Acoustic Frontiers presents "The Bass Optimization eBook"

Nyal Mellor

Industry Expert
Jul 14, 2010
591
0
0
SF Bay Area, CA, USA
#1
Please have a read of my first eBook! It's about optimizing bass, and covers a number of areas including room construction, acoustic treatment, multiple subwoofers and equalization. It's not intended to be a super technical and definitive "manual", more a high level overview of the things that are in my opinion important. With the framework I present in the eBook, you'll be able to identify and research things in more detail to help you get killer bass!

Let me know what you think :)

You can download it by clicking through to this page: THE ACOUSTIC FRONTIERS GUIDE TO BASS OPTIMIZATION.
 

LL21

Active Member
Dec 26, 2010
10,550
2
38
#4
Many thanks Nyal!
 
Jan 29, 2014
983
0
0
Cape Town South Africa
#8
Nyal..excellent..the best free bang for the buck!!!!
I will repost it to our local forums
 

sbo6

Member
May 19, 2014
694
0
16
Round Rock, TX
#9
Very good read. Thanks very much for making it available free.
 
Apr 23, 2016
325
0
0
#10
Please have a read of my first eBook! It's about optimizing bass, and covers a number of areas including room construction, acoustic treatment, multiple subwoofers and equalization. It's not intended to be a super technical and definitive "manual", more a high level overview of the things that are in my opinion important. With the framework I present in the eBook, you'll be able to identify and research things in more detail to help you get killer bass!

Let me know what you think :)

You can download it by clicking through to this page: THE ACOUSTIC FRONTIERS GUIDE TO BASS OPTIMIZATION.
Nyal, what's your opinion on the proper crossover point in a HT if all of your speakers go below 40 HZ? Thanks
 
Dec 13, 2010
253
1
18
#11
Nyal,

Let me know what you think :)
allow me some comments:

1. You refer to research by Welti and Geddes. I’m sure some readers would like to know more and read the technical papers themselves, so the bibliographic data would be quite handy.

Welti, “How many subwoofers are enough”, Audio Engineering Society preprint 5602 (2002)

Welti, “In-room low frequency optimization”, Audio Engineering Society preprint 5942 (2003)

Welti, “Low-frequency optimization using multiple subwoofers”, J. of the Audio Engineering Society 2006, p.347

Welti, “Optimal Configurations for Subwoofers in Rooms Considering Seat to Seat Variation and Low Frequency Efficiency”, Audio Engineering Society preprint 8748 (2012)

One other paper that is certainly most interesting in this context is

Fazenda et al., “Subjective preference of modal control in listening rooms “, J. of the Audio Engineering Society 2012, S.338

where different subwoofer configurations are compared for a single listening position using music as test signal.

2. In chapter 1 you refer to optimum room dimensions. The goal of the various optimization methods is to arrange all of the modes evenly on the frequency axis. Therefore, in order to experience the benefits of optimum ratios, all of the modes must be excited, simultaneously and at equal levels, and the listener must be able to perceive all of them, again simultaneously and at equal levels. This is possible only when source and listener are positioned in corners. As you say later in chapter 3, it makes a difference where you place of subs and seats, because the interaction with the modes is changing.

Since nobody will put the subs and himself into corners, which would actually be rather ridiculous, why optimize for an arrangement of subs and seats which nobody will ever use?

As for stacking modes on top of each other, listening tests have shown that rooms where 2 or even 3 dimensions are the same are not necessarily bad, simply because the result depends on where you put the subs and the seats.

As Floyd Toole has put it: “So it is not that the idea of optimum room ratios is wrong, it is simply that, as originally conceived, it is irrelevant in our business of sound reproduction.”

You have measured peak-to-dip variances of 15 dB+. To that Toole said in a conference paper back in 1990: „It has long been puzzling that music and speech can sound as natural as they do in rooms that are horrendously flawed by numerous resonances. The explanation seems to be that room modes are generally medium- to high-Q phenomena. In steady-state measurements, such as these, modes are very much in evidence. However, when excited by the sounds of speech and music, which are mostly transient or discontinuous events, they are not always as apparent to the ear as the measurements suggest.”

3. In the passage relating to room mode cancellation you say: “The effect of a subwoofer on the level of a room mode depends on where you put the subwoofer. If you put it in the center of the room at the null there would be minimal energy coupling between the subwoofer and the standing wave.”

