Recording Magazine’s Room Acoustics Series Part 8

Recording Magazine sends out a newsletter to its subscribers every few weeks. The newsletter is (coincidentally) titled “Sound Advice” and this month it features the eighth in a series about room acoustics. Room acoustics is one of the biggest concerns for Recording Magazine readers. I know that this is also a big issue for those of you in the voiceover world. I asked permission to reprint this newsletter (and will ask to reprint the others in the series as well) so that those of you with home studios can also benefit from the information. I want to personally thank Brent Heintz, VP/Associate Publisher for granting permission, allowing me to share this great information with you.

Please visit Recording Magazine‘s website and their Facebook Page.

Catch up on the series: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7.

Here is the eighth newsletter in the series on Room Acoustics:

Welcome to Sound Advice on Acoustics! Last time we discussed how reflections from walls and ceiling and floor at the front of the room could be spotted and damped; now it’s time to look at the rest of the room, and one critical piece of studio furniture…The potential problem in having a live rear end of a control room is that too much reflected energy may be directed straight back at the mix position. To avoid this, various techniques have been used to diffuse, or spread out, this reflected sound. One of the simplest and most traditional is the use of a curved rear wall (see Figure 8A).

This is a convex surface, which (once again, in accordance with the “angle of incidence” rule) redirects reflections, distributing them throughout the room, instead of letting them beam straight back to the listening position. Even a slight curve will have a useful effect here. This curved surface is called a polycylindrical diffusor, and if there is a cavity behind it, it can also help with low frequency absorption.The thing to avoid most is any concave rear surface, which would focus reflected sound at a particular spot in the room (see Figure 8B), exactly the opposite of the desired result (that may be great for parabolic mics, but not for the back of a studio!).

Another approach is to create an irregular surface on the rear walls, to send reflections in many different directions, effectively diffusing the ambient sound field. A wall covering consisting of various sized blocks or grooves can be employed to accomplish this; even a bookcase filled with different-sized books can be pressed into service in a pinch. However, the best results will be achieved when this kind of surface treatment is not just random, but specifically designed to diffuse sound most effectively at a wide range of frequencies.

More recent computer-assisted designs of this type are able to not only distribute reflections around the room, but also deliver an optimum balance and distribution of all the diffused reflected frequencies. There are many types of these diffusors available. One good example of this approach is a quadratic residue diffusor. This is a panel made up of what appears to be a series of blocks or grooves. They’re not just randomly arranged to simply spread out reflections—their arrangement and depths are determined mathematically by a quadratic residue sequence (there’s a mouthful!), to provide the greatest degree of diffusion and the most even distribution of reflected sound.

Fortunately, for those who tend to cringe at anything more than long division, diffusors of this type are available commercially. Though not cheap, they are very effective and recommended if the budget allows.

Room boundaries are not the only surfaces that can generate reflections. How about that great big console right in front of you? Reflections off the console or work surface and other studio gear like racks can also contribute to a loss of clarity.

Console reflections in particular can be problematic, since they originate right in front of the listening position and therefore can be quite strong. If the speakers are wall- or soffit- mounted, an absorbent “hood” can sometimes be placed behind and over the meter bridge to prevent sound waves from hitting the console surface. Reflections from console-top monitors are more difficult to eliminate, but at least choosing monitors with narrow, controlled vertical dispersion and angling them carefully will beam less high-frequency sound down to be reflected off the console surface.

Keep other gear out of the direct path of sound from the speakers—use lower racks, or position tall racks to the rear of the mix position. The sharp edges of most racks diffract sound (diffraction is a change in direction of a sound wave caused by an obstacle)—rounding off such sharp edges helps diffuse such reflections. After the room is initially set up and tuned, checking the acoustics as additional pieces of gear are installed can help to identify and prevent new problems from being introduced.

Another approach to minimizing the problematic effects of strong early reflections is to set up the mix position no more than 3 feet or so from the speakers, placing the listener in the speakers’ “near-field.” Theoretically, in this zone, the direct sound from the speakers should be predominant over reflections from other surfaces, providing a more neutral response even in a room with less than optimal acoustics.

This approach is called Near-Field Monitoring™, a concept that was trademarked by industry veteran Ed Long. It is usually implemented via the use of console-top monitors. This does work fairly well, the sound in this near-field area is often somewhat smoother than sound from the same speakers in the far-field (at greater listening distances) of the same room.

This type of setup is often recommended for studios where, for one reason or another, room treatments are not able to completely control reflection issues (which includes most small studios). However, near-field monitoring is not a panacea for acoustic problems—standing wave effects will still be present, and as noted above, the possibility of close strong reflections from the console itself can still potentially compromise monitoring accuracy.

Next time we’ll wrap up our introductory discussion with a look at live rooms for tracking and a list of references you can look at. See you then!

Voiceover Processors – Hardware vs Software Revisited

There is a lot of talk about mic modelers in social media today. A while back, I was asked the question in regards to voiceovers, which processors (i.e.- compressors, EQ, etc.) are better: hardware or software? Well my answer to that question, also applies to mic modelers. The tools available in both formats have advantages and disadvantages. While nothing can truly substitute for the real thing, this doesn’t mean that a simulation can’t be useful, effective or even very good. Whether we are talking about microphones or other audio processors, there are hardware models that sound great and perform exceptionally well, and then there are some that do not. The same is true for software versions.

Consistency is critical in voiceover work, so a clean unprocessed signal during recording is usually ideal. As a voice talent you generally will not, or should not, be using processors when recording, and if you are it should be very minimal and the same every time. Therefore, you really don’t need a great deal of choices.

