Goodacoustics are often the dividing line between professional and personalstudios. After all, the gear that’s used to record and mix much ofwhat we hear on the radio these days is quite similar to what can befound in a well-appointed personal studio. Largely, it’s thosebig, beautiful, acoustically accurate rooms for recording and mixingthat separate the big guys from the little.
But by giving a little attention to the acoustics in your ownstudio, you can improve the quality of your mixes so that they competewith the best the majors have to offer.
BASIC CONSIDERATIONS
Before getting into acoustics, make sure that there are no weaklinks in your monitoring system. Monitoring systems for criticallistening must have a fairly flat frequency response from about 60 Hz(or lower) to 16 kHz (or higher). The power amp should also have asflat a frequency response and as low a distortion spec as possible.Fortunately, most of the studio-grade near-field monitors and poweramps on the market today meet these specifications. Therefore,selecting the "right" system is often just a matter of personaltaste.
The monitoring system must be set up symmetrically within the room.The distance between the speakers should be the same as the distancefrom each speaker to your ears, thus forming an equilateral trianglewith your head. For near-field monitoring, your speakers should beabout two to four feet apart, depending on their size and dispersionand what is most feasible ergonomically. Also, the center of thisequilateral triangle should be equidistant from the room’s sidewalls.
Unfortunately, falling short of sonic accuracy is common, even whenhigh-quality gear is placed symmetrically in the room. The overallsound is often boomy and muddy, the bass is too loud or too soft, thehigh-end is dull or harsh sounding, and the imaging is blurry andundefined. Room acoustics can play a significant role in creating, andreducing, these problems.
In pro studios, room acoustics are considered a top priority.Typically, owners spend lots of money on professional consultation,premium construction, and first-rate sonic treatments, sparing noexpense to achieve problem-free, acoustically "neutral" monitoringenvironments. However, overcoming acoustical problems is not outsidethe financial realm of the personal studio owner. You should expect tospend at least the same amount of money for acoustical treatment as youdid for your monitors.
The acoustical problems that occur most commonly in small monitoringrooms are room resonances (standing waves), speaker/boundaryinterference, early reflections, and poorly diffused late reflections.These problems can be overcome in three easy steps.
Step 1: Controlling Resonance and Reflections
The first step deals with low frequencies–from 20 Hz to 500 Hz.This frequency range affects the smoothness of the bass and low mids:if the room’s acoustics are balanced, the bass and low mids willbe full and warm; if the room has significant frequency boosts in thisrange, the sound will be boomy or muddy; and if the room hassignificant frequency dips in in this range, the sound will be thin andhollow. The goal in Step 1 is to flatten out the room’slow-frequency response so as to avoid erroneously mixing music tocompensate for the boosts or dips caused by the acousticenvironment.
Resonance and standing waves. The way low frequencies behavein a room is dictated largely by the room’s dimensions. Certainfrequencies, due to the lengths of their respective sound waves, arereinforced as they move between the room’s boundaries (walls,floor, and ceiling), creating resonant boosts in volume at thosefrequencies. These resonances are commonly referred to as standingwaves.
You can estimate the most prominent resonant frequencies of a roomby using the following equation:
f1 = 1,130/2L = 565/L
In this formula, f1 represents the resonant frequency,and 1,130 represents the speed of sound in air under "normal"conditions, which are defined as one atmosphere of pressure at sealevel at 21 degrees Celsius. L represents the length of the room infeet. For example, if the room is ten feet long, there will be anatural resonant volume boost in the room at 56.5 Hz. In addition,natural boosts in volume will occur at multiples of this frequency:f2 = 113 Hz, f3 = 169.5 Hz, f4 = 226Hz, and so on.
These resonances become more closely spaced and their volumesdiminish as you move up the frequency spectrum. Therefore, in smallrooms, resonances are typically not as problematic above 200 Hz.
Speaker/boundary interference. Because low frequencies areomnidirectional by nature, they reflect from all nearby roomboundaries. These reflections adversely affect low-frequency response,making the bass sound as though it’s coming from differentdirections.
These slightly delayed reflections of the original signal causecomb-filtering peaks and dips within the range of frequencies above themodal resonance range (typically 200 Hz in a small room) to an upperlimit of approximately 500 Hz. The increasingly directional nature ofsound above 400 Hz makes speaker/boundary interference less of aproblem for mid and high frequencies.
