Thursday, May 24th, 2018

Acoustician Art Noxon on the theory and practice of acoustical design

By editorJune 1, 2012

ACOUSTIC DESIGN

Art Noxon, President of Acoustic Sciences Corp/TubeTrap

“I built and remodeled loudspeakers as a hobby for many years.  After a stint in Aerospace in LA I finished a master’s in Mechanical Engineering, thesis in Acoustics, moved to the Oregon coast and worked as a junior college science teacher.  But I just couldn’t think about waves clearly enough yet so back to school for another 5 years and I graduated with a master’s degree in Physics from the University of Oregon, in Eugene.  It was then, when I was an unemployed wanna-be acoustic engineer that I invented the TubeTrap in my basement.

TubeTrap

It was a cylinder shaped bass trap with a built-in treble diffuser.  It was discovered and marketed by Monster Cable and quickly became the world wide, must-have icon in room acoustics.  What luck.  The TubeTrap gave me entrée into the world of professional audio recording and audiophile playback environments.  That was 25 years ago and today I still own and run Acoustic Sciences Corp, which was originally created to manufacture the TubeTrap.  I’ve always been a prolific inventor and have created hundreds of new acoustic products for a wide variety of sonic applications.  I continue to study, research and perfect my understanding of high-power high-performance audio playback rooms and recording environments and have honed my ability to revoice poor sounding rooms or to design/build new and well voiced rooms.”

“So, let’s have a look at some of the factors, besides money, that limit the sonic performance in today’s version of home recording studios. 

Ceiling Height

“Over the years the ceiling height for real studios has become standardized and it’s about 12 feet.  One of the biggest problems with home studios is the ceiling height.  They are all too often just too low.  With an 8’ ceiling, the talent’s voice is between 2 and 2.5 feet from the ceiling and a hard bounce.  This creates comb filter coloration, starting with a self cancel of the sound coming out of the talent’s mouth in the range of 110 Hz to 140 Hz.

“Take note that 2” glass is completely rolled off and non absorptive at this frequency range.  Probably needs 4” thick fiberglass sound panels overhead to begin to quiet the bounce back down into the 100 Hz range.  Things work better if the talent sits on a stool, instead of standing up, which keeping the head farther away from the ceiling.  What never works is a 7’ basement studio ceiling.”

Professional Studio Built Inside a Tractor Barn

Out Buildings

“Today people are building personal studios.  Ideally this is a studio built inside an outbuilding, a barn or metal building on a concrete pad, located outside of the house.  A garage studio can also work, if the 8’ ceilings are removed and higher ceilings put in.  

“A big problem with studio construction is the building inspector.  Asking the building department to approve of recording studio blueprint and call for inspections throughout construction just doesn’t work very well.  What people usually do is to build a workshop that does not have a sheet rock finish, but is wired and plumbed to meet code, easy to build and easy to pass inspections.  When it gets signed off, that’s when the recording studio project begins.

“If there is room, recording studios should be designed with the live room in the middle of the building, surrounded by a hallway and another set of rooms lining the exterior walls of the building.  The live room needs absolute quiet.  The control room is the only room right next to the live room and the hallway begins and ends at either end of the control room.  The other rooms include the machine (pro tools) room/repair room, bathroom, kitchen/lounge, load in and storage, mechanical room, entry/lobby, tape library, and office.  Notice, no booths or iso-rooms?  “Bands that play together, stay together” seems to be today’s mantra.  Sometimes we do see a small 100% dead soundproof amp room which doubles as a coat room. 

“The roof can slope on all sides but does need to be 15’ high in the middle section, above the live room.  This leaves a good 3’ air cavity between the ceiling and the roof.   If you don’t have the floor space, room, dump the hallway and keep the same list of service rooms around the perimeter of the live room, pretty much all opening into the live room. 

Typical Songwriter Studio

“Musician studios tend to be one large project room with a second catch all room that provides kitchen, bath, lobby, office, machines, storage and load in.  Tracking is over headphones and mixing is done later, out in the open in a corner of the live room.”

HVAC

Big HVAC Isolation Muffler for Recording Studio under construction

“One of the most difficult aspects of small studios is the noise from air conditioning which includes the ducts transmitting sound between rooms.  There are five aspects of air in smaller rooms.  One is air movement.  Air has to move to feel comfortable in a room, otherwise the air seems to feel stuffy.  Another is air turns, changing out the air in a room has to happen maybe 3 times an hour, minimum.  The air temperature and humidity needs stabilization and lastly, we need to have some fresh air.  Normal conditioning does all these jobs without us even having to think about it, but HVAC is way too noisy in the live room, or where ever a mic is going to be opened up.”

“Real studios have huge HVAC mufflers built into their walls.  But the new wave of small, under-budget rooms aren’t doing that.  They just turn the HVAC off before recording starts and turn it back on between takes.  The industry has acclimated to this process, it actually adds a sense of excitement to set the stage for recording; clear the live room, dim the lights, kill the air, roll tape and you’re on.  And the first thing that comes on, after the take, is the air.  To do this it’s best if the live room has its own air conditioning system and the rest of the building has a shared system, otherwise the whole building gets shut down.” 

