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Room acoustics - Part II

Taming your bottom end

[room proximity problems]
[Italian version]

Product: DIY solutions to niggley acoustic problems
Price: your enjoyment of music
Bodger: Mark Wheeler - TNT UK
Researched: during 4 decades
Typed: Spring-Autumn 2009

The previous treatise on domestic acoustics asserted that there is as much bunkum written and spoken about home audio acoustics as there is about bits of wire, if not more so.
Creating or treating a room for your audio pleasure is no mystery and we noted that the better rooms for sitting and chatting in comfort are often the better for audio too. The audio den filled with old audio junk is as audibly displeasing as they are visually distracting.

"But where is it written that a room full of amplifiers and assorted audio gizmos aren't as heart warming and charming as a big chintz covered chair?" asked the late audio maverick Harvey "Gizmo" Rosenberg, "Do women know that the editors of Architectural Digest are deaf! And what about the absurdity of designing a living room that doesn't give highest priority to the position of those glorious gigantic monkey coffins? Loudspeakers create beauty while a couch or a lamp, well, they just sit there gathering dust, doing nothing to open the door to existential mystery."

"CAN AN AUDIOPHILE BE HAPPILY MARRIED?" continues the Gizmo, "In my case NO!. Do you think that I would be into this level of insanity if I had a wife? Let me make my politically incorrect feelings clear: the entire last three decades of the women's liberation movement has no integrity, as far as I am concerned, because I have never heard one of its leaders advocate the importance of creating a state of the art home audio system. How can any women reach higher states of consciousness without learning how to bias the output stage of a push-pull amplifiers?

Your humble scribe knows this'll invite further brickbats from the lunatic fringe, but the Gizmo was completely wrong on this. Almost every word of the foregoing is so wrong it's difficult to know where to start. To begin with, many women, with their greater sensitivity to higher frequencies, will find no merit in push-pull amplification and only bother to learn how to bias the output stage of a single ended amplifier. However, perhaps Harvey already knew he was using the structural linguistics of a testosterone fuelled patriarchy (i.e. talking b*ll*cks) as the rest of that article went on to describe how the answer to the pursuit of audio Nirvana while maintaining domestic harmony was to use active crossovers to make minimonitors work. In this he was correct. Systems with active crossovers are invariably better than those with passive crossovers.

The Gizmo's implication, however, that a domestic listening room is fine filled with old speaker cabinets, half gutted amplifiers and assorted audio junk is wrong. The UK craze for "Single Speaker Demonstration Rooms" in dealerships during the 1980s and 90s attested to the need for spaces uncluttered by other resonant systems (like speakers) to make the most of the product being demonstrated. Rumour and hearsay spread that unsuccessful Linn demos were dismissed by their guru as being invalid due to participants wearing digital watches with inbuilt speakers. Telephones were banned from demonstration rooms and listening rooms at home because the little speaker in the earpiece was enough to destroy the quality of the tri-amped 'briks. In bass terms, only a reviewer ever has any excuse for more than one pair of speakers in the listening room

"Hey, what about multi-channel systems?" Demand plebs, stage left;

"Hey, this is a real stereo website; you don't think I'm gonna find 5 speakers and a subwoofer acceptable do you?" counter-demands the old scribe, "But it is good to get the ancient Greeks back after they were displaced by the Gizmo".

In fact the chintz covered chair, so belittled by Harvey, might be a valuable and inexpensive tamer of standing waves, diffuser of low frequencies and absorber of high frequencies.

Big lumps of furniture like chairs and sofas do a very effective job preventing the room from causing uneven bass response, if they are placed correctly. Sadly the popular layouts where the focus of the room is a fireplace or TV screen tend to be worse for audio than those sensibly designed around the speaker placement. It is perhaps no coincidence that Monty Python's famous sketch on the Spanish Inquisition featured the well known torture devices of "The soft cushions and the easy chair" do you?

