We all want the perfect sound , that perfect minimum phase tight bass, that overwhelming feeling of being enveloped in another space, along with high speaker sensitivity levels (for now just understand it as to how loud a speaker can play), pleasure of feeling that smooth natural frequency and phase response even at high volumes (it just correlates to how your speaker performs in reproducing the playback material in terms of frequency spectrum extended down to the lowest lows and highest highs and in time domain as well, even when the bass guitar and the kick of the track pushes and pulls the speaker dome within a fraction of milliseconds). Can all of this incorporate into one system?
It’s frustrating when you finally find the perfect shirt in the store and they don’t have your size. You don’t just buy a size up because that wont fit you. Before we proceed any further, let’s clear this confusion out about room dimensions as well. there’s no one size fits all rule over here, no unique set of room dimensions that would be suitable for any number of seat and for any kind of audio system.
In a real world, things are always limited , less than perfect thus limiting options for you in terms of number of seat, screen size, resolution, viewing distance etc.
First thing you need to understand and no matter what anyone tells you there’s no other way than what I’m about to say, Trying to control something that you can’t actually see(sound) and with vague industry standards developed over the years some things are for sure :
- Sounds > 250-300 Hz are easy to control and manipulate as they behave somewhat like rays , where ray physics can be applied to easily apply the angle of incidence = angle of reflection rule , which makes things a lot easier for us to deal with. This cutoff frequency is also called the Schroeder Frequency(curious readers can find out more on this on wiki). All the geometrical modelling and predictive analysis done by complex computer acoustical modelling software is helpful & in fact very accurate in the region of the spectrum but it fails to predict anything close to accurate below this cutoff frequency. The frequency spectrum of the sound reaching your ears in this region is dominated by the speaker , it’s capabilities and placement of those thin acoustical treatment materials in order to create a uniform sound field.
Around above 300 Hz wavelength, sound starts to become comparable to the size of the objects in the room i.e. now it can be and will be affected in different manner as we go up the spectrum by the people in the room, seating, furnishing, surface density of materials, angle of incidence of sound ,all matters in this case.
- Sound < 250 Hz are the real problems in a room , in any room in fact until its big enough ( Large Auditoriums, Large theatres, Large churches , mind that Large is mentioned every time, small auditoria or theatre still tend to have some problems related to this region.) Every room is small compared to that size. so every room is considered a small room. There’s a hell lot of problems with small rooms. Sounds in this region are very large compared to the size of the room itself, causing different kind of problems to be dealt with which can’t be explained by rays optics as here the angle of incidence is not equal to the the angle of reflection because of the large wavelength of the sound.
Your room is the dominant factor here, it’s acoustic signature will be imprinted on the frequency spectrum being reproduced by the speaker and finally reaching your ears. So no matter how expensive your speakers are and how much care is taken is finding the perfect electroacoustic system for playback, your room is finally going to change the timbre of the frequency spectrum below 200 Hz.
Here’s the bummer , This is region we all crave for .what we all want , THAT BASS . All the bass frequencies in the track are in this region. No matter what kind of final equalization you put as the final device for correction or extra taste, this region is not actually getting better with you messing around with the Frequency response of your system(take note here it is also going to mess up the phase response severely), your room will change it finally until you make changes to your room itself. This is why even your subwoofer placement and your listening position in the room matters to overcome the acoustical defects of the room below 200 Hz
Always remember the more equalization you put at the final stage and change the response of the system itself rather than correcting the room, Firstly you are actually changing the musical timbre of the played track itself not the problems , not the acoustical defects, the reason why you bought flat frequency response speakers was so that you could recreate the program material as it is. But then again you are putting heavy amounts of equalization and changing the frequency response of the speaker to change the played music itself so that it could behave and compensate correctly for the defects of your room. Why would you come this far down the road just to do something that was already there in a defected room with inexpensive audio system and no detail attention to placement of speakers and listeners. Now you don’t want to recreate the exact same experience as it’s played in the music or movie or any other material , you want to change it to compensate for your room’s defects.
Feel the paradox here!!!????
Secondly , no matter what you do it to fix problems electronically might feel like a change when you heavily equalize the signal but it’s just getting worse, you just feel it as a change , a good change? Definitely not.
Getting back on track, to provide enough bass energy in the room along with dealing the acoustical defects of the room to a very small extent one subwoofer might not be a good idea.
A single subwoofer even if able to provide enough high sensitivity (enough loudness) can only take care of problems for a single listener, for a group of listeners 2 or more subs are desired and their placement in the room matters.
Before we move ahead, Keep in mind all the notes below are for frequencies above 100 Hz.
A normal room, reflective in nature distinctly shows behavior of flutter echoes between and among all the walls (call it the empty room sound).
these can be and are often managed by just scattering sound , no absorption required. Bookcases, storage ,cabinets as well as wall to wall carpeting help in reducing the empty round sound. Heavy drapes chosen to be acoustically effective, pleated to less than 1/2 fabric length and hung around 4″ from the wall.
