question about small vocal booth

#1

hi everyone, I created a vocal booth based on the bold parameters. The vocal booth is 130cm wide, 200cm long and 290cm high.
For the acoustic treatment, I filled all the walls with 15 cm of 50 kg density rock wool, over the rock wool I put a thin plastic and finally the camira cara fabric. In one of the walls (the 130 cm one) I created a slat wall in order to absorb mainly the frequencies from 150 to 300 hz.

Question number 1: I took a cue from john sayers' vocal booth for acoustic treatment, but I noticed that following that type of project the decay time t30 is under 70 ms seconds, ebu tech recommends having a decay time greater than 120 ms, so does it matter to increase the decay time? it seems that john doesn't make almost everything absorbent in small rooms, although putting the plastic will increase the decay time only on the high frequencies.

Question number 2: Should the decay time t30 be flat? I was able to get a fairly flat response by removing the plastic but it sounded too boring, I like more to raise the treble above 6000 hz, also it seems that it sounds more pleasant by increasing the bass a bit, so I feel less effect annoying of the mids around 2000 hz

Many will say that a small vocal booth does not work, I have seen john's vocal booth sound very good, but I cannot get the same results, what is wrong with my vocal booth? I am attaching the graph

#2

Hi there grignacarbo, and WELCOME to the forum!

A very belated welcome, as I have been out of action for a while for health reasons. Hopefully you are still around, and still interested.

but I noticed that following that type of project the decay time t30 is under 70 ms seconds, ebu tech recommends having a decay time greater than 120 ms,

For a very small room, such as yours, it is very hard (practically impossible) to get a good acoustic response with decently long decay times. Even 120ms is not very long, for vocals. The problem is simply related to the physics of how sound moves in a small space. Unfortunately, there is nothing you can do to change the laws of physics! So for very small rooms, the only real option is to do what you have already done: Make it as dead as possible, with some slight return of highs from the plastic, and some slight return of mids from the wood slats. Then sing or speak close to the mic, and add a touch of ambience "in the mix" with some type of reverb plug-in. Its not ideal, but it can work.

what is wrong with my vocal booth? I am attaching the graph

From the MDAT file, you have done a pretty good job of treating that place! Very good, in fact. There is nothing wrong with it, and it should be quite usable as a vocal booth, as long as you use close-micing techniques for vocals. There won't be any "room sound" in your mic, which is a good thing, because small rooms have a "boxy" dull sound of their own. You have quote flat frequency response, but even more important, you have achieved the "Holy Grail" of small rooms, by getting the decay times very much under control, and smoothly even across the spectrum. Except for the very low end, but you will probably have a low-cut filter on your mic in any case, for vocals, so that should not be an issue.

Overall, I would say: congratulations! You have done a god job with a not-so-good room. Part of your success is the excellent ceiling height: at 2.9m, that's great. If you would have had a low ceiling in there, then I doubt you would have got the same results. You can be proud of what you have achieved: the room is about as good as it can be, acoustically: now it is going to be all about choosing the right mic, putting it in the right place, putting the talent in the right place, then using careful EQ and careful effects to get the final result you are looking for.

- Stuart -

#3

i hope you are fine now.. an acoustic designer who works for RPG suggested me to make the vocal booth more reflective with speakers, he told me that when a room is too dead, if you introduce reflective materials you can perceive the defects more, while with a longer decay time a masking effect is created. So I don't understand, I see that john sayers completely deals with vocal booths I presume they have a very low decay time, so I don't understand if you need to increase the decay time or keep the vocal booth as dead as possible.

#4

it's the balance - you would start with a fully absorption space (and most rooms even with lots of absorption will not be anechoic across all frequencies - esp low frequencies) and add the reflective support to make it comfortable without causing anomalies like comb filtering and hard reflections. this way the person singing or speaking in the room "feels" the room but it's not interfering with the recording. and with additional bits like gobos or hanging panels, you can increase the reflections if needed or with dual sided gobos, add more absorption or some isolation. note: windows, music stands, tv monitors, foldback speakers, music instruments (piano, guitars on stands, etc) all contribute to the reflection surfaces so due consideration for those effects is necessary as well.

