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Question for Richard BassNut Greene

12.30.32.2

Posted on July 9, 2003 at 09:58:19
OK, I've read Floyd Toole's papers, and I know you're a big advocate of equalizing out the room modes. Mr. Toole seems to imply that if you can exactly counteract a room mode by careful adjustment of a parametric equalizer, then the time response problem will also be fixed because "a room mode is a minimum phase phenomenom" (page 17 of part 3).

Now, I've tried to understand minimum phase, but all I've learned so far is that I shouldn't have partied myself out of EE many years ago.

But intuitively, it seems as though a standard equalizer cannot fix the time issues. That's why products like the Tact are being developed.

I intend to run some experiments later when I get time and motivation, but I'm curious about your thoughts on this.

 

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Re: Question for Richard BassNut Greene, posted on July 9, 2003 at 11:01:43
Rives
Manufacturer

Posts: 179
Location: Maryland
Joined: January 4, 2003
This is actually a fairly complex and controversial subject. I will give you my thoughts on the subject, but recognize that there are absolutely divergent opinions by many highly regarded people in this field.

I agree with Floyd on the ability to correct the amplitude interaction of standing waves with a parametric, it is for this reason that we developed the PARC (parametric adaptive room compensation system). I also agree that it does create a phase issue that can be measured under labratory conditions. I do not agree that the phase in these frequencies when ATTENUATING for room mode interaction has any significant effect on the sound. This was a concern of ours, particularly since we use an analog system for signal purity. What we found was that boosting the signal more than 5 or 6 db (and narrow Q factors) did result in perceptible phase artifacts, but attenuation did not.

Also, one should be aware that the majority of phase distortions are caused by room interaction. If you try to correct for these types of distortions (across the full bandwidth--not just low frequencies) you can only be successful at one location in the room. How significant this is in subjective listening remains another controversial issue, but even the people at TacT don't put a lot of faith into the phase issue. It's simple enough to have the ability to change the phase in the digital domain--so why not have it. I would be interested in your results and any others that may exist. I would not be surprised if Harmon has done this research, but I have not seen it published or printed.

 

Re: Question for Richard BassNut Greene, posted on July 9, 2003 at 11:22:05
Yes, there's no doubt that things can sound much better when a parametric eq is properly used. But Richard Greene (and Mr. Toole also, I think) advocate putting the woofers in a corner and equalizing that! My own experience, in every room, with every speaker, is that corner placement results in a boomy mess, bacause all room modes are energized. So now I'm wondering if I would simply need to address each of those modes with a parametric channel, or is it that each mode takes some time to build up to equalibrium (for a substained note) and then decay to zero (in which case I'd want to energize as few modes as possible through proper placement, and apply the equalizer to these few remaining ones).

 

Re: Question for Richard BassNut Greene, posted on July 9, 2003 at 11:37:27
Rives
Manufacturer

Posts: 179
Location: Maryland
Joined: January 4, 2003
Corner placement allows the reinforcement of all modes equally. For a room with excellent modal spacing and plenty of modes (relatively large room) this can work to your benefit as the woofer does not have to work as hard and thus has less distortion. For smaller rooms or rooms that don't have good modal distribution, I agree with you and it generally lends itself to accentuating those modes and creating the "boom". In those cases I try to space the woofers at the 1/4 wavelength null point of the most prominent frequency (unfortunately, this can sometimes be in an aesthetically or otherwise bad location). If you can place it here you can use nature to help cancel out the booming frequency(s). Then there may be remaining ones that can be attenuated with a parametric.

 

Answer from Richard BassNut Greene ( long), posted on July 10, 2003 at 07:15:22
Click on hot link for info on "minimum phase"

Long-winded "speech" follows:

About 80-90% of bass energy under 80Hz. is reflected off one or more
room surfaces on the way to your ears. Only 10-20% comes directly from the subwoofer to your ears.

Under 80Hz. the frequency response at your listening position is greatly influenced by in-phase reflections between opposing room surfaces (aka standing waves, or room modes).

After a small delay for standing waves to form (typically 15 to 45 milliseconds in a home listening room), the bass reflections will combine with the bass energy still coming from the subwoofer.

The bass frequencies involved are determined by room dimensions.

Standing waves do not require parallel walls to develop --
they just need opposing surfaces that reflect bass energy.
The strongest standing waves are between two opposing surfaces (aka "axial").

There will be fixed (aka "standing") locations within a room where the bass reflections are out of phase (nulls) and other locations where the reflections are in-phase with bass energy still coming from the subwoofer.

In actual rooms standing waves tend to be 3 to 10Hz. wide (measured using -3dB points) with 5Hz. wide being typical for plasterboard
wall / plywood floor rooms.

In a typical home listening room there will be about five axial standing waves under 80Hz. (sometimes six in large rooms).

If a room happens to be square, some of the standing waves will be stacked at the same frequency.

If the room is near-square, some of the standing waves will be at adjacent frequencies.

