Some home theater design suggestions from Matt Polk
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Hello,
The following is a response Matt sent to a Polk owner who wanted some help in designing a room suitable for a home theater. I thought Matt's response would be enjoyed by the Forum members, so I asked if I could post it. Enjoy:
"Okay, here's some general advice with the caveat and disclaimer that no set of specifications can guarantee good performance. Some rooms are fundamentally better than others. However, I have never encountered a room that I could not make sound quite good using common sense and experience.
I hope the following will help.
-msp
1. Room shape and dimensions: First, do not worry about non-parallel walls or other weird shapes in an attempt to avoid room resonances. All rooms have resonances. But, in choosing room dimensions you are trying to achieve an even distribution of resonances and to avoid strong single fequency resonances above 50Hz. At Polk Audio, for years, our listening rooms were always around 19 ft. x 27ft. with a 9 ft. ceiling. These dimensions worked consistently well and have good modal density. The reason square rooms should be avoided is because all the resonances, or modes, tend to be concentrated at the same frequency. Also, alcoves, bulkheads, etc., anything that creates a break between one part of the room and another should be avoided. For example, suppose your room ends up with an 18" square airconditioning chase running across the ceiling near one end of the room. The area of the room on the other side of that chase will resonate indepently of the rest of the room, usually at higher and more annoying frequencies. Keep the shape more of less rectangular with walls and ceiling free of structural obstructions.
2. Room size: Rooms with either horizontal dimension less than 12 feet can be a problem. On the other hand rooms that are too large can be a problem for different reasons. Keep these points in mind. sound travels about 1 foot per millisecond. Reflections that arrive at your ears less than 5ms after the direct sound from the speakers will tend to blur and color the sound. So, for optimal results you will want your speakers at least 4 feet away from each side wall and about 2.5 feet away from the wall. You'll also want to sit at least 2.5 feet away from the wall behind your chair (I prefer 4 feet) and at least 4 ft. from the side walls. So, If you want your front speakers 8 feet apart your front wall needs to be at least 16 feet wide. If you want to sit 10 feet away from the speakers your room needs to be at least 15 feet deep. I would not go less than 17 feet for the short dimension. The long dimension should be no less than 1.4 times the short dimension and probably not more than 1.65 times. The front speakers will almost always sound best on the long wall. On the other end of the scale reflections that arrive more than 50ms after the direct sound from the speakers may be perceived as echos. So, if your room is more than 50 feet long you could have an echo problem.
3. Ceiling Height: Generally, higher is better but anything over 9 feet seems to work well for most speakers.
4. Construction: Well, sturdy is good. Mainly, you don't want anything rattling but at the same time you don't want your walls to be too rigid. A concrete box is not a good idea. The walls need to have a little give so as to control the room resonances and soak up the sound bouncing around the room at a controlled rate. (see below) Standard stud and drywall construction can work just fine if beefed up a bit. I have used 2x6 studs and double thickness drywall with good results. Fill the walls with fiberglass or rigid foam as if you were insulating an exterior wall. Also, any direct air path, no matter how small, going out of the room will conduct and amazing amount of sound into or out of the room. Seal all joints. Make sure electrical and switch boxes are well sealed. Get a good heavy door that seals at the bottom and all around when closed. Make sure air conditioning ducts do not connect directly to adjacent rooms.
5. Controlling Room Acoustics and Reflections: A bare room with nothing but hard walls, a pair of speakers and a chair won't sound very good. With nothing in the room to absorb sound the acoustic energy keeps bouncing around the room for quite a long time muddling up the sound from the speakers that you're trying to hear. So, every good listening room needs enough sound absorbing stuff in it to soak up sound at the proper rate. Too little and the room is bright and harsh sounding. Too much and it seems dead and lifeless. In general, if the room is comfortable to be in and live in, it will sound good. Another way to think about it is to imagine having a small cocktail party in the room. If you can have a bunch of people in the room all talking at once and still have a conversation with someone without shouting you've probably got just about the right amount of absorbtion. Carpets, sofas, art work and bookshelves are all good sound absorbers. You will also want to want to place things so that you avoid "hard" reflections at your favorite listening area. Hard reflections occur when sound from the speakers bounces off a wall and directly to the listening area like a beam of light off a mirror. You can break up these types of reflections with strategic placement of bookcases or other things that will absorb or scatter the sound. A practical way of figuring out where you'll need to put something is to have some move a tall mirror down the walls while you sit in one of your listening chairs. When you can see one of your speakers in the mirror that's where you need a bookcase or something else to breakup a hard reflection. btw - There are many excellent commercial products specifically designed to help control room acoustics. Column like sound absorbers located in the corners are very effective.
6. Noise and HVAC: Nothing drives me nuts like a loud A/C vent in a listening room. Tell your architect that you want your HVAC system to achieve an NC-20 noise rating. This is quiet enough that you may not be able to tell when the A/C or heat are running. Actually, this is a really good idea for the entire house. It's not significantly more expensive to achieve but must be designed in at the beginning of the project. It's almost impossible to fix a system that's too loud after the fact. Have your architect hire an HVAC engineering firm to specify a system that meets this specification. Then write it into the contract with the General contractor and the HVAC subcontractor. Then have a conversation with the architect, the GC and the subcontractor where you tell them how important this is to you. Then, don't pay the subcontractors final bill until you've verified that this spec has been met. This is the voice of experience."