#5

i had read somewhere that soundman2020 said that the decay time must be in a range of 100 ms for there to be some linearity, but i noticed that gervais rod (see dark pine studios) created rooms with many slat walls, i i tried to replicate something similar (actually i didn't even put many slat walls) and the rt30 results were very unbalanced, here is an example:

so i am wondering, is it really important to have a linear rt30? I suppose that the projects of john sayers or gervais rod are by no means linear in terms of decay time rt30

#6

grignacarbo wrote:Source of the post i had read somewhere that soundman2020 said that the decay time must be in a range of 100 ms for there to be some linearity

It depends on the size of the room. There are equations and "rules of thumb" for figuring out what a good decay time would be for any given size of room, and also for the purpose of the room. For control rooms, for example, there is a very tight range of useful decay rates, and they all need to be within very close tolerance, across the spectrum, with only small differences between adjacent frequency bands (ideally, not more than 50ms difference between adjacent bands). That's because control rooms must meet the requirement of sounding neutral, natural and transparent, with no sound of their own. But live rooms, performance halls, lecture halls, classrooms, rehearsal rooms, and vocal booths all have very different needs, and they don't necessarily have to have strictly controlled decay rates.

However, as Glenn mentioned, they all do need to be balanced, in the sense that no one area of the spectrum is hugely different from any other area. That's part of the challenge of treating such spaces: controlling the balance.

The problem with very small rooms is that it is very, very hard to do that! There is a physical limit to what can be achieved, and it sort of makes sense if you think about it in terms of "wavelength", rather than frequency. (You probably already know that there is a direct relationship between frequency and wavelength.) For wavelengths that are a lot smaller than the dimensions of a room, it is easy to create multiple reflections that build up to create that "reverberant field", but for waves that are BIGGER than the room, it is pretty much impossible. So, for example playing the drums in a small room sounds terrible, because the wavelengths for the kick, snare, and toms are bigger than the room, or comparable in size. The kick often has a fundamental around 80 Hz, for example, which is a wavelength of 4.3 meters. You need quite a large room before you can build up a reverberant field for that. The snare often has a fundamental around 220 Hz, where the wavelength is about 1.5 meters, son once again that is similar to the dimensions of a small room, and doesn't sound good. The same snare in a larger room, will sound fine.

So what about reflective surfaces? Yes, you can add "life" to a room by providing surfaces that will reflect some of the sound energy back into the room. Except that there is a basic "rule of thumb" in acoustics: A surface needs to be about the same size as the wavelength, in order to reflect that sound. So, to reflect the sound of that snare, you would need a surface 1.5 m wide, and 1.5 m high! That's the problem... in a small room, there isn't enough space to properly reflect low frequencies, so there is no reverberant field for low frequencies. That's why you will often see acousticians say that there is no such thing as RT60 in a small room. Technically, they are correct: you cannot have true reverberation for low frequencies in a small room. Now, this is where it gets more complex: you can still have a decay for low frequencies, even though it is not a reverberant field that is decaying! So even though you there is no reverberant field at 220 Hz, your room can still show a decay for that frequency! Even though the wave does not "fit inside the room", it is still there, and can still have some form of resonance.... but not reverberation. And that's where our hearing comes in: Pure resonance doesn't "sound" very nice, whereas reverberance does "sound" nice. That's a subjective opinion, not an objective measurement! Most people prefer the sound of a (for example) 300 ms decay of a true reverberant field at 220 Hz, as compared to the sound of a purely resonant field with the same decay rate and same frequency.

Which gets back to the problem with small rooms: an undamped small room will always sound "boxy" and unpleasant, just because of the way the sound fields interacts physically with the room itself. You can improve that by adding some reflective surfaces for the high frequencies, to give the room a little more life, but there is nothing you can do to reflect low frequencies, because the room is just too small for that. So the best thing you can do for lows, is to mostly absorb them. Which is also hard to do, because absorption for lows also takes up a lot of space! And even if you could completely absorb all the lows, the room sill would still sound terrible, because, as Glenn mentioned, there is no balance. In your case, you have reasonable balance for a vocal booth, but probably not for many instruments.

