Room Acoustics

Room Acoustics. Page 1

Every room has its own acoustic which affects anything that transmits sounds in that particular room. Outside in a free open space, only the ground will affect the sound and only in a limited way. With snow on the ground, the reflections will be further dampened and, predominantly, only the direct sound will be transmitted. If you want to play or speak outside or in a big room, always see that you have a wall behind you to help you to amplify your sound. The wall behind a musician or a speaker is the most important one, but is often overlooked in concert halls The poor musician is normally placed in a free open space, at the edge of the usually very big stage, without any acoustic support at all.

Always remember that the room acts like an amplifier for the sound. The frequency curve for the room is important in that it will directly influence how correctly and evenly the room will enhance all sounds. With an uneven curve, some frequencies or tones will predominate over others. The situation is the same as that of an uneven frequency curve in a loudspeaker. The ideal is to have a nearly flat response. In big rooms other factors will also affect the response - among other things, the treble will always be dampened (by the friction of sound waves against the air over longer distances).

One can conclude that the Berwald Hall has big problems in its frequency response and this is very audible as one sits in the hall. The cello-section will have a weaker sound, as the hall is a poor amplifier for the instruments. The Concert House has a relative straight curve and is a better hall in this aspect, but is limited to the relative low amplification of 1,8 seconds, which is on the low side for an orchestra.

I will illustrate some basic facts about how the sound reflections affect the perception of sound in the room. The sound consists of direct sound, first reflexes and reverberation. The latter often called echo, but that is not what it really is.

It is of utmost importance that the direct sound and the first reflexes will be heard clearly in a room. This fact is often misunderstood or just overlooked. These are patterns from the Berwald Hall and the Stockholm Concert House as captured in a computer program. From these images one can conclude that the Berwald Hall has very big problems with missing first reflections in the very important time space of 1-50 ms.

These are actual measurements from the two halls. You can see the same thing as in the computer-images above, but the information is displayed in another way. There is a strong reflex from the balcony in both halls, but then nearly nothing happens in the Berwald hall. The first reflections will give the sound its details and live feeling. In addition, in a big concert hall, it is important that the first reflexes come from the sidewalls. This behaviour is called lateral reflections. This will give the listeners the impression of being surrounded by sound and immersed in the music. In the Berwald hall, you can see that the actual sound from the hall is of a mono-type quality, which is also clear if you sit in front of the orchestra. It is not a “stereo-sound” that you will hear. If you visit the hall, I recommend that you sit on the side of the hall in the first row, so as to enhance your experience.

Besides Lateral reflection and Reverberation time, other acoustic factors normally taken in consideration while building a hall are: Deutlichkeit, Clarity and Schwerpunktzeit. But the material of the walls, volume and relative dimensions of the different walls of the hall, play a large role in the sound environment.

(The never built concert Hall in Sollentuna) Page 2