Attenuation - historic building with outside facing windows

[Heist]

I've recently purchased an old bank that was built in 1912 which I plan to use primarily as a recording studio.

My biggest challenge is the main live room which features two large windows and two small windows. The largest is a seven piece arch window with dimensions of 3.6m wide at the base x 3.02m high at the top centre point of the arch.

The room dimensions are 7.93m x 6.13m with a 4.6m ceiling height.

The building is red brick construction and has an external wall thickness of 369mm. I'm confident that it's a cavity wall construction, meaning 330mm brick, with plastered walls internally, leaving approximately 35-40mm for the cavity.

The room also has a large external double wooden door 1.82m wide x 2.2m high.

The room sounds fantastic and is very easy to tighten up by adding movable gobos for dampening.

I understand that I can only attenuate as well as my weakest point and therefore I'm seeking advice regarding the appropriate glass thickness preferably laminated to attenuate external noise to same extent as is achieved by the brick wall.

I fear I may be restricted with regard to double glazing as the large arch mahogany window frame if very much structural built into the keystone arch design of the window. This frame has a depth of 68mm of which replacement glass could be fitted and therefore probably not enough room to allow for sufficient air gap between two panes of glass.

The third and final challenge is vents at each corner of the room close to the ceiling. I plan to close two of these off and use the remaining two as inlet and outlet vents to circulate fresh air into the room via an externally mounted cooling system.

I have attached photos to give you some idea of the room, it's unique attractive features but also its challenges.

Could someone please review the information above and advise on the appropriate window glazing options?

The door and ventilation matters don't appear to be unique to this project, so unless there is something glaringly obvious that I need to be aware of, I'm sure I can find those answers elsewhere on this forum.

Kind regards,

Damien

[Soundman2020]

Hi three Damien, and Welcome!

I'm not sure how I managed to miss your initial post for so long, and I'm hoping the reply isn't too late... sorry about that!

That's a rather nice stained-glass window. More reminiscent of a church than a bank.

First question: Can you take it out. Is there any restriction on removing that window and replacing it with something else? For example, if that is a historical or heritage listed site, or local regulations about changing the "look" when renovating, etc. If you are allowed to take it out, then your best bet would be get two thick panes of laminated glass custom made to fit that shape, and separate them with a large air gap. Expensive, but effective.

The building is red brick construction and has an external wall thickness of 369mm. I'm confident that it's a cavity wall construction, meaning 330mm brick, with plastered walls internally, leaving approximately 35-40mm for the cavity.

It's might be what is called "brick veneer" in Australia: external brick facade, with internal facing done as drywall on framing. When you thump on that inside wall with your hand, does it sound solid like brick, or hollow?

I understand that I can only attenuate as well as my weakest point and therefore I'm seeking advice regarding the appropriate glass thickness preferably laminated to attenuate external noise to same extent as is achieved by the brick wall.

How much isolation do you need? In other words, what is your target transmission loss, in decibels?

This frame has a depth of 68mm of which replacement glass could be fitted and therefore probably not enough room to allow for sufficient air gap between two panes of glass.

You might need to build a new, deeper window frame then... You could probably figure out a way to keep the mahogany trim, if you work carefully.

The third and final challenge is vents at each corner of the room close to the ceiling. I plan to close two of these off and use the remaining two as inlet and outlet vents to circulate fresh air into the room via an externally mounted cooling system.

Fine, but you'll need to put silencer boxes on them to provide the level of acoustic isolation that you need, leading to ducts that lead to a fan... and all of that will need to be dimensioned correctly to provide the correct volume of air (flow rate) for that room at the correct speed (flow velocity). You'll also need to figure out how to provide the correct amount of cooling and humidity control: Maybe a small mini-split system?

The door and ventilation matters don't appear to be unique to this project, so unless there is something glaringly obvious that I need to be aware of, I'm sure I can find those answers elsewhere on this forum

I'm sure you can! That's what the forum is here for!

That said, the big issue with HVAC is doing the calculations right to get your flow rates and speeds correct at the registers, and that implies sizing the ducts and silencer boxes correctly too.

- Stuart -

[Heist]

Hi Stuart,

I've been so crazy busy with my first child on the way (due in 3 weeks), and completing the renovation on my old home which I needed to sell to finance the studio fit-out, that I completely overlooked your extremely considerate and comprehensive reply.

To answer your questions.

