John Sayers' Recording Studio Design Forum Archive

Hi everyone,

I've been reading about acoustics in handbooks and online for quite some time and this forum has been the single most helpful ressource that I came across when it came to bridging the gap between theory and practice.

All that thanks to a group of people that decided that sharing their knowledge was more constructive than trying to capitalize on it.
I am very grateful that you did.

So, got my new appartment and for the first time I have enough space to consider setting myself up a decent mixing environment.
I have a good idea of how I'd ideally be treating the room, but some features of this appartment leave me with question regarding the proper acoustics of the space.

The room is roughly 31' x 10.5' x 9' it is perceptually divided in two by small pedestals with columns sitting about halfway of the long side. The columns go up to support a flattened arch, the centerpoint of which sits at 8' above the floor.

The idea is to use one half for my bedroom and study, and the other for the "control" room.

First (newbie) question:

Is it completely ridiculous to expect SOME amount of transmission loss from a heavy curtain dropping from ceiling to floor?

I understand that without proper mass and sealing, no real isolation is possible, but my goal for this one is mainly speech attenuation.
Do you think those curtains would perform reasonably well for this purpose?

My understanding is that it would indeed absorb some of the mids ans highs, but would do nothing to stop the low mid and bass speech components (essentially vowels) from passing through. So are curtains COMPLETELY useless when it comes to isolation?



My next question is a little bit more complicated.

The room in question connects directly to a hallway 4' in width and then to a space of rougly 27' x 10.5' x 9' .

Since the connection between these spaces is acoustically direct (air-loose) and that the partition walls are rather flimsy (oldschool plaster walls on woodstud framing), shouldn't I be doing the math taking into account the other room and the corridor aswell? What about the kitchen, at the other end of the corridor, it is connected also... oh my god! and the bathroom, and the cupboards and the other room... Aaaaargh!

Where does this madness ends? - Will modal issues be less significant due to the acoustical connection between the rooms, thus creating a bigger acoustical space? In this case, shouldn't I be treating the other rooms aswell?

Or, considering my limited budget and lack of proper isolation, should I just treat my first reflection points, put up superchunks in the corners behind my speakers, install a few gobos behind me and leave it at that?

Thank you for your attention!

Hi,

A very interesting space your'e dealing with! Haha.

My first suggestion would be to make your control room the section of the room that doesn't connect to the rest of the house (if this is possible). This connection (the hallway) could prove to be troublesome in that it will create some interesting delays...

I am all for the idea of a divider between the two rooms, not only for the acoustics but also for the sense of separating your bedroom from your workroom (a lot more important that some people realise!).

But perhaps instead of a simple curtain, hang some acoustic foam - parting in the middle like a curtain - on a sliding rail (those that you see on patio and sliding wardrobe doors for example) We used this concept to design the doors of a sliding cupboard in our drum room. It worked very effectively. It meant however that we had to make a frame for the foam to set them in the tracks of the sliders. Perhaps you're looking for a less permanent option...?. Perhaps you could utilise that arch that divides the room? eg, if its wood, drill and screws will be a very useful ally.

Separating the hallway, if you have to, could also be done by constructing a large gobo that swings in/out like a door.

Other than that, I have no real idea (the above advice is just from experience, I am no acoustic engineer).

However, bearing all this in mind, as long as you know what the sound is doing in the room (rev. times, room modes, reflections, etc), you can be conscious of it while mixing. An outboard graphic eq can be handy with problematic room modes, and could be cheaper than proper sound treatment.

Another tip to dealing with a problematic room, mix on a nice pair of headphones as well!!

Hope this has been some help to you, Tillman.

An outboard graphic eq can be handy with problematic room modes, and could be cheaper than proper sound treatment.

Sorry, but that's a myth. You cannot fix modal issues with a graphic equalizer. Modes are time-domain problems, and graphic equalizers are frequency-domain tools. Trying to use EQ to fix modes is like trying to use a screwdriver to screw in nails: can't be done.

This is simple to understand: room modes are standing waves that "store" energy at a certain frequency related to one or more dimensions of the room. After the tone that excited the mode stops, the energy carries on bouncing around the room a few more times, following the standing wave path, until it eventually dies away. So think about this: the speaker has stopped moving! The cone is dead still, yet the mode is still active. So the equalizer can do nothing at all to fix that! EQ can only do things to sound that is coming out of the speaker, but when the speaker is silent, it is useless. So you have sound bouncing around the room still at a certain frequency, and you have a useless tool that cannot help.

