Hey guys, at the time of this writing, i'm in a hotel in Orlando at 2:30am. Not the best time to do the thinking, but i'm bored. Part 1 is here if you're interested. It really has more to do with the basic stuff about noise reduction. Part 2 is more abstract and really more useful if you've already finished doing Part 1. I wrote this section because i've noticed there's almost zilch in SPCR or here or anywhere about sound absorbtion.
Acoustic Management Guide Part II: Acoustic Absorption
This section is going to come together slowly. Anyways, its all about absorbing sound and preventing it from reverberating. One thing should really be clarified, the goal is to absorb the sound, not to block or reflect it and prevent it from going through.
In the article, i will discuss different techniques for absorbing and also blocking sound, most of these techniques are borrowed from home building and are used on a large scale. I will also discuss different ratings systems and how they work. This article should take a few weeks to finish =p
Ok so here we go...
First, sound absorbing material is rated and classified in many different standards, but the most important to us is the NRC and the STC.
NRC
The NRC or the noise reduction coefficient is a value between 0 and 1. 0 being perfect acoustic reflection, and 1 being perfect acoustic absorption. Its is the average of 4 values, the coefficients at 250Hz, 500Hz, 1000Hz, and 2000Hz. While these 4 frequencies span most of the noise we normally hear, there are a few exceptions. Human hearing spans from 20Hz to 20kHz. Some comprehensive sound absorbing foam will actually have ratings for up to 4000Hz. I have also seen NRC's of over 1, but how that's possible, its totally beyond me.
Most computer noise is high pitched. You rarely hear bass coming from a HSF =p. Therefore, you should find materials with good NRCs around the higher pitched ranges.
STC
STC or sound transmission class is generally used in house building, it is a measurement of how well a wall or celing absorbs sound. STC is based on sound loss in dBs at 16 frequencies between 125 and 4000 Hz. The values are then fitted to a transmission loss curve.
STC is measured in dBs and are harder to use in a computer's context because computers rarely generate the same frequencies and intensity of sound generated by a family of 4 in a townhouse. However, they may be used as a qualitative measurement, as to which methods of soundproofing are more effective. This is because acoustic absorption techniques are rarely measured in NRC, but instead in STC.
Alright... lets move on to techniques for reducing sound reflection.
It'd be ideal if you had a sound meter for measuring frequency and intensity, but since most of us don't have professional grade equipment that costs several thousands... well we just try to estimate the sound range.
Foam
Foam is generally a very popular way to reduce sound reflection. They are measured with NRC, and can range in price from several dollars to several hundred. Foam normally comes in large quantities, but it is possible to buy from a reseller for a smaller roll. Most cases will use foam as a preliminary acoustic absorber. Foam can come in many different shapes and sizes. Different sizes and shapes are designed for absorbing different frequencies of sound. The egg carton shape is very popular, as is the wedge. They both have their own respective frequency specialties.
The shape has a huge effect on absorption, but material also has a very large effect aswell. Open-celled foam is almost ALWAYS required for sound absorption. Generally, the more flexible the foam, the more it can absorb. I am not sure about the actual density of the foam, as if you move to really loose foam, it will leak sound, while really dense foam may reflect too much of it. this is where NRC ratings come in handy.
mass-loaded vinyl
Let me start of by saying mass-loaded vinyl is NOT the same as vinyl flooring. MLV is doped up with large amounts of heavy metals such as barrium (health hazard? 0.o) The principal behind MLV, as it is with all sound absorption, is to convert sound energy and vibrations into heat and then disperse. MLV's effectiveness is questionable, but should work nicely as a backing for foam (more on that later.)
fiberglass
Fiberglass is ITCHY. It will get into your skin and make everything very itchy. No joke, you must have professional experience with fiberglass before you try using it. Ontop of that, you must trap the fiberglass fibers in some kind of epoxy resin, which coincidentally releases organic fumes that will probably bond your contact lenses to your corneas or something equally traumatizing. The effectiveness of fiberglass is questionable, although it is employed in some high end speakers as insulation. I really know not much about fiberglass. I don't suggest working with it unless you're really adventurous. Use at own risk!! A safer alternative would be to use something with very fine fibers. Sort of like what the mutemat does, just you can make your own or something.
complicated pathways
Lets say you want to make a maze for your fan airflow, it'll probably completely kill the sound, provided you make the proper angled turns and line the edges with foam. Even just slightly angleing two walls together can create a dramatic effect in reducing sound. This is really more of an experimental kinda thing.