That being said it is not hard to achieve optimal mid frequency Reverberation time(we can talk about this for days and it still wont be enough, all we wanna do is bring this value under our control) for a furnished room.Any material placed in the room helps in somehow controlling this value, you just have to figure out where and how.
Remember room reflections are necessary for so many reasons this post would fall short, as far as it concerns you in the most simplistic way, right amount of reflections improve speech transmission . Remember sitting at the last bench of the class , sometimes it was hard even to hear syllables properly of each word used by the teacher standing at the opposite end of the class.
So don’t fall into the trap of too much absorption even if your consultant or installer advices, don’t just put too much absorption panels. That’s just stupid!!!
Absorption of first reflection points is a choice in home listening rooms, though not recommended until there isn’t any space left in the room to place absorption due to aesthetics & other factors.
Attenuated first reflections reduce the ASW & Diffusivity in a stereo setup. Multi channels are able to cover a large horizontal dispersion in the room so they mask the first reflections anyway.
Placing a carpet at the point of first reflections on the floor is recommended because most of the projects due to budget constraints or any other factor for that matter don’t have the right type of floor and for some it doesn’t matter.
Video & your Room
2/3 distance from the screen is the sweet spot for viewing angle of around 45°, there. You have you cheat sheet. If the screen height ratio is preserved for large screens then it can be upto 56° for 1:2.35 aspect ratio.
For a 16:9 [1:1.85] The screen size will be :
Screen Width = Diagonal x 0.872
Screen Height = Diagonal x 0.49
Screen Diagonal for 45° = distance x 0.95
For different viewing angles for different rows
45° = Screen Width x 1.2
36° =Screen width x 1.54
30° = Screen width x 1.85
Last row viewing angle should be no less than 26°.
Prime location viewing height should be eye level to 1/3 of the screen height from the floor or 6-8″ above head
Speakers & your room
We need speakers that can deliver strong direct sounds otherwise that feeling of space will be degraded. With L,C,R setups in multichannel if they happen to have a wide dispersion angle then preserving those frist reflections would increase the ASW. The amount of propagation loss would be a deciding factor as you move away from the sweet spot.
A wide Front sound stage is preferred no matter the screen size, bigger the better though for a large number of seats itself. If you are going to have multiple rows in your personal home theatre the viewing angle the the sound reproductions will not be the same for each row and your decision of having all seat on the floor or staggered. No matter what the case, don’t place the LF & LR or your speaker setup right beside the screen, remember a large ASW is needed. Middle seat in the middle row is given as the preference for the sweet spot. As you move far propagation losses will be encountered so try to be symmetric with the seating plan along the central axis of the room.
Speakers with narrow horizontal dispersion i.e. behaviour of undesired spectral variation as you move away from the on axis 0° response (both on axis and Listening window) should be aimed towards the listeners for a better response of direct on axis sounds but this also has the disadvantage of reducing the ASW somewhat to a little extent .
Wide horizontal dispersion front speakers can be placed flat against the wall where horizontal dispersion requirement extends to ±50° unlike when aimed at the audience ,it scale down to satisfactory ±30°.
The direct sounds from side/rear speakers ,span over ±75° which is really really hard for any of the speaker on a budget even in today’s world. The one’s that do, well even I don’t have those at my place.
Thus aiming the side/rear speakers towards the audience would be a smart decision in this case and it would also help in case of front-rear speaker symmetry which is pretty common in most rooms.
Propagation loss would be more effective in the room and thus audible when speaker with not enough constant directivity are placed flat on the wall.
Normal forward firing speakers don’t behave so good at large off axis angles which are responsible for the lateral side reflections. These speakers tend to have a smaller ASW compared to constant directivity speakers. It would be good to use the first reflections of the room in this scenario, thus leaving some portion of the side wall reflective for a increased ASW.
This problem is solved by speakers having wide horizontal dispersion with constant directivity thus placing them flat against the wall would give an impression of a larger ASW in that case side walls should be treated for reflection area centered at the ear level with at least 4″ thick absorbers.
ROOM ACOUSTICS & speaker placement
Speakers with constant directivity and strong on axis response upto ±30° which is usually seen by cone/dome or horn design speakers.
For centre channel avoid a MTM(midrange-tweeter-midrange) design and add a real midrange driver , the first reflections range for CF is ±50° to ±70°.
LF & RF when aimed towards the audience will produce first reflections at a wider angle thus attenuating mids & highs of the first reflections by a significant amount depending upon how constant directivity the speaker have which is pretty much high at large angles for any speaker & those will be masked by surround speakers anyway. Thus side wall absorption would be necessary in this case.
Large panels should be subdivided into smaller panels to avoid directional properties of panels to have a flat smooth response overall.