Pushing Air- the art of acoustic design


Acoustics is usually regarded (or often disregarded) as a black art that is practiced with no discernable science involved, with a great reliance on ears for measurement and blind faith as a method statement. Nothing could be further from the truth in the professional study of sound, with many practitioners furnished with several degrees and a wealth of experience. There are a few basic acoustic truisms that deserve a robust explanation.
1. What is the best way to sound proof a room?
Many people confuse sound isolation with room acoustics. Isolation is created by the barrier effect of the room surfaces- their mass, stiffness and internal damping. Absorption within a ‘source’ room has very little effect on these values. For example doubling the absorption in a control room may halve the reverberation time but it will only reduce the total, monitored sound level by about 1.5dB. This is because the source energy, directly radiating from the speaker, will be largely unaffected by absorption. The perceived amount of sound that leaks out will depend far more on the reverberation in the ‘receiving’ room, as that energy becomes another virtual source, radiating from any surface common to both rooms. Sound travels well in solid walls so the new virtual source may well be in another room or even another building!
  1. Why can I hear voices through otherwise well insulated walls?
This is a rather annoying consequence of modern constructions in plasterboard and glass that both resonate at frequencies at which speech is most intelligible and our hearing most sensitive. The resonance is caused by waves travelling along the surface of the material, which literally bends in sympathy, according to its stiffness and density. That is why serious designers go the great lengths to research such fundamental properties as Young’s Modulus and Poisson’s Ratio.
3. Why can’t I hear any bass?
This is a common complaint in control rooms, especially ones with larger monitors! There are several reasons why this happens and usually it is a combination. The most obvious is the modal response of the room where the mixer is sitting in a node or null. All rooms have fundamental modes or ‘standing waves’, which correspond to the dimensions of the room. Their sound pressure is maximum at the room boundary and minimum in the centre and in small control rooms they often occur between 40 and 60Hz. It is galling to spend thousands on a state of the art monitor system and then find half an octave missing! Another reason can be a large reflection from the wall behind the mixer. If bass from the monitor travels past the engineer, hits the wall and bounces back it will arrive out of phase with the direct sound and cancel it at certain frequencies. This produces ‘comb’ filters and to make matters worse they vary with position, making life hell when working a large console.
4. Why not just use near-field monitors?
In a simple universe this would be a no brainer; they are cheaper and you just plonk them on the console or screw them to the nearest surface. The clue is in the question in that, acoustically speaking, the near field is a peculiar place to listen. At small distances speakers radiate very differently according to their size. Bass tends to radiate spherically and so obeys inverse square law. Even at a distance of 1m the ‘wave-front’ of radiated sound will be 13m2 and so very little reaches the ear directly. In fact 99% of the energy fills the room and is effectively lost. The opposite happens at higher frequencies where a small, baffled, wave-guided speaker is very directional in the near field, hence the expression ‘in your face’. The result is a sound balance that bears no resemblance to the real world of music and natural acoustic balances. This can be an advantage to some and many will care not a tinkers cuss but to those of us who cherish the more balanced mixes of yore.
5. So what about so called midfields?
These monitor systems are much in vogue in 5.1 mixing rooms and with good reason. A free standing speaker with a decent balance between size, directivity and power will give the optimum will deliver accurate results no matter where it is placed in the room provided that room is correctly balanced in itself. This opens the door to a good 5.1 arrangement as long as the geometry of reflections, standing waves and symmetry are obeyed. If so done each speaker will be phase coherent with its neighbours and critical mix decisions can be made with confidence. Once this is done it becomes easier to use bass management and other monitoring tools because you can be confident that if it sounds right it is right.
6. What is the best way to set up 5.1 monitor systems?
As it is easy to see there are so many variables in the stereo domain that progressing to 5.1 can be a complete nightmare. Normal statistical analysis with pink noise and real time analysers is fine for a final balance, as Dolby carry out very well in film theatres but in television things are more immediate, variable and frankly, less critically judged. The best way to make sure everything works is with a phase based analysis. It is only by measuring time delays, frequency response and phase at the same time that a confidence point of 100% can be reached. The description of such a method is beyond the scope of this article and, frankly, requires a good working knowledge of such matters if the results are to be trusted.
  1. What about sub woofer placement?
This is a problem in small rooms as the modal response has a big affect on the outcome. Place one in the corner and you will excite every mode and load up the bass with a horn effect created by the walls and floor. The result will be 10dB or more low bass and that may be a good thing if it can be tamed! Place the boom box in the middle of the room and the opposite occurs and you will rely on the direct sound, except that the acoustic impendence may be so low that the speaker just flaps around trying to find some air. Naturally the best results are somewhere between the two extremes and trial and horror will prevail. In film mixing rooms we always build sub woofers into the front wall and cluster several to obtain ‘mutual coupling’. This is not a deviant practice between engineers but a way to obtain high efficiency and radiation so that the bass travels cleanly through the room.
  1. What is the X Curve all about?
This is an equalisation curve applied in film theatres to provide compensation for the build up in bass and the absorption of higher frequencies at a fair distance from the screen. In a perfect scenario the X curve would make the main speakers sound the same as a near-field monitor placed at the point of measurement. If all things were equal the X Curve would be the same in every theatre but there are so many variables that a bit of judicial tweaking is often applied to obtain the sound an experienced mixer will be comfortable with. Dolby has gone a long way to standardise the setting of the curve but with film mixing now carried out in smaller rooms and with less directional speakers a note of caution is required. The best-engineered sound quality is all well and good but the end result must be compatible with a 3000 seater in downtown Mumbai!
In the final analysis all sound mixes must serve the purpose for which they are intended and broadcasters must adapt their facilities accordingly. If 5.1 is to thrive in the world of television it need to employ film experience and acoustic skills. The current standards, with notable exceptions and some great productions, can be woefully poor.

