Studio Acoustics Part 2

Studio Acoustics Part 2: Standing Waves askaudiomag.com by Joe Albano March 18, 2015

This mini-series sees Joe Albano demystifing the science of studio acoustics. In

Part 2 we look at low-frequency standing waves which can get in the way of

getting a well balanced mix.

In the first installment of this series, I outlined a number of room acoustics

issues that can get in the way of achieving good recordings and mixes. In this

article, Ill start going over the specifics, beginning with one of the most common

causes of problems with getting a good mix balance, and the ability of a mix to

travel wellto sound good in other locations. That issue is low-frequency

standing waves.

Stand By Me

Whenever sound waves occur in an enclosed space, they interact with the room

boundariesthey can either reflect off them, be absorbed by them, or pass

through them. Standing waves, a.k.a. room modes, are a function of reflected

waves. When mid and high-frequency waves bounce around a room, they can

either result in a pleasant sense of ambiencelivenessor cause unpleasant

artifacts, like flutter echoes. But when low-frequency waves reflect off room

surfaces, they manifest themselves a little differently.

http://www.askaudiomag.com/articles/studio-acoustics-part-2-standing-waveshttp://askaudiomag.com/articles/studio-acoustics-part-1-an-overview-of-room-issues

Fig 1 A

typical Studio Control room layout

Without getting into the physics of it too much, all audio waves have a particular

fhrequencythe rate of vibration of the sound-producing object, measured in

vibrational cycles-per-second, or hertz. The wave itself is a series of air pressure

variations (higher than normal pressurecompressions, and lower-than normal

pressurerarefactions), that emanate out from the source and propagate

through the room. When one of these waves meets a room surface (wall, floor,

ceiling) it will reflect back into the room, bouncing from surface to surface. At

mid and high frequencies, this can be subtle, but low frequencies present a

different case, because of their wavelengths.

The Long and Short of It

Every wave has a wavelengththe physical distance that wave travels in the

room is the time it takes to complete one cycle of vibration. Since low-frequency

waves vibrate more slowly than mids or highs, their wavelengths are longer.

Mids and highs may have wavelengths of anywhere from a few inches to a few

feet, but lows often have wavelengths that approach and exceed the room

dimensions themselves. When such a wave reflects between two parallel surfaces

in a room, it doubles back on itself, causing interference, in the form of

http://macprovid.vo.llnwd.net/o43/hub/media/1130/10169/Fig_1.png

reinforcements and cancellations, at the particular frequency associated with

that wavelength.

When this happens with mids and highs, these cancellations and reinforcements

are distributed throughout the room. However, with longer, low-frequency

waves, the cancellations and reinforcements are localized to specific areas in the

room. The result is that the bass response of the room is uneven at certain

frequenciesthere will be too much bass at a particular frequency in some spots

in the room, and not enough at others.

Fig 2 Standing waves reinforcing and cancelling a particular frequency and its

harmonics at various locations in a room

If the engineer/mixer, or a speaker, is in one of these spots, then the sound

heard in the room will be a false picture of what the recordings low-end is really

like. Typically, this leads to EQ decisions that compensate for that one rooms

low-frequency issues, rather than for actual issues in the recording itself. When

the resulting mix is heard in other rooms, which dont share those exact same

low-frequency irregularities, they sound baduneven low end, either too thin or

too tubby, overall.

Mapping it Out


To deal with this issue, the first thing thing that must be done is to determine

what frequencies and what locations in a given room will be affected.

Fortunately, for a particular room, the specific frequencies at which standing

waves will occur, and the locations of the problem areas, can be calculated based

on the dimensions of the room. Im not going to go through all the physics

formulas for thistheres no room here, and they can be found in any number of

books on studio acousticsbut I will mention one or two of the most basic

calculations that can be done.

Standing waves occur between all parallel room dimensionswalls (length and

width) and floor & ceiling. There are three typesaxial, tangential, and oblique

modes. The only ones that can really be determined easily are axial modes, and

theyre usually the most prominentthe most problematicso Ill only cover

them.

When the wavelength of a particular frequency is exactly a multiple of a room

dimension, a standing wave will occur at that frequency. Additionally, since

complex musical waves all have harmonics, which are multiples of the

fundamental frequency, then the harmonics wavelengths will also be multiples

of the same room dimension and will also result in standing waves. But the

cancellations and reinforcements for each harmonics standing wave will occur

at different spots in the room.

Dont Fear the Formula

This can be calculated and mapped outwithout any need for test equipment or

special physics knowledgewith the simple formula 11302L (where 2L = room

dimension x 2, and 1130 is the speed of sound). This gives you the frequency at

which a standing wave will form in that roomstanding waves will also form at

whole-number multiples of that frequency (the harmonics).

