Designing your home cinema doesn’t have to feel like another responsibility adulthood dropped on you but it’s not that simple either. At present in the mass home theatre consumer market 5 to 7 channels with a subwoofer namely, 5.1 and 7.1 systems are preferred. There are products in the market making big sales even without providing any technical information of their speaker and how it’s suppose to work.
This is where it all begins, this ratio is just a standard number and it can possibly be anything up to 22.2 what I have witnessed. It all depends on your room’s capability and the number of listeners in the room.
Lets start our long journey of acoustics together and pay attention to what really matters and how to find your #onelove.
I’ll try not too get too excited about it and stay on track ,you too. alright?
Few terms you should understand first and how they translate in real life scenarios. These are not actual definitions, just a rough explanation for those who need to go through this journey without having to take a graduate program in acoustics.
- ASW-Apparent source width: This roughly translates to how big of a front sound stage is active for the listeners. It needs to feel like a concert stage if you are listening to a live concert on that vinyl. Or when you have giant screen for projection and your front sound stage make it seem kind of smaller than it is.
The Front L,C,R and their placement with respect to the room symmetrical axis is responsible for 80% of the ASW for all the listeners in the room(until you incorporate those discrete SIDE channels with proper placement).
Note to take from this: Pay Close attention to the placement of your front L,C,R speakers for a bigger sound stage (~ASW)
- LEV-Listener envelopment: All of this mess, multichannel, stereo , 5.1,7.1,9.4, everything just to make the listeners feel enveloped in a different space. Its all about increasing the measure of the LEV scale for all the listeners present in the room.
Surround speakers mostly are responsible for most of the LEV the listeners experience(again, only if not incorporating those SIDE channels.)
How does is matter : Your surround (rear speakers) are what’s responsible for the envelopment of that space you feel while listening to your favorite track or watching that movie. You should know WHERE NOT TO place them in order to make your small room imperfectly give you an impression of that Big concert hall sound.
- Direct & reflected sounds: Your speakers are not actually a single point source(a dot), they have a horizontal & vertical dispersion angle over which they radiate sound called the solid angle.
The sounds that travel directly to the your ears from the speakers is called the direct sound & ones that tend to bounce of the walls and other surfaces in the room , get reflected and then reach your ears are called reflected sounds. Both direct & reflected sounds are important for different aspects of that sound to completely reach your ears for a complete sense of envelopment, which is why the discussion about multichannel started in the first place.
Point to take from this : Your direct sounds & reflected sounds should have enough energy & time separation between them to fulfill the requirements for all the listeners in a small room.
- Phantom Image : It is a pseudo image created by 2 speakers between them .When 2 speakers are placed equidistant from the sweet spot i.e. in the vicinity of another active speaker & having equal distance between the speaker and the listener , a perfectly aligned phantom image is formed. It’s not a bad thing, its a fact and quite resourceful in recreating that ” listening space”
Any two speakers when placed within a certain distance they will react with each other to create a phantom image between them.
What to take from this: Any two speaker placed “close” to each other create a phantom image between them when the signal fed to both the speakers is exactly the same. Look out for how it is perceived at the sweet spot.
- Prime Location/Sweet Spot- Words of a wise man Dr.Toole ,”money seat”. This is the spot all the money is being spent for ideally. Managing acoustics of a small room in a serious manner without comprising the aesthetics too much and on a budget is quite a task.
Quick check: Play pure tones in your room and hold a spl meter A- weighted get around 83 dB RMS SPL for a quick loudness check. Don’t touch the volume knob after that. Now walk around the room and notice the dips and peaks in loudness you encounter.
The point is , the same sound is different in different positions of the room. It means by fact every seat in the room will sound different. All the measurements are done for a single seat at first, the Prime location/ Sweet spot. As you move away from it the sound starts to degrade in terms of the corrections made for that money seat. There’s a limit to what can be done for other listeners in the room but this sweet spot is where magic happens. Adding more speakers in a specific manner helps in correcting this problem for a group of listeners, that’s why we need more speakers in the first place.