If the room is well shaped, the standing wave frequencies will be at different frequencies not too close together ... or too far apart -- that allows our ears to apply their one-third octave smoothing effect.
If we're lucky, no single standing wave will be noticeable.
This is common in very large rooms where there are more than
five or six axial standing waves under 80Hz. ... and in ordinary sized rooms above 100Hz. where standing waves are more dense
than below 80Hz.

In ordinary listening rooms, it's very likely a listener will be too close to one or more standing wave-related bass frequency peaks and nulls. The nulls tend to be very narrow, so just moving the listening position one foot or two in the right direction will move your ears away from the deepest portion of a null. But the peaks
are much wider, so moving your seat is unlikely to get away from all of your room's "bass booms". Of course there are limitations to where you can move your listening seat, as you have to give top priority to the mid-range and treble frequencies to hear the proper stereo image.

If you hear / measure one or more bass frequency peaks at your listening position, the peak SPL can be reduced using parametric EQ (typically 1/6 octave bandwidth).

EQ applied to each of the three groups of axial room modes and
the side effects on alternative seating positions:

(A) For room modes between the front and back walls
(a very common acoustics problem, and the only group of axial room modes not affected by using dipole speakers):

- EQ that works for the listening position will also work well for people sitting to the left side or right side of the listening position (i.e.; people whose ears are located approximately the same distance from the front wall)
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(B)For room modes between the floor and ceiling:
(a fairly common room acoustics problem, especially in rooms with cement floors and low ceilings):

- EQ that works for the listening position will also work well for people sitting anywhere in the vicinity of the listening position whose ears are located approximately the same distance from the floor.
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(C) For room modes between the side walls:
(Bass frequency peaks caused by these room modes are not a very common problem for two-channel stereo because the main listening position is usually half-way between the side walls, which is a null for many of these room modes ... and the left and right speakers are placed on different sides of the room, so they are out of polarity and can't excite many of these room modes)

- EQ that works for the listening position will also work well for people sitting behind the main listening position whose ears are located approximately the same distance from side walls. Would also work well for someone sitting right in front of the main listening position ... but we can't allow that!
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From the Adire Audio website (click on "Paper that includes ... link):

"The Hilbert transform is a method of deriving the acoustic phase from the acoustic magnitude (the typical frequency response curve shown in most all literature and design programs). There is one caveat: the Hilbert transform is valid ONLY for what is called a minimum phase system. What is a minimum phase system? The actual way to calculate one is also beyond the scope of this paper; however, for our purposes, a subwoofer driver operating in a box, in it's linear mode is a minimum phase system. Operate beyond the linear limits of the driver (where distortion, power compression, suspension compression, and other nonlinear issues come into play) and the system is no linger minimum phase, and hence cannot use the Hilbert transform."

http://www.adireaudio.com/tech_papers/sub_gd.htm
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From a Tom Nousaine post at the google.com link below:

"The major problem at low frequencies is a relative lack of room
modes. Standing waves (room modes) occur at all frequencies between
any pair of opposing wall surfaces. If you play a 1000 Hz tone
(wavelength of roughly a foot) you can hear the modal changes when
you move your head a few inches. However because of the short
wavelength you get the same effect anywhere in the room.

At low frequencies with long wavelengths the main problem is that the
pressure peaks and valleys occur in multiples of several feet.
Therefore the energy in the room will be unevenly distributed.

Remember that the modes are a function of the room dimensions. So the
ONLY thing you can do is FAIL TO EXCITE a mode. Thus moving a
subwoofer around in your room will simply fail to excite some modes.

And the PROBLEM is a shortage of modes. For example below 100 Hz
there will be only 3 axial and 3 2nd harmonics in an average 3000 ft3
room. If you fail to excite ANY of them you will worsen the
distribution of energy in the room. Even if you find the optimal
listening location you'll still fail to excite one or more of those 6
modes and worsen the situation if you choose ANY subwoofer location
than a closed (a full wall and at least 5-feet of wall to either
side) corner.

Even using a corner won't give you even distribution of SPL at low
frequencies BUT it will give you most uniform distribution you can
get. The next job is to find the listening location that takes
advantage of this. In several rooms that I've response mapped I've
generally found that in a shoebox shaped room a spot centered on the
short wall and about 1/4 of the length distance from the back wall is
optimal."

http://groups.google.com/groups?q=nousaine&start=350&hl=en&lr=&ie=UTF-8&scoring=d&selm=a6082001peo@enews1.newsguy.com&rnum=357

 

I don't advocate placing woofers in a corner except maybe Klipschorns, posted on July 10, 2003 at 07:56:17
Moving woofers so they are less than 3-4 feet from room corners boosts upper bass output (80-200Hz.) and makes male voices sound unnaturally heavy (bad). Subwoofers should have very little output above 80Hz. (male voices not audible) so placing one in the vicinity
of a room corner will not have that effect.

I advocate placing subwoofers in a position where they are well integrated with the main left and right channel speakers so they appear to be sonically invisible. That's the top priority.

