Category Archives: Design And Consultancy

The conventional movie theater is giving way to a huge mall, with plenty of retail outlets, and the top floors of such places are usually dedicated to a number of small halls replacing the conventional movie theater experience. The large theater had a very different acoustical characteristic and these small halls require a fairly different approach for acoustical treatment.

I’ve worked on the PVR malls at Orion mall, and now at the Vega Mall, and some issues seem to be common across both projects. Let’s begin by looking at common acoustical issues at the smaller halls. (The larger halls are vanishing, and those that are remaining, can be treated similar to large auditoria – coming up in a different post!)

Sound Isolation

This requires maximum consideration. The biggest problems I see frequently are noise coming in from the food courts, gyms sharing walls and floors, air-conditioning chillers on top of the building, and noise from adjoining halls, in some cases! Thankfully, with most of these being located on the higher floors, traffic noise is never an issue. But I know one place where the door to one such hall was separated from the external facade by 6 feet of corridor space, and the glazing was insufficient to contain noise from airplanes passing by!

Reverberation

This is what most people think acoustical treatment is all about. Yes, these spaces need to be optimized for clear speech, as well as music, and of course, deep bass. I’ve watched movies in some really dead sounding spaces.  The speech sounded great, but the songs, the accident, all sounded very superfluous. It didn’t help that the movie was a crashing bore.

While we’re discussing wall treatment, no, absorptive materials on the wall will NOT prevent transmission of sound. Care must be taken to check sound transmission between walls of adjoining halls, and then design acoustical treatment required.

Ringing

This is the other most commonly seen issue. The hall sizes are large enough to allow plenty of LF in the room, but it is a task to ensure they’re sufficiently diffuse,  and it is essential to ensure that room resonances don’t unnaturally colour the sound. There are 5D movie theaters coming up these days!  It’ll be a lovely to write a separate blog post on them, some day soon.

Seating

Actually, the line-of-sight accuracy is all taken care of by the architects, and if the raking is correct and the headrest is not too high, one can ensure there’s good amount of direct sound reaching each row. But we’re more interested in the seating in terms of upholstery. Usually, these places are carpeted, and the seating provides a GOOD amount of absorption. It’s so significant, that the treatment on the walls must be carefully calculated to ensure that the space doesn’t sound dead after the furniture is moved in.

AV

The coverage of the speakers must be plotted to ensure that no place is receiving more than the intended amount of sound. The front seats have it the worst usually. Using logarithmically curved line array speakers can help, but care needs to be taken to add the right number of speakers in each array, else the sound becomes too directional.

Now here’s the other thing about audio. You DON’T NEED to hear it that loud. And if you’re a regular with movies in such places, I strongly advise you to carry a pair of earplugs. Choose how deeply you want to plug them in, and you will hear everything – the HF, the LF, the mids. Just not as loudly as intended. Honestly – I’ve always been most comfortable sitting through a movie when I have my ears plugged. I’ve attended a healthy amount of concerts, and have always maintained comfortable loudness levels by using earplugs. Sounds absurd? I know. Please heed my advise. I speak out of collected experience and foresight. You don’t want to be speaking these words someday, out of bad experience. 

Now there are plenty of other parameters we acoustical engineers calculate, to ensure the right balance of sound, etc. But these spaces lie between glorified home theaters and large auditoria. The importance for each parameter is therefore, determined by the size of each hall.

These spaces are leading to even more novel concepts – a person I know has turned a space into a home theater area. He has a few rooms, and each room is a dedicated home theater. Groups of friends, or family can book a room all for themselves, food is catered, and you can watch a movie with first class acoustics and audio, with only your loved ones! The acoustical challenge is to ensure that there is sufficient isolation between rooms, and of course, to even out the bass response in small rooms. There’s just so much happening on the audio/video front in Bangalore alone! Interesting times ahead!

I’ve only just started freelancing on my own. Unlike the initial  romanticised impression of acoustics being the perfect mix of music, maths and physics, I really spend most of my day listening to noise :). So I thought I should get my hearing tested for two reasons:

  1. To have a point of reference for a ‘before’ and ‘after’ scenario of my hearing sensitivity.
  2. To understand what frequencies I am sensitive to, and any hearing losses I may have for other frequencies – so that I can accurately tune listening spaces.

I went to this institute near my place and found out two things that didn’t suit my needs.

  1. I could hear road noise very clearly inside their listening room! This is past 3 doors, no less – the entrance door, the door to the audiologist’s room, and the door of the listening booth.
  2. They test only 5 frequencies from 250 Hz to 8 kHz, because this is primarily for old people to hear speech.  For my purpose of tuning spaces for music, I need to be tested for a much wider range – 50 Hz to 18 kHz.