1. Can you take it out. Is there any restriction on removing that window and replacing it with something else?

No, as long as I am sensible. I have met with the local planning authority and I am somewhat fortunate that previous modifications have been done poorly to the external frame. They are satisfied if I change the glass thickness and only use timber frames and mouldings. The window frame mouldings were done very poorly and therefore there is some weather damage that requires replacement and restoration.

2. When you thump on that inside wall with your hand, does it sound solid like brick, or hollow?

It's solid as the plaster render only comes down to part way under the skirting boards. I removed the skirting boards when I painted and it was solid brick on the inside wall behind the plaster.

3. How much isolation do you need? In other words, what is your target transmission loss, in decibels?

This is the advice I really need. As I want the windows to attenuate as well as the wall structure (no more, no less). Given the wall construction and thickness, I'd like to work out what two laminated glass thicknesses would be ideal for the glazing and the appropriate depth of the large air gap between them. My local glazier has quoted me for 8.5mm and 6.5mm laminated glass, with an air gap of 90cm but I am not sure if the thickness of the glass or the air gap suggested with attenuate sufficiently for my needs.

4. You might need to build a new, deeper window frame

I expect this will be the case.

I apologise for not responding sooner. I can't thank you enough for your time so far.

Damien

[Soundman2020]

I've been so crazy busy with my first child on the way (due in 3 weeks),

!! Congratulations on that! I'm sure you've hear this a lot, but that's a life changer, for sure!

I have met with the local planning authority and I am somewhat fortunate that previous modifications have been done poorly to the external frame. They are satisfied if I change the glass thickness and only use timber frames and mouldings.

Excellent! That's good news.

I removed the skirting boards when I painted and it was solid brick on the inside wall behind the plaster.

That's also very good news. You can potentially get rather good isolation. The weak points will be the windows (already identified), the doors, and possible the ceiling.

3. How much isolation do you need? In other words, what is your target transmission loss, in decibels?

This is the advice I really need. As I want the windows to attenuate as well as the wall structure (no more, no less).

Here's the thing: you really need to figure out a number! It's very different to tell you how to isolate your place for 40 decibels vs. 60 decibels. That's "only" 20 dB difference, at first glance, but since the decibel scale is logarithmic, that difference of 20 decibels actually means that you'd need to block one hundred times more sound.... And if it were a 30 dB difference (say, 70 dB instead of 40 dB), then that would mean blocking a thousand times more sound. Each time you go up ten points on the scale, that's ten times more sound to be blocked. As you can imagine, it gets much harder to attain higher levels of isolation, and also much more expensive.

So it's important that you decide on what level of isolation you want. It's no use me telling you how to isolate for 70 dB if you only need to isolate for 30 dB, as you'd spend huge amounts of money on extreme overkill to do that.... and on the other hand, it's no use me telling you how to isolate for 30 dB if what you really need is 60 dB, as you'd be very unhappy with the result.

There's two things you need to know here: number one is "how loud are you?", and number two is "How quiet do you need to be?". Both of those expressed in decibels. The second one is probably easy to find out: check with your local municipality to find out what the legal limit is: how many decibels are you allowed to produce before the cops show up on your doorstep with a piece of paper that you won't want to read... If there is no fixed number in the local bylaws, then you can measure the real ambient level outdoors at the quietest time of day, with a proper hand-held sound level meter. So for example if you measured 40 dBC at 3:00 AM, then that's the "how quiet do you need to be" number.

The first number is something you can measure with the sound level meter. Just measure a typical session that you think you'll be having inside your studio. If you can't set up such a session, then go find one! There must be some place in town where someone is doing what you want to do, so take your meter along and measure them, close-up.

For both of the above, set your meter to "C" weighting and "slow" response.

With those numbers in hand, just subtract the "quiet" number from the "loud" number, and... Bingo! You have your answer. That's how much isolation you need.

Given the wall construction and thickness, I'd like to work out what two laminated glass thicknesses would be ideal for the glazing and the appropriate depth of the large air gap between them. My local glazier has quoted me for 8.5mm and 6.5mm laminated glass, with an air gap of 90cm but I am not sure if the thickness of the glass or the air gap suggested with attenuate sufficiently for my needs.

Your glass will need to be thicker than that. You will need laminated glass, which is a "sandwich" of two layers of glass plus a PVB "interlayer". You will need two panes made like that, with the air gap between.

