In an enclosed space sound is reflected by the room's boundaries. At certain frequencies, two or more reflected sound waves can superimpose themselves on one another in such a way where stationary locations can be found where that frequency becomes very loud or very quiet. This is called a mode. In every enclosed space there's a certain frequency at which the sound field can be devided into two parts. This frequency is called the Schröder frequency. In common living rooms it's about 200–300 Hz.
At higher frequencies, above the Schröder frequency, modes are not a big problem because there are so many. Two ears and a brain are very good in suppressing modes that otherwise would lead to coloration of sound (see Bilsen). Music has a more transient nature at higher frequencies and other effects play a major role: Soundwaves need time to travel through the room (about 34.4 cm per millisecond). If the difference between the direct sound and reflected sound is bigger than about 1 ms then our brain is capable of separating the two. However, only one sound sensation is perceived (precedence effect). If the difference is smaller than 1 ms then the two sounds are melted into one (summing localization). Only in the latter case sound coloration is perceived.
At frequencies below the Schröder frequency modes become a problem. There are fewer modes and they are more spaced relative to each other. Low frequency modes lead to the typical sensation of "booming" or "one-note" bass or even no bass at all. Our hearing needs a couple of cycles before it can determine a sound timbre. So in a small room the reflection becomes part of the sound almost instantly because wavelengths are in the range of a room's dimensions (e.g. 40 Hz = 8.6 m). Additional sound sources influence the frequency response of a mode. Because we can't localize sound sources below 150 Hz the use of additional subwoofers can smooth the frequency response at a given listening position or over a certain listening area. Studies have shown that only 2 to 4 additional subwoofers are necessary to smooth the low frequency response (see Welti/Devantier).
At higher frequencies, above the Schröder frequency, modes are not a big problem because there are so many. Two ears and a brain are very good in suppressing modes that otherwise would lead to coloration of sound (see Bilsen). Music has a more transient nature at higher frequencies and other effects play a major role: Soundwaves need time to travel through the room (about 34.4 cm per millisecond). If the difference between the direct sound and reflected sound is bigger than about 1 ms then our brain is capable of separating the two. However, only one sound sensation is perceived (precedence effect). If the difference is smaller than 1 ms then the two sounds are melted into one (summing localization). Only in the latter case sound coloration is perceived.
At frequencies below the Schröder frequency modes become a problem. There are fewer modes and they are more spaced relative to each other. Low frequency modes lead to the typical sensation of "booming" or "one-note" bass or even no bass at all. Our hearing needs a couple of cycles before it can determine a sound timbre. So in a small room the reflection becomes part of the sound almost instantly because wavelengths are in the range of a room's dimensions (e.g. 40 Hz = 8.6 m). Additional sound sources influence the frequency response of a mode. Because we can't localize sound sources below 150 Hz the use of additional subwoofers can smooth the frequency response at a given listening position or over a certain listening area. Studies have shown that only 2 to 4 additional subwoofers are necessary to smooth the low frequency response (see Welti/Devantier).
I have heard people talk about "hit you in the chest" bass. In my home I have not experienced this. There is bass that I can feel, but its more like the shudder of the lowest notes. I have felt a very strong "wallop" from a kickdrum type bass in a car system from a small sealed truck box 10" woofer behind the seats -not the long droning booming that most people think of when it comes to bass from cars.
I believe that I had read somewhere that the actual frequency range for the impactful bass was like the 100-300 range or basically slightly above traditional subwoofer territory.
Given all of the room issues, I know its a complex problem. Today I wondered if a big piece of this aspect was the smaller sealed environment of the car. And that my rooms have been open to the rest of the house, maybe this is a key thing to have for that kind of effect. Am I way off here?
(I have yet to properly setup multiple subs per the Geddes method though. I made a feeble attempt and never went back to do it correctly.)
Thanks.
Tony
I believe that I had read somewhere that the actual frequency range for the impactful bass was like the 100-300 range or basically slightly above traditional subwoofer territory.
Given all of the room issues, I know its a complex problem. Today I wondered if a big piece of this aspect was the smaller sealed environment of the car. And that my rooms have been open to the rest of the house, maybe this is a key thing to have for that kind of effect. Am I way off here?
(I have yet to properly setup multiple subs per the Geddes method though. I made a feeble attempt and never went back to do it correctly.)
Thanks.
Tony
Hey Tony
My experince is that this can only be achived in a well sealed room or at very high SPL like at a concert. Its actually easy to do in a closed car cabin, and I can do it in my well sealed HT, but in any room that has the typical loses to the rest of the house this is going to be very difficult. It would take a lot of SPL.
From what I've heard and measured, I'd like to suggest that the most impactful bass energy (mostly hits of kick drum in pop music) is more or less centered at 50~60Hz range. Impacts of 100-300Hz would be too 'thin', together with even higher frequencies, they should be the leading edges of the impacts but not the 'foundations'.
