Audio Speakers

We can make some interesting and not really discussed observations about our audio speakers. Lets keep our discussion in the playback domain of our recorded source material. Our playback system will need to be set up in a room with 4 walls, ceiling, and floor.

Our audio speakers will be a two channel system operating in stereo mode. Our first observation must deal with the reflections which occur from the sound generated by our audio speakers and the walls, floors, and ceilings. Primary, secondary, and tertiary reflections all confuse the direct energy which is the energy that reaches our ears in the shortest time frame. Sound that travels in a straight line from our audio speakers to our ears is called direct sound. Sound that is reflected off of room boundary surfaces is called reflected sound.

One of the sonic consequences of these reflections between our audio speakers and the room is a term called coherent interference. This is the name for the sound issues that result from all the room boundary reflections bombarding the listening position with all this “interference”. This coherent interference affects the lower mid and upper bass regions of our sound presentation. Vocals are smeared and blurred and instruments in this region may not be heard.

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Recording Studios

Recording studios must deal with the acoustic of the room in order to make a living or generate income from their studio. Wall reflections and room modes must all be addressed in order for the engineer to hear the sounds produced from the musicians instead of the room sound. The engineer must separate the room sound from the mixed sound.

The primary, secondary, and tertiary side wall reflections in recording studios must be controlled in some acoustical manner in order to minimize their impact at the monitoring position. The engineer wants to hear the sound that comes directly out of his speakers and reaches his ears before the side wall reflections interfere with this wanted direct sound. Similar room boundary reflections must be controlled for the sides and rear channel recording.

In the recording studios I have seen, special care is given to placing the entire monitoring system in a place in the room that is free from room mode issues. Room modal issues occur between two parallel surfaces and their intensity varies with room volume. Room modes can hide or over emphasize certain frequencies. We want to hear everything at the mix position. We don’t want the room hiding or producing anything extra for us to have to deal with. Dealing with artistic, sensible, individuals can produce enough “sound” of their own let alone having to deal with the room mouthing off.

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Sound Deadening

Sound deadening is an interesting blend of words people use in Goggle search engines. I guess this phrase sound deadening refers to a process of making sound “dead”. Lets explore how we would do this.

We all know from our introductory physics class that energy really can’t be destroyed. We all remember that energy can be transformed into another energy type but not destroyed. If we are going to have a sound deadening process introduced in our rooms, we must change the electromechanical sound that comes from our speakers and change it into some other energy form in order to minimize its sonic impact. Absorption of sound is a process where sound energy is converted to heat through the use of some type of sound absorbing material. Once sound energy is converted to heat through the absorption process, it is lost forever.

The sound absorption process can be accomplished with foams, fabric type materials, activated carbon, and numerous other material types that have the necessary physical characteristics to absorb sound energy. When we absorb sound energy we change its form to heat and all sound energy is reduced that goes through the sound absorbing device. Did we make our sound go dead? Our ears will think so if we use too much of any sound absorbing material type.

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Foam Sound

Foam sound is a term that people use in search engines, especially Goggle. It is an odd choice and arrangement of words. It is a blend of science and culture. This misuse of terms happens a lot and one can find these interesting search words and phrases by using Goggle keywords and phrases search engine. It is fun sometimes to see how people perceive the science of acoustics and how they use the science nomenclature in unique ways and blend it with culture.

I try and figure out what someone is looking for when they use the search term ” foam sound”. They probably want some type of lightweight, economical, foam material to reduce unwanted room reflections. Maybe the room is too “loud” and they seek to lower the noise floor. I don’t know how they would ask for it at the store. Would they ask for foam sound or sound foam?

They may mean the type of sound that is created when sound strikes the foam. Sound energy does take on the “sound” of certain material types it strikes. If you strike glass with sound energy such as in our vehicles you get “glass sound”. When you strike wood, you get “wood sound” . Wood sound is much better than glass sound. You can test this by sitting and playing your car stereo system for a period of time. Don’t drive the car and perform this test. Stay static, so you don’t have tire, engine, and wind noise created. Stay still, it won’t take long and it definitely will hurt.

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Sound Panels

The search term “sound panels” is another example of how people take things they are familar with and put them together to try and communicate something. Unfortunately, this process only confuses the issues.

I think people use the term sound panel or sound panels to mean a structure that reacts to sound in some way. My best guess is that sound panel means some sort of panel or structure that does something for or to sound. To an engineer, it means a panel that produces sound energy. I think to most people it means a panel that absorbs sound because their environments they are in are too noisy.

Sound panels could be a panel with speakers in it as is used in sound reinforcement venues such as concerts. A PA or public address system could be referred to as a sound panel. An electrostatic speaker could be a sound panel. An electrostatic speaker is a panel type structure that produces sound energy through a vibrating membrane constructed of screen wire which an electrical current is passed through.

Electrostatic speaker manufacturers would cringe at their technology and sound producing processes as using screen door wire. I know something of their pain when I hear the term sound panel.

