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New ideas in acoustic treatment

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New ideas in acoustic treatment

Postby Watchmaker » Sun Mar 10, 2019 1:31 pm

I'll just put this here in case anyone's curious about "advances" in materials sciences. Curious to hear any thoughts on this. I'm not convinced it has practical studio implications other than making quiet ducts much easier to build.

https://phys.org/news/2019-03-acoustic-metamaterial-cancels.html
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Re: New ideas in acoustic treatment

Postby blinddrew » Sun Mar 10, 2019 1:39 pm

Interesting, will be curious as to how it scales up in practice.
I'm thinking air-movement systems primarily but potentially also as porous room dividers.
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Re: New ideas in acoustic treatment

Postby ef37a » Sun Mar 10, 2019 1:44 pm

Maybe a silent but still zippy computer fan?

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Re: New ideas in acoustic treatment

Postby Martin Walker » Sun Mar 10, 2019 1:55 pm

The associated video IS impressive! :clap:

https://www.youtube.com/watch?v=Fd1D42dVxS0


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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 2:38 pm

Not sure how technical the reporter's background is: "Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note. If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away" (my italics). :headbang:
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Re: New ideas in acoustic treatment

Postby Martin Walker » Sun Mar 10, 2019 2:44 pm

Wonks wrote:Not sure how technical the reporter's background is: "Standing in the room, based on your sense of hearing alone, you'd never know that the loudspeaker was blasting an irritatingly high-pitched note. If, however, you peered into the PVC pipe, you would see the loudspeaker's subwoofers thrumming away" (my italics). :headbang:

Yep, that made my eyebrows go up and down as well ;)

I suspect that sentence may have been written by some marketing bloke who didn't really understand the mechanics of loudspeaker reproduction :beamup:


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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 3:25 pm

Of course, one could postulate that speaker was producing a frequency matching the resonant frequency of the open tube, and that the bung interfered with that resonance.

I'm a born sceptic. A demo using a noise source would have been far more impressive and practical - but maybe it would have shown up significant weaknesses. Does it only work on one frequency? Quite a few of the 'noise cancelling' products I've seen

Also, there was no indication that there was any airflow through the duct - so once you use the duct as it was supposed to be used, what happens then? How much noise does the device itself generate (undoubtedly more broadband than the test noise)? Does moving air carry noise through with it?

There are already many good ways to reduce noise in HVAC systems. One of which is to correctly balance the air supplies - which we do (generally fairly well) in the UK but is only just starting to catch on in the US. There are also very effective in-line silencers.

Most HVAC noise is generated on ductwork bends or on terminal devices/outlet grills where turbulence is created, so it's not simply a case of fitting one device by a fan and all your problems are solved.

Also, that ring was significantly smaller than the circular duct. With airflow, that means both increased resistance and creation of turbulence/noise. As you'd have to have many of these added to a standard system, that would either mean you need increase duct sizes to reduce air velocities and overall pressure loss (both far more expensive and often not practical or physically possible) or else use a bigger fan (which uses a lot more energy and create more noise in the process!).

There is also the fact that in offices, a reasonable level of background noise is required to help mask conversations by others. If offices are too quiet, it becomes hard for people to concentrate and so white noise generators get fitted or a low level of noise is mixed in to any PA or voice alarm system fitted.

Obviously recording studios and performance halls benefit from low noise, but there are existing ways to deal with those ( I've worked on the HVAC control systems of quite a few performing venues, so I know how quiet they can be made).

There may be some applications where it could possibly be used (maybe alongside roads to reduce noise), but it would still depend hugely on cost. There's huge resistance to putting up conventional noise-reducing fences alongside motorways (the M4 near us is getting two hundred metres of fence but it really needs to be a few km long to benefit us).

Reading the linked article, it looks like it's a fairly high-frequency noise cancelling system, with the emphasis on making it go higher, not lower - and we know that it's low frequency sound that causes most noise issues.

It's also currently made from plastic - something we obviously need to use a lot more of!

To me it does rather look like a proof-of-concept device where they're looking for funding for research to take it further, or else use it for a start-up proposal, but will probably take many years and a lot more development to see anything worthwhile. It also needs input from applications people working in the real world, not just research, if it is to have any practical applications at all.
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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Sun Mar 10, 2019 4:00 pm

It's a clever technology, but its sound transmission characteristics appear essentially to be an interference-based notch filter as shown in the graphs in the papers (see below). According to the paper, the prototype device reflects back frequencies between 1 and 2.5kHz.

acoustic reflection filter.jpg


So it's potentially useful in applications where the unwanted source noise is narrow-band and with stable pitch -- such as drone rotors or small constant-speed fans -- but of no general use in typical audio applications. It also appears that it would need to be insanely large to deal with the LF noise from typical HVAC systems.

