Double doors showing the door drop, liner finger and rubber seals. The brown panels are for acoustic absorption.
The next challenge for my home theater was installing the doors, HVAC and electrical systems. I mentioned in my second post in this series that any small holes or significant air leaks from either the inside or outside walls will significantly reduce its ability to reduce sound transmission. The biggest of these holes is going to be the door.
The Door(s)
In my case I needed the wall construction to be capable of a sound reduction index (SRI) rating of at least 60+dB, at least equal to or better than the wall rating. So it is no use installing either a single door or just two standard doors. You need the door opening to perform as well as the walls. Otherwise there would be no purpose of going through all this construction only to be let down by poor door performance.
The door blanks that I used came from the Prime Craft door range and were custom made by Prime Line Inc. Malvern, Arizona, who also offered to machine the doors to take the hinges and lock sets. This I left up to my finish carpenter as I trusted him more than I trusted my measurements. The doors I used are composed of two ¾” high-density MDF blanks bonded together to make a 1.5” door. Each weighed in at 200lbs. Supporting this is no mean feat so I ensured that my carpenter, who was to hang them, knew exactly what was required and that the framed opening was almost a perfect fit to the door liners so there would be minimal air spaces from shimming that needed to be filled with foam.
The Door Acoustic Seals
Getting the door framing and door blanks is only half of the sealing and acoustics problems. You now need to hang the door on some very robust hinges and create an airtight seal when they close.
I elected to work with Zero International of NY who specializes in acoustic doors and sealing systems. They provided the stainless steel ball bearing hinge assemblies, the door perimeter seals and the drop threshold sealing systems for each door. I decided not to use cam hinges as they would have presented my carpenter with some additional challenges in the doors alignment, and in my case would not have benefitted the acoustic seal.
The Door System
Door Cross Section and Acoustic Seals
The door liners were set to leave a 1/8” air gap between the inner and outer walls that was left un-caulked but filled with a small amount of Roxsul but not over packed. This ensured that the inner and out walls did not touch, preventing any acoustic bridging. 1” of rigid fiberglass was then used to line the resulting opening to deaden the space between the doors and absorb any sound leakage.
The doors were to be kept closed using pressure handles but I was unable to get any that met the required access/building codes. In desperation I installed conventional latch locks and added appropriate caulking and fillers to offset the effects of boring holes into the doors; sacrilege.
High Volume Air Conditioning – HVAC
A separate, acoustically isolated and ducted system was installed under the room on resilient TICO pads, used oversized lined ducts, and was provided with a 10% fresh air bleed as the room was virtually air-tight. Installing the air handler above the room was never to be an option due to access, leaks and vibration issues. The sends and returns fed oversized plenum chambers to absorb much of the air noise. These five chambers were formed by the rack room and the front and rear LHS/RHS corner lined spaces behind the main speakers.
In order to keep air noise noise as low as possible I did not use any vent grills and I was looking to achieve a vent velocity not exceeding 1ft/sec. Any grills, no matter what type, and what air velocity would introduce air turbulence and hence noise. so all ducts were left open feeding the plenum voids. Air distribution around the room and in the rack space was left up to the air to find its own way out of the three incoming plenum areas and filtering itself into the main room .
I did not feel that I would require any duct silencers. This turned out to be true for the front and rear plenum chamber feeds. However, it was determined that the incoming air path to the rack was too short resulting in the fan noise, NOT air noise, being audible from the rack recess. This issue was fixed by adding some flexible ducting to increase the air path and adjusting the ducts path so that the fan didn’t drive air directly into the racks vent (see pictures below).
The power to this entire system, the air handler and compressor, are fed separately from the house main board in order to reduce any problems with electrical noise, interference and voltage drop.
Original Basic HVAC Installation
This is a relatively small room being only 1890 cubic feet, so I did not require a large volume of air exchange. The maximum heat load for the room with three people would require just under 1TON of cooling. This requires approximately 300CFM to ensure that the air handler doesn’t ice up. Unfortunately for reasons I will not go into now, the HVAC Company installed a 1.5TON unit; luckily I still managed to get the air handler to operate without icing at 380CFM. This gave me an air velocity at each inlet of approximately 2 ft /sec, higher than I wanted but with all the large plenum chambers and method of room dispersal, it did not create an overly intrusive noise problem, with the rack air flow just being audible at the MLP. However, the room cooled down too fast. This cooling issue was later rectified by the addition of an air handler bypass duct , frost stat and delay timer, see below.