The typical modern living room, arranged for convenient worship of TV almighty, tend toward room arrangements wholly unsuitable for two channel music reproduction. The furniture is usually arranged for unobstructed viewing of that one eyed deity, so the loudspeakers tend to be placed either side if that hollow glass resonating CRT (cathode ray tube) or flat glass specular reflector (be it warm plasma or cool diode). Anything between the speakers is a bad thing, but most of us end up with an equipment rack there to guarantee our gear is exposed to the worst possible airborne and structure borne vibrational hell as possible, but a CRT is the absolute lowest pit of acoustic hell. I used to conduct an experiment in the most difficult room I have set up, where a group of music lovers would be brought into the room and subjected to their usual eyes closed "Can you hear a difference?" exercise. They assumed they were being asked to hear some new interconnect or power supply modification, but the closed eyes were happily accepted as a means to avoid prejudice. On this occasion I would simply lift a small (35cm screen) portable television from a tiny low platform (50cm x 25cm) midway between the speakers, take it from the room and leave it outside the door. Listeners would all comment on the dramatic difference as though the soundstage rushed in and became whole. Those who had complied with the closed eyes injunction, and even when opened again would rarely notice the absence of TV as they would all be seeking a change in the audio equipment, all assumed that a major audio equipment change must be responsible for this scale of effect.

How to look ridiculous as an audiophool:

  1. Play some music with strong bass
  2. Go and put your head at the intersection of wall and floor
  3. Try this all around the perimeter of your room
  4. Move both your loudspeakers close to the wall and repeat the exercise
  5. Move one of the speakers to the centre of the room and leave the other by the wall and repeat again
  6. Hope no one enters while you're doing this

This is especially hilarious if you share the experience with another audiophool friend

The CRT (or reflective flat screen) effect is not the worst offence committed by the presence of a TV in the Real Stereo living room; it is the way that particular piece of electronics demands placement of all the other furniture in an arrangement that inspires worldwide conflict (OK, room-wide conflict) with any stereo friendly arrangement. Move the TV to the kitchen or dining area. Move house. Anything other than share the TV viewing space with the audio. Audiophiles without partners may dispense completely with the infernal device (watch TV on your laptop if you must) and those of us with partners & families must choose our houses to have a large enough (20m^2) audio room and a separate family room where TV, PS3, wii, Xbox, etc may be enjoyed. Monopoly, Cluedo, Risk and chess may be enjoyed in the audio room without disturbing the audiophilic impetus.

[Nodes and antinodes]

The lesson you should have learned while performing the above experiment is that there are bass hot spots throughout the listening room and there are also bass deserts. Tunes like Lou Reed's Walk on the Wild Side are particularly useful for this as Herbie Flower's double tracked bass part (double tracked double-bass and Fender bass so he got paid twice for one session, a popular ploy in the day) covers a wide range and is played with beautifully even intensity and articulation; On The Run by Pink Floyd (on DSOTM) is good for flurries of long low tones; anything featuring Leland Lee Sklaar ('king of the whole note' - semibreve to us Europeans) will serve well too. You might also have noticed that one note's hot spot is another note's desert...

"What's the old fool on about now?" enquire bemused plebs, stage left, "how do puddings affect bass quality? Are we now to place various sweetmeats around the room to control eigentones?"

"Not desserts, - deserts, those arid places lacking in life - like multinational eateries and coffee emporia," quoth the old scribe, reasserting his authority.

The reason bass is intense in some places and absent in others is twofold and how much of each of the two factors applies in your room depends on construction. First we shall consider a solidly constructed listening room as these are generally more prone to this effect, but in other ways superior in every respect. Masonry walls have various advantages compared with lightweight drywall (also stud and plasterboard or gypsum-board, stud and wood-panelling less than 12mm thick), whether the masonry is gypsum plaster skim over render over brick, gypsum plaster skim over plasterboard glued (over all its area) to brick, likewise over a breeze-block, insulation block or cement block substrate, and best of all lime plaster over stone. Likewise masonry floors are invariably better than suspended wooden floors, although Haydn demanded his employer replace the marble floor of the performance chamber with wood, but this was solid hardwood not pine floorboards suspended on joists. A future article will explain the non-bass reasons for the superiority of solid construction, but today we will discus only bass.

