John Sayers' Recording Studio Design Forum Archive

G'Day,
I'm sorry for the length of this post. It gets a bit technical.
If the answers are already in the forum, I'm sorry for asking again, and please redirect me.

My build is in progress (frames up, some materials purchased) and I've found what seems to me to be conflicting information in the forums on what to do with my air exchange.

Purpose of the room:
The objective of my room is to keep the loud musos inside from annoying my neighbours (...through noise transmission that is. Once outside the room I'm sure my muso mates can annoy my neighbours in outer ways). My goal is to try to get an unamplified snare hit and kick drum kick inside (~100db?) reduced to approximately the loudness of the outdoor unit of the split system air con (~40-55db) when standing a couple of metres away from the outer wall of my room.

I want the room specifically to allow a band matching volume to around acoustic drum kit levels. I have maximised the size of the room for the space available. I'm happy to treat the room internally once built. I don’t intend to use it for tracking anything more than capturing song writing ideas. I'd like it to be reasonably pleasant to be in. But I'll work with what I can once it’s up.

Construction type:
It’s a double leaf on separate stud walls on a concrete slab in a large tin shed. 250mm air gap (insulated) with two layers of 16mm gyproc and Quiet Glue Pro (can't get GG here anymore) between gyproc, on inner and outer skins. Ceiling and roof are separated similarly with MDF flooring used on roof. No windows, back to back doors. Split system for cooling, fan with DIY silencers for air exchange (fan not yet purchased).

Current Problem - air exchange silencer design:
The finished inner room will be just under 50m3 and I have the opportunity to include the air exchange silencers within the ceiling space between the inner and outer leaves. (this will increase storage space over the room which I would like, but don't need. I'd rather optimise the silencer design than prioritise storage space). It’s the design of these that is giving me serious Forrest Whitaker eye. Key details:
Room volume: 50m3
Desired air exchange rate: 6 times per hour = 300m3/hr = 5m3/min = 0.083m3/sec
Desired speed of air at register: 1m/s (or less)
Using a duct calculator with these figures I get:
Round duct required = 326mm diameter
or Rectangular duct required = 289x289mm (if it doesn’t hurt, lets round this up to 300x300mm or 12x12 inch)
Single Fan to be located on the supply side only to provide positive room pressure (helps with door seal).

Silencers:
I would like to not take up any space in the room with silencers if I can avoid it. So I plan on putting them within the ceiling space and on the top outer layer as well if needed. I have about 1m clearance available up to the sheds' roof.

I think I understand the concepts behind silencer design and I've looked into the physics of it, but I'd really appreciate some guidance from you guys with a lot of experience in the real world before I commit to building them. Please let me know your thoughts on this.

As I understand it, the objective of the silencer design is to stop transmission of sound through the open air path of the silencer system for incoming fresh and same in the outgoing exhaust air silencer. Constructed in a manner that resists it allowing sound transmission through its sides from external and internal sound sources e.g. equivalent large mass construction as of the walls of the studio: double sheet with GG and resiliently mounted etc. For the purposes of this silencer design discussion, let’s assume that the silencer construction materials and method is sufficient to ignore sound transmission problems through its sides. I am only really concerned with the design for controlling the musicians sounds passing through the open air path of the silencer. Not outside noises or the fan noise either.

Duct lining and baffle box turns:
The duct lining and baffle turns are to reduce transmission of sound through absorption (mostly in the upper and mid frequency ranges). The duct lining absorbs some sound waves and the baffles create an inline obstacle for the sound waves that air moves past more easily. Therefore it seems that an increase in length of the baffle boxes and the number of turns would allow more absorption to occur (at the expense of air resistance seen by the fan).
Q1) When designing our baffle boxes should we aim at making them as long as possible with as many turns as possible (while maintaining cross sectional area along the air path within the whole baffle box)? Or is there an optimum number of turns and length we should go for? Is more better? If so, I expect that at some point the specs of the fan needed to overcome the air resistance will limit bang for buck in baffle box maximum size. What should we aim for in a design that’s not restricted by size?


Create points of acoustic impedance change:
Doubling cross sectional area; A common guidance in these forums is to change the cross sectional area of the path of the air by doubling (or greater) the cross section e.g. change from 6 inch duct to 12 inch duct. This will change the acoustic impedance of the air at that point. From what I can gather this has the effect similar to that described by a sound wave in an open pipe which causes reflections back down the pipe resulting in a reduction of amplitude of the sound wave passing through the pipe (See diagram in attachment 1 from link below)
http://www.animations.physics.unsw.edu. ... edance.htm This acoustic impedance is frequency dependent. It seems is can be designed to have a specific an effect on low frequencies which are not well addressed in absorption of duct lining. (is this all correct so far?)

Q2) if this cross-section-change sizing affects the impedance frequencies, then provided the air exchange requirements are met, there would also be an optimum size to make these ducts. Conversely I expect there would also be a very poor performing size that has little or no effect on low frequencies. Can anyone on here give guidance as to what to aim for and what to avoid for sizing these regarding frequencies and not airflow?