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Re: New ideas in acoustic treatment

Postby Sam Spoons » Sun Mar 10, 2019 10:01 pm

Not sure how placing one beneath the rotors on a drone would affect lift generated by said rotors........ It certainly wouldn't be improved :headbang:
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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Sun Mar 10, 2019 10:15 pm

Lift is about airflow close around the prop, so provided it doesn't affect the local air flow too much it should be fine... And it's selling point is that it doesn't impede air flow, so the application would seem a reasonably practical one in theory.
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Re: New ideas in acoustic treatment

Postby Folderol » Sun Mar 10, 2019 10:53 pm

Don't think that would work. You'd need two per rotor, one above one below, and the rotor would have to be contained which would dramatically reduce the sideways maneuverability as you wouldn't be able to use the slip action with just a slight tilt.
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Re: New ideas in acoustic treatment

Postby Wonks » Sun Mar 10, 2019 11:24 pm

It will certainly impede air flow. Axial fans don't produce a huge amount of differential pressure, and even a small increase in the pressure loss across it can have a huge impact on the air volume passed by the fans.

And those rings will also add weight and sideways air resistance. Meaning you then need bigger fans and bigger batteries, which adds more weight, or you suffer greatly reduced range performance, and battery life. And I really don't know how directional the sound from a drone's prop is as to whether there would be any real benefit.
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Re: New ideas in acoustic treatment

Postby Watchmaker » Mon Mar 11, 2019 11:24 am

Wonks, you summed up my initial reaction and went well beyond it in scope. HVAC noise is pervasive here in the US and the few studios I've been to with adequate handling have spend small fortunes on duct work.

Thanks all for the discussion. I'm still working out how practical this feat of engineering will be. Maybe watching the subwoofer go back and forth will become more popular than watching the wash go round and round.
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Re: New ideas in acoustic treatment

Postby ef37a » Mon Mar 11, 2019 2:59 pm

Wonks, indulge an old bottle jockey a while?
Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?
Same principle but instead of hotness you have a big cold lump. I know of course that "cold" does not radiate but the lump could be positioned such that convection would carry cool air around or some very quiet internal fans?

I am sure you are ahead of my thinking that the cold could be stored with off peak juice and then no noisy AC running in studio time. I also understand that if "I" can think of this, others must have and either kept quiet or found it nonsense. Or of course, maybe thermo-dynamics just doesn't work that way?

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Re: New ideas in acoustic treatment

Postby Hugh Robjohns » Mon Mar 11, 2019 3:29 pm

ef37a wrote:Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?

Not perhaps called 'storage coolers' but they definitely exist. That's why so much marble is used on the floors and walls of buildings in hot countries, and why they traditionally have large vats of water in food storage cellars...

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Re: New ideas in acoustic treatment

Postby Wonks » Mon Mar 11, 2019 5:30 pm

ef37a wrote:Wonks, indulge an old bottle jockey a while?
Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?
Same principle but instead of hotness you have a big cold lump. I know of course that "cold" does not radiate but the lump could be positioned such that convection would carry cool air around or some very quiet internal fans?

I am sure you are ahead of my thinking that the cold could be stored with off peak juice and then no noisy AC running in studio time. I also understand that if "I" can think of this, others must have and either kept quiet or found it nonsense. Or of course, maybe thermo-dynamics just doesn't work that way?

Dave.

Part 1.

First, cold does 'radiate'. As I mentioned in another fairly recent thread, anything above absolute zero radiates energy. It's just that warmer things radiate more energy than colder things, so whilst there is an exchange of energy between hot and cold items, the hot items will always radiate more energy than the cold items (based on the 4th power of the absolute temperature). But if you are next to a cold window in a warm room, you can feel it is cold because the side of your body facing it is radiating warmth away to it, whilst receiving far less heat back from it than other areas of the warm room.

Many offices are now being built with chilled ceilings. Not massively chilled. but maybe down to 18°C. This does a mixture of cooling hot air by convection and also acting as a colder surface seen by your head, and so providing a measure of 'radiative' cooling.

The big problem with storing 'coolth' (as it's known in the HVAC industry) is that of allowable temperature difference. With hot water, you can happily heat it to 80°C, giving the best part of a 60°C temperature difference between it and internal building temperatures (a bit more in say a house where with storage heaters you may only be heating a bedroom to say 17°C).