The frequencies of all the standing waves will have reinforcements at both

wallsthese are called antinodes. The primary standing waves will also be have a

cancellationa nodehalfway between the walls. The next harmonics standing

wave will have cancellationsnodesa quarter of the way out from each wall,

with antinodes in between. The third harmonic will have alternating nodes and

antinodes a sixth of the way from each wall. The diagram in Fig 3 shows how

these first three standing waves affect the balance in the room at those

frequencies. Audio example 1 is what you might hear if you walked from one wall

to the opposite wall in a room with standing waves.

Fig 3 The distribution of the nodes and antinodes of the first three (of one set of

axial) modes in a room.

Audio example 1 How standing waves affect the sound: A repeating bass part is

heard, while the listener gradually walks from one wall, through the center of the

room, to the opposite wall; the tonal variations heard are due to the nodes &

antinodes, as shown in Figs 2 and 3.


The first three or four axial modes, at the lowest frequencies, are usually the

most problematicabove 300 Hz or so the nodes and antinodes are so close

together that they average out for a more even response at those higher

frequencies. But remember, these nodes and antinodes occur for each of the

three parallel room boundarieslength (front & back walls), width (side walls),

and height (floor & ceiling).

What to Do

While its easy enough to determine where in the room the response may be the

most uneven, fixing the problem can be a bit more challenging. Commercial

solutions include a variety of products, like bass traps, that are placed against

walls, or, more likely, in corners, to break up the standing waves, and restore a

more even balance to the space. These must be tuned to the particular room, so

the companies that sell them usually provide a way for their customers to enter

room data, which is used to calculate the best products at the correct sizes to

deal with that particular location.

If youre involved with the initial construction of the recording/mixing space,

from the beginning, then cavities can be designed into the walls themselves to

counter the effects of standing waves. This does require a bit more math &

physics, though, again, the methods and calculations are well documented in a

number of books on studio acoustics. The biggest drawback is that some square

footage will need to be sacrificed to allow for effective treatment.

The Golden Mean

One way to minimize the negative effect of standing waves is to construct a room

with a set of whats called golden mean dimensions. These are specific

combinations of width, depth, and height that insure that there wont be any

overlap of (at least axial) standing waves between those three dimensions at

which they form, and that the nodes and antinodes that do develop are spaced in

the room so as to not interact with each other, and balance each other out, for an

acceptably even low-end response throughout the room. This means no surface

dimensions that are multiples of each othera room thats 24L x 16W x 8H

would not be ideal, because 24 and 16 are multiple of 8, so harmonics of one set

of standing waves would coincide with different harmonics from the other(s),

making the low-frequency unevenness two or three times worse. A cube would

be the worst possible shape! Some traditional golden mean dimensions are listed

in Fig 4.

Fig 4 Some established golden mean room dimensions

The Standing Wave Shuffle

Even if you dont have the option to apply any of the treatments mentioned, the

least that can be done is to insure that the engineer/mixers sweet spotthe

primary monitoring positionand the locations of the speakers, are not right in

the middle of a node or antinode. That means that having the speakers up

against a wall is probably best avoided, unless those speakers are specifically

designed for that placement. Even though the bass response may have less

oomph with a console or desk-top placement, the low end will likely be more

even, and thats much more important than bone-rattling bass. If you map out

the positions of the strongest standing waves, as in Fig 5, you can position the

sweet spot in-between nodes and antinodeswhile this doesnt eliminate the

problem, it will provide a more even reference for mixing.

Fig 5 Top) A studio with speakers and engineer/mixers sweet spot coinciding

with standing waves nodes & antinodes (problematic); Bottom) The speaker

position and sweet spot relocated to avoid the nodes & antinodes of the most

prominent standing waves (better)


Most important, you need to get to know the sound of the room, so as not to

make EQ choices that will only be valid in that room, and will make a mix sound

worse everywhere else (Audio Example 2).

Audio Example 2 A 4-bar passage is repeated 4 times: A) The original un-EQd

mix, as it would be heard in a room with no standing wave issues; B) The same

mix as it might sound in a typical room with an uneven low-frequency balance

from standing waves; C) EQ applied to compensate for the room imbalances; D)

What that mix might sound like when heard in a different room, that doesnt

have the same standing wave imbalances

To do this, assemble a collection of good, commercial recordings, and use them

to get to know how the low end sounds in your room with mixes that are known

to have a proper low-frequency balance. Then use that as your reference for what

the low end balance in your own mixes should sound like in your room, and,

whenever possible, check your mixes on other systems, in other rooms, before

finalizing them. Its certainly possible to make good, well-balanced mixes, even

in a room with a less-than-perfect response due to standing waves issues, as long

as you know the room well enough to not let it trip you up.

Next time, Ill continue this series with a look a mid-and high-frequency

reflections.

Studio Acoustics Part 2

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Transcript of Studio Acoustics Part 2