little case to be made for "group delay" being responsible for a subjective sense of bass tightness or fastness, or whatever. I believe the impression of tightness arises from the perceptual masking of note decay, which in turn relates to the relative levels of simultaneous frequencies. There could be a timing component here, but it would be secondary. (Masking, both spectral and temporal, is a very interesting but arcane subject, best left for another thread, I think.)

6- On the other hand, the damping, ("Q"), of a speaker is readily audible. Exactly why remains unknown, and we quickly get back to armchair speculation. Is it only a frequency response issue? Does overshoot tend to push a woofer towards excursion limits? Is it related to image sources in the room? It's impossible to fully isolate these things in an experiment, so we are left with trying to find best practices.

- k

[speaker dave]

Now, if we are talking about the audibility of filter corners, low pass filters seem to be another matter. With every old preamp that I have played with that had a record noise filter, the pitch of the cutoff corner is very distinct. By that I mean if a preamp has a 3 position noise filter (my Mac C20 and Citation do), you hear a distinct musical pitch for each corner.

These are typically 2nd order low passes and fairly sharp Q. Is this a group delay ringing issue? Would it go away with Bessel filtering? Would it be an issue with infinite slope crossver filters in the middle of the audio band?

David

[soundminded]

The ability of home high fidelity music reproducing systems to accurately reproduce the tonality of acoustic intrurments at low frequencies is generally awful. One of the things that always strikes me when I hear live music is how much more bass there is than you hear from most home hi fi systems. Even a performance by a high school orchestra in a typical high school auditorium demonstrates that fact. This is usually true in the two lowest octaves of audible sound and especially in the lowest octave. I'm not just talking about pipe organ pedal notes, I'm talking about double basses, cellos, tubas, and drums. Many sound systems in real rooms in a home can produce substantial bass at slightly higher frequencies say around 100-120 hz but fail badly in those lowest two octaves. The droning exaggerated false bass in the third octave is extremely irritating if you listen critically to music produced by acoustic instruments.

The ability of loudspaker systems to reproduce very low frequencies with flat frequency response and low distortion at sufficiently loud levels with or without equalization in an anechoic chamber is just the beginning of a well engineered sound system IMO. Public spaces for musical performances are invariably considerably larger than rooms at home. Their acoustic cutoff frequencies are below the audible range. Home listening rooms having shorter dimensions with much higher cutoff frequencies give no acoustic reinforcement below those frequenciesm instead they effectively attenuate them. Just as bad if not worse is that 1/2 inch sheetrock on 2 x4 studs 16" OC which is how most modern homes in America are constructed flexes and absorbs a lot of bass energy. There is also a lot of it that is transmitted right through the walls and ceiling. Unless these losses are compensated for, even speakers that measure flat in anechoic chambers will sound bass shy. Because of the small size of home listening rooms, FR due to cancellatins varies enormously within the listening room. Bass response as heard by a listener is very much subject to where he listens. Just standing up from a sitting position can make a big difference. And then there are the variables of the recordings themselves. Most modern home hi fidelity sound systems cannot adjust or compensate for these variables, all the end user can do is move the speakers around to where they seem to sound best on one or a few particular recordings. If group delay is audible at all, it is among the least of the problems that compromises bass performance.

[speaker dave]

The ability of home high fidelity music reproducing systems to accurately reproduce the tonality of acoustic intrurments at low frequencies is generally awful.

I remember when the "Shefield drum record' came out (now I'm dating myself!) I heard it at a dealer who had a multispeaker comparitor and lots of speakers. As he switched from speaker to speaker there was a different tonality for each speaker. In fact, each speaker gave the drums a unique pitch. Maybe that is the LF counterpart to the HF noise filter I mentioned above.

Just as bad if not worse is that 1/2 inch sheetrock on 2 x4 studs 16" OC which is how most modern homes in America are constructed flexes and absorbs a lot of bass energy. There is also a lot of it that is transmitted right through the walls and ceiling.

