The Cases for & Against Helmholtz Resonators
The temptation for small studio owners, or home studio owners, is to look at multi-million dollar commercial facilities and froth at the mouth at any number of acoustic implements.
23 April 2012
Story: Andrew Steel
The temptation for small studio owners, or home studio owners, is to look at multi-million dollar commercial facilities and froth at the mouth at any number of acoustic implements. It’s studio porn. As much as you might want to run your fingers over the ridges of a quadratic diffuser, or stroke the front of a Helmholtz slot resonator, building them at home is a lot of work, a lot of time, a lot of maths, and often times, they’re not suited to small rooms. Let me explain.
For those unfamiliar with the concept, it refers to air resonance in a cavity, like the phenomenon of blowing across the top of an empty coke bottle. Hermann von Helmholtz, a forefather of acoustic science, used similar devices to identify various frequencies. These days Helmholtz resonators are used in acoustic design to absorb specific frequencies, due to their high ‘Q’. For instance, a Helmholtz resonator could eliminate an annoying machine whir by sympathetically resonating with the offending frequency. With acoustic absorption implemented in the design, the resonators become more broadband.
The thing about them is, if you’re going to try and cover everything from 200Hz down effectively, you would have to make a lot of them, because the most effective resonators (and therefore absorbers) are very high Q. But you might not have enough room for them all. So instead, you damp your device, and it makes it nice and broadband, but it also limits its absorption, so you have to put more in to get enough absorption, it’s a catch 22. It works, but you have to be careful. Don’t assume it’s going to be a quick and dirty solution. Also, how you figure out how many you need for a given room is very difficult. That said; a couple of broadband resonators are better than nothing. The same goes for membrane absorbers, where you would have a box with MDF on the front, and instead of tuning the port you tune the mass with depth and thickness.
If you’re really trying to notch out a specific frequency, you will have to tune them, because the formula won’t put you that close. Mainly because the formula works by treating the vessel, or the space behind, as a spring — and the air in it is the mass. If you’ve put absorption in, it’s a damped spring. And it does allow for a little air to escape out each side of the whole, because it’s a plug moving in and out of the hole. But because air is the mass, all sorts of other effects come into play that aren’t in the formulas; temperature, humidity, the area around the hole… plenty of parameters. So you’ll get close, but it won’t be on the money, you’ll have to tune it. Which is okay, just don’t build something fixed before you can get it into the room and tune it.
+ A NOTE ON QUADRATICS
If you build a quadratic diffuser, and the designed frequency is 500Hz, its diffusion would look like the tail end of a suspension bridge. It would peak at 500Hz, with subsequent peaks at 1k, 1500Hz and so on. But if you build two the same next to each other, they start to defeat each other. So they need to be different to each other. And there’s maths for modulating those sequences, but it’s a complex science and a lot of work. It doesn’t matter how you make them, there’s a lot of making.
Also, they’re not that effective in small rooms. They sound really weird up close. The common number is to be about 3m away for it to start making a diffuse field.
Whereas the right section of a simple circular curve (not some other strange variant) will act as a broadband frequency diffuser that operates from about 350Hz to 3kHz. It serves its purpose, to get rid of flutters. Often times the diffusion in the room is not for the critical listening position, but the flutter echoes at the client couch. — Andrew Steel
John Sayers has developed quite an affinity for Helmholtz resonators. They’re a common theme through his designs, and for good reason, here’s why Sayers uses them.
John Sayers: “They’re my baby. The thing about Helmholtz resonators is that they operate in the frequency of 150-500Hz — the low-mids — and every major room has problems in the low-mids. Even in the container studio that we built, taking out the low-mids is what gave the room its clarity. Plus they keep the highs going, because you’ve got reflective timber in a diffuse way due to all the angles off the wood. So they also act as diffusers and keep the top end nice and bright while taking out the low mids. They just work as far as I’m concerned.
Mark Davie: Have you ever measured their response in a controlled environment or is that knowledge gained from practical experience?
JS: I’ve never measured it, but I’ve heard the effect. When I was building Left Bank I was actually living above the studio as we were building it and I was in one of the booths while they’re were putting the Helmholtz resonators in and I could hear the difference just in the voice of the guys working. as soon as it started to operate all the low end came clean and all the top end brightened up, in what would have normally been a very dead room.