I've also encountered some very impactful bass at some events. At the very beginning, I thought to myself, Wow! what a bass! The slams were so strong, the contrast of dynamics was so huge, and they were not "slow" at all, nor did they drag long decays, very fast and powerful indeed.
However, listened more carefully, I found the notes of electric bass were gone (or submerged in the 'shock waves' of kick drum). I knew there was an electric bass playing on the stage, I saw the player was playing, or I know that piece of music well enough to expect some 'melodies' of bass, but there's no sound of it. Except the drums, I couldn't hear any other bass instruments with recognizable pitches, let alone their individual tones and textures. Overall, the bass sounds were fast and strong, they were still 'one-note-bass' -- that kick drum overwhelmed almost everything else.
Back to my own place with my own gears, I can have similar 'performance' by boosting the 50~63Hz way up and play it loud. Exciting, yes, but not enjoyable at all in the long run, let alone the "fidelity'' I want. I'd like to have all sounds in the low frequency range - their pitches, tones, textures, melodies, rhythms... etc., instead of being punched all the way.
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Whew. What a thread. It's taken days to read through it.
I have done that experiment. It works quite well to suppress modes.
I have a garage / workshop that is about 1/4 full (that is, about 1/4 the actual room volume) of stacked storage crates (cubes about a foot square). The stacks extend floor to ceiling. I have a small system in one corner for music while I work. With the crates stacked flat against the walls, room modes are very evident. By stacking the crates in rows to form "bays" against the walls, plus some walls in the middle of the room, the modes disappear and it sounds as if the speakers have gained a subwoofer.
I suggested to my wife that we adopt a similar treatment in the lounge. All these years of marriage, and I never realised she knew so many strong words...
I have done that experiment. It works quite well to suppress modes.
I have a garage / workshop that is about 1/4 full (that is, about 1/4 the actual room volume) of stacked storage crates (cubes about a foot square). The stacks extend floor to ceiling. I have a small system in one corner for music while I work. With the crates stacked flat against the walls, room modes are very evident. By stacking the crates in rows to form "bays" against the walls, plus some walls in the middle of the room, the modes disappear and it sounds as if the speakers have gained a subwoofer.
I suggested to my wife that we adopt a similar treatment in the lounge. All these years of marriage, and I never realised she knew so many strong words...
Viaually a wall is a wall, but there is no reason that it can't "look like" a normal wall and yet be fully absorptive. I've done this lots of times. Easiet is to just hang very heavy curtains over the wall - lust looks like a window with the curtains drawn.
Peoples wives always object when they want to do somethjing outrageous - they should - just do it reasonably and there is usually no problem.
Hi, very interesting thread, and though I didn't nearly read the whole thing, it has certaily got me thinking.
One question I have however...
The Geddes approach dictates 3 or 4 (if you're counting the mains as a source as well) subwoofers, each of which have their own phase, level and low pass controls (according to this page).
This would typically be the case if you go out to a brick and mortar store, and buy three active subwoofers of the variety sold by most manufacturers. Since these are active subwoofers that each come with their own plate amp, there are usually a couple knobs on the back for adjusting some parameters.
However, my question is the following:
Do you still benefit from having multiple (3 or 4) small subwoofers if they are all driven from one large single amplifier, and cannot be seperately tweaked for level, phase and frequency?
This thread has me thinking about making four 8" woofers, but it seems rather wasteful (or should I say, expensive) to buy four 100 watt plate amps, when you could for about half the price get an external 500 watt amplifier.
So, gedlee & other experts, what do you think? Will four 8" woofers placed randomly around the room (up near the ceiling, in corners, under bed, etc), and yet controlled by a single amp, be an improvement over a single 12 or 15"?
One question I have however...
The Geddes approach dictates 3 or 4 (if you're counting the mains as a source as well) subwoofers, each of which have their own phase, level and low pass controls (according to this page).
This would typically be the case if you go out to a brick and mortar store, and buy three active subwoofers of the variety sold by most manufacturers. Since these are active subwoofers that each come with their own plate amp, there are usually a couple knobs on the back for adjusting some parameters.
However, my question is the following:
Do you still benefit from having multiple (3 or 4) small subwoofers if they are all driven from one large single amplifier, and cannot be seperately tweaked for level, phase and frequency?
This thread has me thinking about making four 8" woofers, but it seems rather wasteful (or should I say, expensive) to buy four 100 watt plate amps, when you could for about half the price get an external 500 watt amplifier.
So, gedlee & other experts, what do you think? Will four 8" woofers placed randomly around the room (up near the ceiling, in corners, under bed, etc), and yet controlled by a single amp, be an improvement over a single 12 or 15"?
The spectral variance will benefit, yes but you probably won't get the result you're looking for. Please read http://harman.com/EN-US/OurCompany/Technologyleadership/Documents/Scientific Publications/13680.pdf
Best, Markus
Not my cup of tea but definitely US-WAF compatible:
An externally hosted image should be here but it was not working when we last tested it.
