Acoustics for 26*19 dedicated HT

[samcruise]

Hi guys, I am planning to set up my dedicated HT which is of measurement 26*19. My idea is to get the room acoustically well done since its a one time thing(electronics keep changing but acoustics not much). I am from Hyderabad and any help in this regard is highly appreciated. Please let me know if you need more details. Thank you

 

[manoj.p]

Can you give little more idea about what you have on mind? Like what kind of speaker configuration (7.1/Atmos/how many ceiling speakers), screen size you are looking for, how many rows of seating etc? Most important, how far you willing to go to build and the budget.

 

[samcruise]

Can you give little more idea about what you have on mind? Like what kind of speaker configuration (7.1/Atmos/how many ceiling speakers), screen size you are looking for, how many rows of seating etc? Most important, how far you willing to go to build and the budget.

Hi, I am looking for a 7.2.4 configuration and a 150inch diagonal screen. Two rows of seating is what i am planning for now. The total budget for acoustics is around 3-4lakhs and i want to get it well done both acoustically and aesthetically as well

 

[manoj.p]

Okay. good to know.

Have you identified the subwoofers? Your room is huge, approx 500 cuft volume assuming 10 ft high ceiling. I think two subs may not be adequate.

I have posted a response in another thread by Rajesh. There is some nice discussion going on, check out this post and subsequent about room modes and speaker placement to avoid them.
http://www.hifivision.com/home-theater/62099-dedicated-home-theatre-2.html#post686231

 

[samcruise]

Okay. good to know.

Have you identified the subwoofers? Your room is huge, approx 500 cuft volume assuming 10 ft high ceiling. I think two subs may not be adequate.

I have posted a response in another thread by Rajesh. There is some nice discussion going on, check out this post and subsequent about room modes and speaker placement to avoid them.
http://www.hifivision.com/home-theater/62099-dedicated-home-theatre-2.html#post686231

I am looking at 2 subwoofers, either velodyne eqmax 12 or svs pb2000(have to audition svs yet though). I am hoping 2 would be enough. And i have checked out the thread that you mentioned and will go through the axial room modes.
I have few questions regarding the ceiling and paneling.
1) Should the ceiling be absorptive or reflective? Some turn key guys are mentioning it should be reflective and some are mentioning that it should absorb the sound. Which idea is correct?

2) Is synth or poly wool coupled with pinewood acoustic absorptive panel required or just polywool would do the job of absorption on the walls? What i got from those people was (in the order of arrangement) wall->synthwool->pine acoustic board(20-25mm thick)->pu foam(12mm thick)->fabric. All the synthwool,acoustic board and foam would sit in a wooden paneling of 2ft*4ft made by ply of 2inch thickness running across the length of the wall.

Could you please throw some light on these, Thank you

 

[sdurani]

If you blow across a Coke bottle, you can make the air inside resonate (make that booooooh sound). Enlarge that small chamber to the size of your home theatre and the air in the room will still resonate (of course at different frequencies than the Coke bottle). Resonances are also known as standing waves or room modes.

For your 26-foot long room, the first 4 length modes will be at 22Hz, 43Hz, 65Hz and 87Hz. Those are the frequencies that will have loud peaks and quiet nulls along the length of your room.

The graph below shows how man feet from the front wall each of the nulls are. Each colour represents a different modal frequency.

Low frequency peaks aren't much of a problem because EQ can pull those down. By comparison, there's not enough amplifier power in the world to boost your way out of a null. Boosting a cancellation frequency (making it louder) just makes the cancellation worse (null gets deeper).

So best thing to do is avoid sitting in nulls. If you look at the graph above, you'll notice that nulls fall at even divisions (half, quarters, sixths, eighths) of room length, which means you should place the listeners' ears at odd divisions (thirds, fifths) of room length.

For example, if you line up your two rows so that the listeners' ears are at 3/5ths and 4/5ths of room length, then every seat will enjoy smoother frequency response (neither row will be at a location of a null or peak). Plus, you get a good 5 feet between rows.

Fixing acoustical problems with placement means you rely less on treatment and equalization. Best part: it's free (doesn't cost anything to move seating or subwoofers).