MD: What variations of the Helmholtz resonators are you using these days and why?
JS: Well there’s the straight-on-the-wall Helmholtz resonator, where the depth is static. I also build them off the wall at angles to give some angular diffusion in the room. Plus it expands the Q factor and broadens the absorption, so instead of it being centred around one frequency it actually works as a broadband around that centre frequency.
I also use them as angular devices to break up parallel walls in studios.
Then there are the module units that I have built at a joinery company. A router cuts out all the slots and I just designed three or four of them that all interact with each other and cover a broad range. There’s some that go down low, really low, below 100Hz and others that fill in between 400 and 500Hz. Putting four of them covers a room nicely.
MD: Do you find Helmholtz resonators difficult to tune?
JS: Not really, if you put them in the right places for the right reasons. The main principle is that you don’t let anything come back at you from behind. And that’s why this whole business of everyone wanting to have a diffuser on the rear wall of their control room is not a good idea, unless you’ve got a decent-sized control room.
The reason they first appeared was because people were building big control rooms where the rear couch was 4-5m away from the console. And by the time the sound had gotten to the back of the room it had lost a lot of its energy. So they put a diffuser in to brighten it back up again. I worked in a Brisbane studio where they had a huge diffuser across the back of the room. It was their big claim to fame. But if you walked around that control room, the bottom end was all over the place.
The aim of a good control room is to smooth the bottom end so it stays even wherever you walk. I don’t feel a need to use diffusers.
So long as the section of the room from the front to where you’re sitting is symmetrical to the left and right, that’s the most important factor. It doesn’t matter too much if it changes at the back because the bottom end likes to kick around a little bit.
TECH TIP
A Helmhotz resonator with a port in it is okay for tuning out a problematic frequency. Basically, it would be a box with a tube in it. Always put the tube in too long, and if it’s at too high a frequency, chop a bit off and measure it again. Using an analogue frequency generator, keep sweeping it until you get a peak amplitude reading. An easy way to tell the frequency is to delaminate a piece of tissue to half thickness and stick a flap over the edge of the Helmholtz port. Have a big speaker with a tone generator and start at 30Hz and sweep upwards. Once you get to the resonant frequency, the tissue will flap about like a bride-to-be. And then you pass the resonant frequency, and it does nothing again. Then you know what frequency it’s tuned to.
Obviously, building a slat or slot resonator is harder to tune, but you could build a box with one slot in it and tune that, as each slot repeated in the final design will function the same.
TECH TIP
The normal practise is to route a solid core door around all four sides, but since we’re in the biz of sound proofing and mass is good, we generally keep the solid core door, cut a piece of 16mm MDF to the interior dimensions and stick it on the back with Green Glue and a couple of screws. What you get is a thicker, heavier door than you can buy as standard, it’s damped with Green Glue, and you don’t have to try route anything out, you just re-cut the MDF to size, and you’re done.
25 YEARS OF RECORDING ARCHITECTURE
Roger D’Arcy has been one half of a driving force behind Recording Architecture for a quarter of a century. Over that time, Recording Architecture has been involved in exotic Greek Island getaways, Alberts in London, Ronnie Woods’ bunker, even Studios 301 over here in Sydney, and too many more to count. He’s seen it all, and written a book about it too. After 25 years, Recording Architecture will finally be calling it quits after the final touches are put on their latest build. Roger kindly took the time to pass on a bit of his knowledge to the budding crop of studio designers.
Mark Davie: What is the most common misconception about what you do?
Roger D’Arcy: There is often surprise that our work is not particularly focused on the equipment – other than the need for a general awareness of the size of a console, the focusing characteristics of a monitoring system or the quantity of outboard gear to be housed in racks, the design process (and the physical realisation of that design) is predominantly architectural. The acoustic performance, and ergonomic and operational requirements must be fully understood and strategically accounted for at an early stage, but it is a job for designers (including suitably experienced architects), but not necessarily recording engineers or acousticians – though they can often make a valuable input as part of a team. I think the name we chose originally says it all – Recording Architecture.
MD: What is the most difficult aspect of getting acoustics right? And why?