Anyway, I went ahead and turns out my hearing is normal, but I suspect it won’t stay so for long. I am a tad sensitive to vocal high pitches – the Lata Mangeshkar types – playing those at normal volumes can grate on my ears. But I did have difficulty in listening to low pitches – the 250 Hz thing. This would’ve been critical information for me to have found out – except that I will have to take it with a pinch of salt, pending more accurate tests. There was plenty of low noise infiltration through the doors, and I could hear them despite the on-ear headphone.

Anyway, the point is, that critical spaces such as these must not have any type of noise coming in. They had even used thermocol for acoustical isolation. Thermocol, styrofoam are easily and cheaply available, but they are not acoustical materials at all – they can at best be used for impact isolation.  There was also some masking effect happening due to road noise intrusion. I could clearly tell the sound of an auto, a bike, a bus, and some local vegetable vendor hawking at a loud voice.  This is also because the glazing they had used was rather thin. It is critical for listening booths to have glazing because they are to be closed, and the only way for the audiologist to know that you are able to hear a certain test frequency, is when you raise your hand. They have to be able to see you. Glazing itself is not a problem, but the right thickness must be used. Also for such applications, either in-ear earphones, or supra-aural earphones must be used. On-ear headphones are not very effective for blocking out sound.

These and many other defects can be solved right at the design stage.  Eventually, the kind of hearing aids that are prescribed are primarily influenced by the extent of the patient’s hearing loss, and the budget.  The first factor – the extent of the patient’s hearing loss can be accurately gauged if external interference is zero. In this case, it is critical to avoid masking effects.

The right technical advice will take into not just that, but also the hum of your HVAC, the type of earphones you must use, the influence of nearby buildings and their DG sets, etc.  It should also prescribe materials from a rough-use point of view (for instance, the rubber linings of the door had peeled off – leading to a compromise in sound isolation). For a place primarily meant for accurate testing of hearing loss, the noise constraints posed by the location can be easily overcome with correct acoustical diagnosis, testing and recommendation.

Studios are critical listening spaces. The noise levels have to be as low as 10-20 dB.  The hum of the HVAC, the rumble of the ceiling fan, the structural vibrations of all the houses/buildings nearby, the rumble of trucks on the road, the cross talk through the HVAC duct, the hum of the machines, all make a rather big difference to the quality of the sound. Also, since most studios are rather small spaces, the the bass response must be carefully evened out.

I’ve seen studios that have sound leaking in and out of the rooms, studios that are somewhat reverberant because the glass wool sagged over the years, and studios that were about 50m from the nearest bus stop but you could hear the squeal of the breaks and even the horns all the time, even if there wasn’t an obvious air path from there.

The purpose of outlining this scenario is to mention common issues faced by a lot of studios in this area. Structural vibration is the biggest grouse. And increasingly, pavements are laid joining the road and residential areas. This, despite there being a mandate not to do so for longer than a certain width ( wide enough for your car to roll out of your house). This leads to added coupling.

That said, in India, the advantage is that most structures are still brickwork/RCC, unlike gyp/wood partitions in the west. Internal walls are usually 4 inches wide, and external walls are 8 inches. Older houses have thicker external walls.  So, a decent amount of isolation exists between rooms, if you ignore flanking.

All this is was about the sound isolation bit. Now when it comes to reverberation treatment, and the frequency response of the room,  studios again are the most critical sound spaces.  It is vital to even out nulls and peaks for bass frequencies, and diffusers must be optimally used.  If the absorbers are carefully and specifically designed, one gets much more benefit out of the treatment than by randomly experimenting with off-the-shelf stuff. Faulty room acoustics leads to the sound engineer falsely believing the sound spectrum to be something it is not, and equalizing to correct what they think they’re hearing.

Sound technical planning should make sure that you avoid all these pitfalls, and the expensive corrections they entail. The right treatment should make the ambient sound crisp and clear, with accurate and predictable colouration from the room.   The quality of sound you hear can then safely be subject only to the quality of the equipment. I have a personal affinity towards sound studios, because these test so much of our skills, and these are also the places where some of the best works of art are immortalized. As someone whose interest in music primarily led her to acoustics, I do have a thing for these little rooms.  🙂

 

That gap on top of this post is intentional. Air Gaps are your friend when it comes to soundproofing!

Every band dreams of practicing freely in a room without neighbours dropping in with a scowl on their faces. Similarly, every studio hopes to record the most subtle aspects of music without worrying about the surrounding urban noise.  These two places need soundproofing for opposite reasons – jamming rooms must prevent sound from going out, and studios need to prevent sound from coming in.

