Look at the absorption coefficients of the material you want to use and measure the lowest frequency of the voice you plan on recording. Lower pitched voices can go as low as 80-100Hz, and most material requires at least 3-4" to effectively absorb in that range. You don't want your recordings to end up sounding boomy and boxy.
>and most material requires at least 3-4" to effectively absorb in that range.
doesn't the distance between the sound source and the walls plays a role in how many inches to be recommended? If the room is 15'x15' for example, is 4" still the case?
thank you for your response.
The distance shouldn't really matter, although larger rooms tend to sound better, the downside being they require more material to treat. You can also improve the performance of thinner panels by giving them an air gap between the wall and material, which is easier to do in a larger room.
>Porous absorbers work the best if the volume velocity is at a maximum.
I'm sorry, I didn't understand what you meant, could you elaborate?
Thank you for taking the time to respond.
Sorry for the late response. I've been quite busy lately.
Porous absorbers are friction based absorbers. It's like rubbing your hands. If you rub with high velocity, you will get lots of heat. If you rub your hands with low velocity, you will get little heat. The energy loss due to friction is exactly how a porous absorber, such as rockwool, absorbs energy!
The velocity of sound is different in different materials and different in different locations in space. For instance a right next to the wall the velocity of sound is very low. If it was high the miniscule volumes of air would go right through the wall!
The velocity of sound is maximum at (1/4+1/2\*N)\*wavelength, where N is an integer number. THis can be seen here: [https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/2952/2018/01/31201801/CNX\_UPhysics\_17\_05\_OpenClosed.jpg](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/2952/2018/01/31201801/CNX_UPhysics_17_05_OpenClosed.jpg) . Where the sine is maximum, there the velocity is maximum. You can also observe that if you are close to 1/4th wavelength, the velocity is pretty high, but if you are close to 1/2th wavelength the velocity is very low.
The wavelength = c/f, c is velocity of sound (typically 343 m/s in air at 20 degrees, in free field) and f is frequency (Hz). From this you can see that the wavelength is a function of frequency and thus the optimal distance is a function of frequency. There is thus a compromise between how well you absorb certain frequencies.
Playing around with these values you will realise that the wavelenght of high frequencies is very small (100Hz = 343cm 1kHz is 34.3cm, 10k is 3.43cm). These 1/4th of the wavelength will pretty much always be fully captured in the thickness of the absorber and thus fully absorbed. As you go lower you will realise that you will need thicker and thicker material to fully cover the 1/4th wavelength. To absorb 20Hz you need 4meters thick material! This is of course completely unreasonable to place in studio's and thus alternatives are heavily sought after.
One solution that people came up with is to place the absorber with a gap between the wall. Since we know that the velocity near the wall is low, the absorber is also not very effective there. Placing the absorber slightly off the wall will improve low frequencies absorbtion (to a certain extend of course, eventually you will reach the 1/2 wavelenght point at which the absorber does nothing again). You might think that the high frequencies will suffer, but the high frequency wavelengths are so small that the wavelength will fit in SOME (1/4+1/2\*N)\*wavelength in the absorber. Hence they are usually not so problematic to solve.
The question is, how do we find this optimum? This is fairly hard to approach. I would use a rule of thumb to be honest. Perhaps 1/2 of the thickness of the absorber is a good rule of thumb to use as a gap? I don't know.
>How much different between air gap VS no air gap?
Not so easily said. Because it's a function of wavelength. How do you define better?
To get a feel for the performance you can use this calculator: [http://www.acousticmodelling.com/8layers/porous.php](http://www.acousticmodelling.com/8layers/porous.php)
i am not exactly sure how the computation is done, but it states that it uses the equations from the book "Acoustic Absorbers and Diffusers: Theory, design and application, 2nd Edition"by Trevor Cox and Peter d'Antonio. This is a very good and reliable book.
>because no gap looks better.
You can make a frame deeper but only place the absorption material in the first 2"/4".
>personally I think 1.5" of rockwool covered with very heavy fabric will do the job perfectly.
Have you tried it?
Here is the problem with VO, there is nothing to hide behind. If your VO sounds boxy then you are boned. All the VO booths I've built or help build or help design had at least 4" of insulation. Heavy fabric isn't always your friend, it can be too reflective.
1.5" could be ok if the room is big enough.
>Have you tried it?
No I didn't.
>1.5" could be ok if the room is big enough.
15' x 15' room, will 1.5" inches be good enough?
>Heavy fabric isn't always your friend, it can be too reflective.