That's about all i can think of interms of sound absorption, now onto the sound blocking.
vacuum
Sound needs a medium to travel in. Remove medium, remove sound. 100% effective if you can pull off an absolute vacuum 0.o Not sure what effect a partial vacuum would have. I assume its a better way to stop sound waves?
Changing densities
Basically sound waves lose energy as they travel through different types of material. If you manage to sandwich many different density materials together, then sound will be diminished.
One way is actually to put sound absorpative material ontop of sound reflective material. Theoretically, it should force the sound waves to travel through the material twice, hence more absorption. I'm not sure about this though, never seen it really implemented anywhere.
Sound reflective material
Certain materials with very low NRC ratings will reflect sound waves. That's basically the point of a double paned window in a sound studio. Glass is a horrible medium to absorb sound with. It's really a better bet to reflect it back and let the other material handle it instead.
active noise cancellation
I've heard alot of people talk about this, none of them try it. Personally i'm very skeptical about the entire thing, plus not very enthusiastic about the piggy-bank breaking cost. If any of you brave souls out there wanna give it a try, document it for us.
I think that's all about sound blocking techniques.
Many building techniques use a combination of multiple walls, soft mounted walls, staggered walls, and insulation. look it up, some of them look promising but impossibly hard to do.
Theres more that i thought of on the 12 hour bus rides, but i can't remember any of it right now. I'll add if it comes to me.
In the next section i'll be actually much more step-oriented, and it'll really be more of a "guide". I'm going to construct my optical drive cage. This should be interesting. It should also take a long time since my exams are coming in a month heh. If it gets a bit too technical, give me a shout and i'll try to explain better. Unfortunately, i might start to use more complicated tools, since i have 2 machineshops at my disposal. Don't worry, i'll suggest suitable alternate tools for you guys.
There is one section i need some help with. I need an interface between a 5"1/4 IDE drive to USB cable. I also need an adaptor between a 120v wall mount and a 4pin molex power connector. I COULD buy a brand new 5"1/4 enclosure and then rip it all out, but it kinda seems like a waste to me at 50cdn... so i'm open to any suggestions. Don't mind paying around 25 cdn for it... lol my bank account is taking such a huge hit this month...
Before i forget! i also need a suggestion for a cheap way to measure the dBs my computer is making, and also if possible the frequency range. That way i'll know which foam to purchase. Thanks guys.
Acoustic Management Guide Part II: Acoustic Absorption
This section is going to come together slowly. Anyways, its all about absorbing sound and preventing it from reverberating. One thing should really be clarified, the goal is to absorb the sound, not to block or reflect it and prevent it from going through.
In the article, i will discuss different techniques for absorbing and also blocking sound, most of these techniques are borrowed from home building and are used on a large scale. I will also discuss different ratings systems and how they work. This article should take a few weeks to finish =p
Ok so here we go...
First, sound absorbing material is rated and classified in many different standards, but the most important to us is the NRC and the STC.
NRC
The NRC or the noise reduction coefficient is a value between 0 and 1. 0 being perfect acoustic reflection, and 1 being perfect acoustic absorption. Its is the average of 4 values, the coefficients at 250Hz, 500Hz, 1000Hz, and 2000Hz. While these 4 frequencies span most of the noise we normally hear, there are a few exceptions. Human hearing spans from 20Hz to 20kHz. Some comprehensive sound absorbing foam will actually have ratings for up to 4000Hz. I have also seen NRC's of over 1, but how that's possible, its totally beyond me.
Most computer noise is high pitched. You rarely hear bass coming from a HSF =p. Therefore, you should find materials with good NRCs around the higher pitched ranges.
STC
STC or sound transmission class is generally used in house building, it is a measurement of how well a wall or celing absorbs sound. STC is based on sound loss in dBs at 16 frequencies between 125 and 4000 Hz. The values are then fitted to a transmission loss curve.
STC is measured in dBs and are harder to use in a computer's context because computers rarely generate the same frequencies and intensity of sound generated by a family of 4 in a townhouse. However, they may be used as a qualitative measurement, as to which methods of soundproofing are more effective. This is because acoustic absorption techniques are rarely measured in NRC, but instead in STC.
Alright... lets move on to techniques for reducing sound reflection.
It'd be ideal if you had a sound meter for measuring frequency and intensity, but since most of us don't have professional grade equipment that costs several thousands... well we just try to estimate the sound range.