This post discusses the soundproofing requirements of jamming rooms, and lists some loopholes to watch out for. Soundproofing for studios requires a separate post.

Jamming Rooms generate noise levels equivalent to around 100-110 Db – close to twice or thrice the loudness you hear at a noisy traffic signal. This can get distressing because by definition, practice implies long hours. Involuntary listening for long hours can get disturbing – leading to annoyance, irritation, headaches, raised heart rates, and raised blood pressure. A sustained elevated heart rate is good for a workout – staying in such a state for long causes a dip in your longevity.

How much soundproofing do these rooms need?

The good news is, there’s no need to have a zero output concept here. Urban noise is expected to be at least 60 db during daytime, and around 43 db during night time in residential areas. What’s even better news, our ears perceive loudness logarithmically, and so the decibel scale is logarithmic. That means if you reduce the sound even by 10dB, you perceive it as only half as loud.

Soundproofing projects are always more challenging than those which require acoustical treatment for reverberation, or fine tuning. This is because heavy mass is needed to block sound, and most venues do not afford space. A room within a room is the best option, but when that’s not possible due to structural constraints (or in the absence of permission to make structural changes to a building), alternatives must be carefully calculated.

The common mistake here is that while gypboard constructions can be used for isolation, the calculations must be for that purpose alone. Expecting gypboard configurations that work for absorption, to also work for isolation, is a folly here – one that I see all too often around me. Rockwool or glasswool will do nothing to absorb sound at low frequencies if you don’t stop them first.

A word of caution to DIY guys.

  • All soundproofing materials come with numbers that tell you how much transmission loss they give. Do not assume that simple arithmetic will give you accurate results. The materials come with a clever line “All joints to be properly sealed and caulked”. Here lies the whole game.
  • Also, the materials are tested under lab conditions – the numbers will vary rather widely under different site conditions.
  • Further, they come with instructions on how to construct. If structural constraints or budget force you to make variations, discuss them with an acoustical consultant, not the vendor.
  • While heavy absorption helps in transmission loss if carefully designed, careless use of absorption materials will make your drum kit sound dead. A drum kit produces a wide range of frequencies – 50 to 15kHz. The high frequencies will sound dead almost immediately, while low frequencies will painfully linger on if you provide too much absorption in the room.  

The other important thing to watch out for are leakages due to site constraints, or plain human error. Every soundproofing project has a leakage fixing phase. A 1 mm gap is enough to turn a 99 % success into a 100 % failure. Also, the most important thing to watch out for is leakages due to structural contacts. You may come up with a clever design, but if you drive nails or screws through to hold things together, that’s a short circuit route. Acoustical caulks must be carefully chosen. I know carpenters who use metal paste to block leaks. It’s rather difficult to convince them of the structural coupling it provides.

Lastly, a note about budgeting. Soundproofing materials are more expensive than absorption materials. All choices must be based on calculations, with generous discounts on the claimed performance, keeping site conditions in mind. While budget constraints are understandable, it is vital to have a talk with an acoustical consultant to understand what part of it can be DIY, and what part of it must be professionally installed.

The funny part is, road traffic noise can be a blessing for such projects. I have two precisely opposite cases on hand – one project next to a noisy road, and one located in a quiet residential layout. Needless to say, the sound reduction needed in the latter was huge, and with structural changes not being an option, I had many sleepless nights reading quoted transmission loss numbers and separating the grain from the chaff.

If there’s a choice of location, try to choose a noisy location, so that you don’t stand out. Low frequencies and structural vibrations are the toughest to isolate, but there are clear laws defining how things work, so in the right hands, your project can be a decent success even with budget constraints. Happy Jamming!

Unfortunately, there’s only one design a building is going to have, and it has to include everyone’s conveniences and dreams – the client’s, the architect’s, the HVAC, electrical and plumbing people, and last but not the least, the acoustic consultant’s.  The correct interpretation of  ideas into engineering requirements, and their faithful implementation will form the bridge between dreams and conveniences. This is why it is vital to call in your acoustical consultant sooner – during the design phase. The laws of physics won’t change at the last minute, during the commissioning, and the acoustical consultant will usually have no good news to give you if you bring them in just before the inauguration party.

Walls are walls, glass is glass, a rose is a rose, etc…And one usually can’t take the place of another in the general scheme of things. A large part of the value of an acoustical consultant lies in their ability to prevent a problem before it occurs.  This effort and expertise may go entirely unnoticed, because good sound is taken for granted, just like air to breathe. One does not notice the acoustical environment unless it is disturbing. 🙂 That’s a job hazard acousticians live with.  🙂