Standing waves and speaker/boundary interference can causefrequency-response deviations as high as 15 dB. This amount of levelvariation could keep you guessing about proper levels for all the bassfrequencies you mix.
Identifying the problems. The best way to tell if you have aproblem with standing waves or speaker/boundary interference is througha combination of listening and measurement. First, listen to a finelyengineered CD through your monitoring system at a decent mix level. TheCD you select for this exercise should have a tight bass sound andminimal reverb. Some of my favorites are Tchad Blake’s mixes onCrowded House’s Woodface album and Sheryl Crow’s two latestalbums, Sheryl Crow and the Globe Sessions. Tchad Blake usesinteresting imaging and minimal reverb, which makes his mixes great forcritical listening exercises. (But then, that’s the style of musicI mix; you may prefer something else).
As you listen to these CDs through your system, notice whether thebass sounds tight, smooth, and consistent in volume. If the mix soundsfull and warm, then your room naturally promotes good bass response.Larger "small rooms," rooms with lots of windows, and rooms withlightweight walls tend to balance bass frequencies nicely.
However, if the room rumbles and booms with the music, or if thebass either sounds mushy or alternates between high and low levels,then you may have a problem with resonance or speaker/boundaryinterference. At this point, it’s a good idea to take a precisemeasurement of your room’s resonant characteristics.
Simple measures. Measuring for room resonance andspeaker/boundary interference requires a high-resolution frequencyanalyzer, rather than the usual octave or 1/3-octave real-timeanalyzer. Octave and 1/3-octave analyzers average out too muchinformation to be useful for this task. Fortunately, some new softwareprograms allow you to perform high-resolution acoustical analysisaffordably from a computer (see the sidebar "Acoustical Programs for the PC").
For example, Figure 4 (in the print version of this article, fromFebruary, 1999) depicts a high-resolution, low-frequency response graphmade with AcoustiSoft’s ETF 4.0 software. Measurements such asthese can help you better identify the problem areas in your studio.Given the high resolution of the measurement, narrow notches in theresponse aren’t that bad, but you should pay attention to thegeneral frequency-response trends. Notice, for instance, that theaverage signal level is around -16 dB, with a boost of 8 dB centeredaround 300 Hz, a 12 dB boost around 125 Hz, and one sharp 16 dB boostaround 55 Hz.
Based on this measurement, you can guess that our monitoring systemwould sound muddy in this room because of the 300 Hz boost, too bassybecause of the 125 Hz boost, and too boomy because of the peak at 55Hz. Given that the specifications for the loudspeakers used in thistest are flat throughout their low end, it can be assumed that theselow-frequency response deviations result from room influences.
Fixing the problems. A good way to smooth out resonance andboundary-reflection problems is by optimizing the location of thespeakers and the listener in the room. Resonance and boundaryreflection are less pronounced in certain areas of a given room. Infact, changing the location of the loudspeakers and listening positionoften results in a drastic change in sound from the previouslocation.
Optimizing speaker and listener locations used to be a process oftrial and error. Now, however, there are several PC-based programs thatcan model the acoustics of your room and help you find a location whereroom resonance and boundary reflections are minimized.
I used a speaker/listener optimization program on the samemonitoring system shown in Figure 4. Then, after changing the locationof the listening station, I remeasured the low-frequency response. Theoptimization program worked well: the average signal level is stillabout -16 dB, but the significant boosts shown in Figure 4 have beensmoothed out. The only exception is that the boost at 55 Hzremains.
The frequency response still appears to have a lot of variationbecause of the high resolution of the measurement. If measured using alower resolution of 1/6-octave or 1/3-octave, however, the frequencyresponse would appear as a nearly flat line.
After optimizing the placement of the speakers and listener, you cando several other things to further reduce any problematic boosts in thelow frequencies. Applying normal acoustical foam works well to dampenhigh- and mid-frequency energy but doesn’t adequately absorb lowfrequencies. You can absorb low frequencies by using a bass trap, whichis any acoustical device that absorbs low-frequency energy in aroom.
Often, the best place to put bass traps is in the corners of rooms,because that’s where low-frequency energy collects. As thelow-frequency energy is absorbed, the various peaks are reduced,resulting in a smoother bass sound overall. The average listening roombenefits from having about 1 percent of its total volume dedicated tobass trapping.
Many companies manufacture broadband bass traps, but one inparticular, Acoustic Sciences Corporation (ASC), also offers affordableacoustical consulting for treatment of low frequencies using a testthat they developed called the M.A.T.T. (Musical Articulation TestTones) test.