Tags: acoustic | iss036 | audio design | audio | sound proofing | insulation | bass | near filed monitors | midfields | 5.1 monitor | sub woofer placement | x curve | N/A
Contributing Author N/A

Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

Related Interviews
  • Linear Acoustic at IBC2011

    Linear Acoustic at IBC2011

  • The Telos Alliance at IBC 2016

    The Telos Alliance at IBC 2016

  • Glensound show Beatrice the AES67 Network audio intercom beltpack at IBC 2018

    Glensound show Beatrice the AES67 Network audio intercom beltpack at IBC 2018

  • Latest audio product updates from TSL Products at IBC 2018

    Latest audio product updates from TSL Products at IBC 2018

  • Nugen Audio Loudness Meter, Halo Upmix and Halo Downmix at NAB 2018

    Nugen Audio Loudness Meter, Halo Upmix and Halo Downmix at NAB 2018

  • 7.1.2 Dolby Atmos support by Halo from Nugen Audio at NAB 2017

    7.1.2 Dolby Atmos support by Halo from Nugen Audio at NAB 2017

  • Nugen Audio at IBC 2016

    Nugen Audio at IBC 2016

  • AudioVideo BrandBuilder Corp at NAB 2016

    AudioVideo BrandBuilder Corp at NAB 2016

  • Nugen Audio at NAB 2016

    Nugen Audio at NAB 2016

  • Minnetonka Audio at NAB 2016

    Minnetonka Audio at NAB 2016

  • Nugen Audio Loudness Toolkit at BVE 2016

    Nugen Audio Loudness Toolkit at BVE 2016

  • Nugen Audio Halo Upmix at BVE 2016

    Nugen Audio Halo Upmix at BVE 2016

  • NUGEN Audio at IBC 2015

    NUGEN Audio at IBC 2015

  • NOA Audio Solutions at IBC 2015

    NOA Audio Solutions at IBC 2015

  • NUGEN Audio at BVE 2015

    NUGEN Audio at BVE 2015

  • Nugen Audio at NAB 2014

    Nugen Audio at NAB 2014

  • Audio Network on BroadcastShow LIVE at IBC 2013

    Audio Network on BroadcastShow LIVE at IBC 2013

  • NOA Audio Solutions at IBC 2013

    NOA Audio Solutions at IBC 2013

  • NuGen Audio at IBC 2013

    NuGen Audio at IBC 2013

  • NUGEN Audio: Loudness Toolkit at NAB 2013

    NUGEN Audio: Loudness Toolkit at NAB 2013

  • Nugen Audio at IBC 2012

    Nugen Audio at IBC 2012

  • Audio Developments at ProVideo2011

    Audio Developments at ProVideo2011

  • Audio Developments at BVE North 2011

    Audio Developments at BVE North 2011

  • Junger Audio at IBC2011

    Junger Audio at IBC2011

  • Nugen Halo Upmix and AMB Updates at IBC 2017

    Nugen Halo Upmix and AMB Updates at IBC 2017

  • Canford at BVE 2017

    Canford at BVE 2017

  • Telos Alliance at BVE 2017

    Telos Alliance at BVE 2017

  • Glensound at IBC 2016

    Glensound at IBC 2016

  • Studer Glacier at IBC 2016

    Studer Glacier at IBC 2016

  • Wohler at IBC 2016

    Wohler at IBC 2016

  • Video Devices PIX-E at IBC 2016

    Video Devices PIX-E at IBC 2016

  • Sound Devices Wingman app at IBC 2016

    Sound Devices Wingman app at IBC 2016

  • Sound Devices 688 at IBC 2015

    Sound Devices 688 at IBC 2015

  • Video Devices PIX-E Series at IBC 2015

    Video Devices PIX-E Series at IBC 2015

  • RTS at BVE 2015

    RTS at BVE 2015

  • Glensound Dante at IBC 2014

    Glensound Dante at IBC 2014

  • NOA Archive Solutions at IBC 2014

    NOA Archive Solutions at IBC 2014

  • RTW at IBC 2014

    RTW at IBC 2014

  • RTW at NAB 2014

    RTW at NAB 2014

  • Sound Devices at NAB 2014

    Sound Devices at NAB 2014

  • Studio Technologies at NAB 2014

    Studio Technologies at NAB 2014

