It’s not easy to summarize my years of knowledge into a single page of words and images, still I can say my instructions here are more free from any commercial bias but it’s still one man’s word against another.
You could say it’s not bias free afterall.
Damn!!! Another paradox!!!! I’m getting tired of these.
The goal here would be to identify the involved parameters and develop some understanding about the auditory relationship of these parameters to the actual physical acoustics of the room itself. In other words, what’s actually needed to be taken care of when choosing the right speaker system for your room and how to place the chosen speakers in your room accounting for the acoustics of the room itself and the listening position.
-What the speaker needs in order to fulfill the two criteria:
- What needs to be delivered to the listeners to give the complete sense of directional and spatial impressions of the source.
- Deciding an arrangement for speaker layout in the room that can successfully recreate the directional and spatial impressions.
-Deliver high quality sound
Even after all the conditions are matched to provide the listeners with adequate levels from each speaker to fulfill the above requirements, the quality of the reproduced sound is another issue There are a million ways about judging quality subjectively but to actually quantify it in actionable terms is where you need a professional to guide you. Simply put , it’s the statistical understanding of the sugar and salt.
-Do’s & Don’ts of room Acoustical Treatments.
Collecting all data about the room geometry & it’s acoustical behaviour and the characteristics of the electro acoustic systems, number of seats & their listening positions etc and implying specific targeted recommendation about room furnishing and acoustical treatment for satisfactory directional, spatial impressions & quality.
Multichannel Speaker System
A basic 5.1 channel system can be expanded to 6.1 or 7.1 channel but it all depends upon whether your source material is made for such arrangement or not. Still large number of seats can benefit from more speakers that provide adequate coverage & loudness.
- Localization : Perception of direction. Front speakers are rarely used for this phenomenon , it’s the job of the side and the rear speakers mostly to provide the ambient level of sound as well as the short localized impulses.
- Distance : The delayed reflections of the program material provided in the recordings create an impression of distance making the room seem acoustically bigger or smaller.
- Spaciousness & Envelopment : Listening to a concert recorded live should make you feel that sense of live concert space around you. Being in a different space surrounded by ambient sounds
- Play adequately loud with a wide coverage area.
Envelopment requires strong sounds delayed by 80 ms or more reproduced through suitable surround speakers. All the important sounds to reproduce the perception need to arrive at the listener ears from the sides.
- Two channels doesn’t come close to a multi channel setup when talking about adequate envelopment.
- Symmetrical front back arrangements contribute nothing to the sense of envelopment. it’s all about the delayed reflections from the sides.
- Centre rear speaker may provide some localized sound effects but does nothing to increase the envelopment.
- Four speakers behind the listener are not preferred over four speakers in front
- 5 speakers at 0° , ±30° & ±120° is a well defined starting point.
Thing to remember , all this is just applicable to the sweet spot as we move away from this location various degradations increasing with distance are experienced. More speakers can’t deal with problem of propagation loss.
Bass problems can be dealt with 2 or more subwoofers.
Layout of the room
- Video: Right sized screen for the viewing distances that are possible in the room and arranging seats to avoid any obstructions in viewing angles horizontally or vertically.
- Audience & seats : Reclining large comfortable seats takes a lot more area than normals seats. It’s best to decide what kind of seating you want at an early design stage.
- Speaker Layout : Front speakers aligned with the screen/display with the surrounds setup in a way to provide directional & spatial cues with required acoustical treatment to deal with speaker boundary. interactions depending upon your mounting options.
- Bass management is the most difficult to achieve in the right amount of budget for the right amount of listeners in a room with sufficiently right dimensions. Figuring out the nulls of the room can help manipulate the room dimensions or listening positions.
Video Concerns
- Right size display for all seats.
- Deciding upon seats and layout. If having multiple rows , it may require reconsidering the screen size.
- Finally arranging seats for right angles.
Audio Concerns
- Speaker Placement with right crossovers and hi resolution measurements.
- Front LCR should be at same height almost not higher than 60 % of the screen height.
- Closer they are to the ear height level the better.
- Front speakers aimed at the sweet spot with minor adjustment for other seats
- Surround speakers roughly 2 ft above seated ear height.
- Using on wall mounting options , its best to have a flat surface behind and around the speakers.
Speaker directivity & Acoustical Treatment
L,C ,R & Side Walls
- The first side LCR reflections degrade speech, induce some sort of comb filtering or mask any other reflections that in turn provide more spaciousness(ASW) to the listener. But there’s a high regarded belief in the sound industry that sound engineers,musicians have a higher sense of critical listening ability which might not be even perceptible by average listener.
