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[Steve M Smith]

Not particularly relevant, but the start of Deep Purple's Fireball features the studio's air conditioning starting up (intentionally).


Steve.

[Alex Donkle]

There are two types of "balancing" in the HVAC world. One is to balance the fan wheel much like you balance a tire to reduce the vibrations from it. The other type is to balance the air by installing a diffuser screen, a piece of metal with perforations in it, which will reduce the turbulence in the air. Neither of these two methods are going to lower your noise floor much.

IF the noise is coming through the ductwork itself, and is not bleed over from the mechanical room,  duct installed sound attenuators or silencers will greatly reduce the noise level passing thru from the motor and fan.  I'm not sure what the claimed reduction is off the top of my head but if you stand on one side of a 3 foot section of this and talk into the "silencer" at a normal conversation level, no one will hear you on the other side.

These are not that expensive to buy or very difficult to install but they will necessarily lengthen your ductwork and can reek havoc on the unit's performance which in turn is going to require a larger fan/motor and/or different evaporator coil. If the sound reduction requirement is not factored into the original unit's configuration I imagine it's a real PITA to retrofit them in.

My day job is working for one of the world's largest HVAC companies. I work in the commercial air handling manufacturing division and we install these sound attenuators in our custom products weekly.  Occasionally we have customers spec out sound testing on the units before they take delivery. That testing often is 3-4 times the cost of the actual unit and in some cases a single test order exceeds my generous annual salary.

Scott-
Your experience differs from mine. Duct silencers are typically not integrated into large HVAC equipment that we deal with for new buildings, since that puts them too close to the fan wheel for a laminar airflow through the silencer, which greatly increases the static pressure, causes the fan to spin faster / louder, and defeats the purpose. Occasionally we have large discharge and return plenums added to the built-up AHU connections as those don't add significant pressure drop, but that's it. Silencers are more often installed 4-6' down the duct away from the AHU, ideally centered in the mechanical room wall.

Balancing HVAC can affect high-frequency noise (the whooshing sound at diffusers) if the flow isn't evenly divided between all the diffusers in the system. Air diffusers noise is directly related to air volume (cubic feet per minute) and how smooth the airflow is. If you have 2 diffusers in a room and one is receiving significantly more air than the other (caused by poor HVAC system balancing), the diffuser receiving more airflow than designed will generate more noise. Low frequency noise is rarely affected by system balancing, unless the system has so much excess pressure drop that the fan has to run at higher RPM to compensate.

[Ivan Beaver]

Not particularly relevant, but the start of Deep Purple's Fireball features the studio's air conditioning starting up (intentionally).


Steve.

I always wondered what that sound was-thanks

[Scott Olewiler]

Scott-
Your experience differs from mine. Duct silencers are typically not integrated into large HVAC equipment that we deal with for new buildings, since that puts them too close to the fan wheel for a laminar airflow through the silencer, which greatly increases the static pressure, causes the fan to spin faster / louder, and defeats the purpose. Occasionally we have large discharge and return plenums added to the built-up AHU connections as those don't add significant pressure drop, but that's it. Silencers are more often installed 4-6' down the duct away from the AHU, ideally centered in the mechanical room wall.

The size units we're building are 25 to 45 ft long with tunnels as large as 12' by 12' to begin with and our silencers are usually downstream of the supply fan a good  10-20 ft in the unit itself.  Often we see them upstream of the fans as well. Sometime both upstream and downstream. We don't design here, but just build to what is ordered, so I don't understand all the rationale.  I have noticed silencer usage in the units themselves seems to have dropped with an uptick in energy recovery wheels and have often wondered if they don't have a significant noise reduction quality as a result of their design.

[Jeremy Young]

How did this turn into an HVAC thread?  My bills are also paid through residential HVAC/Hydronics work (design, wholesale sales and technical support) and a lot of the tools I've learned from this site have actually helped that business.  For example, air-source heat pumps with a specified dB(a) @1m half-space sound rating can sometimes be a higher number than is "acceptable" for some old-timer friendly neighbourhood bylaws or stratas.  some quick use of the ISL to approximate the noise level at the property line (where the bylaw measurement is taken) can quickly aid me in a sale and keep those people off my back.  Thanks PSW forums!

The mechanical trades balancing I'm most familiar with is that which Alex mentions; trying to achieve balance between diffusers (or heat emitters of any kind).  In live audio, I'm blessed with some conveniences.  For example, if I take two monitor wedges (same model, identical electrically for all intents and purposes) and run them on the same amp channel in parallel, even though one might be twice the distance from the amplifier, i would expect to see the exact same voltage applied to both cabinets.  With matched sensitivities, I would expect the "throw" (perceived loudness, which is our goal) to be identical.  Now let's replace that amp with an air-handling unit, the speaker wire with a spiral duct, and the speakers with diffusers.  assuming identical diffusers, i will most certainly not get the same "throw" (velocity of air) at both diffusors.  since the greater duct run creates a higher friction loss (static pressure), we can add friction (almost like an L-pad for a high-sensitivity tweeter) to the shorter run with a mechanical damper partially closed.  Even coverage of the room with equal supply temperatures is typically the goal of most systems designed for human comfort.  Like anything perceived, it's more about comparison than nominal values.