Best, Markus
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You will get a benefit but not as much as you will get with individual control over each source. The amount of difference in the two approaches will be very room dependent, so there is no one "general" answer. To be safe against "Murphy's Law" I would always go for independent control. But I don;t recommend plate amps as I have found them to be totally unreliable. Use a rack amp and a DCX2496.
Thanks for the article, I have read it, and based on my understanding of it, I think I can conclude: Simply being able to adjust the position of the subwoofers can halve the mean spectral variance. (Level, delay and single-band EQ controls could halve or even quarter it further beyond that).
Comparing the images in Figure 6 and 7, four strategically placed subwoofers would seem to give a significant improvement already.
Thank you. This both confirms my own understanding of the subject based on the article I just read, and my initial assumption / hope.
I think (assuming optimal placement in all cases), a single large woofer would be better in low end response, multiple small woofers would be better in variance throughout the room, and multiple small independently controlled woofers would be even better with regard to variance.
I did some experiments recently, and I found that with a 57Hz sine wave playing in my room, the point to point difference in sound levels while moving about the room is over 20dB! Meaning you can go from head hurting to almost quietness simply by walking a few steps.
I believe that seeing as multiple proper independently controlled fullrange subs are out of my budget at this point, 4 small subs would have a much better chance of eliminating this horrid variance that a single large one would. And even if this comes at the cost of losing the 16-32Hz octave, I think given the amount of content at that level, it's a trade-off worth making.
I'm sorry to be derailing this theoretical thread with my own practical applications, but I think one theoretical conclusion in here is that more subs is better than one, and even if you can't control them individually, it's a step in the right direction.
EDIT: Another thought I just had, in theoretical nature, but based on my own listening setup:
The longest standing wave possible in my listening room (by my understanding) would be around 50Hz. It's quite easy to reach 40Hz in a small enclosure (1 cu ft or less). So my idea is - bass management, twice. Everything below 100Hz or so is fed to the LFE channel, which is distributed according to the setup criteria detailed in this thread among 3 or 4 small "subwoofers" placed strategically in the room. However these woofers are highpassed at 40Hz (either acoustically or electrically), and everything from 20Hz (maybe 10Hz if you want to be really ambitious) to 40Hz is reproduced by the standard "huge box in the corner", seeing as modes aren't a problem and it relies purely on room pressurization.
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klankymen
First you need to determine if you want to optimize a single listening position or multiple seats.
A single sub plus EQ might be perfectly fine for one seat. A sub in the near field might be an even better solution.
Multiple subs minimize the variance (and smooth the frequency response to some extend) but this doesn't help at all if all seats show a similar bad frequency response. So you need to have control over the single sources (EQ, level, phase, crossover).
Based on mode canceling techniques there's another approach that shows very good results IF the room is rectangular AND the walls are very rigid: Double Bass Array (patented by Genelec).
See AES paper "Controlled Acoustic Bass System (CABS) A Method to Achieve Uniform Sound Field Distribution at Low Frequencies in Rectangular Rooms" by Adrian Celestinos and Sofus Birkedal Nielsen
You're German so you might want to have a look at Double Bass Array ? Wikipedia
Best, Markus
First you need to determine if you want to optimize a single listening position or multiple seats.
A single sub plus EQ might be perfectly fine for one seat. A sub in the near field might be an even better solution.
Multiple subs minimize the variance (and smooth the frequency response to some extend) but this doesn't help at all if all seats show a similar bad frequency response. So you need to have control over the single sources (EQ, level, phase, crossover).
Based on mode canceling techniques there's another approach that shows very good results IF the room is rectangular AND the walls are very rigid: Double Bass Array (patented by Genelec).
See AES paper "Controlled Acoustic Bass System (CABS) A Method to Achieve Uniform Sound Field Distribution at Low Frequencies in Rectangular Rooms" by Adrian Celestinos and Sofus Birkedal Nielsen
You're German so you might want to have a look at Double Bass Array ? Wikipedia
Best, Markus
Unfortunately, I'm trying to optimize for 3 different positions - a nearfield position for the mains (sitting at a desk) and two farfield positions, for watching the television set.
Well, I could certainly find a single amp with parametric EQ, and phase and crossover control for a lot lower of a price than 3 or 4 such amps (even if they have a lower power rating), so I think that is within the budget.
I've read about the double bass array on both English and German speaking forums, and curiously enough I was doing some measurements today regarding the necessary placement of drivers (seeing as I do live in a cuboid rectangle), and it seems the only realistic setup would be a 2x1 array, from side to side (front wall is all glass, can't really hang a woofer on a window). Not sure how effective such a basic form of the array would be, but certainly a consideration.
I meant with independent control and without. But you mean Todd's does what I do since I was doing this long before he was.
This is true, but it results in the classic "boomy bass".
This is not so far from what I actually do with my "Broad-band" subs and my ULF sub. The big sub is tuned very low and sits in a corner. Then the broad-band higher tuned subs are spaced arround the room. All the HP and LP you describe isn't really necessary since it all works out when the subs are setup in the room.