With cold water, or any other cold substances, you run into the problem that the moment you expose them to air and the temperature of the cold water drops too low, you get condensation, which is generally something you try and avoid. So unless you can fit condensation trays and drains around any form of cold storage heater, you really need to keep the temperature above 10°C (unless you are actively monitoring room conditions and calculating the dew point of the air and keeping the cold setpoint a couple of degrees above the dewpoint temperature, as often happens with chilled ceilings as you don't want rain in the office).

You also really don't want condensation as you loose some of the stored coolth's 'energy' in turning water vapour into liquid ('latent' cooling). It's more efficient to not condense when cooling and keep all the cooling as 'sensible' cooling.

So if we say 10°C as the starting temperature of the cold item, you are left with a temperature difference of only 15°C, given a 25°C room temp. So whatever is used to store the cold needs to be at least 4 times the size of an equivalent heat storage system.

Obviously there are problems with putting large items into rooms that probably need to go against walls and can't have things against them and also weigh a considerable amount. Storage heaters themselves are much larger and bulkier than radiators, so imagine something 4x to 6x larger in each location.

But as the cold storage starts to warm up and the space cool down, a 5°C temperature gain has a much bigger effect on the ability of the cooling system to cool the space down compared to a 5°C loss in a storage heating system to warm the place up. Which really makes it necessary to make the cooling storage system even bulkier so that there's more effective cooling.

Which then leads us on to the method of actually cooling the storage medium down, which in realistic terms, means some sort of chiller system. Yes there are electronic cooling effects (like reverse Peltier effect) that could be used, but unless all your energy comes from renewable sources, you are still trying to minimise electrical power usage, and those methods are electrically inefficient.

Modern modern variable speed chillers with electromagnetic motor bearings are pretty efficient, with a COP (coefficient of performance - ratio of cooling energy out to electrical energy in), often in the region of 4:1 to 6:1. the chiller, like a big fridge, uses a motor and a compression and expansion system to take heat out from one medium (normally water) and transfer it to another (usually air).

There are various things that affect the efficiency of a chiller. Some are:

1. The colder the chilled water leaving temperature is, the less efficient the chiller is.

2. The condenser water/refrigerant gas circuit (taking the heat away from the cooled medium) needs to be kept above a certain temperature, but the hotter it gets above that value, the less efficient the chiller gets. So as the ambient temperature rises, you need to use more cooling fan power to keep those temperatures down.

3. The refrigerant type used in the chiller. Unfortunately the really efficient ones, R11 and R22, turned out to seriously destroy the ozone layer, so are now banned. There have been various blend refrigerants used which are quire efficient, but most of these turned out to have a high greenhouse gas value, so they are being phased out. I'm now a few years out of date, but ammonia was probably the main remaining contender for large chillers, but as a gas it's deadly in not too high a concentration. Chillers will leak refrigerant slowly over time, so good ventilation is essential for ammonia chillers, but if the condenser cooling system goes horribly wrong (and it can do) then gas pressures in the chiller can go beyond the safety point and the chiller equivalent of a safety valve - a 'bursting disk' should rupture and let the gas escape to atmosphere, not something you want to do with a large quantity of hot ammonia gas. So really not viable for many situations.

So you've got some benefit in having a chiller produce water at 8°C (to cool a medium down to 10°C) compared to a more normal 6°C.

But you would end up having to pipe a lot or water round in insulated pipes, running secondary pumping systems, which all becomes rather expensive for a not very efficient form of cooling system. Its a lot simpler to install DX (direct expansion) or VRV (variable refrigerant volume) system in small spaces and small offices, and full AC with maybe FCUs (fan coil units) on walls or in ceilings in larger offices. These can provide both heating and cooling functions.

There was a form of cooling storage used on main chilled water systems for buildings that was quite popular in the very late 80s and early 90s, called 'ice storage'. It came about when there were cheap off-peak electricity tariffs available. By storing coolth in the form of ice, and creating this at night when it was cheaper to run the chillers, the stored coolth could be used to 'peak lop' the cooling demand on the chillers during the day, meaning that the chillers themselves didn't have to be so large - capital and operational savings. The ice was stored within sealed plastic balls in a large storage tank.

There were several drawbacks:

1. To store a reasonable amount of coolth in the form of ice, you need to have a lot of storage capacity. The delta-T in the system is small (normally 0°C to 6°C) so you need to have a lot of balls. Yes, you benefit from the stored phase change energy from turning 0°C ice into 0°C water, which helps. But it still wasn't a compact means of storage. You needed a large well insulated tank, which will weigh many tonnes and you need a place to put it in, which normally meant a large basement plantroom. Which meant that you had to have a building that was big enough to have such a space to spare.