  • Clear-Com ProGrid at NAB 2014

    Clear-Com ProGrid at NAB 2014

  • Clear-Com HelixNet Partyline at NAB 2014

    Clear-Com HelixNet Partyline at NAB 2014

  • Glensound Atomic Copper at NAB 2014

    Glensound Atomic Copper at NAB 2014

  • TSL Products SAM1 MADI at BVE 2014

    TSL Products SAM1 MADI at BVE 2014

  • Glensound Cub at BVE 2014

    Glensound Cub at BVE 2014

  • Calrec on BroadcastShow LIVE at IBC 2013

    Calrec on BroadcastShow LIVE at IBC 2013

  • Wohler Technologies on BroadcastShow LIVE at IBC 2013

    Wohler Technologies on BroadcastShow LIVE at IBC 2013

  • Nexidia QC on BroadcastShow LIVE at IBC 2013

    Nexidia QC on BroadcastShow LIVE at IBC 2013

  • TSL Products PAM1, SAM1 and PAM PiCo at IBC 2013

    TSL Products PAM1, SAM1 and PAM PiCo at IBC 2013

  • Glensound with the USB Commentary Mixer at IBC 2013

    Glensound with the USB Commentary Mixer at IBC 2013

  • TSL Products at IBC 2013

    TSL Products at IBC 2013

  • Calrec at NAB 2013

    Calrec at NAB 2013

  • Sound Devices at NAB 2013

    Sound Devices at NAB 2013

  • TSL Products at NAB 2013

    TSL Products at NAB 2013

  • Thear Technology at BVE 2013

    Thear Technology at BVE 2013

  • TSL Systems at BVE 2013

    TSL Systems at BVE 2013

  • TSL Products at BVE 2013

    TSL Products at BVE 2013

  • AC Entertainment Technologies at BVE 2013

    AC Entertainment Technologies at BVE 2013

  • Pinknoise at BVE North 2012

    Pinknoise at BVE North 2012

  • Shure Distribution at BVE North 2012 LIVE Show

    Shure Distribution at BVE North 2012 LIVE Show

  • RTW at NAB 2012

    RTW at NAB 2012

  • Sonifex at NAB 2012

    Sonifex at NAB 2012

  • Sound Devices at NAB 2012

    Sound Devices at NAB 2012

  • Emotion Systems at NAB 2012

    Emotion Systems at NAB 2012

  • Clear-Com Eclipse MADI Card at NAB 2012

    Clear-Com Eclipse MADI Card at NAB 2012

  • Glensound at NAB 2012

    Glensound at NAB 2012

  • TSL at NAB 2012

    TSL at NAB 2012

  • Triveni Digital at NAB 2012

    Triveni Digital at NAB 2012

  • Petrol at NAB 2012

    Petrol at NAB 2012

  • Qualis at NAB 2012

    Qualis at NAB 2012

  • Calrec at NAB 2012

    Calrec at NAB 2012

  • DK-Technologies at NAB 2012

    DK-Technologies at NAB 2012

  • Glensound at BVE 2012

    Glensound at BVE 2012

  • Calrec at BVE 2012

    Calrec at BVE 2012

  • Sonifex at BVE 2012

    Sonifex at BVE 2012

  • DiGiCo at BVE 2012

    DiGiCo at BVE 2012

  • 4HM at BVE 2012

    4HM at BVE 2012

  • Thear Technology Limited at ProVideo2011

    Thear Technology Limited at ProVideo2011

  • 4HM at BVE North 2011

    4HM at BVE North 2011

  • TNP and DSMB at BVE North 2011

    TNP and DSMB at BVE North 2011

  • The Mic Store at BVE North 2011

    The Mic Store at BVE North 2011

  • Sennheiser at BVE North 2011

    Sennheiser at BVE North 2011

  • Source Distribution and Genelec at BVE North 2011

    Source Distribution and Genelec at BVE North 2011

  • HHB and Soundfield at BVE North 2011

    HHB and Soundfield at BVE North 2011

  • Riedel at BVE North 2011

    Riedel at BVE North 2011

  • Solid State Logic at IBC2011

    Solid State Logic at IBC2011

  • Dolby at IBC2011

    Dolby at IBC2011

  • Murraypro at IBC2011

    Murraypro at IBC2011

  • Calrec at IBC2011

    Calrec at IBC2011

  • TC Electronic at IBC2011

