If you find your expensive hifi system does not sound right when played loud, or you have just moved it to another room, added some new costly upgrade with disappointing results, the chances are it is the room acoustics that are the cause.

Room treatment is normally at the bottom of, or not even on the list of the typical audiophile's 'must have' list. Years of marketing emphasis on sources, amplifiers, interconnects and speakers have left the last remaining audio frontier, the listening room, sadly neglected.

This article is intended to apply to rooms of a domestic size for critical listening situations. The reader should be aware that typical guidelines for auditoriums, theatres or other types of halls are not applicable in this application.

Generally, we do not listen to music through speakers at a distance of less than 1 metre. 2 metres is a more typical distance and at this point it is generally accepted that the reverberant sound field dominates the direct radiation from a traditional speaker such as a two or three way unit including a dome tweeter.

Room measurements of this kind of speaker in the reverberant field always show a significant treble roll off beginning at around 1500Hz with a typical slope of 3dB/octave. The response is controlled by the room absorption characteristics hence the requirement for a predictable room response in critical applications.

Another alarming effect is over-reverberent room conditions is the rise and fall times of transient signals, especially at lower frequencies. Resonating bass notes may take 200 - 300ms to rise (and even longer to decay). This must have a deleterious effect on music reproduction.

DIPOLE SPEAKERS AND RIBBON SPEAKERS

Room measurements of speakers with ribbon (and membrane) drivers do not display the treble roll off characteristics of dome tweeters. This somewhat surprising result is due to the fan like distribution of sound from long, thin drive units. The overall effect is to make the speaker characteristics in the active region of the driver more independent of room characteristics, generally a desirable characteristic.

Is this also partly why dipole bass speakers are preferred ? The answer is undoubtedly yes. The polar characteristics of a dipole give a forward and reverse gain of 4dB placing the listener in a more intense near field and reducing the domination of the room reverberent field.

REVERBERATION TIME
The time taken for the sound level in a room to drop to1/1000^th or 60dB below its initial value.

Of the three main categories of room effects, LF resonances, Incorrect room reverberation times and Room colouration can be investigated partially by calculating and measuring reverberation times in rooms.

In practice, there is no one value for a room since it changes markedly with frequency.

This looks relatively easy to measure with simple equipment i.e. a sound generator, microphone, and an oscilloscope to monitor the decay time. However, as always with practical measurements, there are a number of complications that throw a few spanners in the works.

Fig. 1 Floor to ceiling decay	Fig. 2 Wall to wall decay
Fig. 3 Combined decay	Fig. 4 Linear oscilloscope measurement

The effect is more apparent in figure 4 which is a linear scaled oscilloscope waveform where we see a discontinuity in the decay rate of the waveform. This leads us to 3 more conclusions:

Calculations shown below highlight the potential range of variation in a typical room.

It is clear that the lower range indicated of below 0.25s is not normal in an untreated domestic room even at frequencies above 500Hz. Bass reverberation times will also be much higher.

CALCULATING REVERBERATION TIMES FROM SOUND ABSORPTION DATA

This is a very simple process as developed by Sabine late in the 19^th century. In most cases, absorption coefficients are easy to understand. For example 25 mm of rockwool absorbs approximately 80% of the incident sound over the 1000 – 4000 Hz range. Its absorption coefficient is therefore defined as 0.8 Sabines per square meter. Its effectiveness falls off below 500 Hz and so the coefficient reduces accordingly.
An upholstered auditorium chair has an absorption of 3 Sabines which is a combination of the area of the chair and the coefficient. If we can sum all the absorption rates for the items in a room, the reverberation time can be calculated using Sabines formula:

RT(60) = 0.16 x V/S where V = volume of the room in cu m, S = total absorption in Sabines

As stated above, this formula works best with diffuse sound, evenly spread absorption across all surfaces and with relatively low average absorption coefficient (<0.2). The simple formula gives a shorter RT(60) than measured.

There are a number more complex calculation methods that take into account different conditions in each axis and where absorption coefficients are higher. They are all based on Eyrings more general formula.

RT(60) = 0.16 x V/(S x ln(1-a) ) where S=total surface area of the room, a = mean absorption coefficient, ln = natural logarithm

This formula is used in the calculations that follow.

We will now consider a room dimensions 5 x 3.3 x 3 m with solid brick walls and floor, carpet tiled floor and lath and plaster ceiling. This represents our listening room in the basement of a large Victorian property with 15” walls and no windows. It is perhaps an extreme case with little absorption, when completely empty, despite carpet tiles on the floor.

Clearly this is totally unsuitable. Even if listeners and upholstered chairs are added, the room is too lively, has major modal peaks at 30, 60 and 90Hz and all the expected problems.