Recirculation

Free Standing Air Recirculation System

“One way to manage HVAC noise is how big studios do it, which is to all some kind of large diameter, low speed specialty HVAC muffler, typically 3’ diameter and 8’ long and install one in each inlet or outlet line of the live room.  

“On the other hand, the HVAC process can be subdivided and by doing that, you can maintain air quality while keeping air on.   Imagine separating air circulation, air exchange, humidity control, fresh air and temp control. 

“Get silent air movers like the modern ionic blowers or Bermuda fan to get air movement.  Make sure the motor of the Bermuda fan does not make noise. 

“Air exchange is maybe 2 times an hour in lightly used rooms.  A live room might have 4000 cubic feet of volume, which means 8,000 cubic feet of air exchange per hour or 130 cubic feet per minute, or 2 cubic feet per second.  Air speed should be no more than 5 feet per second and that means we want an air exchange duct that is 2/5 square feet in cross section or about 8” diameter. 

“Machine rooms are kept pretty cold.   These days people are using split systems, a heat exchanger outside and a small hole in the wall and a head or air fan/heat exchanger unit inside.  These run pretty quiet.  However, there is no fresh air here.  Just temperature regulated air.  Usually we want something like 10% fresh air, which means we want about 0.2 cubic feet per second of air exchange with the outside in addition to circulation. So if we are exchanging air into a room that is being cooled with a split system, we need to exchange some room air and that means a hole in the wall, a fan and possibly some mufflers.  Since the fresh air is going in and out of the machine room, we can use small diameter higher speed air, which reduces the amount of outdoor noise getting in. 

“DIY recording studio books usually have a decent section on air handling.  What most people think is that after they are all done, they still will have to turn it off during recording.  And so they end up doing almost nothing about it.”

Goboes

“Bands seem to be returning to the good old days of recording, which is the whole band in the room at one time, or something pretty close to that.   This means no iso rooms, but a big live room and lots of goboes, or roll around sound isolation panels.  Goboes are very difficult to get right.  If you make a sound absorbing barrier goboe (2” fiberglass sound panel on ½” plywood) the treble is absorbed by the sound panel but the bass is reflected back towards the instrument and mic, producing a bass heavy sound. The plywood quivers as well which also colors the sound.   On the other side of the goboe we have no treble but we do have bass bleed-through, because the plywood moves under bass pressure. Deeper bass simply goes around the goboe. 

“If you make just a sound absorbing goboe (2” fiberglass sound panel) then both bass and treble are not reflected off the panel back into the mic.  Sound absorbing panels absorb treble very efficiently but absorb bass very poorly.  Treble might be 30 dB down on the other side of a panel but the bass is only down maybe 5 dB.  We have bass bleed-through.

“Both goboe systems block treble effectively but only slightly block the bass.  Of the two, the sound absorbing barrier type makes for heavy bass on the incident side of the panel.  We usually don’t want extra heavy bass near the mic, so it’s best to make sound absorbing goboes, not bass blocking goboes. 

“The weakness in goboe construction is the uncontrolled vibration of the separating panel.  Even if the panel is 100% sound absorbing material, it can easily vibrate, and thereby pass extra bass through.  Nearfield sound baffles must be heavily reinforced so they do not vibrate under the bass pressure.”

StudioTraps being used as a Gobo

“We have developed another type of gobo setup, made out of cylinder shaped StudioTraps.  One side of the StudioTrap is treble diffusive and the other side is treble absorptive while the entire surface of the StudioTrap is a bass trap.  This system produces isolation without bass coloration.  Bass is not reflected back towards the talent because the traps absorb bass and the spaces vent bass.  The treble diffuser side can be rotated towards the talent to bring into play the Haas or sound fusion effect or rotated away to give the mic a dry sound. Notice that what leaks through the gobo setup is full bandwidth, and is not bass heavy or bass light, just a lower volume level. 

Room Acoustics

“We think that the surface of a room is comprised of a floor, 4 walls and a ceiling.  Wrong!  When it comes to smaller sized audio rooms, the surfaces of the room are the floor, 4 walls, the ceiling and the 12 corners of the room.  We tend to forget how powerful corners are in the realm of room acoustics.  Walls, floor and ceiling are flat surfaces which is where treble range sound absorber/diffusers do their best work.

“Adjoining the flat surfaces of the room are the corners of the room.  Corners act like megaphones in reverse, collecting sound and building up bass pressures some 6 to 9 dB louder than out in the room.  Corners are the best place to locate bass traps that require bass pressure to operate.   

“There is an old saying in acoustics: Sound is stored in the volume of the room and absorbed on the surfaces of the room. This is pretty much true in big rooms but in small rooms we have to make a new saying.  Sound is stored in the volume of the room but only treble is absorbed/diffused on the surfaces of the room.  Bass is best absorbed in the corners of the room.