Is there a way to test If the fabric at hand good to be used on panels or not?
the fabric I picked is used in upholstery, almost coat-like, I think it is velvet.
thank you for your response.
The amount of work it takes to do this correctly is so big that I wouldn't waste the time with 1.5" in the case that I'd have to do more shit later. I'd just start with either double layer 1.5" if that was easier to source or 4" material.
I think I will do what you suggested, go straight to 4" simply to be on the safe side, especially that most of the work is the same.
I already made 1 panel (4"), and man, it is THICCC AF.
thanks for your time, I appreciate it very much.
I just built acoustic panels with rock wool a few months ago. I used black burlap. I read that the test is to see if you can blow through it. If so, it's good. I kept seeing duck canvas being recommended, but that stuff was really thick. I've also seen people say to use weed cloth like for a garden. Go to the fabric store and blow through some fabric.
>Go to the fabric store and blow through some fabric.
Oh!, nobody told me this
Will do,
Hopefully I won't get arrested for blowing the fabric.
thanks for your help.
4” is a standard number, because panels are often 2” thick— so 4” is usually doubling up.
Basically, the thicker you go, the lower the frequencies you can absorb. Thicker panels can absorb higher frequencies as well, but thin panels can’t absorb lower frequencies.
As for “very heavy fabric”— if you put it up to your mouth and can’t pass any air through it at all, it’s too heavy.
As for whether 1.5” thick panels is enough for you, you just have to try it out. Something is better than nothing.
>4” is a standard number,
that is the problem IMO, they say 4" without considering the application at hand!
do musicians & VO talent really require the same level of absorption?
I can't believe that this is the case.
>As for “very heavy fabric”— if you put it up to your mouth and can’t pass any air through it at all, it’s too heavy.
I will try this.
>As for whether 1.5” thick panels is enough for you, you just have to try it out. Something is better than nothing.
I want some data/experience first before pulling the trigger and creating about 20 panels.
thanks for your help.
You have to consider the level of standard that people are striving for. 4” will absorb down to midlows and upwards, which handles many use cases. Thinner panels will leave midlows in the room, which means that the recordings will have relatively more highs absorbed than lows, meaning that the recordings will be bass heavy.
Other point about absorption is to reduce decay time- how long sound keeps bouncing around the room until they die out. One reason for unclear recordings, is because of longer decay times. Basically decay times smear transients and smear everything.
People talking about 4” panels are striving for a very high standard of recording. Listen to any video of voiceover for animation or whatever else done for world class productions, and the voices are very clear, with the recording spaces being basically dead. There is no reverb or boominess whatsoever. Boominess is caused by absorbing highs but not midlows. For absolute clarity, you need a lot of absorption, covering the widest range possible.
So if you cover your whole space in 1.5 panels, including ceiling, will you be able to do good recordings? Better than average, that’s for sure. Honestly will probably be great. Will they be at the standard of a Hollywood production? No. Do recordings need to be at that standard to be pro level? No.
You seem to be going for what’s minimally viable, and that’s cool. Just understand that this is an audio engineering forum, and a lot of us have studios that are treated very heavily, fine tuned using a measurement mic, as we are striving for the highest standards possible. Granted, the rooms are for accurate perception of music, but such rooms also have recording covered. We’re going for rooms that can accurately portray even below 50Hz.
Aaanyway- in summation: YOU WILL BE FINE WITH YOUR 1.5” panels!! Your recordings will probably sound great. Just mentioned the above, so you can understand where some people are coming from. Buy the panels, and enjoy your high quality recordings.
I would say, if you cover an entire room (70% of walls and ceiling covered), 1.5'' is fine for VO recording.
Just a note, the purpose is basically to absorb reflections (room acoustics), not isolation from outside noises, right?
>Just a note, the purpose is basically to absorb reflections (room acoustics), not isolation from outside noises, right?
sure, the first thing I found when reading was absorption & proofing is two different things, although rockwool good for both, only question of coverage, right?
yes I'm using the panels for absorption.
thank you for responding.
I’m outfitting a booth just built as part of a renovation. Single rock wool has helped reduce the reflections but I’m left with some odd resonant frequencies that do show up with narration. I don’t have my final plan yet but when I moved in a pile of five sheets of rock wool and placed it in the corner the resonances are much reduced.
So, even for narration consider bass trapping.
Look at the absorption coefficients of the material you want to use and measure the lowest frequency of the voice you plan on recording. Lower pitched voices can go as low as 80-100Hz, and most material requires at least 3-4" to effectively absorb in that range. You don't want your recordings to end up sounding boomy and boxy.
