I'm sorry but "comb filtering" at such LFs in a small room is simply not a factor.
Multiple Subwoofer Placement
- Thread starter Ron Party
- Start date
I'm sorry but "comb filtering" at such LFs in a small room is simply not a factor.
I disagree. This is a fundamental and basic principal of wave physics. When you have two or more sources of sound, their outputs will sum and cancel at a repeating interval of frequencies (result is identical to a comb filter's response). This effect is easily visualized with laser light, where you have two sources of the same wavelength of coherent light. Where they converge on one another, they form distinct bright and dark bands of light as the result of interference patterns.
In a room with poor acoustic treatment, walls and other surfaces act as parasitic radiators by reflecting sound. The secondary arrivals cancel the primary direct sound from the speakers at regular frequency intervals, the result being a frequency response curve that looks like a comb. This is known throughout the acoustics industry as comb filtering.
The same thing happens when you have two or more woofers spaced apart so that they act as separate, distinct sound sources. Whether it be a wall reflection or a second woofer, the effect of comb filtering is the same--peaks and dips in the spectral response as frequency is swept from one end of the spectrum to the other.
It is particularly nasty with woofers because it can cause whole bass notes to disappear in a fixed listening location. In a different location, different bass notes are diminished to near inaudibility. The opposite happens with other intervals of frequency, where sounds from the two woofers reinforce and you get excessive emphasis of certain bass notes (different at varied listener locations).
Clustering the woofers all together greatly reduces this effect, since the array can now behave as a single source. Also, due to mutual coupling, and the effects of adjacent baffle faces, the sensitivity of an array of woofers is greater than the sum of all those woofers, spaced apart.
Well we will have to disagree then. The whole idea of "comb filtering" in a small room at LFs when there are a multitude of reflections - and hence a multitude of "sources" , all within a single wavelength, seems totally absurd to me. Unless you have some analysis or some data to support your position I would contend that it is just conjecture. Having done my PhD thesis on "The Sound Field in Small Rooms at Low Frequencies", I have more than enough data to support mine.
If I recall correctly, the wavelengths of bass frequencies are so long that it's pointless to worry about inches in different placement differences of subwoofers as they would affect each other. I thought it was the room dimensions and modes/nulls that were the major contributors to bass response irregularities (along with the ability of the woofer to play the notes....).
Mark and Earl,
Perhaps you gentlemen could appear in our Debate forum with a discussion of woofer setup, placement, and effects on output and frequency response....????? PM me if interested.
Lee
Mark and Earl,
Perhaps you gentlemen could appear in our Debate forum with a discussion of woofer setup, placement, and effects on output and frequency response....????? PM me if interested.
Lee
I guess I really don't understand. My subs are crossed over at 80hz (with a VERY steep filter). An 80hz signal is about 14 feet long. If one sub is, for example, 8 inches closer than another, I can not image how that can affect the audible experience. Change 8 inches to 20 inches and I still don't see how it can affect what I hear. Change 20 inches to ?
Where is either the anecdotal or scientific data to back up that one can hear/experience such small distance differences with such long wave lengths?
Last edited:
Unless your listening room is a closet, I would think we're not talking about inches here, but about feet or yards of spacing between mains and subs. And that you CAN hear and feel a difference with bass percussion.
There is a nice article on Comb Filtering and the Power Alley Effect here.
and Subwoofer Info, here.
My room is around 8,000 cubic feet and the differences in woofer arrangement when changing from a split config near the corners to a unified row of woofers in the front were nothing short of dramatic. While a few inches might not be audible to most folks (even though there are folks around here who claim they can hear a difference between two speaker cables), we can't rule out that small differences in placement do not make a measurable difference. In fact, a simple impulse test with Room EQ Wizard will demonstrate massive differences in frequency response and impulse response between the two configurations.
Since relocating all of my LF drivers to a row cluster, the LF response uniformity improved immensely and the system sensitivity below 100Hz increased by almost 6dB.
Unless we're talking about frequencies greater than the room's wavelength (which would be very low, even subsonic), for most practical musical bass frequencies, the concept of woofer clustering is effective and valid.