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Sound Proof Foam

The term sound proof foam is another one of those misnomers. There is no foam on the market that is soundproof. I know this to be true because I have been manufacturing foam for 10 years. Lets pull apart these terms “sound proof foam” and examine them individually.

The term soundproof means to keep sound from entering a room from any outside source and to keep sound in the room from leaving the room and entering adjacent structures. Foam does not possess the physical properties necessary to stop this type of sound transmission. Foam is not dense enough or rigid enough to provide the mass and structural stability to stop middle and high frequencies, let alone have any impact on bass energy. It would take a piece of foam 11 feet thick to absorb all the energy of a 100 Hz. wavelength.

The terms sound proof and foam should never be used together. Foam is not used to sound proof anything. There is no such thing as sound proof foam. Foam is an acoustical tool that can absorb energy within a contained room. It can be used to control unwanted room boundary reflections, so the listening position is free from unwanted reflections and our ears can focus on the direct energy from our loudspeakers. Foam can be used to absorb and minimize the unwanted energy from another individual who is telling you to turn your stereo down. A better approach would be to place that individual in a soundproof closet.

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Home Theater Room

What must our home theater room have in it to make it a home theater room? It must have a video display unit. It could be a projector with screen, a lcd or plasma flat screen, or simply a crt television. It must have a receiver to process audio from our dvd, amplify this audio and send it to numerous channels which are all represented by speakers. It must have a sub woofer to provide low energy output into the room as to try and acoustically recreate explosions and car crashes.

Our home theater room must also have the ability to control external light from out side, so as not to interfer with the video display inside. The room must be of adequate size in both width, depth, and height to allow for proper video display sizing. The room must also be large enough to be able to position side and rear channels to allow for this energy to be utilized correctly at the listening position. Low frequency energy from the sub woofer needs the most room volume it can get to be heard as realistic.

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Home Recording Studio

Home recording studios are prominent today. With the digital technologies available today, one can have most of the processing power of larger studios in a much smaller space requirement. A home recording studio is usually set up in a spare bedroom or some other space not utilized by the current occupants. The science of small room acoustics must apply here.

Our first consideration in a smaller space must be the low frequency or bass energy. Small room volumes play havoc with low frequencies, or should I say low frequency energy plays havoc with smaller home recording studios. A great deal of attention must be paid to bass energy whether recording, monitoring or playback is being used , so that the bass energy does not smother the mids and highs. Low frequency absorbers are a must.

Reflection control in recording rooms must be controlled, so that the microphones are “hearing” the music and musicians and are not recording the room sound. Our monitoring position must be a realistic balance of absorption and diffusion technologies to allow for more direct energy from our speakers to reach our ears first, before the relected energy from our room boundary surfaces. This greater direct/reflected wall energy at the mixing position allows for a more realistic sound balance and thus a better mix.

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Sound Proof Materials

Sound proof materials is a bit of a misnomer. There are really no specific materials that are labeled sound proof materials. There are materials that have good sound proofing qualities, but no material, no matter what it is called by itself is a sound proof material. Materials of different types must be assembled in a specific way to create a sound proof wall, door, etc.

Most sound proof materials have a high density or weight per square foot that is heavier than most other materials. If the density of the specific material, say drywall, is not enough, one can add multiple sheets to create a larger surface density number. One can also use MDF or other composites to achieve proper density numbers.

These multiple material sheets should be isolated from each other by some type of damping compound. It is customary when designing and building sound proof materials, to separate materials of different densities physically, with another material type, from each other. This arrangement reduces vibration transmission through the material being constructed and thus noise transmission.

It is the density of the materials used and the way in which they are assembled together that makes for good sound proof materials. Now, one must decide where to place the materials to achieve the desired acoustical objectives. That process is entirely another issue.

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How To Sound Proof a Room

I get asked everyday about the process of how to sound proof a room. It is a difficult question to answer because it has so many variables to consider. It is even more difficult to accomplish especially when the client sees the cost.

In order to answer the question of how to soundproof a room, first, we must define some parameters. Sound proofing a room means that we must build the room in a way that keeps the energy produced within the room from whatever source, speakers, music, voice, etc., inside the room itself and not let it “bleed” outside of the room or into adjacent spaces. Secondly, we must keep the sound energy created from outside sources, busses, airplanes, motorcycles, out of the sound within the room.

If we were going to write a book on how to soundproof a room we would use absorption technology inside the room to reduce the pressure levels of low frequency energy within the room. We would also use absorption technology for the excess middle and high frequencies within the room. To keep outside generated energy from entering our room, we have to use barrier technology in the construction of the room. We must create a “room within a room”, similar to the Russian toy doll which has a large doll with a series of littler dolls in side in incrementally smaller sizes. This process mechanically decouples each structure from the other and thus reduces the pathways for vibrations to travel and create noise.

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