Since you refer to “a room mode” the reader will think that the following is true for all room modes, which obviously is not the case, since only the odd-order modes have a null at the room center.

4. Quarter-wavelength cancellation from the wall behind the speakers: I did an experiment to see how this sounds, placed a small active speaker on a board with wheels, attached a string to the board, placed the board against the wall, played a 80 Hz sine tone, slowly pulled the board up to about 2 m distance from the wall, listened. On paper the cancellation should occur at about 1.07 m distance from the wall, but no, nothing, no cancellation, not even partially, nothing at all. Looks as if theory and practice are different worlds.


Klaus
 

Nyal Mellor

Industry Expert
Jul 14, 2010
591
0
0
SF Bay Area, CA, USA
#13

Nyal Mellor

Industry Expert
Jul 14, 2010
591
0
0
SF Bay Area, CA, USA
#14
Nyal,



allow me some comments:

1. You refer to research by Welti and Geddes. I’m sure some readers would like to know more and read the technical papers themselves, so the bibliographic data would be quite handy.

Welti, “How many subwoofers are enough”, Audio Engineering Society preprint 5602 (2002)

Welti, “In-room low frequency optimization”, Audio Engineering Society preprint 5942 (2003)

Welti, “Low-frequency optimization using multiple subwoofers”, J. of the Audio Engineering Society 2006, p.347

Welti, “Optimal Configurations for Subwoofers in Rooms Considering Seat to Seat Variation and Low Frequency Efficiency”, Audio Engineering Society preprint 8748 (2012)

One other paper that is certainly most interesting in this context is

Fazenda et al., “Subjective preference of modal control in listening rooms “, J. of the Audio Engineering Society 2012, S.338

where different subwoofer configurations are compared for a single listening position using music as test signal.

2. In chapter 1 you refer to optimum room dimensions. The goal of the various optimization methods is to arrange all of the modes evenly on the frequency axis. Therefore, in order to experience the benefits of optimum ratios, all of the modes must be excited, simultaneously and at equal levels, and the listener must be able to perceive all of them, again simultaneously and at equal levels. This is possible only when source and listener are positioned in corners. As you say later in chapter 3, it makes a difference where you place of subs and seats, because the interaction with the modes is changing.

Since nobody will put the subs and himself into corners, which would actually be rather ridiculous, why optimize for an arrangement of subs and seats which nobody will ever use?

As for stacking modes on top of each other, listening tests have shown that rooms where 2 or even 3 dimensions are the same are not necessarily bad, simply because the result depends on where you put the subs and the seats.

As Floyd Toole has put it: “So it is not that the idea of optimum room ratios is wrong, it is simply that, as originally conceived, it is irrelevant in our business of sound reproduction.”

You have measured peak-to-dip variances of 15 dB+. To that Toole said in a conference paper back in 1990: „It has long been puzzling that music and speech can sound as natural as they do in rooms that are horrendously flawed by numerous resonances. The explanation seems to be that room modes are generally medium- to high-Q phenomena. In steady-state measurements, such as these, modes are very much in evidence. However, when excited by the sounds of speech and music, which are mostly transient or discontinuous events, they are not always as apparent to the ear as the measurements suggest.”

3. In the passage relating to room mode cancellation you say: “The effect of a subwoofer on the level of a room mode depends on where you put the subwoofer. If you put it in the center of the room at the null there would be minimal energy coupling between the subwoofer and the standing wave.”

Since you refer to “a room mode” the reader will think that the following is true for all room modes, which obviously is not the case, since only the odd-order modes have a null at the room center.

4. Quarter-wavelength cancellation from the wall behind the speakers: I did an experiment to see how this sounds, placed a small active speaker on a board with wheels, attached a string to the board, placed the board against the wall, played a 80 Hz sine tone, slowly pulled the board up to about 2 m distance from the wall, listened. On paper the cancellation should occur at about 1.07 m distance from the wall, but no, nothing, no cancellation, not even partially, nothing at all. Looks as if theory and practice are different worlds.


Klaus
Thanks for reading and detailed thoughts.

2. I maintain that if you have flexibility there is STRONG worth in optimizing room dimensions, it just produces a better starting position for the room, with less room mode issues due to better SPACING of the modes. I've had experience trying to improve rooms with "stacked modes" and they are PITA to deal with. Much easier to start with a room where the modes are regularly and evenly spaced.