    TC Electronic at IBC2011

  • SADiE at IBC2011

    SADiE at IBC2011

  • RTW at IBC2011

    RTW at IBC2011

  • Prodys at IBC2011

    Prodys at IBC2011

  • DK Technology at IBC2011

    DK Technology at IBC2011

  • Sound Devices at IBC2011

    Sound Devices at IBC2011


Related Shows
  • The future of audio with BBC R&D at BVE

    The future of audio with BBC R&D at BVE


Articles
Protecting the continuity of transmission
Lorna Garrett Your viewers love you. You consistently bring them their preferred channels 24/7. They’ve come to rely on you for their viewing pleasure. They never miss cheering on their beloved sports teams. They’re the envy of their friends as they watch live concerts of their favourite bands. They gather the family around and catch up on their must-see shows. They don’t have a bad word to say about you.
Tags: iss134 | garland | gpl | streaming | artel | disaster recovery | Lorna Garrett
Contributing Author Lorna Garrett Click to read or download PDF
Accelerated Workflows with eGPU
Mike Griggs From the UK’s National Trust to magazine publishers to manufacturers, digital content creator Mike Griggs has a wide and varied portfolio of clients for whom he creates 3D art, motion graphics and multimedia exhibits. A typical day might involve sampling birdsong near Virginia Woolf’s country estate or creating 3D animations for VR. To keep on top of these demands, Griggs wanted to take the full power of the GPU computing revolution on the road.
Tags: iss134 | sonnet | egpu | amd | post production | editing | Mike Griggs
Contributing Author Mike Griggs Click to read or download PDF
State of the Nation - November 2018
Dick Hobbs - new There is an interesting seminar called Size Matters at the KitPlus Show – organised by the publishers of this fine magazine – at MediaCityUK in Salford on 6 November. It’s a talk by cinematographer Alistair Chapman on the way that camera technology is changing, and in particular the size of the electronic device which creates the image is growing.
Tags: iss134 | cmos | 35mm | AJA | Arri | Blackmagic | Canon | Datavideo | GoPro | Grass Valley | Hitachi | Ikegami | JVC | Kinefinity | Nikon | Panasonic | Red | Sony | jpeg2000 | Dick Hobbs - new
Contributing Author Dick Hobbs - new Click to read or download PDF
Giving Welsh sport a global audience
Adam Amor From the Ospreys Rugby Union team, to the Football Association of Wales, as well as national cycling, swimming and boxing coverage, Port Talbot based Buffoon Film and Media has been heavily involved in putting Welsh sports on the world stage.
Tags: iss134 | blackmagic | atem | buffoon | micro studio camera | Adam Amor
Contributing Author Adam Amor Click to read or download PDF
University and Mental Health
Rhiannon Jenkins University study and mental health has been in the media quite a bit over the last year, and I’m sure there are many people wondering what is going on? The issues are complex, and I suppose the focus of employability off the back of a degree course has raised the stress stakes for a lot of young people. I’m only qualified to talk about this from my perspective, and my story began when I joined a course not knowing I had a mental health condition.
Tags: iss134 | portsmouth uni | mental health | student | tvfutures | Rhiannon Jenkins
Contributing Author Rhiannon Jenkins Click to read or download PDF