 

[samcruise]

If you blow across a Coke bottle, you can make the air inside resonate (make that booooooh sound). Enlarge that small chamber to the size of your home theatre and the air in the room will still resonate (of course at different frequencies than the Coke bottle). Resonances are also known as standing waves or room modes.

For your 26-foot long room, the first 4 length modes will be at 22Hz, 43Hz, 65Hz and 87Hz. Those are the frequencies that will have loud peaks and quiet nulls along the length of your room.

The graph below shows how man feet from the front wall each of the nulls are. Each colour represents a different modal frequency.

Low frequency peaks aren't much of a problem because EQ can pull those down. By comparison, there's not enough amplifier power in the world to boost your way out of a null. Boosting a cancellation frequency (making it louder) just makes the cancellation worse (null gets deeper).

So best thing to do is avoid sitting in nulls. If you look at the graph above, you'll notice that nulls fall at even divisions (half, quarters, sixths, eighths) of room length, which means you should place the listeners' ears at odd divisions (thirds, fifths) of room length.

For example, if you line up your two rows so that the listeners' ears are at 3/5ths and 4/5ths of room length, then every seat will enjoy smoother frequency response (neither row will be at a location of a null or peak). Plus, you get a good 5 feet between rows.

Fixing acoustical problems with placement means you rely less on treatment and equalization. Best part: it's free (doesn't cost anything to move seating or subwoofers).

Thanks a lot Sanjay, thats a superb explanation. Can i also place the seating between 16 and 19 as well?

 

[sdurani]

Can i also place the seating between 16 and 19 as well?

Yes, around 17 feet would be 2/3rd room length, which would be good if you were doing a single row. If you put your main row at that distance, where will you put the second row?

 

[samcruise]

Yes, around 17 feet would be 2/3rd room length, which would be good if you were doing a single row. If you put your main row at that distance, where will you put the second row?

I was thinking first row at 17 feet and second row at 20 feet distance. Does that work?

 

[sdurani]

I was thinking first row at 17 feet and second row at 20 feet distance. Does that work?

Works IF the people on the second row have no legs (3 feet between rows too tight). Was there something wrong with the suggestion of 3/5 and 4/5 room length?

 

[samcruise]

Works IF the people on the second row have no legs (3 feet between rows too tight). Was there something wrong with the suggestion of 3/5 and 4/5 room length?

Nothing wrong in your suggestion, i am just trying to place the seating as far as possible to the screen for better experience but i guess 3/5th of 26feet would also give a good distance from the screen, thank you

 

[sdurani]

i am just trying to place the seating as far as possible to the screen for better experience

What makes being far away a better experience?

 

[samcruise]

What makes being far away a better experience?

I am planning for a 150inch screen and i just wanted to make sure that i sit at a good distance from the screen to get a good viewing experience

 

[raghupb]

Sam,
A quick and dirty way of judging the min distance for a 150" screen size would be to conduct a simple expt.
For 16:9 aspect ratio the screen dimensions are approx 11'x6'
Take a newspaper with full page ads (Times of India has many pages like this)
Stick them up at the corners of the 11'x6' rectangle (on wall) and a few randomly within the rectangle.
Walk away until the edges are in reasonable focus without having to move your head (it is OK to move your eyeballs; because we always do it while watching a movie).
This will give you the minimum comfortable distance for the first row of seats.
26 x 3/5 = 15+ft as per Sanjay's suggestion, not an unreasonable distance to view the center and periphery without discomfort.
Remember, the lens of the movie camera is mostly focusing on a smaller rectangle, say 70-80% of the full screen at max. The rest is pushed to periphery.
Cheers,
Raghu

 

[sdurani]

I am planning for a 150inch screen and i just wanted to make sure that i sit at a good distance from the screen to get a good viewing experience

You're approaching it backwards compared to how every professional home theatre designer does it (at least the competent ones). They never place seating based on screen size. What if screens size forces one of the rows to be at an acoustically terrible location? That entire row of listeners is stuck with bad sound.