Balancing is typically a mechanical engineer specified step of commissioning any commercial system before it is considered complete during new construction.  Re/Re however has very few strict "rules" about what is acceptable and what is not.  Ivan, I'm sorry to hear the HVAC companies in the areas you've experienced have been unable to keep their duct velocities to acceptable levels.  Larger diameter ducting will extend the life of the blower (if ECM type, also reduce amperage draw), as well as reduce noise. 

With variable volume systems, this really needs to become an automated process to achieve complete control, but that's for another thread.

Sorry, i promise not to compare HVAC and Audio again! (today).

To bring this back to point for the OP, I couldn't imagine requesting a venue to stop making drinks, or providing space conditioning for it's ticket holders based on the required noise floor for a quiet passage of a performance. personally i would find a lack of human comfort more distracting than some low level constant noise during a quiet passage in a performance.

In your shoes I would take the opposite approach: stating that the system provide a specific minimum of dynamic range above the noise floor of the venue.  That way if the "quiet" passage is meant to be 10-20dB (just a random number, please be nice) below the louder passages, and let's say you wanted the quietest passages to be 10dB above the noise floor, then you could say you need a system that can adequately provide a minimum of 30dB above the noise floor of the venue.  That way you get your comparative levels you wanted, specific to the loudness of the venue. 

However, I've been in busy bars during sound check for a band, where the audience sound level was 90dB at mixing position.  Give people some beer and a hockey game on mute, and they'll make more noise than any walk-in cooler or unbalancing ceiling fan you can find.  If I needed 30dB above that, we would have other comfort issues at hand unless i had a large box of earplugs handy for the audience. 

That's my two cents on the matter!  Good luck!

[brian maddox]

The Dolby Labs screening room in San Francisco is set up to provide simulated HVAC noise through the sound system. If you're in the movie business...

--Frank

Funny I happen to BE in SF.  where is this screening room you speak of.....

[Jason Raboin]

I have worked in venues and with artists that have specific needs outside the realm of normal.  The only way to ensure success is to have management alert the agent to the needs and have them agreed upon during the booking process.  If it is really that crucial, it should be discussed by the agent with the venue buyer and not just written in a rider.  By the time it gets to the advance it may be too late.  A venue may agree to suspend wait service, or not make drinks at the bar during the set, but doing so may result in a lower guarantee to compensate the venue for a potential loss of income.  Once the contract is signed a venue is less likely to agree to stipulations that may impact their income or impact the comfort level of their patrons. 

My primary artist will not play venues with a bar inside the theatre.  Seating is only allowed between songs.  There are to be no servers or vendors walking in the venue during the show.  This is all in the rider and agreed upon before it gets to me to do the advance.

As to the actual question - PA system noise, I have not found this to be an issue in most modern sound systems.  I assume that the show is pretty quiet, so there are many places in the system you can turn it down and reduce the noise floor.  I guess I would stay away from moving light rigs, as the fans can be a distraction.

During the advance and in the rider you could ask that amp racks be placed as far off stage as possible to reduce amp fan noise.

[George Dougherty]

I had a quirky, well-known folkie for an intimate concert.  When the promoter brought him to the hall for sound check, he wouldn't come near anyone without asking, "Do you have cats?  I'm allergic to cats!"  When he found out I had some at home, pretty much refused to be on stage unless I stayed behind the board, even insisting that I move FOH from the optimum position to the back corner of the room.

He gave his concert without much further fuss.  I thought it strange that he had no adverse reaction to sitting in the middle of the oriental carpet I had placed on stage as I had brought it from my living room...where the four of my cats loved to roll and play on it.

Two one-hour sets on a cat carpet and no allergic reaction...but he wouldn't shake anyone's hand.

CATS!!!

Sounds like an over reaction but I have a friend that ended up in the hospital from visiting a house that "used" to have cats. It's rare but some people do have very strong reactions.

[Frank Koenig]

Funny I happen to BE in SF.  where is this screening room you speak of.....

It's at the Dolby corporate headquarters at 100 Potrero Ave. They hold AES San Francisco Section meetings there now and then (thank you Dolby). We got a demo of the simulated HVAC noise at Dan Dugan's very interesting presentation on nature sound recording a couple years ago. It was along the lines of "now everyone be really quiet and describe what you hear". And we're all like "it's pretty quiet but I hear some rumbling, sounds like HVAC". Then they turn it off and tell us that it was coming out of the speakers.

I suppose that to demonstrate to clients what various cinema processing schemes sound like you need realistic movie theater background noise. -F

[ProSoundWeb Community]