(This article is part of the series:
"How to design and build your own recording studio, home theater, music room, band rehearsal room, or listening room" )
"How to design and build your own recording studio, home theater, music room, band rehearsal room, or listening room" )
(If you haven't already done so, first read: INTRODUCTION TO DESIGNING YOUR OWN STUDIO, and also Step 1 and Step 2, then come back here to read this).
After reading this far, you can move onto Step 3, which is:
Decide on how much isolation ("soundproofing") you need
After going through "Step 1" and "Step 2", you now have a pretty good idea of the general form that your studio will take, and now it's time to get down to details. This is usually the point where people get all enthusiastic, and want to rush ahead: to start "doing stuff".... but the problem is, you don't yet know what to do! You don't even know how much isolation or "soundproofing" you need. You can't build something unless you know what it is supposed to do!
You might already be thinking: "Well, I can skip this step because I don't need any isolation: I don't have neighbors nearby, and I live alone, so I won't disturb anyone!". So you are SURE you don't need isolation? Hmmm... There is nothing in your area that could trash your recording sessions or annoy you as you try to mix? No thunder, rain, hail, or wind? No aircraft or helicopters flying over? No sirens from ambulances / police / fire engines? No trains? No cars arriving / leaving / driving past? No dogs barking outside? No neighbors with a lawn-mower or leaf-blower? And nothing in the house either, such as water running in pipes, fans, pumps and other motors, people walking on floors, doors closing, people talking, vacuum cleaners, washing machine, radio, TV, furnace.... There are hundreds of possible sounds that could destroy a good recording, if they get into the mics in your live room, or break your concentration as you try to mix that delicate part.... Are you CERTAIN that your studio will not get any of that?
You need to take all of that into account when deciding if you need isolation, and how much you need. It is important to consider sounds going in both directions, and set your goal to deal with the loudest one. The "loudest one" might well be your music going out, but it also might be something out there that you don't want to let in... A few years ago I was approached by a potential client in the city where I live, who wanted to build a high-end recording studio just a few blocks from a major sports stadium, that is often also used for rock concerts and other music festivals. On one occasion when I went to visit the potential studio site with him, there was a band in the stadium doing a sound check for a concert that night... the sound levels at the studio site were over 90 dBC! Deafening! It sounded like we were standing on stage, right in front of the speaker stack... yet we were several blocks away. It became apparent that the client would need major isolation to stop loud music GETTING IN to his studio, rather than sound getting out.
So, even if you think you don't need isolation for your studio, you should still go through this process just so that you are aware of the potential issues.
The basic issue here is that you first need to determine IF you need any isolation, and if so, then HOW MUCH you need. The answer will always be measured in "decibels". To be more accurate, it will be "decibels of Transmission Loss", which is the technical term for isolation. Usually abbreviated "dB TL", or just plain "TL". (Acousticians do not use the term "soundproofing" very much, first because there is no such thing! It is impossible to "proof" something against all "sounds": there is no possible barrier that can stop all conceivable sounds: not even if you use the entire planet! Earthquakes, for example, are basically just sound: vibrations in a transmission medium. There is no way you can stop an earthquake getting from one side of the planet to the other. So "soundproofing" is not even possible. But the word itself is a problem, because it has no technical definition: "soundproofing" means different things to different people. So when we talk about attenuating sound as it passes through a wall, ceiling, door or window, we don't call that "soundproofing". Rather, we call that "sound isolation", or in some cases "sound insulation". I prefer to avoid "insulation" to avoid confusion with the fluffy materials of the same name! So I pretty much always use the term "isolation" to refer to stopping sound getting in and out, and we measured that in decibels.)
So, you need a number for your studio: a number that describes "how much isolation I need". And that number will be in decibels: it will NOT be in "STC"! (See here: Why STC is not a good way of measuring studio isolation.)
But why do you even need a number anwyay? Why can't you just explain what you need, in words? Many people just say "I want enough isolation so that I can play music late at night without disturbing my family / the neighbors / the cops". And that's a good start! But it doesn't go far enough. Acoustics is a science: it is based around math, and numbers. There are equations, and charts, and tables, and pictures, and graphs that show how to get a certain amount of isolation with different building methods, but you wont find any of those labeled "enough to not annoy the neighbors" or "enough that the cops won't give me a fine"! Those are subjective evaluations. What you need is a real-world number that represents your isolation goal. Something like "I need at least 40 dB of isolation". You can look up "40 dB" on the tables and charts, or plug it into an equation, and easily find methods and materials for building a studio that will get you 40 dB of isolation. But you can't plug in "enough isolation to keep my wife happy", or "won't wake the kids" to an equation! You need a number.
But how do you figure that out?