Foam
Foam is generally a very popular way to reduce sound reflection. They are measured with NRC, and can range in price from several dollars to several hundred. Foam normally comes in large quantities, but it is possible to buy from a reseller for a smaller roll. Most cases will use foam as a preliminary acoustic absorber. Foam can come in many different shapes and sizes. Different sizes and shapes are designed for absorbing different frequencies of sound. The egg carton shape is very popular, as is the wedge. They both have their own respective frequency specialties.
The shape has a huge effect on absorption, but material also has a very large effect aswell. Open-celled foam is almost ALWAYS required for sound absorption. Generally, the more flexible the foam, the more it can absorb. I am not sure about the actual density of the foam, as if you move to really loose foam, it will leak sound, while really dense foam may reflect too much of it. this is where NRC ratings come in handy.
mass-loaded vinyl
Let me start of by saying mass-loaded vinyl is NOT the same as vinyl flooring. MLV is doped up with large amounts of heavy metals such as barrium (health hazard? 0.o) The principal behind MLV, as it is with all sound absorption, is to convert sound energy and vibrations into heat and then disperse. MLV's effectiveness is questionable, but should work nicely as a backing for foam (more on that later.)
fiberglass
Fiberglass is ITCHY. It will get into your skin and make everything very itchy. No joke, you must have professional experience with fiberglass before you try using it. Ontop of that, you must trap the fiberglass fibers in some kind of epoxy resin, which coincidentally releases organic fumes that will probably bond your contact lenses to your corneas or something equally traumatizing. The effectiveness of fiberglass is questionable, although it is employed in some high end speakers as insulation. I really know not much about fiberglass. I don't suggest working with it unless you're really adventurous. Use at own risk!! A safer alternative would be to use something with very fine fibers. Sort of like what the mutemat does, just you can make your own or something.
complicated pathways
Lets say you want to make a maze for your fan airflow, it'll probably completely kill the sound, provided you make the proper angled turns and line the edges with foam. Even just slightly angleing two walls together can create a dramatic effect in reducing sound. This is really more of an experimental kinda thing.
That's about all i can think of interms of sound absorption, now onto the sound blocking.
vacuum
Sound needs a medium to travel in. Remove medium, remove sound. 100% effective if you can pull off an absolute vacuum 0.o Not sure what effect a partial vacuum would have. I assume its a better way to stop sound waves?
Changing densities
Basically sound waves lose energy as they travel through different types of material. If you manage to sandwich many different density materials together, then sound will be diminished.
One way is actually to put sound absorpative material ontop of sound reflective material. Theoretically, it should force the sound waves to travel through the material twice, hence more absorption. I'm not sure about this though, never seen it really implemented anywhere.
Sound reflective material
Certain materials with very low NRC ratings will reflect sound waves. That's basically the point of a double paned window in a sound studio. Glass is a horrible medium to absorb sound with. It's really a better bet to reflect it back and let the other material handle it instead.
active noise cancellation
I've heard alot of people talk about this, none of them try it. Personally i'm very skeptical about the entire thing, plus not very enthusiastic about the piggy-bank breaking cost. If any of you brave souls out there wanna give it a try, document it for us.
I think that's all about sound blocking techniques.
Many building techniques use a combination of multiple walls, soft mounted walls, staggered walls, and insulation. look it up, some of them look promising but impossibly hard to do.
Theres more that i thought of on the 12 hour bus rides, but i can't remember any of it right now. I'll add if it comes to me.
In the next section i'll be actually much more step-oriented, and it'll really be more of a "guide". I'm going to construct my optical drive cage. This should be interesting. It should also take a long time since my exams are coming in a month heh. If it gets a bit too technical, give me a shout and i'll try to explain better. Unfortunately, i might start to use more complicated tools, since i have 2 machineshops at my disposal. Don't worry, i'll suggest suitable alternate tools for you guys.
There is one section i need some help with. I need an interface between a 5"1/4 IDE drive to USB cable. I also need an adaptor between a 120v wall mount and a 4pin molex power connector. I COULD buy a brand new 5"1/4 enclosure and then rip it all out, but it kinda seems like a waste to me at 50cdn... so i'm open to any suggestions. Don't mind paying around 25 cdn for it... lol my bank account is taking such a huge hit this month...
Before i forget! i also need a suggestion for a cheap way to measure the dBs my computer is making, and also if possible the frequency range. That way i'll know which foam to purchase. Thanks guys.
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