- Room curves : Some of you might have wondered how the same song on same speakers sound different in different rooms , that it because your room is the final instrument giving the final polishing timbral signature.
Every Room will have its own fingerprint on the soundtrack being played at any moment of time(humidity and temperature matters).
This is what we measure and try to eliminate via different means.
You need to remember: Having a good Frequency & Time response from your speakers that are calibrated to perform in cohesiveness with the room is the most important part at the end. All the money that you spent buying all the expensive hardware will be worth nothing if no calibration according to the room curves is made.
We’ll talk about Room Modes & small room problems later on.
The industry standard ±23° to ±30° arrangement of LF & RF for stereo. For a clear phantom center and right amount of localization of panned images , listener must be on the symmetrical axis between the speakers forming an equilateral triangle from the center of both the speakers being ear height, meeting right behind the head of the listener.
A wise man once said. In the beginning , there was mono. we came a long way from that to being able to replicate sound from a source & being able to do that with 2 channels called “Stereo”.
As long as the adjustments were to be done for a single listener. It worked fine but L,C,R speaker arrangement is required to establish a front sound stage for a group of listeners. The number of channels needed to completely replicate the performance of an orchestra recorded in a big auditorium will depend upon the size of the listening room it is being replicated in & size of the frontstage to be recreated in that room.
In order to recreate the experience the side wall reflections of the auditorium needs to be encoded in the record channels as well. Delivered by several speakers placed around the room and fed by a single processor with suitable time delays for each speaker.
- Hi & Low Pass filters for appropriate crossovers in the surround processor. Pay attention to how the subwoofer & tower speakers superimposed within the crossover region. As both are in different location with respect to the listener, there’s few complications in making the two perform as intended in that crossover region cohesively.
- Decorrelation between left & right surround channels. There are some differences in the signals of both the channel in terms of timbre and frequency response. This function is already taken care of by your AV processor so look out for this one on the data/spec sheet.
- Equalizing or simply EQ the final sound to alter the frequency response of speaker to perform at the best.
- Wide horizontal dispersion speakers using bi-directional in-phase (bipole) or out of phase(dipole) or monopole using selectable directivity.
My take on this :
The dipole designs have some issues of its own until addressed very carefully at the design process of the speaker. Manufacturers input on how to use the speaker is crucial if not the data sheet itself. Basically bipole configuration is easier to build and dipole takes some considerations into account that should be specified by the manufacturer on how to use the speakers finally in a room.
If you don’t feel like going through the whole journey without knowing what the end looks like. Here’s a small glimpse
What actually are surrounds for?
- Localization of subsidiary sounds : The perception of having sound being localizable i.e. coming from one direction & not ambient (all around you) in nature. The amount of localization is limited by the number of discrete channels provided by the AV processor , this means no two pair of speakers running parallelly. Beyond that phantom images is what’s responsible for filling up the left out space in the room.
Remember as we move away from the sweet spot the , distortion can be heard & very much evidentially be seen in the phantom images.
This highly depends upon the number of listeners in the room , as we said the phantom center degrade away from the sweet spot.
Hard truth. I’ve made my peace. you take your time.
- Distance: To create the impression of large distances in small rooms, These are technically well taken care of at the time of recording and mixing the soundtrack by the engineer, they just need to be appropriately encoded and decode by your AV processor with the usage of proper delays fed to each channel separately.
This actually could be the most difficult to replicate even after everything’s setup as this need proper time aligning your side/rears to your subs. It’s hard enough to differentiate distance from the perception of being enveloped in a different space so like I said pay attention .
- Spaciousness & envelopment : Creating that sense of being present in a different ambience in your room itself. Your own live concert with you in the sweet spot. Man! This feature of the surrounds is what they came into existence for.
So envelopment huh? How do I get more of that?