Firstly, you need a sound level meter. A decent one, like the one above, not a Chinese toy meter! Not one of these:
You could also use an app on your cell-phone, but that won't be very accurate. I'm working on putting together a set of photos that show several cell phones with the SAME app measuring the SAME sound, alongside a proper professional, calibrated sound level meter, to show the differences. Cell phone apps can easily be off by many decibels! I've seen some people say that cell phone apps are fine for measuring and tuning studios... even people who should know better! But that isn't really true. You might get lucky, and find your cell phone with a good app is spot on... but probably not. And how would you even know, if you don't have a calibrated professional meter to check it against?
That said, if you don't have anything else, and can't get a meter because you are on Covid-19 lock-down, then you could use a cell-phone app to get a rough idea. Even though your cell-phone might not show the correct absolute numbers, it will probably still be able to show a valid DIFFERENCE between two levels, and that's what really matter for this measurement. In other words, if your meter always over-reads by 8 decibels, that that's no use for figuring out what the real sound level is, but it IS useful for know that the level in one location is 30 dB louder than the level in another: Because in BOTH locations it will be showing a number that is 8 dB too high, so the DIFFERENCE will therefore still be accurate. Thus, a calibrated meter might show 70 dB in one location and 100 dB in the other, so the difference is 30, while your meter will show 78 dB in the first location and 108 dB in the second (because it always shows 8 db too high), so the difference is once again 30 dB.
And that's what we need here, to figure out your isolation needs: the difference between two levels, rather than the exact absolute levels.
So, this is one place where you can use a lousy app on your cell phone! But it's the only place in your studio building and tuning process where it is valid: later, you will need a real meter, so you might as well get one now.
Now for the process:
It's actually very simple! You need to answer just two questions:
- "How loud am I?" and
- "How quiet do I need to be?".
To answer the first question (How loud am I?), go to a typical session at the studio you are using right now, or a friend's studio, or any other studio, and measure the level with your meter. If your future studio is going to have a live room / tracking room / rehearsal room / isolation room, or if it is going to be a home theater or listening room, then measure the levels in a typical scenario that you envision will also occur in your place. If you plan to track heavy metal, then go measure a heavy-metal tracking session somewhere. If your thing is jazz, then go measure a jazz session. If it is plain old rock / pop, then measure a rock session. If your room is a home theater, then go visit some other home theater with your favorite move in one hand, and measure that. Etc. Try to find the loudest session, playing the loudest song, and measure it.
VERY IMPORTANT! Set your meter to "C" weighting and "Slow" response! This is critical! Don't get it wrong.
There are different "weighting" scales that are used for measuring sound pressure levels, including A, B, C, D, E, and others. But you will NOT see most of those on meters today. Commonly, meters these days only have "A", "C" and maybe also "Flat".
These weightings are just different methods for adjusting how sensitive the meter is to different frequencies. "C" is much more sensitive to low frequencies, "A" is very INsensitive to lows. There are other possible weightings too, but A and C are the most common. Here's a graph that shows the difference: As you can see, there's a very big difference in the low end! If you played a tone at 50 Hz, for example, and had your meter set to "A", then it would show that it was a whopping 28 dB lower than if you set it to "C"! That's a HUGE difference...
Why are there two scales? Why not just have one?
We humans hear things in ways that you would not expect. The sensitivity of our ears changes, depending on how loud the sound is! For very loud sounds, we hear all frequencies at about the same level, but for quieter sounds we tend to hear the mid range louder than either the lows or the highs. Our ears and brains are more sensitive to frequencies within a couple of octaves either side of 3 kHz, for quite sounds. Here's a chart that shows human sensitivity for several levels of loudness. For very loud sounds, the top line applies. That shows our PERCEPTION of how loud each frequency is, when the music is payed at 100 dB SPL. As you can see, most people would judge that a 100 Hz tone and a 2 kHz tone are at about the same level, with maybe just a couple of dB different. But if you turn down the volume so the music is now very quiet, only 20 dB SPL, the situation is very different: that's the second last line on the graph down at the bottom, just above the red line. At this level you would hear the 100 Hz tone about 25 dB quieter than the 2 kHz tone! Stated another way, you would have to turn up the 100 Hz tone 25 dB before you would say that it is exactly as loud as the 2 kHz tone.
This graph is called the "Equal Loudness Contour Graph", and each curve shows how loud each frequency would have to be, so that it sounds as loud as other frequencies. Or said another way: every point on one of those curves sound exactly as loud as every other point n the same curve. This was first documented by a couple of scientists by the names Fletcher and Munson, so you also often see this graph referred to as the "Fletcher-Munson curves". That isn't really correct, since further research has been done over the years, and the best results today are a bit different from the original Fletcher-Munson curves. Their original graphs were pretty close, but not fully. Here's a comparison: The blue lines are the original Fletcher-Munson curves, while the red lines are the modern "Equal Loudness" curves. The curves are now referred to by their acoustic level, in terms of "phons"... yet another unit of measurement to confuse you even more! But basically you can think of "phons" as just referring to the curves on this graph. A sound that is measured to have a level of 100 phons, would be on that curve some place, depending on frequency. If the frequency was 1 kHz, then 100 phons = 100 dB. But if the tone was at 4 kHz, then 100 phons is about 92 dB, while if the tone were 50 Hz, then 100 phons would be about 112 dB. In real-word terms, that means that you if you played a 50 Hz tone at 112 dB and also a 1 kHz tone at 92 dB, you would judge that the seemed to be at the same volume. Because the are both playing at 100 phons, even though they are at different dB levels!