________________________________________
The following is a response Matt sent to a Polk owner who wanted some help in designing a room suitable for a home theater. I thought Matt's response would be enjoyed by the Forum members, so I asked if I could post it. Enjoy:
"Okay, here's some general advice with the caveat and disclaimer that no set of specifications can guarantee good performance. Some rooms are fundamentally better than others. However, I have never encountered a room that I could not make sound quite good using common sense and experience.
I hope the following will help.
-msp
1. Room shape and dimensions: First, do not worry about non-parallel walls or other weird shapes in an attempt to avoid room resonances. All rooms have resonances. But, in choosing room dimensions you are trying to achieve an even distribution of resonances and to avoid strong single fequency resonances above 50Hz. At Polk Audio, for years, our listening rooms were always around 19 ft. x 27ft. with a 9 ft. ceiling. These dimensions worked consistently well and have good modal density. The reason square rooms should be avoided is because all the resonances, or modes, tend to be concentrated at the same frequency. Also, alcoves, bulkheads, etc., anything that creates a break between one part of the room and another should be avoided. For example, suppose your room ends up with an 18" square airconditioning chase running across the ceiling near one end of the room. The area of the room on the other side of that chase will resonate indepently of the rest of the room, usually at higher and more annoying frequencies. Keep the shape more of less rectangular with walls and ceiling free of structural obstructions.
2. Room size: Rooms with either horizontal dimension less than 12 feet can be a problem. On the other hand rooms that are too large can be a problem for different reasons. Keep these points in mind. sound travels about 1 foot per millisecond. Reflections that arrive at your ears less than 5ms after the direct sound from the speakers will tend to blur and color the sound. So, for optimal results you will want your speakers at least 4 feet away from each side wall and about 2.5 feet away from the wall. You'll also want to sit at least 2.5 feet away from the wall behind your chair (I prefer 4 feet) and at least 4 ft. from the side walls. So, If you want your front speakers 8 feet apart your front wall needs to be at least 16 feet wide. If you want to sit 10 feet away from the speakers your room needs to be at least 15 feet deep. I would not go less than 17 feet for the short dimension. The long dimension should be no less than 1.4 times the short dimension and probably not more than 1.65 times. The front speakers will almost always sound best on the long wall. On the other end of the scale reflections that arrive more than 50ms after the direct sound from the speakers may be perceived as echos. So, if your room is more than 50 feet long you could have an echo problem.
3. Ceiling Height: Generally, higher is better but anything over 9 feet seems to work well for most speakers.
4. Construction: Well, sturdy is good. Mainly, you don't want anything rattling but at the same time you don't want your walls to be too rigid. A concrete box is not a good idea. The walls need to have a little give so as to control the room resonances and soak up the sound bouncing around the room at a controlled rate. (see below) Standard stud and drywall construction can work just fine if beefed up a bit. I have used 2x6 studs and double thickness drywall with good results. Fill the walls with fiberglass or rigid foam as if you were insulating an exterior wall. Also, any direct air path, no matter how small, going out of the room will conduct and amazing amount of sound into or out of the room. Seal all joints. Make sure electrical and switch boxes are well sealed. Get a good heavy door that seals at the bottom and all around when closed. Make sure air conditioning ducts do not connect directly to adjacent rooms.
5. Controlling Room Acoustics and Reflections: A bare room with nothing but hard walls, a pair of speakers and a chair won't sound very good. With nothing in the room to absorb sound the acoustic energy keeps bouncing around the room for quite a long time muddling up the sound from the speakers that you're trying to hear. So, every good listening room needs enough sound absorbing stuff in it to soak up sound at the proper rate. Too little and the room is bright and harsh sounding. Too much and it seems dead and lifeless. In general, if the room is comfortable to be in and live in, it will sound good. Another way to think about it is to imagine having a small cocktail party in the room. If you can have a bunch of people in the room all talking at once and still have a conversation with someone without shouting you've probably got just about the right amount of absorbtion. Carpets, sofas, art work and bookshelves are all good sound absorbers. You will also want to want to place things so that you avoid "hard" reflections at your favorite listening area. Hard reflections occur when sound from the speakers bounces off a wall and directly to the listening area like a beam of light off a mirror. You can break up these types of reflections with strategic placement of bookcases or other things that will absorb or scatter the sound. A practical way of figuring out where you'll need to put something is to have some move a tall mirror down the walls while you sit in one of your listening chairs. When you can see one of your speakers in the mirror that's where you need a bookcase or something else to breakup a hard reflection. btw - There are many excellent commercial products specifically designed to help control room acoustics. Column like sound absorbers located in the corners are very effective.
6. Noise and HVAC: Nothing drives me nuts like a loud A/C vent in a listening room. Tell your architect that you want your HVAC system to achieve an NC-20 noise rating. This is quiet enough that you may not be able to tell when the A/C or heat are running. Actually, this is a really good idea for the entire house. It's not significantly more expensive to achieve but must be designed in at the beginning of the project. It's almost impossible to fix a system that's too loud after the fact. Have your architect hire an HVAC engineering firm to specify a system that meets this specification. Then write it into the contract with the General contractor and the HVAC subcontractor. Then have a conversation with the architect, the GC and the subcontractor where you tell them how important this is to you. Then, don't pay the subcontractors final bill until you've verified that this spec has been met. This is the voice of experience."
________________________________________
Post edited by [Deleted User] on
Comments
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Thank you
engtazengtaz
I love how music can brighten up a bad day. -
Thanks for posting that Ken. It's always good to read what the big man has to say.Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk -
Has he ever experimented with lowering the listening position / floor?~ In search of accurate reproduction of music. Real sound is my reference and while perfection may not be attainable? If I chase it, I might just catch excellence. ~