If a "subwoofer" has a lot of output over 80Hz., I'd call it a "center woofer" -- it should be placed somewhere between
the left and right speakers (not near a corner), although it
does not necessarily have to be half way between the left and
right speakers to preserve the correct stereo soundstage.

For real subwoofers with little output above 80Hz. (which I'll
define as down at least -6dB at 80Hz. and down at least -24dB at 160Hz.) the subwoofer does not have to be placed somewhere between the left and right speakers.

A single subwoofer can be placed in the area behind / to the outside of one of the main speakers -- I call that "in the vicinity of a room corner" because it usually is.

But unless you have time delay for the main speakers,
the subwoofer driver should be located approximately the same distance from your ears as the nearby main speaker woofer.
Fortunately even without time delay, locating a subwoofer driver up to three feet further away from your ears seems to integrate well with the main speakers in most rooms (assuming your ears are at least nine feet away from the main speakers). That means you can usually move a subwoofer driver three feet closer to a room corner than the nearby main speaker without ruining its integration with the main speakers.

A single mono subwoofer in a room tricorner fully excites all room modes and provides the best possible distribution of energy throughout most rooms. But the subwoofer driver can be located up to
1/4 of the way from the corner without a large effect on the excitation of the important axial room modes. Beyond 1/4, the excitation of the axial room mode drops rapidly until a null is reached at the 1/2 point between opposing walls.

It would be best to damp all room modes equally using a room full of
bass traps. Next best is reducing the SPL of specific bass frequencies that are too loud at your listening position.

The only room modes that really matter are those heard / measured at your listening position. There will almost always be one audible bass frequency peak at your listening position - sometimes two or three. Bass instruments will sound more natural if the one (or two or three) bass frequency peak is reduced using narrow band parametric EQ.

 

Re: I don't advocate placing woofers in a corner except maybe Klipschorns, posted on July 11, 2003 at 03:55:26
Sorry, I didn't mean to put words in your mouth regarding corner placement.

I'm familiar with much of the info in your posts. For the record, I use dipoles, bass traps, placement control, and parametrics and it sounds mighty fine (I'm a bass freak too).

But I'm trying to understand a fundamental issue and I'm having a tough time getting a definitive answer. The issue is: A room mode takes some time to build up to equalibrium amplitude when driven by a steady state signal, and time to decay when the driven signal is removed. If this mode, with its response boost, is exactly counteracted electronically (such as a parametric eq tuned properly, ie. Q and amplitude and frequency all correctly set), will the time abberations get corrected too? Mr. Toole seems to be saying that they will. I feel that this is not true, but what do I know? I imagine that anyone who is well versed in analyzing resonant systems (with physics and math instead of intuition) could answer this. I suppose it comes down to whether an electrical tank circuit behaves the same way as air in a room.

Of course, my goal in all this is better bass.

 

Re: I don't advocate placing woofers in a corner except maybe Klipschorns, posted on July 11, 2003 at 06:02:55
Room resonances take roughly 15 to 45 milliseconds to build up, depending on how far apart the two opposing walls or floor & ceiling are.

That means using equalization cuts bass output 15 to 45 milliseconds too early, so the initial direct ouput from the bass speaker is not as loud as it ought to be. That could be a problem except for two facts unique to bass frequencies under 80Hz.:

(1) The attack of bass transients (pluck of a bass guitar, or slap of a kick drum) begins in the mid-range frequencies which are not being equalized (assuming you use a dedicated subwoofer equalizer)

(2) Only 10-20% of the bass energy under 80Hz. goes directly from a
forward-firing subwoofer driver to your ears. 80-90% bounces off one or more room surfaces on the way.

On the decay side of a bass note after equalization:
The listening room will still resonate too long at room mode frequencies -- 200 milliseconds before room mode frequencies attenuate 30dB is typical. If you use a parametric EQ to cut bass output 6db for a specific room mode the 200 milliseconds of room resonance will be less annoying because it starts at an SPL 6dB lower than w/o equalization (at subwoofer frequencies -6dB is subjectively half as loud = big difference).

Bass traps would not effect the direct bass energy from the bass drivers to your ears unless you placed them between the speakers and your ears. Bass traps would cause the room resonances to decay
by 30dB faster than the typical 200 milliseconds w/o traps.

The primary problem with bass traps is they are big, ugly, expensive except DIY, and not very efficient. You need lots of them to thoroughly damp a listening room. Perhaps 2% of a typical home listening room's volume would need to be filled by tube traps, for one example.

 

Thank you Richard. nt, posted on July 11, 2003 at 08:18:01
nt

 

Positioning the subwoofer, posted on August 8, 2003 at 06:31:24
atico
Audiophile

Posts: 2
Joined: August 8, 2003
I am new at home thater. Would anyone assist me in explaining whether it is possible to position the subwoofer with built-in amplifier on a piece of furniture such as a buffet. Thank you so much in anticipation.

 

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