When a group of drivers are operated in close proximity to one another, they behave as one radiator at wavelengths longer than the radius of each driver, instead of as many sound radiators. Anyone remember the "Sweet Sixteen" speaker systems you could build back in the '50s with 5" PM speakers?
I think perhaps Earl disagrees based on different perceptions of frequency and room size. He's right, but the room would have to be exceedingly tiny in his case. In the example in the power alley article, a mere 8 feet spacing produced combs with dips down to 70hz. See the linked articles for more detail and frequency response measurement data.
There is a nice article on Comb Filtering and the Power Alley Effect here.
and Subwoofer Info, here.
My room is around 8,000 cubic feet and the differences in woofer arrangement when changing from a split config near the corners to a unified row of woofers in the front were nothing short of dramatic. While a few inches might not be audible to most folks (even though there are folks around here who claim they can hear a difference between two speaker cables), we can't rule out that small differences in placement do not make a measurable difference. In fact, a simple impulse test with Room EQ Wizard will demonstrate massive differences in frequency response and impulse response between the two configurations.
Since relocating all of my LF drivers to a row cluster, the LF response uniformity improved immensely and the system sensitivity below 100Hz increased by almost 6dB.
Unless we're talking about frequencies greater than the room's wavelength (which would be very low, even subsonic), for most practical musical bass frequencies, the concept of woofer clustering is effective and valid.
When a group of drivers are operated in close proximity to one another, they behave as one radiator at wavelengths longer than the radius of each driver, instead of as many sound radiators. Anyone remember the "Sweet Sixteen" speaker systems you could build back in the '50s with 5" PM speakers?
I think perhaps Earl disagrees based on different perceptions of frequency and room size. He's right, but the room would have to be exceedingly tiny in his case. In the example in the power alley article, a mere 8 feet spacing produced combs with dips down to 70hz. See the linked articles for more detail and frequency response measurement data.
No, not exceedingly tiny, just a typical sized living room.
Lets consider the situation for a moment. A 100 Hz tone has a ten ft wavelength, a one ft difference is a mear 10% shift in phase, it takes 5 ft. at 100 Hz. to get a 1/2 wavelength shift. But thats irrelavent anyways.
One model for room acoustics that is often used is the "image model". In this model every surface creates a source at the mirror image. Below 100 Hz in a typical room there are at least a dozen "image" sources within a wavelength at any point in the room. Are you going to now say that clustering the subs is going to make any difference in this scenario? Even clustered the image sources make them discrete sperated sources.
And what about the numerous studies by JBL and myself which have shown, theoretically and with measurements, that displaced subs always yield the smoother response at LFs? Are these all wrong?
Your arguments are all based on opinions with nothing of fact to back them up. I have studied the LF sound field in small rooms for almost 40 years. The modal response of a typical home sized listening room will always be smoother with distributed subs than with clustered ones.
As to anything regarding "transient perception" this is not a factor at LFs since the ear takes more than a single cycle of sound to even detect a LF signal. This means that at least 200 ms. is required to "perceive" a 100 Hz. tone. Now in this 200 ms. the sound wave has reflected arround the room more than a dozen times, passing the listener on each traverse. There is no way that the ear can detect this as "transient". At LFs we perceive only the steady state sound. Nothing transient has any meaning and "comb filtering" is a transient effect. It does not apply in the modal region of a room.
Lets consider the situation for a moment. A 100 Hz tone has a ten ft wavelength, a one ft difference is a mear 10% shift in phase, it takes 5 ft. at 100 Hz. to get a 1/2 wavelength shift. But thats irrelavent anyways.
One model for room acoustics that is often used is the "image model". In this model every surface creates a source at the mirror image. Below 100 Hz in a typical room there are at least a dozen "image" sources within a wavelength at any point in the room. Are you going to now say that clustering the subs is going to make any difference in this scenario? Even clustered the image sources make them discrete sperated sources.
And what about the numerous studies by JBL and myself which have shown, theoretically and with measurements, that displaced subs always yield the smoother response at LFs? Are these all wrong?