Assuming the preferred solid room condition, there will be standing waves between any solid parallel facing surfaces. The sound-waves emanating omnidirectionally from a perfect point source in the empty void of an unfurnished room will encounter room boundaries. If these boundaries are reflective to those travelling waves, they will be reflected continuing the waveform at same the point in the cycle as the incident wave. The standard schoolteacher explanation (the one I was given anyway) is to consider a skipping rope tied to a hook bolted at the surface of the wall. Where λ represents the wavelength, if the incident wave strikes the reflective surface at λ or λ/2 the reflected wave will be in phase and if the wave strikes the surface at λ/4 or 3λ/4 it reflects out of phase. If the surface were a perfect 100% reflector, the first condition λ results in a doubling of intensity and if the second λ/4 case it cancels or nulls. At the point of most displacement from the straight rope, where the incident and reflected parts of the standing wave coincide in phase it is known as an antinode. Sharp readers will have guessed that the point of least displacement from the straight rope condition is known as a node. If these terms seem counter-intuitive it is simply because when we consider sound waves rather than skipping ropes, we observe that displacement is inversely proportional to pressure; when the displacement is at its peak pressure is zero; when the displacement crosses the zero point the pressure is at its maximum.

The standing waves may be axial (bashing between opposite flat reflective surfaces like a cartoon character) or tangential (zooming round the space in straight lines but equal incident and reflected waves at each surface). As long as the reflected wave coincides with itself at some point, whether adding together (constructive interference) or cancelling itself (destructive interference), the wave is a standing wave. The many standing waves for a given room are the room modes. There's an explanation of room modes here that looks quite accessible, and among the academic posts here are some splendid (copyright) diagrams.

[skip to]

Standing waves not only cause hot and cold spots where particular notes are doubly powerful and others are nulled, they also cause overhang, timing inaccuracies and lumpy bass. Each solid surface reflects a wide range of frequencies, and the wave bounces back across the room to meet the opposite wall (or floor to ceiling). If the wave strikes the opposite boundary at an integer multiple of the first surface reflection's point in the waveform's cycle λ λ /4, λ/2, and 3λ/4 [look up proper definition of standing waves] the incident and reflected waves will either add together or cancel each other out. This produces a comb filter effect at any point between the reflective surfaces. Hence there are nodes of out-of-phase coincidence and antinodes of in phase coincidence. To oversimplify, to the point of inaccuracy: where even multiples of these reflected waves coincide at the listening position, the sound will be bass shy; where odd numbered multiples of the same frequency will be equally disproportionately emphasised. As soon as reflections enter our considerations the wavefront ceases to travel ad infinitum, it becomes a standing wave.

As soon as we enter real audio rooms we notice that they do not consist entirely of bare reflective walls and nor is there a single point source omnidirectional loudspeaker. Reflectors only reflect wavelengths that are small in relation to the size of the reflector. Walls enclosing a room are inevitably as big as any wavelength that it is possible to reproduce in that room. Rooms are only able to contain coherently those wavelengths that can be accommodated in the room dimensions. The lowest frequency that may be reproduced by a loudspeaker in a room is usually that where the furthest distance from the loudspeaker driver to a flat reflective surface is λ/2. This will also usually be the lowest frequency standing wave from that speaker to wall and back.

[Standing waves patterns]

There are exceptions to this, such as rooms with openings to other spaces where those spaces become part of the effective room volume at certain frequencies, especially where there are frequency selective devices (like curtains) over the aperture. The flexibility of large glass areas (picture windows uninterrupted by glazing bars) also allows frequency selective transmission and reflection to the outside. The outside is really big, it goes all the way up to the tropopause, so ultra low frequencies are unrestricted. Large thin areas of glass can flap about permitting transmission of these very low frequencies, while higher frequencies are reflected. Argon filled double and triple glazing mitigates this so effectively that it becomes an irrelevant, and smaller panes do not suffer this problem anyway. The myth that glass is universally bad in listening rooms has no basis in fact. Glass, even in large areas is less of a problem than stud partition walls (known variously as plasterboard, gypsum-board, drywall etc.) or suspended floors (floorboards on joists or sheet materials on joists).