Careful here. Short of extremely large bass traps, the only useful mechanism for bass absorption is wall flexure. Wooden walls are typically verboten in the modern concert hall (unless adhered to solid concrete) because they are trying to keep bass RT as high as possible, but bass absorption is a very good thing in the typical living room. Those that build their home theaters into cinderblock basements usually have the worst standing wave issues and very uneven bass. Rooms always gain bass over the anechoic system response, so giving some up for smoothness is probably a good thing.

David

[soundminded]

I remember when the "Shefield drum record' came out (now I'm dating myself!) I heard it at a dealer who had a multispeaker comparitor and lots of speakers. As he switched from speaker to speaker there was a different tonality for each speaker. In fact, each speaker gave the drums a unique pitch. Maybe that is the LF counterpart to the HF noise filter I mentioned above.

Careful here. Short of extremely large bass traps, the only useful mechanism for bass absorption is wall flexure. Wooden walls are typically verboten in the modern concert hall (unless adhered to solid concrete) because they are trying to keep bass RT as high as possible, but bass absorption is a very good thing in the typical living room. Those that build their home theaters into cinderblock basements usually have the worst standing wave issues and very uneven bass. Rooms always gain bass over the anechoic system response, so giving some up for smoothness is probably a good thing.

David

Drum heads vibrate with many harmonic modes according to bessel functions. It is the upper harmonics (for their range) that give these instruments pitch and if you listen carefully to the decay of a kettle drum, you can often actually hear it change pitch as it decays. Differences in frequency response in the upper bass and midrange will affect the tonality of drums. Bass drums when struck hard should produce a sound like the thud of a heavy object falling. IMO most home audio systems can't produce that sound. AR9 can produce it even in a room that is a slab on grade when properly equalized. So can Bose 901 if there is enough amplifier power available and enough pairs not to overload them.

Rooms for home theaters with cinderblock walls are too small to reinforce bass at the lowest frequencies. Even a room that is 25 x 25 x 8 will have cutoff frequencies at 40 hz and standing waves above 100 hz. By contrast typical concert halls are about 70 feet wide, 40 to 50 feet tall, and well over 100 to 150 feet deep. Typical volume for concert halls varies from 500,000 to over 1 million cubic feet. Reverberation time at low frequencies is around 2.5 to 3.5 seconds. Typical home listening rooms are in the range of 2000 to 5000 cubic feet and have RTs in the range of .25 to .4 seconds. According to Leo Beranek, concert halls should not be constructed of wood unless the wood is at least 2 1/2 inches thick or it will absorb bass. Wood is not a good material for construction of halls, contrary to the expectations of musicians who believe it is because many instruments are made of wood. The wainscotting on the stage of Boston Symphony Hall was a concession to this manner of wrongthinking. Good material for concert halls in the old days was wet plaster on lath (practically a lost art in the US) and today cement. Someone who had performed at Carnegie Hall and was very familiar with it said its acoustics were degraded seriously when the NYC fire marshall forced the owners to replace the oil base paint with latex. Beranek is not impressed with the hall and complains there is serious acoustic shadow under the balconies. Carnegie Hall has a volume of 900,000 cubic feet. I think Davies hall is made of cement. Acoustic baffles, banners, and reflectors are used in many halls to tune the room. Audience absorption also plays an important roll especially in absorption of direct and early arrival of bass frequencies according to Sams Handbook. It adds that this can be compensated for if there is much bass energy in the reverberant field. Typical data for halls show RT vs frequency up to 8 khz with the hall both empty and fully occupied. Usually the audience affects the HF components most obviously.

[genek]

The interior of Davies Hall is "polymer concrete," which is concrete that replaces cement with a thermosetting resin. Basically, it's a very dense plastic not unlike a solid-surface kitchen countertop. And it's reflective as all hell. The problem wasn't the timings of reverberations, but their levels. Too loud. 12 years after Davies opened in 1980 another $10 million was spent to reduce and reshape the volume and install a redesigned "cloud" of diffusing panels to reduce the levels of the reverberations.