3. Agree, I should clarify.

4. Did you measure the room in conjunction with your listening tests to confirm that the expected suckout actually occurred at that frequency
 
Dec 13, 2010
253
1
18
#15
Hi Nyal,

Nyal Mellor said:
KlausR. said:
2. In chapter 1 you refer to optimum room dimensions….
I maintain that if you have flexibility there is STRONG worth in optimizing room dimensions, it just produces a better starting position for the room, with less room mode issues due to better SPACING of the modes.
In 2002 we added an additional living room to our house, so I had the flexibility and choose the dimensions according to Bonello. The builder made a mistake and increased one of the dimensions, without any audible negative effect whatsoever.

Yes, with optimum dimensions on paper the modes are better spaced, but how many modes does one excite simultaneously when playing real program material? On paper in our room the length and width modes are at 20, 37, 40, 60, 74, 80, 100, 111, 148, 185 Hz, for the vertical modes it’s difficult to say because of the stretched-fabric acoustic ceiling.

If of those modes I excite simultaneously for instance the 37 and 80 Hz modes, what does it help to have these two modes better spaced than they currently are? In any case so far I have found only a few tracks which audibly excite the 74 Hz mode, and that’s something I can live with.

Nyal Mellor said:
KlausR. said:
4. Quarter-wavelength cancellation from the wall behind the speakers: I did an experiment to see how this sounds, placed a small active speaker on a board with wheels, attached a string to the board, placed the board against the wall, played a 80 Hz sine tone, slowly pulled the board up to about 2 m distance from the wall, listened. On paper the cancellation should occur at about 1.07 m distance from the wall, but no, nothing, no cancellation, not even partially, nothing at all. Looks as if theory and practice are different worlds.
Did you measure the room in conjunction with your listening tests to confirm that the expected suckout actually occurred at that frequency
No, I didn’t, don’t have any gear for that. However, on Genelec’s website is a whole chapter on this issue, so I did expect to hear something while moving the speaker slowly away from the wall up to a distance of 2 m. On paper all of this might be true, but I did experience otherwise.

Klaus
 
Jun 23, 2015
73
0
6
Brisbane
#16
Klaus - I was wondering how you knew the changed dimension did not effect the audible outcome

on the matter of the speaker rear wall reflection cancellation at 80Hz that would depend on how "leaky" the room was at 80Hz and the radiation pattern of the speaker - I have walked across a relatively sound proof room with and 80 Hz tone and heard complete cancellation at each null - it is an amazing experience

In your earlier post I would wager if O'Tooles natural sounding music in a poor space had been moved to a better performing space it would sound "more natural"
As many have said the room is the most important component of hifi and the physics is pretty simple

Thanks for the work Nyal

Cheers,

Phil
 

Nyal Mellor

Industry Expert
Jul 14, 2010
591
0
0
SF Bay Area, CA, USA
#17
Hi Nyal,
In 2002 we added an additional living room to our house, so I had the flexibility and choose the dimensions according to Bonello. The builder made a mistake and increased one of the dimensions, without any audible negative effect whatsoever.

Yes, with optimum dimensions on paper the modes are better spaced, but how many modes does one excite simultaneously when playing real program material? On paper in our room the length and width modes are at 20, 37, 40, 60, 74, 80, 100, 111, 148, 185 Hz, for the vertical modes it’s difficult to say because of the stretched-fabric acoustic ceiling.

If of those modes I excite simultaneously for instance the 37 and 80 Hz modes, what does it help to have these two modes better spaced than they currently are? In any case so far I have found only a few tracks which audibly excite the 74 Hz mode, and that’s something I can live with.
Klaus....whilst room dimensions are not the be all and end all, they are a very sensible starting point. If you can optimize the room mode spacing then it makes your life a lot easier when it comes to speaker / listener placement, acoustic treatment, EQ, etc. You end with more even spacing of room modes and minimal or no room mode stacking (coincidences). This means the response is flatter to begin with.

For sure, even with optimized room dimensions where you put the speaker and listener changes the coupling to the standing waves and hence the frequency response. However if you have possibility to optimize room dimensions then it's a no brainer not to do it.

I'm not sure I understand your argument about "simultaneously exciting room modes". Your room is already dimensionally well optimized! Compare where your room modes are and the gaps (in fractions of an octave) between them relative to a square room and you'll see what I mean.
 