In order to get the same isolation as your brick walls, the glass would have to be VERY thick, and the air gap would need to be huge! But you probably don't need that: you only need to make your window as good as it needs to be to produce the level of isolation that you need. In other words, the window doesn't need to be as good as the wall: it only needs to be good enough to do the job.

It works like this: The two panes of glass and the air gap act as a tuned system. That system is tuned to a specific resonant frequency, which is very low. Any tone that happens to be at the resonant frequency of that system will not be isolated at all, but tones at twice that frequency (one octave up) and higher will be isolated. The higher the frequency, the better the isolation. Therefore, you need to tune that system such that the resonant frequency is one octave lower than the lowest note you need to isolate. The way you tune it is very simple: it depends on only two things: The surface density of each pane of glass (mass), and the size of the air gap in between them. The higher the mass, the lower the frequency. The larger the air gap, the lower the frequency. So you need lots of mass and a big air gap. If the glass is thin, it won't have much mass, the frequency will be too high, and you won't get the isolation you want. If the air gap is small, then ditto. There's a set of equations that is used to figure this out, or you can look it up in tables, or on graphs.

So all that brings us back to the same point: Once you know the number of decibels of isolation that you need to aim for, then I can plug in that number to the equations and come up with some predictions for different scenarios of glass thickness and air depth, that will give you the isolation you need.

I apologise for not responding sooner. I can't thank you enough for your time so far.

- Stuart -

[Heist]

Thanks again Stuart,

I understand what you need to know.

1. With those numbers in hand, just subtract the "quiet" number from the "loud" number, and... Bingo! You have your answer. That's how much isolation you need.

Under noise regulations, the entertainment “music” noise must not exceed 45dB inside a neighbouring habitable room.

Indoor music venues are also allowed +5dB above the ambient external noise levels in the day/evening and +8dB at night.

The external ambient noise level in the middle of the night is around 40dB (+8dB = 48dB).

To be on the safe side, I’d say the how quiet I need to be number is 45dB.

I will record everything from acoustic performances to rock bands, so my estimate of how loud I will be could be around 110dB at the extreme end.

110dB – 45dB = 65dB.

Of course the other challenge will be external noises entering my studio. As the studio is in a commercial precinct on a main street, I am conscious of noises such as a passing truck interfering with my recording.

I expect that gain on my mic preamps will amplify all noises entering my studio, so with 65dB of attenuation and the noise of a truck outside generating around 85dB, that my mics will amplify the remaining 20dB that enters my room.

Perhaps this second issue is not quite as extreme as I anticipate, as apart from room mics – much of this could be controlled with mic placement.
Since I’m obviously already talking about some serious attenuation, are you able to calculate the laminated glass thicknesses and air gaps required to attenuate by couple of scenarios, say 55dB, 65dB and 75dB if I were to go all-in?

I expect that I can’t go beyond a 20cm air gap without the job becoming too difficult to achieve. No doubt this limitation would be a factor in determining the appropriate glass thicknesses.

2. Your glass will need to be thicker than that. You will need laminated glass, which is a "sandwich" of two layers of glass plus a PVB "interlayer". You will need two panes made like that, with the air gap between.

Yes, I expected as much. I had asked for a quote for much thicker glass but they only came back to me with prices for 8.5mm and 6.5mm. I may need to find another supplier, as the job may be beyond them.

Don’t hesitate to let me know if you require any additional info.

Kind regards,

Damien

[Heist]

Hi Stuart,

Me again - the baby is now four months old and you're right, it has indeed been a life changer! He's doing very well and I'm even getting some sleep now.

I've finally returned to the studio build and have all the other ducks in a row but I just don't want to make a silly mistake with the window system.

I also apologise for asking what would be required for too many scenarios with the previous post. My best estimate of the how quiet I need to be number is 45dB, which to get to will require 65db of isolation.

Given I will record everything from acoustic performances to rock bands, my estimate of how loud I will be - could be around 110dB at the extreme end (I have a large underground cellar for recording guitar amps so I should be able to isolate those pretty well - with drums being the single loudest instrument in my live room that needs to be isolated from the outside world).

110dB – 45dB = 65dB.

I should be able to achieve an airgap of 20cm between the panes of glass, however, if it can be done with a smaller gap that would be ideal.

Thanks for you excellent and thorough explanation about the challenges and what needs to be factored into the equation. It's really great that there's such a great forum with input from people with expertise that I clearly don't have! I will certainly share photos and information here about what I've learnt about the studio build and fit-out once I get it underway (Have included a pic of the control room as it is now).

Kind regards,

Damien