Your arguments are all based on opinions with nothing of fact to back them up. I have studied the LF sound field in small rooms for almost 40 years. The modal response of a typical home sized listening room will always be smoother with distributed subs than with clustered ones.
As to anything regarding "transient perception" this is not a factor at LFs since the ear takes more than a single cycle of sound to even detect a LF signal. This means that at least 200 ms. is required to "perceive" a 100 Hz. tone. Now in this 200 ms. the sound wave has reflected arround the room more than a dozen times, passing the listener on each traverse. There is no way that the ear can detect this as "transient". At LFs we perceive only the steady state sound. Nothing transient has any meaning and "comb filtering" is a transient effect. It does not apply in the modal region of a room.
I would very much like to see some of these JBL studies. If there is anything online, would you please point me to the sites where I can read them?
My only facts are my personal listening experience and the work of professional sound reinforcement engineers in this field of study.
I have observed that placing my subwoofers in the far corners of the front of the room causes transient response to suffer and much cancellation of bass at regular intervals of frequency. I may still have the REW data plots of my before and after room measurements, if I have not deleted them and they are properly catalogued.
It may be your belief that it is not possible to hear the difference in woofer placement, but I can hear and feel the difference with percussion bass. Pipe organ pedal tones are a different story. With that music, I can't tell the difference, so in that narrow realm, I would agree with you.
I'll go a step further: I can even tell from which direction a bass drum or other percussive LF sound originated from, because I can feel the initial burst of energy physically. It's not entirely about hearing--bass is a very tactile experience. That's why I continue to maintain stereo bass processing in my sound system, rather than mono the LF channels together.
At 100Hz, there are multiple dips when two sources are present, in a properly-treated room having minimal reflections. As you observed, these will occur at approx 5' intervals. Take two identical sound sources, operate them in phase, and look at their polar pattern versus frequency along their axis of separation. You will see interference beginning at frequencies whose wavelengths are twice the separation distance and continuing with increasing frequency. You can try this test with a pair of subwoofers. Power only one and measure various points in the room in front of the woofers. Then power both and watch the extra dips appear. Now move the two woofers together and note how they behave as one subwoofer, only louder. These are all easy tests to perform and they bear out what I am talking about here.
My only facts are my personal listening experience and the work of professional sound reinforcement engineers in this field of study.
I have observed that placing my subwoofers in the far corners of the front of the room causes transient response to suffer and much cancellation of bass at regular intervals of frequency. I may still have the REW data plots of my before and after room measurements, if I have not deleted them and they are properly catalogued.
It may be your belief that it is not possible to hear the difference in woofer placement, but I can hear and feel the difference with percussion bass. Pipe organ pedal tones are a different story. With that music, I can't tell the difference, so in that narrow realm, I would agree with you.
I'll go a step further: I can even tell from which direction a bass drum or other percussive LF sound originated from, because I can feel the initial burst of energy physically. It's not entirely about hearing--bass is a very tactile experience. That's why I continue to maintain stereo bass processing in my sound system, rather than mono the LF channels together.
At 100Hz, there are multiple dips when two sources are present, in a properly-treated room having minimal reflections. As you observed, these will occur at approx 5' intervals. Take two identical sound sources, operate them in phase, and look at their polar pattern versus frequency along their axis of separation. You will see interference beginning at frequencies whose wavelengths are twice the separation distance and continuing with increasing frequency. You can try this test with a pair of subwoofers. Power only one and measure various points in the room in front of the woofers. Then power both and watch the extra dips appear. Now move the two woofers together and note how they behave as one subwoofer, only louder. These are all easy tests to perform and they bear out what I am talking about here.
Thank you. Interesting read. I already had the article on my hard drive, as it offered to overwrite existing file when I downloaded this.. However... this is discussing the potential advantages of filling room modes by using subs placed at specific intervals to excite different room modes in the goal of achieving a flatter response in the frequency domain.
While the concept of using multiple woofers to excite various room modes is valid, it still creates other issues of interference between sources, as described in the Power Alley article that I linked to earlier. Not to mention the fact that sound originating from different directions other than the 'stage' area, would be confusing to the observer.