Step 2: Reducing Early Reflections
The second step deals with frequencies of 500 Hz and up. This rangehas a critical effect on the accuracy of the monitoring system’simaging and its mid- and high-frequency tonality. The biggest detrimentto mid- and high-frequency accuracy is the presence of earlyreflections.
Early reflections. When listening to your monitors, you heara combination of the direct sound from the speakers followed by thereflections of the direct sound from the room’s boundaries (walls,ceiling, and other hard surfaces). Reflections that hit the ear within20 milliseconds of when the direct sound is produced are heard as partof the direct sound and are called early reflections. Because soundwaves travel at a rate of about one foot per millisecond, most of thefirst reflections that make their way to the listening position in asmall room qualify as early reflections.
Early reflections often add audible comb-filter distortion to thedirect signal, tainting the frequency response with a variety of boostsand dips. Early reflections also tend to blur the stereo imagingbetween the speakers, making it difficult to accurately hear the exactposition of sounds within the stereo field.
Identifying early-reflection problems. The best way todetermine whether you have a problem with early reflections is tolisten for and measure them. For this exercise, play a well-mixed CDthat has clear and precise imaging, such as one of those mentioned inStep 1.
As you listen, notice whether the locations of the instruments areclearly identifiable in the stereo spread or whether they blend betweenthe speakers. You should be able to hear the various instruments comingfrom specific points in the stereo field. Problematic earlyreflections, however, will degrade the aural clues that help usidentify stereo imaging, and the resulting mix will sound blended andfuzzy.
Measuring for early reflections requires a high-resolution analyzerthat can generate an impulse response, or an energy-time curve, of yourenvironment. Or you can use one of several acoustics analyzer programsmentioned in the sidebar "Acoustical Programs for the PC" to generatean energy-time curve. This kind of measurement will give you a clearidea about any problems you might have with early-reflectionlevels.
The direct sound from the speakers is at 10 milliseconds, with theearly reflections occurring between 10 to 30 milliseconds–a periodof 20 milliseconds. In a balanced acoustical environment, thereflections between 10 and 30 milliseconds would be 15 to 20 dB belowthe level of the direct sound. In other words, early reflections shouldbe virtually inaudible.
The early reflections are about 15 to 18 dB below the level of thedirect sound for nearly 15 milliseconds. Although not perfect, thisparticular room would have good imaging and would be free ofsignificant comb-filtering in the mid- and high-frequencies. Prior tothis test, the room had already been treated with acoustical foam atstrategic points to reduce early reflections. If measurements orlistening tests confirm that your room has problematic earlyreflections, you should consider a similar treatment with acousticalfoam.
Fixing early reflection problems. The goal in fixing earlyreflections is to reduce them to an inaudible level, which is typicallyabout 15 to 20 dB below the level of the direct sound. This is wheresound absorption materials, such as acoustical foam, work wonderfully.Companies such as ASC, RPG, and Acoustical Solutions market a varietyof sound absorption products. Generally, it is best to use productsthat have sound-absorption coefficients greater than 1 and that absorbfrequencies down to 400 or 500 Hz.
You don’t need to cover every inch of your walls with thisstuff: using too much absorption can make the room sound too dry.Rather, determine the best places to put sound absorbers to reduceearly reflections. You can easily do this by using the "mirrortrick."
To perform the mirror trick, sit in the mix position facing thespeakers. Then, have someone move a picture-size mirror flush along thewalls, ceiling, and other surfaces to your sides and front. (You areallowed to turn your head, of course.) Any spot on the walls or ceilingwhere you can see the face of the speaker in the mirror should becovered with sound-absorption material. If you are unable to see thefront of the speaker in the mirror, it is best to leave the surfacealone. Once you get the concept, you can perform this operation withouta mirror. It’s also a good idea to cover the wall space behind andbetween the speakers with sound absorption to reduce any diffractionreflections from the speakers.
There will be a noticeable improvement in your system’s imagingand frequency response once you have reduced early reflections in yourroom. You will immediately hear sounds in your mixes that werepreviously masked.
Step 3: Diffusing Later Reflections
The third step, which also covers frequencies of 500 Hz and up,deals with late reflections. Unlike early reflections, late reflectionsarrive outside of the ear’s integration time and do notnecessarily affect the accuracy of the monitoring system. In fact, latereflections are desirable for creating acoustical "spaciousness" in theroom. Without them, the room would sound like a dry, anechoicchamber.