RD: Achieving balanced control across a broad frequency range is the single most difficult aspect of getting acoustics right. It is essential to appreciate the vast disparity of wavelengths involved – 6.8m at 50Hz down to 68mm at 5kHz – it therefore follows logically and inescapably that it is impossible to adequately control low frequencies with only wavelength (or quarter wavelength) dependent porous absorbent materials and diffusion techniques. The application of foam-based products (or other relatively lightweight elements) to walls and ceilings simply cannot provide a workable solution – it is absolutely necessary to employ techniques that address the pressure component of a sound wave (such as membrane absorption) which is not wavelength dependent.
MD: What particular types of buildings make for great retrofit studios?
RD: A theoretically ideal space would be large and open, with widely spaced (or preferably no) columns, high (over 4m), and with a solid, ground-bearing floor of high loading capacity such as an industrial space. Basements can be ideal but often have restricted height, and are frequently peppered with columns that can severely restrict the planning of appropriately sized/shaped spaces.
If a building is shared it’s sensible to avoid the upper floors of steel framed or otherwise ‘architecturally lightweight’ structures. There are few better alternatives than being able to construct massive sound-isolating shells off a solid ground floor.
MD: What are three common things that you often see people do wrong when it comes to building a studio?
RD: The placing of only relatively thin and (architecturally) low density absorbent materials (or elements) on walls and ceilings in the vain and misguided hope that this alone will create a balanced acoustic – the lone acoustic foam tile placed directly on a wall behind the speaker is perhaps the most extreme, all too frequent and saddest caricature of this approach. It is impossible to overstress the importance of constantly thinking ‘wavelength’.
Failure to achieve architectural/acoustic symmetry about the front-back axis of a control room. It’s the most basic and critical aspect if monitoring on more than one speaker, and generally the simplest thing to get right at no cost! This would include the (mis)-positioning of stereo speakers at different distances from the boundaries (affecting low frequency) to major architectural asymmetry – e.g. a recessed roof light over one half of the console or a window alcove to one side only of the monitoring position (affects mid and high frequency perception – google ‘precedence effect’).
Placing far too much high frequency absorptive material in both control rooms and live spaces. The aim in a larger performance area would generally be to achieve a diffuse, ‘naturally expansive’ but controlled acoustic, which would be appropriate for anything from drums to string sections and vocals. The only real requirement for a completely ‘dead’ space is in an amplifier chamber for guitar or bass. If the acoustic accuracy of the control room can be trusted then the live space can become a (distortion free) palette of acoustic possibilities using nothing more than instrument placement and microphone technique.
MD: What are a couple of functionality pitfalls you come across in studios?
RD: One potential functionality pitfall can be the obsession with forward vision from the monitoring position into a live space – which forces up the height of the main monitors, which in turn necessitates tilting them, which in turn increases console reflections – and which can seriously restrict the zone in which accurate monitoring can be achieved. RA has almost always avoided it – except in situations where faced with remodelling an existing control room – e.g. London’s Angel Studios control room 3 – here, we located the drivers within new, forward side-wall acoustic treatment, which enabled the mid range units and tweeters to be dropped as low as possible within the pre-existing, over-window monitor housings, and reducing the negative acoustic effects to a minimum. At CTS 1, the monitors were inverted (drivers to the top) and the tweeters completely removed from the cabinets and mounted on small ‘floating baffles’ suspended at a lower level over the large front window of the control room.
A second pitfall that occasionally comes up is the need for a large patch bay associated with a large analogue desk. Control room width can often be restrictive – if positioned to the side of the desk, the patch bays should not become so high as to create potential HF reflections at the monitoring position – similarly, if positioned behind the desk then they need to be distributed horizontally (if space permits) to keep the height down or angled back and away sufficiently to divert any reflections.
MD: You’re finishing up Recording Architecture after a good 25-year period. In that time, what have been the boom periods, and have you seen a decline in new studio builds lately?