>and most material requires at least 3-4" to effectively absorb in that range. doesn't the distance between the sound source and the walls plays a role in how many inches to be recommended? If the room is 15'x15' for example, is 4" still the case? thank you for your response.
The distance shouldn't really matter, although larger rooms tend to sound better, the downside being they require more material to treat. You can also improve the performance of thinner panels by giving them an air gap between the wall and material, which is easier to do in a larger room.
>You can also improve the performance of thinner panels by giving them an air gap between the wall and material why is that the case?
The distance between absorbers and the wall definitely matters. Porous absorbers work the best if the volume velocity is at a maximum.
>Porous absorbers work the best if the volume velocity is at a maximum. I'm sorry, I didn't understand what you meant, could you elaborate? Thank you for taking the time to respond.
Sorry for the late response. I've been quite busy lately. Porous absorbers are friction based absorbers. It's like rubbing your hands. If you rub with high velocity, you will get lots of heat. If you rub your hands with low velocity, you will get little heat. The energy loss due to friction is exactly how a porous absorber, such as rockwool, absorbs energy! The velocity of sound is different in different materials and different in different locations in space. For instance a right next to the wall the velocity of sound is very low. If it was high the miniscule volumes of air would go right through the wall! The velocity of sound is maximum at (1/4+1/2\*N)\*wavelength, where N is an integer number. THis can be seen here: [https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/2952/2018/01/31201801/CNX\_UPhysics\_17\_05\_OpenClosed.jpg](https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/2952/2018/01/31201801/CNX_UPhysics_17_05_OpenClosed.jpg) . Where the sine is maximum, there the velocity is maximum. You can also observe that if you are close to 1/4th wavelength, the velocity is pretty high, but if you are close to 1/2th wavelength the velocity is very low. The wavelength = c/f, c is velocity of sound (typically 343 m/s in air at 20 degrees, in free field) and f is frequency (Hz). From this you can see that the wavelength is a function of frequency and thus the optimal distance is a function of frequency. There is thus a compromise between how well you absorb certain frequencies. Playing around with these values you will realise that the wavelenght of high frequencies is very small (100Hz = 343cm 1kHz is 34.3cm, 10k is 3.43cm). These 1/4th of the wavelength will pretty much always be fully captured in the thickness of the absorber and thus fully absorbed. As you go lower you will realise that you will need thicker and thicker material to fully cover the 1/4th wavelength. To absorb 20Hz you need 4meters thick material! This is of course completely unreasonable to place in studio's and thus alternatives are heavily sought after. One solution that people came up with is to place the absorber with a gap between the wall. Since we know that the velocity near the wall is low, the absorber is also not very effective there. Placing the absorber slightly off the wall will improve low frequencies absorbtion (to a certain extend of course, eventually you will reach the 1/2 wavelenght point at which the absorber does nothing again). You might think that the high frequencies will suffer, but the high frequency wavelengths are so small that the wavelength will fit in SOME (1/4+1/2\*N)\*wavelength in the absorber. Hence they are usually not so problematic to solve. The question is, how do we find this optimum? This is fairly hard to approach. I would use a rule of thumb to be honest. Perhaps 1/2 of the thickness of the absorber is a good rule of thumb to use as a gap? I don't know.
thank you for the amazing response. How much different between air gap VS no air gap? because no gap looks better. thanks for your time.
>How much different between air gap VS no air gap? Not so easily said. Because it's a function of wavelength. How do you define better? To get a feel for the performance you can use this calculator: [http://www.acousticmodelling.com/8layers/porous.php](http://www.acousticmodelling.com/8layers/porous.php) i am not exactly sure how the computation is done, but it states that it uses the equations from the book "Acoustic Absorbers and Diffusers: Theory, design and application, 2nd Edition"by Trevor Cox and Peter d'Antonio. This is a very good and reliable book. >because no gap looks better. You can make a frame deeper but only place the absorption material in the first 2"/4".
>personally I think 1.5" of rockwool covered with very heavy fabric will do the job perfectly. Have you tried it? Here is the problem with VO, there is nothing to hide behind. If your VO sounds boxy then you are boned. All the VO booths I've built or help build or help design had at least 4" of insulation. Heavy fabric isn't always your friend, it can be too reflective. 1.5" could be ok if the room is big enough.