Corner Loaded Sound Traps– Quarter Round Style

“If you trap all 4 vertical wall-wall corners and the 4 wall-ceiling corners with a 16” diagonal sound panel, you will achieve a full bandwidth RT60 of about ½ second in a small live room.  Let’s run through the simple version of room acoustics and see how it this works.”

“The live room is 18 x 20 x 12’ high and we have a room volume of 4320 cubic  feet. We have 4 x 18 + 4 x 12 + 4 x 20 = 200 lineal feet of corner in the room.  If we only use the 4 vertical and the 4 ceiling corners we have 140 lineal feet of corner.  If the sound funneling corner is in effect acoustically 3’ wide we have about 420 sq ft of absorption or 420 Sabines in the room.  The room constant is the volume divided by 20, or 216 Sabine seconds.  Divide the room constant by the Sabines to get RT60 of 0.51 seconds.  There is almost nothing worse than room with a short RT that is filled with flutter echo.  So we have to add poly cylinder diffusion on the walls and ceiling to get rid of flutter.  Net result is a clear sounding full bandwidth controlled live room.”

Studio Construction

“The soundproofing equation has 4 terms:  Strength, Damping, Mass and Density.  Mass and Density are the popular soundproofing terms.  Damping is a new one that has become popular in the last few years.  The Strength term is the one that remains under the radar of public awareness.  Soundproofing has become heavily marketed.  People are talking about STC this and STC that…  The problem is that this is a speech bandwidth soundproof rating good only between 100 to 5k Hz, which has a lot to do with apartments and nothing to do with recording studios.  Audio extends 2 octaves above and 2 octaves below the speech bandwidth and it’s always those two octaves below the STC rating that give so much problem, the bass range. 

“STC is not a good indicator for quality studio acoustics.  It pretty much is only sensitive to the Mass and the Density terms in the soundproofing equation.  What’s worse is that both Mass and Density both roll off at 6 dB/octave in the low end.  The good news is that the Strength term rolls on the opposite way, raising 6 dB per octave in the low end.  In the bottom end, it’s only the strength term that blocks deep bass.  Flimsy walls, no matter how heavy, leak bass. 

Vibration Damping Compound – WallDampVibration Amplitude vs Frequency with and without Damping
Vibration Amplitude vs Frequency with and without Damping

 

 

 

 

 

 

 

 

 

 

Cut-away Model of Studio Isolation/Damping Construction

“Damping is used to eliminate resonance between the Strength and the Mass parts of the wall, floor, ceiling and platform construction.  Damping only improves soundproof ratings at resonant frequencies.  We’ve been using WallDamp for about 25 years for structural damping of our studio construction projects.  It has 1/3rd the weight of sheet steel and is self adhesive.  It cannot be squeezed out of compression joints like the modern damping liquids that are heavily marketed today.

Studio during and after IsoDamp Construction

 

“Strong walls, not heavy walls block bass.  The basic soundproofing rule is a two step rule: 1) Block the sound and 2) Absorb the sound you just blocked.  For soundproofing to work you have to do both. 

“Here’s the secret of high performance bass proof walls. 

1) Build very strong walls, so strong they can’t flex under pressure.  Now you have blocked the bass. 

2) Add flexible damped walls in front of the strong wall, on the studio side.  Now you have absorbed the sound you blocked.  

“The interior walls and ceiling of a studio are two things at the same time.  They are composed of damped sheetrock so they don’t resonate, twang or otherwise make any sound at all.  Next they flex under bass pressure and absorb the bass energy.  Use double sheetrock with damping compound between the layers.  Float the sheetrock on resilient channels.  The sheetrock is only attached to the structure at the edges of the wall so that when the pressure builds, the sheetrock bends and energy is absorbed.  We call them IsoDamp walls.  This kind of construction turns the whole room into a giant membrane bass trap that works just great no matter how low the frequency goes. 

“There is no bass roll off with this kind of damping system except if the air cavity behind the flex wall is not deep enough, 15” is good.   A decent sized air cavity has to be kept behind the flex wall, between it and the non-flexing strong wall so the flex wall does not harden up and looses its flexibility.  This DIY deep bass trap only costs a couple dollars per square foot plus labor, a very low cost for such a fantastic deep-bass trap.  The double damped sheetrock insures that the surfaces of the room are non-tympanic, which is very important when it comes to having a neutral recording or playback room.  Inside the wall air cavity is thick mineral or fabric wool to absorb the buildup of clatter inside the wall.  On the outside of this flex damped wall is a very strong wall. 

“Well, I hope you had fun and learned some useful things about recording studio acoustics.  If you want to learn more, just visit our website at http://www.acousticsciences.net/, or http://www.asc-soundproofing.com/ and you can always just call and visit for a while about your current project.”

Art Noxon
President of Acoustic Sciences Corp/TubeTrap
June 2012

 

Thanks to Art for the really excellent combination of acoustical theory and practical advice.

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