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Common Sound Quality Q&A back

Feel free to send me a question and it might end up here with an answer.

Acoustical Performance Acoustics performance contains a very broad range of subjects. The details are sometimes boring or take a long time to describe and sometimes can't be avoided. I'm limiting my scope information to the practical application of acoustics and noise control that I use as it relates to your home.

Website, magazines, books and others generally give good advice on acoustical performance and noise control. Unfortunately, it's almost impossible to distinguish the difference between advertising and fact when trying to decide what might be best for your own application. Very rarely are any two rooms the same and a plan needs to be developed to most efficiently use your materials, time and money.

Q. What size and shape should my room be?
A. I delay answering the question diretly. Almost always there is a limit to the size the room can be or some other restricting element like doorways, windows, other rooms, cost or many others.

There are several room dimension ratios that have been tried and are published. I don't believe they are always valid or practical. The "Golden Ratio" seems to be the most common where the H:W:L ratios are approximately 1:1.6:2.3. I only give it as a starting point. I also use RPG's Room Sizer software to help find room sizes that takes into account rooms dimensions dynamically by calculating iterations of best and worst sizes. It has it's limits also.

How the walls are constructed, doors and widows and seating and speaker locations also figure into a room size I suggest. Often, the sizes I suggest do not follow any of the published shapes. I prefer you find the size limiting factor of the room then give me the information to find the size for you.

Q. Can I sit along the back wall along the long wall with the speakers along the long wall so the short walls are my sidewalls?
A. Absolutely, but you need to aware of the consequences! Somewhere many of us got the idea this is alway a disaster. It does have some up and down sides to it though. When you sit very near (couch on the rear wall) a rear wall the mid bass (<250Hz about) gets louder caused by the boundary or proximity effect. Mid and low bass get louder so you may need to consider using bass shy speakers, acoustically treating the wall behind the seating and speakers with mid-bass absorptive materials or use equalizers. I know audiophiles shun eq's but if they solve more problems than they create use them.

Also, room modes predominate at boundaries. You can expect the widest variations at lower frequencies at the room's mode frequencies (1130(ft/s)/(2* the room width)*(1,2,3and 4) = mode frequencies) at the wall surface.

One big advantage to this arrangement is that the speakers are generally quite far from the sidewalls and you are closer to the speakers. We can pay less attention to the first reflection from the sidewalls due to the speakers' off axis response. You're closer to what a music studio would call a near field speaker system.

Q. Do carpets and a couch count as acoustical materials?
A. Yes, but to a small degree. Most carpet is not very absorptive for anything but high frequencies (maybe >3000Hz). The make a good 3k-20K filter to what the room gives back to you as a sense of the space. Sometimes this makes the aliveness of other frequencies more noticeable. Couches are a little different where thick soft cushions may absorb some sound from a few hundred to thousand Hz. The effectiveness of any acoustical material is directly proportional to the surface area and volume of the room. You need a fairly large proportion of the room covered to make a significant effect. Adding several couches and people could make a significant effect. Yes, people absorb sound.

There are two behaviors that makes some couches and chairs poor choices. A tall backed chair will block sound from the rear of room (reduce ambience) and how well you hear the surrounds in a home theater. Shinny leather chairs seem to reflect mid to high-range sounds while absorbing lower sounds. This can often make the room sound "hotter" or "too warm" in the upper frequencies. The best choice is to keep the chair backs low and the fabric soft.

Q. What do acoustical materials look like?
A. They are mostly ugly unless covered. Mid and high frequency absorbers are usually dense fiberglass or cotton. Lower frequency absorbers are usually of a very few basic types although in practice there are several that expand on the basic versions behavior. Resonant panel (or membrane) absorbers look like a panel or box with the face made of some materials that is wood, vinyl, rubber or metal. Helmholtz resonators are panels, boxes or large tubes that have one large or many small holes in one face. Pressure zone bass traps (or 1/4 wavelength) bass traps have a dense fibrous (fiberglass, cotton) face covering a sealed box with the box partially filled with more fibrous material.

Diffusers take a sound from one direction and scatter the sound into many directions. They can scatter sound in one direction (called 1d diffusers) or two directions (called 2d diffusers). They often have curved or irregular surfaces from about and inch in depth to 9 inches or more. Bookcases, irregular room shapes ,other cabinets and soffit can affect the rooms behavior. Although not traditional acoustical devices they can work quite well if positioned and/or chosen carefully.