HVAC showing front ducts, revised duct connection to rack enclosure and bypass duct.
All the ducts are fully lined and oversized so the final room air noise after feeding the three plenum chambers was just on the threshold of NR25 at the MLP. This was improved when I managed to get the air velocity down to about 1.25 ft/sec with the addition of the bypass duct which also significantly helped with the room cool rate.
With the HVAC off, and no equipment fans running, I am unable to measure the rooms residual noise floor as it is below what my equipment can measure (lowest NR25). The Noise Rating (NR) (Europe) or Noise Criteria Rating (NC) (America) are a measure of the rooms background noise on an octave basis. See below for typical NR curves. Tables of NR and NC ratings exist that recommend the desired ideal background noise levels for control rooms, studios and offices etc.
Examples of typical Noise Rating (NR) Curves.
My AV Room noise floor with and without the HVAC. Green HVAC off. Purple HVAC on. The increase in noise above 10K is a measurement anomaly due to an incorrect sample rate.
As can be seen the only appreciable increase in room noise when the HVAC is on is from 40Hz down. These SPL levels significantly exceed my required NR rating at these low frequencies as they almost hit NR10. It is the instantaneous peaks from 60Hz and up that cause the overall NR rating not to exceed NR25. In reality the noise shown above exceeds my NR25 rating if I ignore the few peaks above 60Hz which when averaged out over time drop by several dB’s. Reducing the air noise below this level would be very challenging, is not necessary for a room of this caliber and is already virtually undetectable at the MLP.
The Electrical System
In my mind, next to acoustics, this is the next most important consideration for the room. It is of prime importance that:
- Any electrical interference from the house, HVAC systems, fridges etc. does not get into the grounding system for the room or it will appear as a signal on the A/V circuits.
- The grounding system impedance is virtually zero ohms or again any interference that gets into the A/V circuitry will appear as noise on any signals.
- The room only uses a SINGLE-phase supply or there is a potential to have 220 VAC between devices. This is not only dangerous but can lead to some horrible hum problems.
- There is minimal voltage drop during peak current demands.
All these issues were solved by providing the room with its own single-phase 100amp 120VAC supply straight off the incoming 200amp house supply. An oversized copper feeder to the new breaker distribution board ensured absolutely minimal voltage drop at peak currents provided by this supply. The new breaker box had its own safety ground associated with it, which was tied to the main house breaker box safety ground at the ground rod bonding point. The neutral to ground bonding screws were removed from the new breaker board in order to prevent ground loops. The rack power strips were all isolated ground type, and a new low impedance isolated technical ground was installed to support all the audio and video technical grounding requirements.
A technical ground is a separate ground system, used extensively in broadcast, which is separate from the safety ground. In my case it was composed of a bonded 6SWG copper cable to the floor rebar metalwork and several grounding rods which was then taken back to an isolated copper ground bus bar in the rear of my rack. . This arrangement would produce a very low impedance connection to ground in order to ’sink’ any interference signals.
Fortunately, most modern A/V equipment is double insulated so does not require a safety ground. This made laying out the grounding system much easier and an isolated technical ground bus bar was added to the rack to which appropriate connections from the equipment were made. This provided the single bonding point for both the safety and technical grounds reducing the effects of any potential ground loops.
The breaker board provides fourteen 20 amp circuits to drive the rack and powered speakers to ensure that during high current demand voltage drops are kept to a minimum.
New 100Amp Distribution Board. The opening to the RHS will become the main access to under the AV room floor.
Power Updates
- All speakers are active and they need to be turned on, and off when not in use. This was achieved using the X10 series 20amp switched outlets with an IR controller.
- When I say this system is quiet…I mean it is quiet at ANY volume with no measurable/audible hum, and just the expected very low white noise from the audio chain.
- Two single conversion UPS/anti-surge units are provided one for the projector (simulated sine wave) to protect the bulb and one for the AVP Receiver (pure sine wave) to protect the electronics. (After a number of ‘incidents’ it now also protects all the DVD players.)
So now I have a room with power, lighting and HVAC. The next post reveals the rooms’ un-treated initial acoustic tests; SRI rating and how it was to be acoustically designed, setup and acoustically treated.