So, in summary: the best you can do in a small room is to keep it quite absorbent, to deal with the lows, then add back some reverberance in the mids and highs, but not too much or you will lose the balance again. That will mean that the decay curve is not linear, and not flat: And that's usually not a problem, in reality. It is impossible to make it flat in any case, for a small room, so just do the best you can. In your case, at first glance it would seem that you don't have enough reverberance at around 300 Hz, but that's actually fine: there isn't much you could do to build that up again any way (wavelength is about 1.1 m), and if you tried then you would also build it up too much in the higher frequencies.

Your room is a vocal booth, so the large build-up in decay time below about 70 Hz does not matter: you will have a low-cut filter on your vocal mic in any case, and you can probably set that to maybe 120 Hz or so without any problem, so the low end just won't be in your recordings. And the rest of the spectrum is reasonable for vocals.

You might be able to improve the balance a bit more, but honestly, I'm not sure that is necessary.

Maybe you could post some pictures of your room, to show how it looks, and what treatment you have? I think many people would find that interesting, to see how you accomplished this.

- Stuart -

#7

the rt30 image I sent you refers to my control room which is a bit bigger, about 5 meters x 4 with a height of 270 cm, the whole ceiling is completely absorbent with 20 cm of rock wool, the the wall where the study desk is located is completely absorbent, the opposite wall has slat walls and the two side walls are almost completely reflective. As you can see, although I have not put many wooden staves, the rt30 shows that an unbalanced response of over 150ms, with a minimum value of about 80ms around 200 hz up to about 230 ms on the high frequencies. So if I see the rooms of gervais rod or john sayers using a lot more wooden slats, I assume that the rt30 will be even more unbalanced, by the way I noticed that john puts plastic on all the treated surfaces, so purposely their projects have a very unbalanced decay time which is perhaps the secret of their sound.

Also consider that although I have a longer decay time in the high frequencies, my control room sounds a little dead to me in the high frequencies. I believe for example the dark pine studios designed by gervais rod have an unbalanced decay time of at least 200ms (or maybe 250ms) due to the fact that all the walls and ceiling have strips of wood, also note that he performed this type treatment in the control room, vocal booth (which is very small) and live room, or john sayers projects that have a more moderate amount of wooden slats, still have more wooden slats than what I did in my control room

so I can't understand why we often talk about decay times within 100ms, if some acoustic designers go too far. I am attaching some photos of the dark pine studios so that you get an idea, also this is the link to see a short video of my control room https://www.youtube.com/watch?v=hS5GyuJO3SY

#8

grignacarbo wrote:Source of the post so I can't understand why we often talk about decay times within 100ms,

Because that's what the experts say is needed, psycho-acoustically, for top quality critical listening rooms.

There are a few documents that lay out the specifications for such critical listening rooms, such as EBU Tech.3726 and ITU BS.1116-3. You can find both of those here on the forum: viewtopic.php?f=9&t=4&p=4 For example, on page 16 of the second one, there is a graph that shows what the decay times should be across the entire spectrum for a critical listening room:

along with the equation for calculating the overall decay time. Many studio designers use these two documents (and other similar ones) as the basis for designing and tuning control rooms.

refers to my control room which is a bit bigger, about 5 meters x 4 with a height of 270 cm,

That's a very decent size for a control room, and falls within the dimension range specified in BS.1116-3. It should be possible to get the results show in that document, in your room, with suitable treatment. For example, here's the results from a we-designed and carefully tuned control room, showing what can be achieved with a lot of effort: Decay times:

Frequency response:

And from a different room, much smaller than the above:

So it is possible to achieve those specifications, and they are necessary for top quality critical listening rooms.

the whole ceiling is completely absorbent with 20 cm of rock wool, the the wall where the study desk is located is completely absorbent, the opposite wall has slat walls and the two side walls are almost completely reflective.

That doesn't sound like suitable treatment for a control room. With reflective side walls, you probably have a lot of early reflections within the critical 30ms Haas time. Also, there doesn't seem to be sufficient bass trapping in there, which would explain the very long decay times in the lowest frequencies and the overall uneven decay times.