Dec 13, 2010
253
1
18
#18
Klaus - I was wondering how you knew the changed dimension did not affect the audible outcome.
I can’t know whether or not the outcome was affected, all I know is that it was not affected in a negative way. And in light of Welti, “Investigation of Bonello criterion for use in small room acoustics”, Audio Engineering Society preprint 7849 (2009), I’m not surprised. My experience hence is that a deviation from optimum does not automatically result in sonic disaster, which makes me raise the question of the usefulness of such optimizing approaches which do not take into account real scenarios at all.

On the matter of the speaker rear wall reflection cancellation at 80Hz that would depend on how "leaky" the room was at 80Hz and the radiation pattern of the speaker - I have walked across a relatively sound proof room with and 80 Hz tone and heard complete cancellation at each null - it is an amazing experience.
I don’t think that for rear wall reflection cancellation possible “leaks” are relevant, and of course, being our kitchen-diner, the room I made the test in isn’t sound proof. Radiation pattern: the 4 inch woofer of my small Genelec starts beaming at about 1110 Hz, so 80 Hz is radiated to the rear, therefore rear wall cancellation should occur to some degree.

Walking across a room and hearing nulls with a 80 Hz playing is most likely not rear wall reflection cancellation but standing waves, a different issue. For the rear wall reflection cancellation at 80 Hz to occur the speaker has to be at a distance from the rear wall that is equal to the quarter wavelength, i.e. about 1.10 m.

As many have said the room is the most important component of hifi and the physics is pretty simple
To my understanding to only genuine room parameter is reverberation time RT60. Every room has modes, but the effects depend on positions of source and listener. Placing the subwoofer in a corner or placing it in the room’s centre will have different audible effects, yet the room remains the same.

Every room has first reflections, but the effects depend on positions of source and listener + radiation pattern of the source. Placing the speaker symmetrically w.r.t the side walls or placing them asymmetrically may have different audible effects, yet the room remains the same.

The only parameter that is not depending on position and type of source is RT60.

Yes, the physics look simple, but what many seem to forget or ignore is psychophysics in general and psychoacoustics in particular, and plain straightforward physics doesn’t tell you anything about those.

Klaus
 
Dec 13, 2010
253
1
18
#19
Nyal,

Nyal Mellor said:
...whilst room dimensions are not the be all and end all, they are a very sensible starting point. If you can optimize the room mode spacing then it makes your life a lot easier when it comes to speaker / listener placement, acoustic treatment, EQ, etc. You end with more even spacing of room modes and minimal or no room mode stacking (coincidences). This means the response is flatter to begin with.
Yes, the modes are more evenly spaced. But why should the frequency response be flatter everywhere in the room? And what exactly is “the” frequency response of a room? At what exact location do you measure it, where do you locate the source(s)? And what does the frequency response of the source(s) look like, is it flat or is it rollercoaster?

For sure, even with optimized room dimensions where you put the speaker and listener changes the coupling to the standing waves and hence the frequency response.
That’s exactly my point! The response depends on where you put source(s) and receiver, so in view of the very different amplitudes of the individual modes there is no reason why it should be flatter just because the dimensions are different.

However if you have possibility to optimize room dimensions then it's a no brainer not to do it.
In view of the above, I obviously beg to differ. With the response depending entirely on locations of source and receiver I consider optimizing room dimensions of little use.

I'm not sure I understand your argument about "simultaneously exciting room modes". Your room is already dimensionally well optimized! Compare where your room modes are and the gaps (in fractions of an octave) between them relative to a square room and you'll see what I mean.
How many of the 20 or so modes below 100 Hz are excited at the same time when playing music? If it’s only one at a time, even spacing of those 20 doesn’t help. If it’s two at a time being 50 Hz apart, even spacing of the 20 doesn’t help, if it’s three with 20 and 40 Hz spacing, even spacing of the 20 doesn’t help.

I just had another look at Bonello’s graphs for the dimensions I had chosen and the dimensions as actually built, and whereas the former is ok, the latter is not, so the dimensions as built are not optimum, as I had pointed out. When looking at the major modes in both cases some modes are stacking, which so far isn’t a problem with only one of those stacks being audibly excited with a handful tracks from my collection.

Klaus
 

artto

New Member
Mar 17, 2014
1
0
0
#20
Klaus, to make one thing perfectly clear, there is no such thing as a (perfectly) optimized room (in regards to room modes) ~ unless ------ ALL of the dimensions of the room, are (preferably) several times longer than the wavelength of the lowest frequency that is going to be reproduced (or produced, as in a live concert).
 

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