Hopefully, I have not confused you very thoroughly, and you have no idea what any of this has to do with deciding how thick your wall has to be!!! But it is rather important, actually.
So, now we an get back to "A" and "C" weighting on your sound level meter, and what it really means!
"C" weighting is a rough approximation of how we humans perceive loud sounds. It is roughly the same as the 100 phon curve. "A" weighting is a rough approximation of how we humans perceive quite sounds. It is roughly the same as the 40 phon curve.
It's that simple. Or that complex!
This is why you need to set your meter to "C" when you are trying to determine how much isolation you need: because we are talking about LOUD sounds. If you set it to "A" by accident then it will show a level that is way too low, for typical contemporary music, which is very much "bass heavy". It could easily show you a level that is 20 dB or more, lower than the REAL level that humans would perceive.
Thus, you sometimes see people saying: "Well, I play the drums at about 80 dB, so I don't need much isolation." Wrong! If you play the drums at 80 dB, then you must be hitting them with feathers! Either that, or you are measuring on the "A" scale, which is very INSENSITIVE to low frequency sounds... such as kicks, toms, snares, .... and the bass guitar roaring out of the cab next to you...
A drum kit played normally by typical drummers, will actually put out more like 105-110 dB. Played harder, it is easily 115 dB. And when this is your drummer, the level can approach 120 dB (I know a few drummers like that... both in playing style, and also looks!.... ).
Thus, the importance of setting your meter right. Set it to "C", to make sure it is showing the way we homo sapiens creatures actually perceive loud sounds.
All that long rant to explain why you need "C"!!! And now we can get back to answering the question. "How loud am I?"
Before I went off on that meandering rant about graphs and curves, we were going to measure the sound level in a typical session that you would probably have in your new studio. So, let's get back on track here, and do that! Go ahead, and measure the level of that typical session, with your meter on "C", and you standing in the room about three feet away from the loudest thing (probably the drums). Measure several times, with several different songs, at several points around the room, and take an average . That number represents "How loud am I?"
But what it your studio does NOT have a live room / rehearsal room / isolation booth? What if it is just a control room, for mixing / mastering, or a home theater, or an audiophile listening room?
The same applies: set up your speaker as you would for a typical mixing session or movie watching session, play music at the level you typically listen at, and measure that level. Then "turn it up to check the bass", as engineers always do, and make a note of that level. Now throw away the first level you measured, and use this one as the REAL level for how loud you mix!
So now you have answered the first question! "How loud am I?". Now you need to answer the other question: "How quiet do I need to be?) This is also simple: Go outside at the location where your studio will be, and measure the typical background around noise level... at the same time of day as you plan to us your studio! In other words, if you want to play the drums at 3 AM, then take your meter outside at 3 AM to measure the background noise level.
Now, you might think that you need to flip the meter to "A" weighting at this point, because you are going to check for a very quiet sound... but nope! The meter needs to stay on "C" weighting! The reason here is simple. Later, you are going to subtract one reading from another, but you CANNOT subtract readings on different scales! Just like you cannot validly subtract kilometers from miles, or liters from gallons, so too you cannot subtract dBA from dBC ("dBA" means "Decibels of Sound Pressure Level measured on the A weighting scale", and "dBC" means "Decibels of Sound Pressure Level measured on the C weighting scale"). You can't subtract 30 dBA from 90 dBC. It is not valid. And there is also no way to convert between the two, because they measure different frequencies with different values! So it is impossible to convert dBA to dBC. You can convert miles to kilometers, sure, and also liters to gallons, but you cannot convert dBA to dBC. Trying to subtract one type of dB from another is sort of like saying: "I have 10 apples. I take away three bananas from that. How many oranges do I have left?". The question and the answer are both meaningless.
Thus, you have to keep your meter on dBC for the background noise measurements, because you need to compare them to the "loud" measurements, which you also did using "C". Even though "A" would be more appropriate, you have to use "C".
So, go outside and measure the background level, taking care that you are not standing close to something noisy! Like the vent from your bathroom extractor fan, kitchen hood, furnace, HVAC compressor, etc. You want only the real neighborhood sounds, at the quietest time of day. Measure in a few locations, over a space of many minutes and take an average, as well as noting the loudest and quiets levels. Some meters can do that for you: they have "max hold" and "min hold" functions.
So, now you have two levels: "How loud am I?", and "How quiet do I need to be?": And they are both on the dBC scale, so all you have to do now is: subtract!
In your calculator (or your head) enter the big number, hit the minus key, enter the smaller number, and hit the "=" key. That number you have now, is how much isolation you need, in decibels.
Stay tuned for the next step...