"But..." some people say "... you can fix that by reducing the level of the problem frequency, so that mode is not as loud". Yes, you can do that, but think about what you just did: you screwed up the frequency response curve of your speakers! They are no longer producing flat response. Instead, you have a dip at the frequency you just tweaked. Even worse, modes occur at exact frequencies, while graphic equalizers can only adjust 1/3 of an octave at a time, so you have now affected not only the problem frequency, but also a whole bunch of other frequencies around it. That's like using a shotgun to kill a fly.

Yeah, I know all about the wonderful software applications that you you can buy/download/borrow to "fix" your room, but as you can see from the above, they do not work, and cannot work, because they are frequency domain tools trying to deal with a time domain problem. At best, they can disguise the problem for one specific location in the room, but move your head a bit to the left or right, forwards or backwards, and you are no longer on that "perfect" spot (which wasn't even perfect anyway).

And even though some people still argue that you can use EQ to reduce a modal peak, they are only looking at half of the issue: how can EQ fix a modal NULL? By definition, a null is a point where the standing wave cancels itself: there is nothing there. So turning up that frequency is useless: you are adding more power to a black hole! The null just gets deeper, with respect to the rest of the standing wave. You can pour as much power as you want into it, all you will succeed in doing it making it even more obvious. It is impossible to "fill up" a null by making adding more power into it with a graphic equalizer.

So no, EQ cannot be used as a substitute for acoustic treatment of modal issues in a room. The ONLY solution for modes is treatment. (And even that does not make the modes go away: it just damps them a bit.)


- Stuart -

Anyway, back to the original post, and the original questions:

Is it completely ridiculous to expect SOME amount of transmission loss from a heavy curtain dropping from ceiling to floor?

You will get some loss, yes, but not enough to be useful. Isolation depends mostly on mass, and unless your curtain is extremely massive (many kilograms per square meter) you won't have any useful effect on the level. Plus, as you also mentioned, you need an air-tight seal to get decent isolation, and of course curtains are everything except air tight. Curtains can be useful as part of your treatment plan, but have very little use in isolation.

My understanding is that it would indeed absorb some of the mids ans highs, but would do nothing to stop the low mid and bass speech components (essentially vowels) from passing through. So are curtains COMPLETELY useless when it comes to isolation?

Welllll..... a qualified "maybe".

Let's look at some real data, to get a better handle on this.

If your curtains were airtight (not porous) happen to weight around 5 kg /m2, then that would get you roughly 5 dB of TL at 60 Hz: Mass law says that this will rise by roughly 6 dB per octave, so you'd be getting 9 dB of attenuation at 125 Hz, 13 and 250, etc. up to about 39 at 8 kHz. Normal speech peaks around 2kHz, and you'd have roughly 28 dB of TL there. That isn't much at all, and normal speech would be quite audible on the other side of your curtain: somewhat muffled, but still audible.

And the above assumes that your curtains are airtight, such as being made from heavy plastic sheeting that is sealed to the walls, floor and ceiling, so the real isolation figures would be much lower, since normal curtains are not like that at all.

The room in question connects directly to a hallway 4' in width and then to a space of rougly 27' x 10.5' x 9' . Since the connection between these spaces is acoustically direct (air-loose) and that the partition walls are rather flimsy (oldschool plaster walls on woodstud framing), shouldn't I be doing the math taking into account the other room and the corridor aswell? What about the kitchen, at the other end of the corridor, it is connected also... oh my god! and the bathroom, and the cupboards and the other room... Aaaaargh!

I think you are concerned about MSM resonance and the triple-leaf and quadruple-leaf effects? So you are asking "when is a triple leaf no longer a problem?".

If so, then don't sweat it too much. The resonant frequency of multi-leaf walls depends greatly on the depth of the air gaps. If your hallway is 4 feet wide, that's a large distance for MSM, so the frequency will be quite low, even for light-weight walls. For example, if the walls are just very thin 10mm drywall, then F0 for a 4 foot wide air gap (your hallway) is down at about 18 Hz, and good isolation starts at about 36 Hz.

Where does this madness ends? - Will modal issues be less significant due to the acoustical connection between the rooms,

Not really. Modes are related to the hard boundaries of the room itself, so as long as the walls are reasonably rigid, massive and solid, then you don't need to worry about other rooms affecting modal behavior. (Of course, if your "walls" are curtains, then the above does not apply ... )

Or, considering my limited budget and lack of proper isolation, should I just treat my first reflection points, put up superchunks in the corners behind my speakers, install a few gobos behind me and leave it at that?

Depends on what you want to accomplish! If you want isolation, then what you describe will have a sum total of zero effect on isolation. If you want to improve room acoustics without caring about isolation, then that certainly is an option.


- Stuart -