Finally, nothing in this article invalidates what I stated about interference between two discreet LF sources. In fact, it doesn't even raise the issue. It focuses upon exciting room modes in an effort to smooth response.
Footnote: the multiple 'all around the room' subwoofer placement great for reproduction of organ pedal tones, since in a real cathedral, organ pipes are often placed at front and rear of the cathedral with multiple ranks in various locations throughout. The four walls placement would approximate this setup nicely. However, when we here a concert orchestra, the bass comes from the stage, not behind us, so I see a problem with this setup for such types of music.
Mark, are you talking about stereo bass instead of dual, quad, etc., mono bass?
Hi
Not much time to fully entertain this immensely entertaining discussion.
The transient in a bass "percussive" sound is due to the higher frequency components of the sound not to the lower ones ... I would surmise ... The Power Alley phenomenon you allude to seems to manifest itself in venues that would not quallify as "small"... Auditoriums and the likes for most home environments even palatial ones we still remain in the province of "Small Rooms" acoustics ...
Not much time to fully entertain this immensely entertaining discussion.
The transient in a bass "percussive" sound is due to the higher frequency components of the sound not to the lower ones ... I would surmise ... The Power Alley phenomenon you allude to seems to manifest itself in venues that would not quallify as "small"... Auditoriums and the likes for most home environments even palatial ones we still remain in the province of "Small Rooms" acoustics ...
Can it not be argued that if you can localize a sub then the system is not properly calibrated for that sub?
I'm referring to both stereo or mono bass configurations using multiple subwoofers.
If I was not witnessing the Power Alley Effect in my own listening space when my woofers were grouped into two distinct clusters, then what was I witnessing?
Can anyone offer a scientific explanation why two or more woofers, spaced apart, would NOT produce interference/cancellation/reinforcement, at least at the upper mid-bass frequencies?
Once there is more than one source, there will be interference patterns. That's unavoidable, except at the very lowest frequencies in the smallest of rooms. I have never seen a situation where adding a second subwoofer magically added power at all frequencies. In practice, it causes more dips and peaks in the response. And not having all drivers located on the soundstage produces tactile and audible confusion about the direction of sound.
Years ago, I used to believe that 'bass is non-directional' and accepted it as 'scientific fact', until I started trying other arrangements of woofers, other than the woofers in the corners arrangements of my earlier sound systems. Once I lined them all up in a row, I was astounded at the immediacy of the LF transient response, particularly with recorded pyrotechnics. For me, such a demonstration caused me to reconsider my formerly rigid belief that 'bass is non-directional.'
If I was not witnessing the Power Alley Effect in my own listening space when my woofers were grouped into two distinct clusters, then what was I witnessing?
Can anyone offer a scientific explanation why two or more woofers, spaced apart, would NOT produce interference/cancellation/reinforcement, at least at the upper mid-bass frequencies?
Once there is more than one source, there will be interference patterns. That's unavoidable, except at the very lowest frequencies in the smallest of rooms. I have never seen a situation where adding a second subwoofer magically added power at all frequencies. In practice, it causes more dips and peaks in the response. And not having all drivers located on the soundstage produces tactile and audible confusion about the direction of sound.
Years ago, I used to believe that 'bass is non-directional' and accepted it as 'scientific fact', until I started trying other arrangements of woofers, other than the woofers in the corners arrangements of my earlier sound systems. Once I lined them all up in a row, I was astounded at the immediacy of the LF transient response, particularly with recorded pyrotechnics. For me, such a demonstration caused me to reconsider my formerly rigid belief that 'bass is non-directional.'
Mark
Not much time to discuss and I apologize .. The directional clues are not from the "bass" they are from the upper frequencies in the signal/sound... Low Bass signals remain non-directional.
It must also be said that symmetrical arrangement of subwoofers tend to create it own problems ... in some rooms ... more later
Not much time to discuss and I apologize .. The directional clues are not from the "bass" they are from the upper frequencies in the signal/sound... Low Bass signals remain non-directional.