So if I see the rooms of gervais rod or john sayers using a lot more wooden slats, I assume that the rt30 will be even more unbalanced,

Not necessarily! Both Rod and John use tuned "slot walls", not just wood slats. Slat walls consist of a series of tuned Helmholtz resonators that are designed to treat either several specific frequencies, or a general range of frequencies, depending on how they are designed (basically, that's related to the "open area" percentage of the slots vs. the total surface area). By carefully designing and tuning a slot wall, it is possible to tune the treatment as needed by the room, so it absorbs some specific frequency resonances, while reflecting others above the tuned range, and absorbing all the others below the tuned range. I prefer to not use slot walls in rooms I design (although I do sometimes), as they can be tricky to design and build right. They do work, when done properly. Helmholtz resonators are a very useful tool in the designer's toolbox.

Also consider that although I have a longer decay time in the high frequencies, my control room sounds a little dead to me in the high frequencies.

I would probably go a little longer in the high end, but it isn't out of place. It's the mids and lows that are problematic in your room, currently. Having such short decay times in the mids can also lead to the impression that the high end is too dead, when really it isn't.

due to the fact that all the walls and ceiling have strips of wood, also note that he performed this type treatment in the control room, vocal booth (which is very small) and live room, or john sayers projects that have a more moderate amount of wooden slats, still have more wooden slats than what I did in my control room

Did you tune your slats specifically? That's the question. Just putting up slats at random over an unsealed cavity is not the same as what John, Rod, and others do with slats/slots. True slot walls are built over sealed cavities, with the depth of the cavity, the dimensions of the slots and slats, and the thickness of the slats all carefully selected to deal with specific frequencies or ranges of frequencies. The insulation inside the cavity is also important, in bot the amount of insulation (coverage), the thickness, the type, the location within the cavity, and the characteristics (specifically, the "gas flow resistivity").

It would be good if you could post photos of your control room, and the actual MDAT file, so we can take a closer look.

- Stuart -

#9

I state that I have treated the control room only to record the voices (I am not interested in mixing or mastering) also consider that I have a good knowledge of acoustic materials since I have been doing many experiments for 2-3 years now. yes obviously my slat walls are sealed and calculated to absorb mainly from 200 to 500 hz, if I wanted to I can make the decay time completely flat to record vocals, but i'm not doing this because i want to understand how the rt30 of john sayers vocal booth / recording room could be, so i'm coming to the conclusion that those rooms have a decay time that is in the shape of a smile, i.e they don't have much rt30 flat and almost certainly exceed the tolerance range of 100 ms, I understand that the vocal booth / recording rooms he designs have emphasis on low and high while tends to absorb more mid-low frequencies, he had stated in his forum.

as for the my control room seems dead on the high frequencies, I could increase the decay time to around 300ms, but the problem is that in a control room (which I don't want to treat as a control room) like mine where there is also the studio desk and other objects that create reflections, perhaps the best idea is to reduce reflections as much as possible. I have attached the measurements of the control room, I had put some panels on the side walls to have a more balanced rt30 but the room seemed too dead, so tell me if you see some harmful early reflections

#10

however by ear my control room does not sound very dead now, I mean it seemed dead when I had put some absorbing panels on the side walls, considering that however the decay response was more pronounced in the high frequencies of almost 50ms, so if it were flat it would be even worse

#11

gullfo wrote:Source of the post it's the balance - you would start with a fully absorption space (and most rooms even with lots of absorption will not be anechoic across all frequencies - esp low frequencies) and add the reflective support to make it comfortable without causing anomalies like comb filtering and hard reflections. this way the person singing or speaking in the room "feels" the room but it's not interfering with the recording. and with additional bits like gobos or hanging panels, you can increase the reflections if needed or with dual sided gobos, add more absorption or some isolation. note: windows, music stands, tv monitors, foldback speakers, music instruments (piano, guitars on stands, etc) all contribute to the reflection surfaces so due consideration for those effects is necessary as well.

hello due to the size of my vocal booth which is too small, I have decided to treat the control room as if it were a vocal booth, so I am not interested in mixing.

I am attaching the measurements of the t30, as you can see the frequencies around 200 -300 hz have a decay time around 70ms. Do you think that if I make the decay time flat around 70 ms it will sound good or would you advise me to increase the decay time?

for me the simplest solution is to put on a wall that is currently reflecting a thin absorption layer in order to make the whole range of freqs linear around 70ms, but I would like to be sure if with such a low decay time a vocal booth

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