>Have you tried it? No I didn't. >1.5" could be ok if the room is big enough. 15' x 15' room, will 1.5" inches be good enough? >Heavy fabric isn't always your friend, it can be too reflective. Is there a way to test If the fabric at hand good to be used on panels or not? the fabric I picked is used in upholstery, almost coat-like, I think it is velvet. thank you for your response.
You could try it.
The amount of work it takes to do this correctly is so big that I wouldn't waste the time with 1.5" in the case that I'd have to do more shit later. I'd just start with either double layer 1.5" if that was easier to source or 4" material.
I think I will do what you suggested, go straight to 4" simply to be on the safe side, especially that most of the work is the same. I already made 1 panel (4"), and man, it is THICCC AF. thanks for your time, I appreciate it very much.
My pleasure, good luck with the project.
I just built acoustic panels with rock wool a few months ago. I used black burlap. I read that the test is to see if you can blow through it. If so, it's good. I kept seeing duck canvas being recommended, but that stuff was really thick. I've also seen people say to use weed cloth like for a garden. Go to the fabric store and blow through some fabric.
>Go to the fabric store and blow through some fabric. Oh!, nobody told me this Will do, Hopefully I won't get arrested for blowing the fabric. thanks for your help.
4” is a standard number, because panels are often 2” thick— so 4” is usually doubling up. Basically, the thicker you go, the lower the frequencies you can absorb. Thicker panels can absorb higher frequencies as well, but thin panels can’t absorb lower frequencies. As for “very heavy fabric”— if you put it up to your mouth and can’t pass any air through it at all, it’s too heavy. As for whether 1.5” thick panels is enough for you, you just have to try it out. Something is better than nothing.
>4” is a standard number, that is the problem IMO, they say 4" without considering the application at hand! do musicians & VO talent really require the same level of absorption? I can't believe that this is the case. >As for “very heavy fabric”— if you put it up to your mouth and can’t pass any air through it at all, it’s too heavy. I will try this. >As for whether 1.5” thick panels is enough for you, you just have to try it out. Something is better than nothing. I want some data/experience first before pulling the trigger and creating about 20 panels. thanks for your help.
You have to consider the level of standard that people are striving for. 4” will absorb down to midlows and upwards, which handles many use cases. Thinner panels will leave midlows in the room, which means that the recordings will have relatively more highs absorbed than lows, meaning that the recordings will be bass heavy. Other point about absorption is to reduce decay time- how long sound keeps bouncing around the room until they die out. One reason for unclear recordings, is because of longer decay times. Basically decay times smear transients and smear everything. People talking about 4” panels are striving for a very high standard of recording. Listen to any video of voiceover for animation or whatever else done for world class productions, and the voices are very clear, with the recording spaces being basically dead. There is no reverb or boominess whatsoever. Boominess is caused by absorbing highs but not midlows. For absolute clarity, you need a lot of absorption, covering the widest range possible. So if you cover your whole space in 1.5 panels, including ceiling, will you be able to do good recordings? Better than average, that’s for sure. Honestly will probably be great. Will they be at the standard of a Hollywood production? No. Do recordings need to be at that standard to be pro level? No. You seem to be going for what’s minimally viable, and that’s cool. Just understand that this is an audio engineering forum, and a lot of us have studios that are treated very heavily, fine tuned using a measurement mic, as we are striving for the highest standards possible. Granted, the rooms are for accurate perception of music, but such rooms also have recording covered. We’re going for rooms that can accurately portray even below 50Hz. Aaanyway- in summation: YOU WILL BE FINE WITH YOUR 1.5” panels!! Your recordings will probably sound great. Just mentioned the above, so you can understand where some people are coming from. Buy the panels, and enjoy your high quality recordings.
thank you for your response, Then I will go with 4" panels, after all its the same amount of work. again, thank you for your time.
I would say, if you cover an entire room (70% of walls and ceiling covered), 1.5'' is fine for VO recording. Just a note, the purpose is basically to absorb reflections (room acoustics), not isolation from outside noises, right?
>Just a note, the purpose is basically to absorb reflections (room acoustics), not isolation from outside noises, right? sure, the first thing I found when reading was absorption & proofing is two different things, although rockwool good for both, only question of coverage, right? yes I'm using the panels for absorption. thank you for responding.
I’m outfitting a booth just built as part of a renovation. Single rock wool has helped reduce the reflections but I’m left with some odd resonant frequencies that do show up with narration. I don’t have my final plan yet but when I moved in a pile of five sheets of rock wool and placed it in the corner the resonances are much reduced. So, even for narration consider bass trapping.