Pilchner-Schoustal Acoustic X

Figure 1: Modal display.

Acoustic design is usually a combination of guesswork, intuition and very hard maths. Paul White tries delegating it to his PC instead.

During the course of my series on basic acoustic design, which finished last month, I've mentioned Acoustic X on more than one occasion, not least because it's the only software I've found that provides serious help with studio design for users with a limited knowledge of acoustics. It was developed by Canadian acoustic consultants Pilchner‑Schoustal with a view to providing the project studio designer with the basic tools necessary to optimise the main acoustic parameters of a small room.

Designed to run on just about any CD‑ROM‑equipped PC running Windows 95, the software requires a password from the manufacturers to activate it once it has been installed. The program itself is divided into four main modules, each of which may be used independently, though in most cases, you'd start at the beginning and work through them in sequence. In Module number one, you enter the dimensions of a rectangular room you want evaluated, and the program checks the modal response of the room, plotting out the axial, tangential and oblique modes, and warning of any problematic conditions. If you haven't yet built the room, you can give the program a set of upper and lower size limits for each dimension and it will work out the optimum room dimensions for you.

Module two works out speaker boundary interference so that it can advise on the best position in the room to place your monitors.

Module three uses ray tracing to track reflections from the studio walls so that their arrival time and level at the listening position can be evaluated. Finally, module four is used to determine how much absorbent material is required to achieve the target reverb time. On top of that there's a useful acoustic calculator that can be called up at any time. The best way to get to know this software is to examine one module at a time.

Modal Response

Figure 2a: Speaker boundary response.

Room modes occur when a sound is present at a frequency such that its wavelength is an exact multiple of one of the room dimensions. As there are many multiples, each mode comprises a whole series of frequencies. How well a room performs for listening is determined by how evenly these modes are distributed, and of course that depends on the room dimensions. In addition to the axial modes that exist between opposite surfaces, Acoustic X also calculates tangential models involving pairs of surfaces in two axes and oblique modes that involve all three axes of the room. The software checks to see if the modal distribution increases with every third‑octave band, which it should, and also checks to make sure that two or more modes don't exist at the same frequency unless there are at least five modes in that particular third‑octave band to help smooth out the effect.

As soon as the room dimensions are entered, Acoustic X displays the modal response, which can be viewed as a graph or as a list of modes. If problems are likely to occur, the software issues a warning detailing the frequencies at which modes coincide. Low frequency modal pile‑up problems can then be tackled by installing a low‑frequency absorber at the problem frequency. The software is also able to construct a three‑dimensional pressure graph of the room — absorbers must be placed in regions of high pressure to be effective, generally in the corners. Acoustic X also includes a library of 'famous' room ratios that have been shown to produce good listening results, though these won't always work well if the room size is scaled up or down from the original as the modal distribution is affected by room volume. Still, it's easy enough to run the software and recheck the results. Various display options are available within this module, and on the whole, it's as simple as entering data and watching the results appear. It certainly beats doing the calculations manually! Figure 1, on page 184, shows one of the display options.

Speaker Boundary Response

Figure 2b: Room view display.

This module calculates what happens when low‑frequency sound from the monitors reflects from the walls, floor and ceiling near to the monitors and then combines with the direct sound. Because the reflections travel further than the direct sound before reaching the listener, they are slightly delayed, resulting in phase additions and cancellations at specific frequencies. This boundary effect is why monitors sound more bassy when they are mounted close to walls or corners. A small database of speakers is included with the program, and you can add your own by detailing the box size and the height of the drivers above the bottom of the cabinet. The database software also asks for the low‑frequency cutoff of your monitors and the low‑frequency crossover frequency.

Once you've told the software whereabouts your speakers are positioned, it will create a graph of the low‑frequency response, as shown in Figure 2a. Acoustic X will also show you where your speakers should be placed for the best results, but if you don't have the flexibility to follow this advice, you can specify a range of limits over which you can move the speakers and the program will give you the best option. Interestingly, the software can also work out the best place for surround speakers. In all cases, an optimum listening area is shown, and the speaker angles in both the vertical and horizontal plane are displayed. A three‑dimensional drawing of the room and the speakers within it is created, also showing the ideal positions for wall/ceiling absorbers and corner traps. This view is shown in Figure 2b.

Ray Tracing

Figure 3: Results of ray tracing.

Here information about room size, speaker placement and absorber placement (as shown in the room view) is used to work out the possible reflection paths from the monitors back to the listener. Each of these is displayed with its delay time and amplitude; you can also see a three‑dimensional room view as the reflections are being traced. How long tracing takes depends on how coarse or fine you set the reflection angle resolution to be, and on how many reflections you want to view. In reality, this particular display soon becomes a mess of coloured lines, but the listed results, as shown in Figure 3 on page 188, are valuable. A Data View mode shows a number of parameters including the room volume, surface area, the average absorption coefficient (Alpha), RT60, the number of reflections used in ray tracing and the total number of traces calculated. In practice, what you need to confirm is that no early reflections are louder than around 10dB below the direct sound. As with the other modules, a number of presentation views are available.