[soundminded]

The interior of Davies Hall is "polymer concrete," which is concrete that replaces cement with a thermosetting resin. Basically, it's a very dense plastic not unlike a solid-surface kitchen countertop. And it's reflective as all hell. The problem wasn't the timings of reverberations, but their levels. Too loud. 12 years after Davies opened in 1980 another $10 million was spent to reduce and reshape the volume and install a redesigned "cloud" of diffusing panels to reduce the levels of the reverberations.

According to the data I've got for the original construction Louise M. Davies Symphony Hall constructed in 1980 at a cost of (only) $27 million has a "ceiling that is a 6" thick concrete slab, the walls are multifaceted precast concrete 4 inches thick, and the balcony fronts and soffits are of 2" plaster"

The RT with no banners deployed starts at 4.0 seconds at 63 hz, drops to around 3 seconds until 1 khz where it begins a linear fall to about 1.5 seconds at 8 khz. With all banners deployed, the RT is around 3.3 seconds at 63 hz, drops to 2 seconds at around 300 hz, and slowly falls off to about 1.1 seconds at 8 khz. This is with the audience half filled. The spectral response starts with a linear rise from 20 hz to about a 15 db peak at 63 hz, falls back linearly to the 20 hz level at 125 hz where it stays until around 2 khz, and then drops off linearly down about 10 db by 8 khz. The banners don't change that by more than about 1 db over the entire range.

"Mid frequency reverberation time in the occupied hall can be varied from 1.6 to 2.2 seconds with retractable velour banners."

"The 3000 seat hall was designed to provide "a perfect instrument for the presentation of symphonic music. Any other uses are of secondary importance....."

I guess there must have been some who disagreed either with the philosophy or the result. As audio systems are the play toys of the not so rich and somewhat rich hobbyists, concert halls are the playtoys of acoustic consultants, engineers, and architects. You have to wonder why with so much money at stake and such high expectations, they don't just take the most successful model like Boston Symphony Hall, MusikvereinSaal, or Concertgebouw and copy it as closely as they can. I guess eveyone wants to make a "statement."

The acoustic consultant was Bolt, Beranek, and Newman.

[genek]

According to the data I've got for the original construction Louise M. Davies Symphony Hall constructed in 1980 at a cost of (only) $27 million has a "ceiling that is a 6" thick concrete slab, the walls are multifaceted precast concrete 4 inches thick, and the balcony fronts and soffits are of 2" plaster"

Yes, but it's not conventional sand-and-portland-cement concrete (though now that I think about it, I really don't remember if the polymer component completely replaces conventional cement or if it's a mix, so maybe think Silestone rathar than Corian). And yes, I forgot about the plasterwork.

I guess there must have been some who disagreed either with the philosophy or the result. As audio systems are the play toys of the not so rich and somewhat rich hobbyists, concert halls are the playtoys of acoustic consultants, engineers, and architects. You have to wonder why with so much money at stake and such high expectations, they don't just take the most successful model like Boston Symphony Hall, MusikvereinSaal, or Concertgebouw and copy it as closely as they can. I guess eveyone wants to make a "statement."

The acoustic consultant was Bolt, Beranek, and Newman.

I've been in Davies Hall both before and after the redo. It sounds more like a classically-designed hall after. Before, its sound reminded me of a THX movie theater. Or in contemporary HT-speak, maybe a setup where the surrounds are turned up too high.

[Steve F]

Steve,

I keep delaying my reply to your question, because I vacillate between very short answers and very long ones! Here are a few comments:

1- Let's do a thought experiment: First, consider an extremely large system , with an Fc of 1 Hz. I think it is clear that resonance-related issues like Qts, group delay, alignment order, etc, will have almost no impact on the sound. Next, consider a tiny system, with an Fc of 1,000 Hz. Here, a lot of musical energy resides directly in the resonance region, and below. Every real-world system is going to fall somewhere between. As Hegel observed, the human mind is attracted to binary frameworks. We all want to align with an alignment, pick our heros, and be done with gray areas. But, I don't think nature always cooperates.