Construction materials in the listening room will be more fully considered again in a later piece. For now we continue on the trail of those modes.

[sound bouncing around]

From the diagram above we can see how the intensity of those constructive and destructive interferences form patterns that tend to have integer relationships with those solid boundaries. Corners, for example are places where at least half of all the standing waves are at their most obvious (head in corner time again) and equally most susceptible to treatment. It only takes a big solid object in a corner to disrupt a whole system of interference. I have known various successful treatments like this using domestic furniture already owned by the audiophile. A large heavy corner cupboard 2m tall (whose action was aided by a 100mm layer of absorbant on its top surface hidden behind the cupboard's cornice) tamed a killer standing wave in a nearly square room in one dramatic example. The owner had considered replacing speakers and reinforcing the floor on reading articles about room acoustics.

Positioning the speakers slightly assymetrically to the room axis can also help as it prevents identical patterns set up between each speaker and near boundaries. Big sofas and easy chairs should be positioned to have most effect. The sofa on which the listeners are seated could be tried at various distances from the back wall (never against it) until the best compromise is reached.

Hard reflective objects should be kept away from close proximity to the speakers or comb effect interferences will destroy any semblance of timbral accuracy, heard as if the instrument is cheap trash and played inconsistently. Large areas of plain wall are a BAD THING. Art is a GOOD THING so big prints and paintings and tapestry hangings are not only visually desirable, they help break up the identical dimensions between opposite surfaces. They are even better of you ensure that they can vibrate or rattle in sympathy with particular bass notes by hanging them on proper picture wire with a felt or cork pad behind each lower corner. I have been known to space the top edge of a picture away from the wall in a troublesome room to increase the benefits. This cured a recalcitrant flutter echo between fireplace and opposite wall for one audiophile I helped.


Arranging the furniture in the listening room for best acoustics will bring immediate benefits in listening quality at zero cost. We must begin with correct speaker placement for bass reproduction, either close proximity to room boundaries if so designed (e.g. Naim or Allison) or as far as practicably distant from walls and corners for the more common anechoically flat alignments. Assuming correct speaker positioning, and this is a BIG assumption, we can do much to ensure even bass reproduction throughout most of the room area. This can be done without resort to expensive specialist acoustic treatments. If readers are engaged in commercial activity that demands particular acoustics (an obvious example being sound recording) then competent professional acoustic treatment is more than justified, it becomes essential. Semi-pro studio operators who imagine that they can cut corners with egg boxes remain semi-pro. However, TNT-audio readers are more likely to be amateur in the best sense of the word (from amo = I love, Latin) in that we love music without being paid.

Every object in the room, and every decorative finish is an acoustic treatment, whether good or bad. If we wish to avoid cluttering our rooms with daft looking objects whose sole purpose is acoustic, we must arrange the speakers and other furniture to suit. The choice is yours: either get it right by arranging everything to suit the audio or spend years and hundreds of Euros upgrading unecessarily and buying problem-solving acoustic gizmos.

If you wish to keep your bottom end in proportion, exercise by moving the furniture around to suit the music!

Music enjoyed during this editorial

Pink Floyd: Dark Side Of The Moon; Q4SHVL 804,
SQ Quadrophonic used in '70s head to floor experiments
Pink Floyd: Dark Side Of The Moon; SHVL 804 remaster 2003,
not used in any potentially embarrassing experiments
Easy Star All-Stars: Dub Side Of The Moon; ES-1012v, colored vinyl
Miles Davis: Tutu
Jimi Hendrix: Martin Scorsessi
Mott the Hoople: All The Young Dudes
Mott the Hoople: Mott
Mott the Hoople: The Hoople
The Clash: At Shea Stadium
Gong: Camembert Electrique, virgin album for the price of a single,
Strong and Streamin Mate
all on vinyl, natch



Mackenzie, G W (1964) ACOUSTICS, Focal

© Copyright 2009 Mark Wheeler - www.tnt-audio.com

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