RD: RA started out in 1987 in the midst of a major UK economic recession – the ’90s were good (for the whole industry) and then everything seemed to collapse again around the turn of the century. This was followed by a few good years and then the current global gloom and doom kicked in a couple of years ago. But we have built some of our largest and most ambitious projects in recent years – Babajim, Al Rayyan, Analogue Baby (shortly to open just outside Liverpool in the UK). It’s just that we have had to travel increasingly further afield to find the projects we want to be involved with. Overall there seems to be fewer professionally put together new projects, but we have always maintained that there will always be room for a studio that is properly conceived and put together, that knows and understands its market and is well managed. So while many studios are continually closing for a huge variety of reasons – a certain proportion will need continual replacing.
MD: Have you noticed any major changes in studio design over the years to adapt to general changes in the music industry?
RD: In the mid ’80s, there was a massive shift (in popular music) away from recording live musicians as a band. Coupled with a corresponding development and incorporation of processing equipment, and multi track capacity, it led to a demand for much, much larger control rooms. This was accounted for in new-build studio design (with a general trend to provide little more than an associated booth – maybe large enough for drums at most) but in existing facilities, this usually involved the substantial remodelling or conversion of live spaces into control rooms with the original (and usually small) control room becoming a booth (e.g. UB40’s DEP International).
The adoption and incorporation of centre and rear speakers for surround monitoring formats was perhaps the next obvious development in control room design. In recent years, there seems to have been a healthy resurgence in ensemble playing and recording with the intention of capturing the ambience and vibe of the performance, rather than simply sampling it. So, we have come full circle really in commercial music facility design terms – but with centre/surround speakers added and generally more accurate and larger control rooms. But the physics and behaviour of sound are unchanging – wavelengths will not get shorter no matter what the technical advances.
Photography: Neil Waving
Roger D’Arcy has been one half of a driving force behind Recording Architecture for a quarter of a century. Over that time, Recording Architecture has been involved in exotic Greek Island getaways, Alberts in London, Ronnie Woods’ bunker, even Studios 301 over here in Sydney, and too many more to count. He’s seen it all, and written a book about it too. After 25 years, Recording Architecture will finally be calling it quits after the final touches are put on their latest build. Roger kindly took the time to pass on a bit of his knowledge to the budding crop of studio designers.
Mark Davie: What is the most common misconception about what you do?
Roger D’Arcy: There is often surprise that our work is not particularly focused on the equipment – other than the need for a general awareness of the size of a console, the focusing characteristics of a monitoring system or the quantity of outboard gear to be housed in racks, the design process (and the physical realisation of that design) is predominantly architectural. The acoustic performance, and ergonomic and operational requirements must be fully understood and strategically accounted for at an early stage, but it is a job for designers (including suitably experienced architects), but not necessarily recording engineers or acousticians – though they can often make a valuable input as part of a team. I think the name we chose originally says it all – Recording Architecture.
MD: What is the most difficult aspect of getting acoustics right? And why?
RD: Achieving balanced control across a broad frequency range is the single most difficult aspect of getting acoustics right. It is essential to appreciate the vast disparity of wavelengths involved – 6.8m at 50Hz down to 68mm at 5kHz – it therefore follows logically and inescapably that it is impossible to adequately control low frequencies with only wavelength (or quarter wavelength) dependent porous absorbent materials and diffusion techniques. The application of foam-based products (or other relatively lightweight elements) to walls and ceilings simply cannot provide a workable solution – it is absolutely necessary to employ techniques that address the pressure component of a sound wave (such as membrane absorption) which is not wavelength dependent.
MD: What particular types of buildings make for great retrofit studios?
RD: A theoretically ideal space would be large and open, with widely spaced (or preferably no) columns, high (over 4m), and with a solid, ground-bearing floor of high loading capacity such as an industrial space. Basements can be ideal but often have restricted height, and are frequently peppered with columns that can severely restrict the planning of appropriately sized/shaped spaces.
If a building is shared it’s sensible to avoid the upper floors of steel framed or otherwise ‘architecturally lightweight’ structures. There are few better alternatives than being able to construct massive sound-isolating shells off a solid ground floor.
MD: What are three common things that you often see people do wrong when it comes to building a studio?
RD: The placing of only relatively thin and (architecturally) low density absorbent materials (or elements) on walls and ceilings in the vain and misguided hope that this alone will create a balanced acoustic – the lone acoustic foam tile placed directly on a wall behind the speaker is perhaps the most extreme, all too frequent and saddest caricature of this approach. It is impossible to overstress the importance of constantly thinking ‘wavelength’.