Single baffle box design:
It can be seen in many baffle box designs in this forum where the sound input opening (hole facing the musician tank) is half the cross section as the internal dimensions and sound exit hole of the baffle boxes. I've also seen designs in this forum where the same baffle box design is connected the other way around with the large hole, the same cross section as the box internals, is facing the musicians and the small hole is the duct to fresh or exhaust air.

Q3) If we are only concerned with reducing the musicians sounds from escaping (not external noise getting in to the room), which way should the baffle box face; Big hole to the musicians and small to the duct, or small hole to the musician and big hole to the duct?
If its big hole to the musicians this doesn't create a doubling of cross section, and is the opposite of the pipe reflection theory. (not trying to be argumentative, just trying to see the physics behind the reasoning).

Q4) Reducing the cross sectional area of a duct increases the air velocity. Does this also create a point of acoustic impedance? I can't find any documentation on this but it is intuitive that it would (from an electrical analogy of transmission line theory).

Inner Baffle box -> duct -> Outer baffle box design:
A lot of advice in the forums for additional isolation is to have an inner and outer baffle box design linked by a resilient duct for the silencer. one silencer for each of the fresh and exhaust points.
In this design I've seen that it is often recommended that the duct be half the cross section as the baffle boxes.
Q5) This seems to orient the baffle boxes with the large hole of the inner baffle towards the musician tank, and the large hole of the outer baffle towards the outside world. Is this a desirable design?

At the outer end of the joining duct, where the cross section doubles to that of the outer baffle box, this is a point of acoustic impedance due to the doubling of cross section.
Q6) Is the entire inner (1st) baffle box and duct to be considered as an open pipe at this point?

Q7) with a silencer design of Inner Baffle Box -> duct -> Outer Baffle Box, is there now an optimum length, number of turns, cross sections etc. to achieve optimum isolation vs fan size?
I have the space above my room in my shed to design very large silencers if this is a good idea. But if it’s not a good idea, I'd rather not waste the materials and potential storage space. What is worth doing knowing my double doors are most likely going to be the next weakest link?

Joining Duct:
In the same weblink as above, under acoustic inertance and compliance, it seems to me that the advice in the forums to reduce the cross sectional area when passing from an inner wall baffle box to an outer wall baffle box via a reduced cross section duct is using the idea of a compact region for assessment of this point of acoustic impedance. The compact regions effect on acoustic impedance is proportional to the volume of the compact region. In that case, not just the cross sectional area of the duct is relevant, but also the length of the duct in relation to the wavelength. These have an effect on the frequency dependence of the impedance. (wavelength is a function of volume).
Q8) Is the compact region worth considering and if the cross section and length of the duct are relevant, what would be the aim of this duct design regarding restricting frequencies? What would you recommend as a design of this duct for length and area?

Q9) Is a design where the joining duct is greater cross section than the baffle boxes desirable? E.g. 12 inch baffle boxes joined by 18 inch duct. Or as in thread from Quint http://www.johnlsayers.com/phpBB2/viewt ... 0&start=30 is it desirable to use an 8 inch duct connecting two 6 inch cross section baffles versus a 4 inch duct?

Registers, diffusers, grills etc. (let’s call them registers in this case):
In the forums I've read that to slow the air flow down to the desired speed to reduce airflow noise in the room, an increase in cross sectional area right before the register can achieve this. I've seen examples of people building timber cones for registers at both the fresh and exhaust outlets to the room where the cone widens from the baffle to the register.
Example From post http://johnlsayers.com/phpBB2/viewtopic ... &start=165
Q10) If there should be a register on both the fresh and exhaust air openings, then can the noise heard by the exhaust inlet being reduced in cross section (air speeding up just after the exhaust register) be heard? What’s best practice in designing the duct with change in cross section from baffle to register? Is it the same for in and out-going air points?

Fans:
Q11) is the supply side currently the best practice for the fan location? (Positive room pressure)
Q12) when choosing a fan, assuming it has the correct capacity, what is a good diameter fan to aim for? Larger is usually quieter (observed from spec sheets) but is 200mm a good target and 400mm overkill? 630mm seems way over the top when I need a register size of about 300mm for my desired velocity.

Gradual vs abrupt transition in duct cross sectional area reduction:/
I had a look in the ASHRAE-HANDBOOK-Sound-and-Vibration-Control.pdf
On page 29 there is a section on fixed duct fittings. There is a very nifty graph (see 2nd attachment) showing the effects on octave band power attenuation and the difference between gradual transitions and abrupt transitions when reducing the duct size from 24x24inch to 12x36inch. It shows that gradual transition is greatly improved over the performance of abrupt transitions for frequencies over 125hz.
Q) has anyone had real world experience on using gradual transitions say in a silencer? Any advice on when making joining ducts between inner and outer baffle boxes if a gradual transition duct would be preferred? Would it be worth the effort?

I thank in advance anyone who has read this far. Hopefully I'm not the only person wondering these things.