2. To create ice, you need a cooling medium that's below 0°C, ideally several degrees below if you want to create it within a reasonable time period (the night tariff didn't last for ever. 7 hours maximum IIRC, though sometimes it was less depending on the amount of electricity taken. Which means that you can't use standard water as the cooling medium or your chiller freezes. So you need to have a fairly high anti-freeze content in the cooling water. This reduces the specific heat capacity of the water, so for an air handling unit cooling coil to produce the same kW of cooling, it needs larger coils and a higher water flow rate. So pumps and pipework all go up a size or two.

3. The chiller setpoint need to be set down to at least -3°C/-4°C, instead of the typical 6°C. This makes the chiller work a lot harder, it's efficiency drops and whilst it uses cheap electricity, it's using more than it would normally. Slightly offset by cooler night time temperatures making it easier to cool the chiller's condenser circuit.

4. You need a very complicated control system to operate a) the ice making and then b) the switch back to normal operation and then c) the blending in of 0°C water with warmer water from the chillers (say at 7°C to get to a 6°C common flow temp). It has to decide when the storage system is full so you don't need to charge it any more, and also when the storage system is basically empty, so you stop passing warm return water (that hasn't gone through the chiller) through it. It also needs to be aware if the storage system hasn't charged (due to a faulty motorised valve). The storage tanks came with their own %storage full output signal to the BMS (building management system) but these rarely worked well so we stopped using them for control and added our own sensors. There were a lot of possible failure scenarios and a practical inability to test the system fully before handover to the client.

Towards the end of the period (and occasionally later when a mech. engineer reinvented the wheel), there was a move to utilise a different phase/change material, which changed from liquid to solid at a lower temperature than ice. This gave a more compact storage solution for the same storage capacity, but meant that the chillers had to operate at even lower temperatures, which really limited the choice of refrigerants as not all could create say -8°C flow temperatures.
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Re: New ideas in acoustic treatment

Postby Wonks » Mon Mar 11, 2019 5:30 pm

Part 2...

And then low night tariffs ended, everyone negotiated their own (much cheaper) electricity and gas rates with the new energy supply companies after the markets were opened up and ice storage was no more. The circuit controls normally got modified to run the chillers as normal if the chillers weren't too undersized and often the storage tanks were removed (you really don't want a huge tank of stagnant water, now at room temperature attached to your CHW circuit as it's a lovely breeding ground for pseudomonas bacteria which in this context produces a brown sludge that gums up valves and cooling coils and is near impossible to remove). Sometimes an extra peak-use chiller was added.

Hope that makes sense. I've probably forgotten some things. Different phase change materials have also been used as higher temperature storage materials in conjunction with ground source heat pumps (an example is installed at the We The Curious museum in Bristol, which was called @Bristol). Here the storage is again done at night, but mainly in order to reduce the museums' electricity demand during the day.

As Hugh has mentioned, thermal mass is used to store both heat and coolth naturally in many old buildings. In new buildings, especially those with natural ventilation forming the main part of the heating and cooling regime, the building fabric is sometimes passively cooled (by opening motorised windows) and sometimes actively cooled by running supply fans at low speed) to bring outside air in to cool down the building fabric overnight. As most office buildings generally need cooling all the time (with perimeter heating during the winter), this can be used for a large part of the year to reduce cooling load/costs. Obviously there are limits to allowable temperatures, so not too cold outside (to avoid condensation risks) and obviously not too hot. You also don't want to over-cool the space so that you actively have to heat the space up first thing in the morning before you then start cooling it down again.

Sometimes there are a large series of concrete ducts with thick walls constructed under the building which are cooled naturally at night (often will low air volume fans), then the cool concrete is used to cool warm intake air during the day.

I've probably forgotten lots, but that's a fair chunk of the stuff I know.
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Re: New ideas in acoustic treatment

Postby ef37a » Mon Mar 11, 2019 5:43 pm

I am suitably indulged! Thanks Wonks MOST interesting.

SOME might say "cool!"

(I'll get me coat....)
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Re: New ideas in acoustic treatment

Postby Folderol » Mon Mar 11, 2019 5:46 pm

Well done that man... err wabbit :bouncy:

Incidentally there's an old mill out in the back of beyond near Reigate that's been turned into luxury flats. I was chatting to the builders at the time it was nearly completed, and what they did was to restore the old millpond and use it as a heat exchanger for both cooling and heating.

Edit.
Forgot to mention there's a public footpath running the other side of the pond, and in winter, when the water is clear you can see a network of pipes.
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Re: New ideas in acoustic treatment

Postby Wonks » Mon Mar 11, 2019 6:56 pm

It's quite rare to see that, as the environment agency normally have pretty strict rules on the amount a system like that can can raise or lower the temperature by. But as I assume the water is flowing at a fair rate, it shouldn't alter it too much.
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