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One Two Training

When Is A Room

Sound Training

sound in enclosed spaces


What makes the places we play in sound the way they do? Andy Honeybone considers the reasons for reverb.

Alan Bennett's play "40 Years On" contains the immortal line, "When one has to go to the lavatory for humour, the writing is on the wall." Be that as it may, the smallest room in the house is where we start this month's impromptu gander at the environment in which we make music.

It's possible to spend any amount of money on kit and have the lot sound rubbishy because of the room in which it's being used. Few of us have the luxury of being able to choose where we practice our art but a few selected ground rules won't go amiss — so it says here.

The urge to sing in the shower is prompted by the ions pouring down on you plus the vibrant tone of your voice bouncing around the tastefully tiled decor. But why does the voice sound so full? Simple: sound can be reflected from a solid object in the same way that a mirror works for light. Multiple reflections occur — floor to ceiling, wall to wall and so on — until the soundwave runs out of energy. Your ear will hear reflected images of the original sound, each arriving after a different journey time. This fattening effect is called reverberation and is distinct from echo which is a recognisable reiteration of the original.

Some materials reflect sound well, others absorb it and pass little on. The "absorption coefficient" is a decimal fraction or percentage which indicates how effective a substance is at mopping up sound. For example, bare floor boards have an absorption coefficient of 0.08 at 1 kHz — in other words they will reflect 92% of sound reaching them. Rubber-underlayed carpet has a figure of 0.8 at the same frequency, so it reflects only 20%.

The majority of building and furnishing materials are most effective at absorbing sound at high frequencies. To achieve attenuation at bass frequencies it is necessary to introduce air behind the absorbing surface, as with double glazing or plywood spaced from the wall by 2in battens.

Where do egg boxes fit into the picture? Leave them at the grocers. True, they trap air, but thermal insulation is about all they'll give. The egg box myth probably stems from pictures of anechoic chambers (special rooms with no reverberation time) which show walls smothered in unearthly wedges of absorbent material.

The "reverberation time" of a room is the time taken for the reverberation to drop to one millionth of the level of the originating sound. It may be calculated from a simple formula, which is 0.05 times the volume of the room (in cubic feet), divided by the total sound absorption. This last parameter has to be worked out by aggregating the sound absorptions of the different materials present with respect to their contribution to the total.

But back to our small tiled room, or "ambience booth" as home recordists would have it. Part of the attraction of singing in such environments is that certain notes really ring out. This is resonance, which occurs when a wall to wall or floor to ceiling dimension is half the wavelength of the particular note.

Remember, a whole wavelength is a figure-of-eight shape, having two anti-nodes (the "hips" of the eight) and three nodes (the "waist"). Half a wavelength is just two nodes and one anti-node, invisibly poised above the bath between the walls as you sustain the first note of "Jerusalem". Assuming that you don't have namby-pamby carpet or polystyrene ceiling tiles there should be three fundamental resonances in every room: floor to ceiling, narrowest wall to wall, and widest wall to wall.

In most rooms, bar the box room, the floor to ceiling distance is the shortest dimension and this equates to a resonance of about 80Hz with weaker harmonics at 160Hz, etc. An 11ft room resonates at 50Hz and the lowest note on a bass guitar (40.2Hz) would send a 13½ ft room crazy in the absence of soft furnishings.

Assuming that you want to be rid of resonances there is a lot that can be done to diminish them. First of all, try to break up large areas of bare wall with shelves, cupboards and other projecting artefacts. Because sound absorbent material is frequency dependent, it is important that materials used to face bare surfaces absorb at the resonant frequency. For example, carpet absorbs best at high frequencies and hence is of little use in avoiding floor to ceiling resonance.

It is possible that a room could have its three primary dimensions the same. This would add up to one enormous resonance. Square rooms are almost as bad as this, and rooms where one width is in direct ratio to another should also be avoided.

Although resonance is unwanted, reverberation has to be controlled to give a balance between warmth and loss of clarity. If an environment is too reverberant, close-miking will boost the direct signal to swamp the reflected sounds. Heavy curtains will also reduce reverberation — test recordings should be made to allow comparison of such introductions.

Reverberation can be created artificially in a number of ways. Historically, a metal sheet was suspended using springs tensioned from a surrounding frame. A driving loudspeaker and receiving microphone were attached, and there you had plate reverb. Reflections on the metal sheet simulated the indirect reverberation paths of the natural effect. Cheaper and smaller were the spring units attributed to Laurens Hammond. In their nastiest form they exhibit pronounced metallic resonances, make sonar-like pings when overdriven, cause all manner of feedback problems in combo amps, and never pass a frequency over 4kHz at any amplitude.

Solid state reverb got off to a shaky start because early units tried to make do with just one delay path. The results were rather like shouting down a large pipe.

More recently we were offered an analogue delay chip which was tapped to give several outputs along its length, each having a different delay time. These taps could be summed to give a highly realistic effect. Digital units (see also page 10) offer room characteristics at a touch.

Using pulse echo techniques it would be (is) possible to analyse a room and generate the reverberation time versus frequency profile to treat sounds recorded dry in the studio. And there's them that think 8-track's overkill.


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One Two Testing - Copyright: IPC Magazines Ltd, Northern & Shell Ltd.

 

One Two Testing - Dec 1985

Donated by: Neil Scrivin

One Two Training

Feature by Andy Honeybone

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