Cheers,
Steve

Or is there an optimum number of turns and length we should go for?

I seldom do more than about 5 or 6 baffles, or the boxes get ummanageably large.

What should we aim for in a design that’s not restricted by size?

Get the interior cross section as large as you can, at all points. That makes the box even longer...

(is this all correct so far?)

Yes

then provided the air exchange requirements are met, there would also be an optimum size to make these ducts.

Duct size is normally set by a combination of: Maximum allowable air flow velocity - available space - need to change cross section as much as possible. If you have very large ducts, then yest you get slow air flow which is great, but it also means you need huge silencer boxes in order to double the area... etc.

Q3) If we are only concerned with reducing the musicians sounds from escaping (not external noise getting in to the room),

Why would there be any difference between the two?

Q4) Reducing the cross sectional area of a duct increases the air velocity. Does this also create a point of acoustic impedance?

Yes, changing the cross sectional area does in deed create an impedance mismatch.

Q5) This seems to orient the baffle boxes with the large hole of the inner baffle towards the musician tank, and the large hole of the outer baffle towards the outside world. Is this a desirable design?

Yes, in general, that is a desirable design. I often end up with silencer boxes where the cross sectional area changes two, three, or even four times. So it doubles where the duct comes into the box, then doubles again due to a 2:2 split, then it doubles a third time just before the final exist path into the room, and then there's the final massive change in area at the register itself, where it interfaces with the actual room.

Q6) Is the entire inner (1st) baffle box and duct to be considered as an open pipe at this point?

No. It is not an open pipe. It exhibits impedance changes that are similar, and based on the same principle, but it isn't an open pipe.

Q7) with a silencer design of Inner Baffle Box -> duct -> Outer Baffle Box, is there now an optimum length, number of turns, cross sections etc. to achieve optimum isolation vs fan size?

Fan size selection depends on three things, basically: the total volume of air flow that you need to move (CFM), the velocity that you need to move it at (FPM), and the static pressure of the complete duct/room/box system. IT is no related ot the length, number of turns, or cross section, except insofar as those have an impact on the static pressure.

Q9) Is a design where the joining duct is greater cross section than the baffle boxes desirable? E.g. 12 inch baffle boxes joined by 18 inch duct.

Well you COULD do that if you wanted to I suppose, but I sure do hope you have a LOT of space available! an 18" duct is pretty big...

is it desirable to use an 8 inch duct connecting two 6 inch cross section baffles versus a 4 inch duct?

I try to use the smallest duct that will do the job, without increasing the static pressure too much. Smaller ducts are cheaper, easier to handle, easier to install, and easier to interface to the silencer boxes. Why wast money using a duct that is large than the box, when there's no real benefit to that?

Q10) If there should be a register on both the fresh and exhaust air openings, then can the noise heard by the exhaust inlet being reduced in cross section (air speeding up just after the exhaust register) be heard?

Why would the reduction in speed be audible? The only time that airflow becomes audible at these slow speeds is if it is turbulent. As long as your register does not create undue turbulence, then there's no reason why the change in cross section would cause a problem.

Q11) is the supply side currently the best practice for the fan location? (Positive room pressure)

Not for me. I prefer to have the fan on the "downstream" end: the point where the exhaust duct meets the real-world, outside the studio. Some people prefer to do it the other way.

Q12) when choosing a fan, assuming it has the correct capacity, what is a good diameter fan to aim for?

Going for one that matches the size of the duct seems like a good idea!

On page 29 there is a section on fixed duct fittings. There is a very nifty graph (see 2nd attachment) showing the effects on octave band power attenuation and the difference between gradual transitions and abrupt transitions when reducing the duct size from 24x24inch to 12x36inch. It shows that gradual transition is greatly improved over the performance of abrupt transitions for frequencies over 125hz.

Take another look at the title of that graph! It says "Velocity-generated sound of duct transitions". It's not what you think it is at all! It's about how much sound will be GENERATED by sudden velocity changes vs. gradual transitions. As you'd expect, a sudden change generates more turbulence, and therefore generates more air-noise. But look at the speeds they are talking about there: around 2000 to 4000 FPM! That's fast-moving air. 4000 FPM is over 45 MPH (70 kM/hr). Stick your head out the car window next time you are driving 45MPH (70 kM/Hr), and see how noisy the air flow is... Those speeds are much, much higher than the speeds that we need for studios.

But anyway, that graph does not show what you think it shows. It does not show how sound is attenuated due to changes in duct section. In fact, it shows the opposite: how sound is generated by fast moving air at duct section transitions.

Q) has anyone had real world experience on using gradual transitions say in a silencer?

Not personally, no, as I would never do that if I could avoid it. If you don't have a sudden transition, then you don't get the sudden change in impedance that you need. The only time I use gradual transitions in ducts, is at take-of points, to keep the speed and pressure constant. But not inside a silencer box. Or I MIGHT do that if I needed to reduce turbulence for some reason, and that was a bigger problem then having a good impedance mismatch. But I would not do that for purposes of improving TL in silencer boxes.


- Stuart -