It must also be said that symmetrical arrangement of subwoofers tend to create it own problems ... in some rooms ... more later
Correct. Audible directional cues come from the upper frequnencies. However, tactile cues come from the leading edge of the LF wavefront. How can I explain this in terms audiophiles with low power systems can identify with... okay, here's an example.. you're vacationing in a third world country and a bomb explodes outside of a market. The frequency energy spectra looks like a sonic boom--a sudden pressure wave with a steep rising edge. The harmonics and the sounds of glass shattering give you the audible cue of direction, but even a deaf person can tell from whence the blast came because he can FEEL the blast wave hit his body from a certain direction on the surfaces facing that particular direction. With hi-fi, to a lesser extent, when when the bass drum is hit hard, you feel the leading edge and your body senses the direction from which it came by the same mechanism as it senses the direction of an explosion.
High end speaker designers put a lot of effort into time alignment of drivers and it is for, what I believe to be, good reason. You don't want the harmonics of the rising edge of what will be a LF transient to arrive much sooner than the LF energy, otherwise the net effect of the impact will be diffuse. The impulse response of a system depends on all drivers being in the proper time alignment.
High end speaker designers put a lot of effort into time alignment of drivers and it is for, what I believe to be, good reason. You don't want the harmonics of the rising edge of what will be a LF transient to arrive much sooner than the LF energy, otherwise the net effect of the impact will be diffuse. The impulse response of a system depends on all drivers being in the proper time alignment.
So you don't read JAES? Todd Welte's paper is quite well known and if you haven't read it then you need to before you make any more claims, because your opinions go against the facts.
Dr. Geddes, in my first post in this thread, which served to open up the discussion, I wrote:
A couple of questions. First did I torture your position or did I get it (essentially) correct?
Second, if I do have it correct that you recommend getting one sub off of the floor, can you explain why?
A couple of questions. First did I torture your position or did I get it (essentially) correct?
Second, if I do have it correct that you recommend getting one sub off of the floor, can you explain why?
Yes, that is essentially correct.
The Welti paper (JAES) is a "must read" if you want to understand the concepts, but Welti missed a few key points that I had already investigated. First his placements were all symmetrical - this was due to a simplification in his model that required this. My models were more general and did not require a symmetrical placement. So while there is nothing (much) incorrect in the Welti paper, it did not go far enough. I was able to show that a more random placement could achieve the same level of spatial and temporal smoothness with three subs as Welti was able to do with four subs in symmetrical locations.
In my study, which was a full 3-D model, I moved one of the subs vertically and found a small improvement from that. So while moving the sub off of the floor is a slight improvement, the most significant improvements came from just using the three subs located as you state above.
Hi mark. my subs are behind me, and one of the 'tell tale' signs that they are not properly integrated IS that the bass does not seem to come properly from the front. Once you have it sorted, there is no change in the perceived direction/source of the bass.
Totally agree with frantz, any directional cues come from elsewhere in the spectrum, surely most have done this?? Play ONLY the subs. Just a horrible tuneless noise, hardly musical.
BUT, I agree with mark also! The FR may measure and look smooth and flat with distributed subs, but if they are not properly time aligned the bodily sensation is odd. You are 'shaken' out of time with the music.
I have often pondered why maybe my system seems to be prone to it? (ie most deny the effect, so maybe mine is particularly sensitive?)
I have a few theories...as mine is controlled by the deqx (of which time and phase is all important) perhaps the foundation in that regard from the mains makes it more critical to get the timing right??
The main theory I have is the type of floor I have. Old house, over 130 years old, walls two feet thick of solid masonry, but wooden floor which is not that 'solid' or non resonant.
A lot of people have concrete floors. So I wonder if my floor has a lot to do with it? If so, then praps stuff like the time for the signal to go thru the air is different than the vibration thru the box, coupled to the floor, transmitted thru the floorboards (that are so old there are no tongues or grooves, ie they would all act 'independently' from each other) then into the seat and then the bum.
So, I use distributed subs but 'have' to pay particular attention to the timing of the 'thumps' from each relative to the mains.The thumps need to arrive at the same time, else it can be clearly 'THUMP thump'.
Similar threads