The problem is that small rooms have such a low density ofreflections that later reflections typically sound sparse and choppy intheir decay. You can improve this situation by increasing the diffusionin the room.
The concept of diffusing late reflections in small rooms proceedsfrom the fact that mid- and high-frequency sounds typically reflectfrom a flat surface at a single angle only. However, when mid- andhigh-frequency sounds strike a diffusing surface on a wall (such as aquadratic residue diffuser), they reflect back into the room at manydifferent angles. This results in a more complex spread of sound, whichis known as diffusion. Spreading the reflections out in space and timealso reduces their volume levels.
The best way to identify diffusion problems is by listening forthem. Sit in the mix position, and have another person clap loudly infront of each speaker. This simulates the sound of transients comingfrom the speakers. Does the room have a noticeable echo at the mixposition? Do the reflections sound well blended, or do they sound harshand fluttery? If you notice echoes, your room would benefit from addeddiffusion.
Fixing diffusion problems. Improving diffusion is generallydone by placing diffusive surfaces along the back wall of the room. Forexample, when sound strikes a cylindrical (as opposed to flat) object,it reflects into the room laterally over a 120-degree arc. This createsa uniform spreading of the reflection back into the room. Diffusers canbe as simple as bookshelves or cylindrical objects, or as complex asprimitive root and quadratic residue diffusers. However, some diffusersare much more effective than others.
The most effective diffuser is the quadratic residue diffuser. Firstconceived and proposed by acoustics researcher Manfred Schroeder, itwas commercially introduced into the audio world by Dr. PeterD’Antonio of RPG Diffusor Systems, Inc. A quadratic residuediffuser is essentially a box that comprises a series of parallel"wells" of varying depths. The depth and width of the wells arecalculated to give an effective diffusion of a specific range offrequencies. In addition, these units reflect sound laterally over a180-degree angle.
The primitive root diffuser is also highly effective. Its wellconfiguration is based on a different mathematical sequence than thatof the quadratic residue diffuser. Both these types of diffusers arecommercially available through RPG Diffusor Systems.
Placing a diffuser at each point where sound first reflects from theback wall will improve diffusion and will result in a more natural and"spacious" decay. Plan on covering about 60 percent of the rear wallwith diffusers if you want to achieve a highly noticeable diffusioneffect in the room.
Diffusion is like icing on the cake for room acoustics: it gives theroom a pleasant, spacious ambience that often makes the room mucheasier to work in. Once you’ve completed this last step, yourroom’s configuration will most likely resemble the one shown inFigure 9.
Upon Reflection
An accurate monitoring system in a balanced acoustical environmentallows you to clearly hear the imaging, tonality, and other nuances inyour mixes. Smoothing out room resonance and boundary reflections,reducing early reflections, and diffusing late reflections are the bestmethods of improving a studio’s listening environment. As you gothrough each of these steps, take advantage of the various toolsmentioned in this article, and don’t be shy about contactingpeople for advice.
In the end, good acoustics will not only make your music and yourmixes more fun to work with and listen to, they will also increase yourefficiency and make the task of mixing easier. Ultimately, you willturn out more reliable and professional-sounding mixes–and more ofthem.
Geoffrey Goacher is the founder of Acoustical ResearchAssociates, which specializes in research and communications on audioand acoustics for critical-listening environments.
Acoustical Programs forthe PC
High-quality acoustical measurement systems have traditionally beentoo expensive for the average audio enthusiast. These systems havetherefore remained in the domain of acoustical consultants and audiodesigners, who have bigger budgets. Recently, however, significantadvances have been made in the availability of affordable measurementsystems for home-studio owners.
AcoustiSoft’s ETF Room Acoustics Analyzer, LibertyInstruments’ LAUD, and JBL’s SMAART are all affordable,Windows-based software programs that will turn your computer into anacoustics analyzer. Each of these companies can give you furtherinstruction on taking and interpreting measurements of your room if youneed help.
Several new programs are also available for optimizing location ofmonitors and the mix position. KB Acoustics’ Visual Ears,Pilchner-Schoustal’s Acoustics-X, and RPG’s Room Optimizerare three PC-based programs that will model the resonances and boundaryreflections of your room and help you find an optimal location forloudspeakers and listeners. These programs usually work very well,provided your room fits the program’s criteria.