Reverb Time

The reverb time module aims to help you determine what types of absorbent treatment are needed, and how much. You can simulate the application of varying areas of different materials from a database to the six room surfaces, and each time you make a change, the reverb time is recalculated for eight frequencies in one‑octave bands running from 63Hz to 8kHz and displayed on a graph. Whenever a material is selected, a chart is created showing the absorption coefficients for that material over the same eight frequencies. If the material you want to use isn't in the database, you can add it to the database providing you have its absorption coefficients.

When starting this module, you can pick from six different room types ranging from a Dolby surround cinema to a user‑defined room; average data for a control room, studio area and isolation booth are included. Once one of these is chosen, it shows up as a reference line on the reverb time graph alongside the calculated line for your room. You can also opt to calculate the reverb time using formulae from Sabine, Eyring, Fitzroy (based on Sabine), Fitzroy (based on Eyring) or feed in the actual measured reverb time of the empty room. While Sabine's formula is the simplest, Eyring is better for rooms that contain a lot of absorptive materials, and the Fitzroy methods are best where the absorptive materials are not evenly distributed. That means that it's probably best to use one of the Fitzroy methods to evaluate a typical control room, which is why it's important that the program knows on which surfaces you've placed your various absorbers. There is also an optimisation routine that lets the program change the areas of certain absorbers within limits set by the user to get the closest fit to the optimum reverb characteristics. Unfortunately, however, this deals only with total areas of absorption and not placement, so if you run this, the Fitzroy options are removed from the menu.

The final accessory is an acoustic calculator that can be brought up at any time by clicking the calculator button on the tool bar. Here you can change from metric to imperial units, work out inverse square law calculations, add levels, convert frequency to wavelength and vice versa, calculate comb filter frequencies from a delay time, and of course you can add up!

In Use

Acoustic X doesn't have the neatest interface in the world, but it's fairly easy to get to know, and other than the ray tracing option, most calculations are virtually instantaneous. Calculating even the basic axial room modes manually can be very tedious, but Acoustic X shows you all three room mode types and their frequencies as soon as you enter your room size. Similarly, the reverb time calculations are fast and easy, with a useful number of real‑world surfaces included in the database. There's even a figure for freshly fallen snow, but I don't know of anyone who's used that as the basis for studio design!

The speaker boundary module is a genuine help in figuring out the best place to put your speakers, and also helps you place absorbers to cut down on reflections, while the ray tracing section shows the severity and timing of early reflections. The handbook takes you through each stage quite logically and in plenty of detail, and it even allows you to print reports if you're doing acoustic design work commercially, though it falls a little short when it comes to telling you exactly to what extent you can rely on the calculated data. The limitation with any such program is that the results for RT60 are only as good as the figures you feed in, and even then, the result is for an empty treated room. Once you add a mixing console (which changes the early reflections considerably), a few racks of gear and some shelving, things are bound to change to some extent.

If this program goes on to be revised, I'd like to see a more graphical approach that allow areas of absorbent material to be dragged onto a representation of the room and then be resized using the mouse, just as you might in a drawing program. The software could then calculate the changes in real time as you made them. Also, for project studio owners, it might be useful to have a more streamlined approach to room optimisation where you, for example, provide the information on room size, surface types, monitor details and what types (and maximum sizes) of acoustic absorbers you have available (curtains, foam tiles, acoustic blankets and so on). The software could then run through the whole optimisation program automatically, presenting the user with a best option for speaker and absorber placement as well as a report on how well, or otherwise, the optimised studio will perform. As it stands, you have to go through each module individually and make decisions that some people might feel unqualified to make. Each of the modules has several more options and facets than I've had room to describe here, but I hope you have a general idea of what the program can do.

Summary

Acoustic X is ideally suited to the project studio owner with a little background knowledge who wants to have a go at improving their listening environment. This includes just about any SOS reader who has followed our series on basic acoustics and would like to try to put a few of the concepts into practice. However, even taking into account the comprehensive handbook, Acoustic X could be a little overwhelming for complete novices. Such users may well be more interested in the forthcoming 'Lite' version, which has a simplified user interface, omits the ray‑tracing module and is expected to cost about half as much as the full version.

The professional studio designer, by contrast, will probably come up against Acoustic X's limitations fairly quickly, or at least have to use it in conjunction with specialised measuring equipment. As pointed out earlier, perhaps the main limitation is that you can't always be certain of the precise acoustic properties of the materials from which your room is built, and there's no way to model the effects of a mixing console or racks of gear other than inputting figures (if you can get any figures) as areas of wall or floor. These limitations aside, Acoustic X is very useful tool that takes all the donkey work out of calculating room modes and reverberation time, and it's so fast that it allows you to explore any number of 'what if' scenarios. To my knowledge, there's no equivalent product on the market, though there's definitely a need for acoustic design software pitched at this level. Despite a few rough edges, Acoustic X is both practical and educational, and though it's not the last word in studio design, it beats the hell out of sitting at a desk with a notebook and a calculator!