Failure to achieve architectural/acoustic symmetry about the front-back axis of a control room. It’s the most basic and critical aspect if monitoring on more than one speaker, and generally the simplest thing to get right at no cost! This would include the (mis)-positioning of stereo speakers at different distances from the boundaries (affecting low frequency) to major architectural asymmetry – e.g. a recessed roof light over one half of the console or a window alcove to one side only of the monitoring position (affects mid and high frequency perception – google ‘precedence effect’).
Placing far too much high frequency absorptive material in both control rooms and live spaces. The aim in a larger performance area would generally be to achieve a diffuse, ‘naturally expansive’ but controlled acoustic, which would be appropriate for anything from drums to string sections and vocals. The only real requirement for a completely ‘dead’ space is in an amplifier chamber for guitar or bass. If the acoustic accuracy of the control room can be trusted then the live space can become a (distortion free) palette of acoustic possibilities using nothing more than instrument placement and microphone technique.
MD: What are a couple of functionality pitfalls you come across in studios?
RD: One potential functionality pitfall can be the obsession with forward vision from the monitoring position into a live space – which forces up the height of the main monitors, which in turn necessitates tilting them, which in turn increases console reflections – and which can seriously restrict the zone in which accurate monitoring can be achieved. RA has almost always avoided it – except in situations where faced with remodelling an existing control room – e.g. London’s Angel Studios control room 3 – here, we located the drivers within new, forward side-wall acoustic treatment, which enabled the mid range units and tweeters to be dropped as low as possible within the pre-existing, over-window monitor housings, and reducing the negative acoustic effects to a minimum. At CTS 1, the monitors were inverted (drivers to the top) and the tweeters completely removed from the cabinets and mounted on small ‘floating baffles’ suspended at a lower level over the large front window of the control room.
A second pitfall that occasionally comes up is the need for a large patch bay associated with a large analogue desk. Control room width can often be restrictive – if positioned to the side of the desk, the patch bays should not become so high as to create potential HF reflections at the monitoring position – similarly, if positioned behind the desk then they need to be distributed horizontally (if space permits) to keep the height down or angled back and away sufficiently to divert any reflections.
MD: You’re finishing up Recording Architecture after a good 25-year period. In that time, what have been the boom periods, and have you seen a decline in new studio builds lately?
RD: RA started out in 1987 in the midst of a major UK economic recession – the ’90s were good (for the whole industry) and then everything seemed to collapse again around the turn of the century. This was followed by a few good years and then the current global gloom and doom kicked in a couple of years ago. But we have built some of our largest and most ambitious projects in recent years – Babajim, Al Rayyan, Analogue Baby (shortly to open just outside Liverpool in the UK). It’s just that we have had to travel increasingly further afield to find the projects we want to be involved with. Overall there seems to be fewer professionally put together new projects, but we have always maintained that there will always be room for a studio that is properly conceived and put together, that knows and understands its market and is well managed. So while many studios are continually closing for a huge variety of reasons – a certain proportion will need continual replacing.
MD: Have you noticed any major changes in studio design over the years to adapt to general changes in the music industry?
RD: In the mid ’80s, there was a massive shift (in popular music) away from recording live musicians as a band. Coupled with a corresponding development and incorporation of processing equipment, and multi track capacity, it led to a demand for much, much larger control rooms. This was accounted for in new-build studio design (with a general trend to provide little more than an associated booth – maybe large enough for drums at most) but in existing facilities, this usually involved the substantial remodelling or conversion of live spaces into control rooms with the original (and usually small) control room becoming a booth (e.g. UB40’s DEP International).
The adoption and incorporation of centre and rear speakers for surround monitoring formats was perhaps the next obvious development in control room design. In recent years, there seems to have been a healthy resurgence in ensemble playing and recording with the intention of capturing the ambience and vibe of the performance, rather than simply sampling it. So, we have come full circle really in commercial music facility design terms – but with centre/surround speakers added and generally more accurate and larger control rooms. But the physics and behaviour of sound are unchanging – wavelengths will not get shorter no matter what the technical advances.
Photography: Neil Waving
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