In other words it’s not the reflections themselves that are bothersome, it’s their effect on spaciousness and the matter of psychoacoustics and your ability to LISTEN. - Front speakers with wide coverage angles (good off-axis response) might sound exceptionally good with reflective surface at first lateral reflection points from front speakers.
- When absorbing those reflections it is recommended to use at least 4″ thick absorbers & 8″ thick diffusers.
Surrounds & opposite walls
- For uniform horizontal dispersion:
surround require a huge almost ±90° to deliver a similarly good direct & reflected sounds to the listeners. Achievable with wide dispersion design , not so much with dipole surround speakers. - Momentary Localizable effects :
No specific acoustical treatment is required for this phenomenon to be experienced. Direct sounds from the speakers is of most importance , when placed correctly in the room and having enough sensitivity levels with constant directivity, precedence effect dominated the directional impression. - Envelopment : The material when recorded and mixed by the engineer for multichannel contains in mostly all the cases, the material contains delayed reflections of sounds coming from the front stage. Reflections from opposite walls helps in increasing the sense of envelopment
Treating the interior
By now you know the front speakers are required to deliver a moderate ±30° horizontal dispersion. Surround speakers require a wider coverage angle to deliver a smoother uniform response to the audience.
Speakers with propagation loss of less than -6 dB/double distance are preferred for surrounds. On wall bidirectional in phase speakers are suited best for surround requirements. Wide dispersion forward firing designs might be good enough if aimed properly for coverage.
Summarized basic needs :
- Floor carpet covered wall to wall.
- Centre of front & rear wall mostly absorbent with diffusors to the sides on rear wall.
- Side walls a combination of absorption & diffusors with some portion reflective where diffusors are placed around ear height.
- Good diffusors would be one’s that are 8″ thick & disperse horizontally. Curved diffusors must have enough thickness.
- Engineered diffusors have enough wavelength separation and variability to be placed next to each other. Where as Geometric shapes should be placed with separation between them.
- Ceiling diffusor must be a engineered surface & positioned to scatter the sound to the sides.
- Wide dispersion Front speakers would be required if incorporation first lateral reflections & making use of reflective surface/blank wall.
Sub, seating & room
- One sub with hi-res measurement & parametric EQ is sufficient for 1 listener.
- Spatial averaging will result in large seat to seat variation .
- Multiple subs are required for multiple listeners in rectangular rooms.
- Few arrangements of 2 or more subs results in most energy concentrated in 3 room modes. Point is to figure out where the nulls lie.
- All subs placed similarly along the room boundaries with proper EQ for each & same output level will have the lowest seat to seat variation.
- Rooms that are not perfectly rectangular or have any kind of opening attached will require carefully measure and addressed sound field management(SFM).
- Amount of LF absorption & damping in the room will also determine the variation in frequency response, which in case would be necessary when incorporating multiple subwoofers.
Speakers
- Speakers with hi-res anechoic measurement data are to be trusted.
- On & off axis curves extending at least 60° or more vertically & horizontally.
- Early reflections , Sound power & Directivity Index curves helps in comparing one speaker to other.
- Speaker mounting options highly decides it’s diffraction properties around the edges of enclosure.
- Be aware of the speaker boundary interference.
- In wall speakers shouldn’t be buried in recess created by treatments.
- For rooms less than 4000 cu ft well designed conventional cone dome speakers should suffice for adequate SPL levels. Larger rooms are best when used with horn-loaded speakers.
- Centre speakers with MTM design (midrange-tweeter-midrange) should be avoided as it provides a very poor off axis response.
- Correction for excessive bass would be necessary for in wall speakers designs which should be placed around ear height.
Magnitude & phase adjustments
Calibrating & tuning your system can be a real time consuming problem when you are not sure how to make it sound good and not just different.
Aim your mic towards the ceiling & get a C weighted measurement for approximate equal loudness. Listen to each speakers timbre & tonal characteristic facing it with pink noise playback.
Speaker that are not the same distance from the prime location must be delayed by the DSP. Remember there is only so much EQ can do below 300 HZ as well as treating the nulls would be hard. Any thrusting peaks are reduced, dips are left alone but depressions are boosted till 6 dB.
Steady state measurements are not that useful above 300 Hz. For rectangular rooms , when arranged in certain manner multiple subs yield special kind of spatial equalization which can be added as cherry on the cake.