The normal approach is to find locations in the room with the best/smoothest frequency response (fewest/smallest peaks & dips). Then you can choose a screen size that gives you the viewing angle you want.

 

[samcruise]

Hi guys...finally i am ready to start the work this month. I just want to make sure i am correct before the work is started. I will list out what i am planning to do according to what i understood so far, please let me know if i am on the right track.

WALLS:

1) I would build 2ft*4ft or 2ft*2ft panels with 18mm ply on all the 4 walls.
2) I would fill the panels with 50mm thick, 1000gsm synth wool.
3) Now i would spot the reflection points with the mirror method and place wood wool boards of 15mm thickness 2ft either side of the reflection point(i.e, total 4ft at each reflection point) and from floor to ceiling.
4) The other areas where i don't place the wood wool boards i would place a 6mm or 9mm ply on top of the synth wool providing a gap of about 1inch between them.
5) 10mm or 12mm PU foam on top of these
6) Fabric gripper system and then finish with fabric.

CEILING:

Use anutone or similar acoustic boards throughout or use ply to make panels, fill with synthwool and cover with fabric (please suggest which is better)

FLOORING:

Laminate is what i am planning for flooring with a decent size carpet for the first reflection point on the floor.

Please let me know if what i mentioned above is a good way forward or if any changes to any particular point is needed. Thank you

Note: I am looking for inwall LCR's behind the screen, so mostly would go with the baffle wall behind the screen.

 

[GeorgeO]

Why two threads covering the same subject?

 

[realactivex]

If you blow across a Coke bottle, you can make the air inside resonate (make that booooooh sound). Enlarge that small chamber to the size of your home theatre and the air in the room will still resonate (of course at different frequencies than the Coke bottle). Resonances are also known as standing waves or room modes.

For your 26-foot long room, the first 4 length modes will be at 22Hz, 43Hz, 65Hz and 87Hz. Those are the frequencies that will have loud peaks and quiet nulls along the length of your room.

The graph below shows how man feet from the front wall each of the nulls are. Each colour represents a different modal frequency.

Low frequency peaks aren't much of a problem because EQ can pull those down. By comparison, there's not enough amplifier power in the world to boost your way out of a null. Boosting a cancellation frequency (making it louder) just makes the cancellation worse (null gets deeper).

So best thing to do is avoid sitting in nulls. If you look at the graph above, you'll notice that nulls fall at even divisions (half, quarters, sixths, eighths) of room length, which means you should place the listeners' ears at odd divisions (thirds, fifths) of room length.

For example, if you line up your two rows so that the listeners' ears are at 3/5ths and 4/5ths of room length, then every seat will enjoy smoother frequency response (neither row will be at a location of a null or peak). Plus, you get a good 5 feet between rows.

Fixing acoustical problems with placement means you rely less on treatment and equalization. Best part: it's free (doesn't cost anything to move seating or subwoofers).

Super explanation.. Please can you explain a generic way of calculating this and inferring the results (sorry novice here).. for eg: if my proposed room is 11*13, where would the nulls be? if i am not asking for too much, would like some education around accoustics (have trolled the internet, but after some point it goes ove rmy head :-( )

 

[hydrovac]

Too often, when we first hear of the supposed benefits of acoustic treatment
we immediately go out and buy stuff, without first diagnosing and evaluating the extent of the problem.

So to find out how bad the acoustics in your room really are, heres what you do:
Walk around the room, clap your hands as loud as you can from every spot, and listen closely to the reverberations that follow.
Next, you need to decide how much wall coverage you need to solve your particular acoustic problem

Then later on, when you begin installing acoustic treatment in your room, use the clap test constantly throughout the process to observe how the sound changes.
With each new addition, that nasty ringing should be becoming less prominent, until it disappears completely.

Getting your room to sound great with acoustic treatment requires of a combination of 3 items:

Bass Traps to absorb the low frequencies
Acoustic Panels to absorb the mid/high frequencies
Diffusers to scatter the remaining frequencies

Seek professional advice if required... spend wisely on Acoustics.

 

[samcruise]

Why two threads covering the same subject?

I just want to make sure i get inputs from as many experts as possible before i start the work