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A Unified Explanation of Room Effects on Speaker Responses
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Summary

It is an accepted fact that the acoustic properties of a listening room have a dominating effect over the performance of high quality speakers intended for audiophile use.

Less well documented is the actual response of different speaker designs and drivers in acoustically treated rooms. This makes the subjective comparison and review of different designs more difficult.

The main negative effects of acoustically treated rooms, especially with high absorption, (reverberation time < 0.2s) is to seriously unbalance the speaker response across the entire audio spectrum. Suggestions are made for simple yet effective tone controls capable of equalizing all speaker designs with varying driver types.

Why acoustically treat listening rooms in the first place ?

1. Echoes from walls, floors and ceilings build up to give a sound field that may have little resemblance to the original recording.

Stereo cues (if there are any genuine ones in the original recording) may easily be confused by early multiple reflections. It is only because of the amazing properties of the ear to ignore many of these extra sound sources that we find the end result worth listening to.

2. Room resonances give rise to large peaks and troughs in the response from 20 – 150Hz. This gives rise to uneven confused bass.

3. There needs to be an accurate balance between reverberant and direct sound for the speakers. This varies according to the recording method involved in the original recording. Multichannel recording methods with close microphoning, probably in a studio, does not yield any kind of ambiance to the recording. This may be added artificially later but a contribution from the listening room is preferred.

Recording methods that record the original ambiance require high absorption in the listening room for best results.

4. Room gain is a term used to describe the increase in bass output below 150Hz as a result of confining the speaker within room boundaries i.e. A typical corner placement. This cannot be overcome by acoustic treatment but it has to be dealt with in order to provide accurate bass.

5. Transient responses of speakers in rooms is dominated by boundary reflections. This is not a well documented or researched subject.

The Speaker response problem:

Fig 1. Idealized speaker response, closed box system, significant room absorption, moving coil drivers, directional HF

Figure 1 above is a suggested simplified model of the response of a speaker, as described, with an excessively directional HF. This may be typical of any single full range driver design. It does not include room bass resonances.

Fig. 1 - Region A

Cause

Effect

Room boundaries boost bass output which is eventually curtailed by the speaker LF response

Inaccurate, boomy , 'slow' bass.

Fig. 1 – Region B

Cause

Effect

Room Absorption starts to reduce from about 400Hz. Wide polar response in the mid range increases the level of absorption and decreases the mid range output.

Mid range inaccuracy, relative unbalance compared with LF and HF responses. As the driver becomes more directional in the upper mid range, the output begins to rise again.

Fig. 1 – Region C

Cause

Effect

Directional HF changes the ratio of reverberant to direct sound.

Gradually rising HF response from about 2kHz

Example 1

2 way speaker, moving coil drivers, reflex bass loading, 25mm dome tweeter.

Room response, relatively high absorption, 1.5m distance.

Semi Near field response (1m) showing reduced room effects.

Example 2

2 way speaker, Dipole bass, 8” full range driver crossing over at 170Hz

Response in a room with relatively high absorption, at 1.5m, of a system that is essentially flat in a less absorptive room.

To be continued, last download 2006-09-07.