This far down the road you actually know the answer. MORE SPEAKERS .
but unfortunately you can’t just put 50 speakers in your room anywhere and except that to work by itself (certain considerations and it’s a worthy experiment). But where there’s will there’s a way. Acousticians and scientists have been having this debate for long and its still adapting to look for a general setup, configuration of speakers that would decide upon the number of channels needed to work for every room period.
For now we carefully take care of what matters and where to put what and by how much. First things first , avoid equally spaced arrays of 4 & 3. i.e do not put 2 pair of speakers on side walls placed symmetrically & even a pair of speakers behind the sweet spot if not decorrelated from the front L,C, R speakers are no good.
Apart from the L,C,R & Rear speakers , another suitable way of solving the problem is adding a pair of SIDE speakers . Extra pair of speakers placed in 60°-120° range from the symmetrical axis performs exceptionally well. Doing so remember to avoid placing of front & surround speaker at equal angles.
Tip : An array of 12, 8, 6 performs the best if the room size allows for it.
We need a diffuse sound field (we’ll talk about this in more detail in the small room discussion post) in order to create similar listening experiences for all the listeners present in the room but those close to the perimeter of the room will still be able to localize the surround speaker closet to them and for that:
- Stereo comes no where close to what’s required.
- Centre rear speaker is not a good choice, I mean NO!! it doesn’t makes sense.
- Front speakers at 30° and another pair within the 60°-135° range.
- Avoid 150° for rear speakers or whatever angle the front speakers are spread at. It’s the whole avoid equal front and rear channels angle fiasco. I don’t wanna talk about it here.
- Four speakers behind the listeners would be less effective as compared to four speaker in front at the same reflected angles.
Considering basic principles taken into account lets talk :
3/2 (5.1) channel setup
- Front soundstage of 0° center and Left & right speakers at ±30° is the very fundamental step.
- Adding Side channels for more envelopment with appropriate delay added to the channel signals.
- For multiple rows of listeners it might be necessary to place another pair of side/rear speakers ,connected in parallel with others. Keep in mind the net parallel impedance of the speakers being connected together or you might blow your amplifier or something worse in some cases.
3/4(7.1) channel setup
Four surrounds opens up a lot of possibilities , like positioning two speakers forward of the listening are for maximum envelopment. ±60° to ±90° would be the range of placement with added delay and some kind of decorrelation among the channels. Remember the whole avoiding the front and rear speaker placement symmetry thing?
In 7.1 systems the side speakers have been pretty helpful in creating that high sense of envelopment for the listeners so now the rear speakers can be placed symmetrically to the front and right speakers, this should be certainly avoided in case of 3/2 systems.
Speaker Directivity requirements
We’ll talk more about speaker directivity in detail and why it matters. What it actually is and how manufacturer’s lie with high numbers in their technical sheet. For now,
- Ensure that all listeners are provided with a strong Direct sounds from all the speakers. This creates some problems when the far away placed rear speakers are to be heard likely the same for all the rows in the room.
- Front left & Right channels angled inwards to the listeners would be a good start.
- Rear speakers placed on mounting brackets can be aimed at listeners for maximum coverage but it comes with another problem of having to deal with boundary interaction.
OK!!!! So, now you can setup your sound system in a fairly good manner than it was. You know where to place them, how to place them in certain conditions and what problems to deal with in each scenario. Not everything can be discussed here in such a small time but this much would be enough if taken care of properly as discussed here to make you settle for just switching your listening position and speaker placement for now instead of buying that new system. You’ll probably be amazed with the results that your sound system you already had was capable of in the right manner.
SO far down the road we have actually seen the process of just throwing money and buying the most expensive hardware out in the market ( even if they are actually putting money where their mouth is) wont actually work until you setup it up for your room . Make it unique to produce the best response possible in that very room and your listening position.
It’s a long way, so fasten your seat belts we’ll gear up in the next section.
For now think about the decisions you were making before about going out and buying that new sound system.