2- I will say that smaller systems probably do favor lower-order systems, for two reasons. Excursion is better controlled below resonance, and the area under the curve below cutoff is greater.

3- The physics and psychophysics of very deep bass in listening rooms is a surprisingly under-researched area, IMO. (There are notable exceptions, like Fielder's work at Dolby.) Most discussions are based on speculation and theory, not experimental data.

4- In my own casual experiments, I find that even the basic behavior of home listening rooms is complex. For example, many room boundaries eventually become transparent as frequency goes down, and do so at varying rates. So, while the response of a given room location is fairly predictable down to 40 Hz, it is not so down to 20 Hz. Even more disconcerting is the fact that it can take many seconds for the steady state response at very low frequencies to stabilize. At these frequencies, there can be reflection paths that are very long... like all the way to outer walls of the building, or the neighbor's building.

5- I knew Peter Mitchell and have debated this subject with him on several occasions. To me, there is very little case to be made for "group delay" being responsible for a subjective sense of bass tightness or fastness, or whatever. I believe the impression of tightness arises from the perceptual masking of note decay, which in turn relates to the relative levels of simultaneous frequencies. There could be a timing component here, but it would be secondary. (Masking, both spectral and temporal, is a very interesting but arcane subject, best left for another thread, I think.)

6- On the other hand, the damping, ("Q"), of a speaker is readily audible. Exactly why remains unknown, and we quickly get back to armchair speculation. Is it only a frequency response issue? Does overshoot tend to push a woofer towards excursion limits? Is it related to image sources in the room? It's impossible to fully isolate these things in an experiment, so we are left with trying to find best practices.

- k

Thank you, Ken. I appreciate your response, and now that I have both yours and David's, I feel better "armed" on this subject, on both sides.

In your no. 2 above, what would you characterize as a "small" system? A 6 1/2" or 8"-type system, like an AR-7/18? Is a 2ax/5--10" woof in a 1.35 cu. ft. box-- small or large? What is your definition of 'small/large'?

PM was a wonderfully pragmatic, relentlessly logical individual, but like all of us, he had very definite ideas about audio and what was/was not audibly important. I'd have loved to have witnessed those debates.

Thanks again.

Steve F.

[kkantor]

Thank you, Ken. I appreciate your response, and now that I have both yours and David's, I feel better "armed" on this subject, on both sides.

In your no. 2 above, what would you characterize as a "small" system? A 6 1/2" or 8"-type system, like an AR-7/18? Is a 2ax/5--10" woof in a 1.35 cu. ft. box-- small or large? What is your definition of 'small/large'?

PM was a wonderfully pragmatic, relentlessly logical individual, but like all of us, he had very definite ideas about audio and what was/was not audibly important. I'd have loved to have witnessed those debates.

Thanks again.

Steve F.

Steve,

Agreed. Fond memories. (Like the time I had to rent a car at Heathrow, because Peter was just not going to make it into my NAD-supplied Renault 5.) Peter is one of the last real "renaissance men" in audio, who had credentials in almost all areas of the field, and who could dissect and explain just about any technological issue.

Of course, what constitutes a "small" or a "large" system is a continuum. Playback level, low frequency filtering and eq must be considered. I used to say that 8" was the dividing line for home use; that 6.5" woofers should never be vented, and that 8" woofers probably shouldn't be. But, then again, it used to be that record warp, acoustic feeback and RIAA infrasonic boost were the biggest threats.

David,

My current job is designing small guitar amps, and these products include agressive high pass filtering. It was an epiphany to me to figure out that the choice of LF cutoff frequency has distinct musical/tonal implications, just as you are explaining.

-k

[Pete B]

All you ever need to know: http://www.archive.org/details/HowtoLis1957