If you are a voice over talent working from your home studio, chances are that software versions will be your best or maybe your only option. Besides not really needing many choices, it just doesn’t make financial sense for someone who only does voiceover work to own a vast array of microphones, hardware or rack gear. When considering processors, hardware boxes will take up valuable space in what are often small spaces to begin with. Another downside is that they give off a great deal of heat (especially tube gear) making tight spaces more uncomfortable.

Does hardware sound better than software? Not necessarily. As an engineer, I would love to have full racks of hardware options at my disposal. Each helps to create different sounds, characteristics and textures within a mix. But would having racks of gear stop me from using software versions? Not likely.

Software versions of processors perform the same functions as their hardware counterparts, and like their hardware counterparts, they also have different sounds, characteristics and textures of their own. These characteristics vary from processor to processor in software versions just as they do in different brands and types of hardware. Software versions meant to emulate specific models of rack gear can come very close to capturing that hardware sound. But they will still have their own sound and character that make them different, not necessarily better or worse, than the hardware they emulate.

If it is great sound as well as a variety of sounds you are going for, then having both hardware and software is the way to go. However, if your space and budget are limited, then having a variety of software versions will probably be just fine for your needs. The bottom line is this, whether your processing options come from hardware or software, the tools are only as good and as effective as the person using them. Get the best sounding tools you can afford, learn how to use them and most importantly… use your ears.

Recording Magazine’s Acoustics Series Part 4

Recording Magazine sends out a newsletter to its subscribers every few weeks. The newsletter is (coincidentally) titled “Sound Advice” and this month it features the fourth in a series about room acoustics. Room acoustics is one of the biggest concerns for Recording Magazine readers. I know that this is also a big issue for those of you in the voiceover world. I asked permission to reprint this newsletter (and will ask to reprint the others in the series as well) so that those of you with home studios can also benefit from the information. I want to personally thank Brent Heintz, VP/Associate Publisher for granting permission, allowing me to share this great information with you.

Please visit Recording Magazine‘s website and Facebook Page.

Catch up or skip ahead: Part 1, Part 2, Part 3, Part 5, Part 6, Part 7, Part 8.

Here is the fourth newsletter in the series on Room Acoustics:

Welcome back to Sound Advice on Acoustics! Last month, we talked about how the relative dimensions of a room can make standing waves (modes) better or worse, and introduced special ratios called Golden Means that will help produce the smoothest bass behavior… but all that just tells us where the modes are, it doesn’t make them go away!

This naturally brings us to the question of what to do about these modal issues, once the room has been analyzed. In the case of new room or room-in-a-room construction, following the Golden Mean ratios (we printed a table of them last time) is an excellent place to start.

If space permits, it’s also recommended that room volume (L x W x H) be at least 1500 cubic feet (for example 16′ x 12′ x 8’=1536 cu’)—this will push the lower-order modes down in frequency and provide greater density and evenness in the useful bass range.

In the case of an existing space, however, adjusting room dimensions is, of course, not usually an option. Sometimes, after mapping out the locations of the nodes and antinodes, simply rearranging the location of speakers and listening position in the room can help to avoid being in the path of the most severe artifacts. Examples: Not establishing a critical listening position right up against a wall; placing speakers in a more neutral location.

But while this can help, even good modal spacing and careful room layout isn’t really an adequate alternative to a truly balanced room response for serious use, so in a professional situation the preferred option is to address problematic standing waves with room treatments.

One approach, frequently used in studio construction, is the splaying (angling) of the walls/floor/ceiling, to avoid having the parallel surfaces which give rise to standing waves. However, this will not really eliminate standing waves, but will simply shift their distribution in the room slightly. This may help to reduce the severity of some modes a little, but it will also make it more difficult to calculate and map them out. Unless you have access to computer software to help you map out the room’s response, it’s probably better to stick with a rectangular shape with predictable modal behavior, and turn to other solutions.

Another way to deal with standing waves might be to absorb them. Unfortunately the absorption of low frequencies (by typical porous absorbers that you could easily apply to walls and surfaces) would require depths that are comparable to the wavelengths of the frequencies to be absorbed, which would be impractical.

In practice, the commonly available sheets and tiles you often see in studios are only effective at mid and higher frequencies, well above those at which standing waves form. To absorb low frequencies, with their room-sized wavelengths, the best approach is to “trap” the low frequency waves via the use of cavities at or behind the walls. This approach has given rise to the term “Bass Traps” for some such cavity-based solutions.

A bass trap is a tuned cavity with a depth of a quarter-wavelength of the frequency at which maximum absorption is desired. Such a cavity could be built into one of the parallel walls that are contributing to the formation of a standing wave. The wave would enter the cavity at its point of maximum pressure buildup, at the wall. Inside the cavity, maximum pressure would develop at the back, resulting in zero pressure at the mouth (opening), countering the normal pressure buildup there.

A bass trap will be effective at 1/4 wavelength and odd-numbered multiples of 1/4 wavelength. This technique can be effective, but of course requires some extra depth to be available behind the walls if built in.

Another approach is based on a design called a Helmholz resonator (an example of this is a long-necked bottle). For our purposes this would consist of an opening or series of openings (like the neck of the bottle) into a cavity (or connected series of cavities). The resonant frequency of the cavity is determined by the length of the “neck” and the volume of air in the cavity. Sound is absorbed at and around the resonant frequency (matched to the frequency of a standing wave).

It’s possible to construct your own resonators, usually consisting of a panel with holes or slots in it in front of a cavity (corner placement is a good choice; these can even be free-standing). The exact size and spacing of the holes is the key to “tuning” such a perforated resonator to a particular mode—the formulas involved are not too complex, but there isn’t enough space to really get into the specifics here.

There are quite a few other types of low-frequency absorbers, and if building your own is not feasible, many commercial standalone solutions are available that work very well.

Wow, all that discussion just to deal with the low end! There’s more to deal with next time; see you then…