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How does a closed room affect music playback? In addition to the room modes that have been discussed many times, there is an effect that is often underestimated in practice, the so-called room gain.
Situation in automobiles
Admittedly, the headline sounds a little strange, but eye-catching titles are ultimately meant to attract attention. And since this is a little-discussed topic, a little attention can't hurt. I first noticed this phenomenon when I was writing tests and taking measurements for the then magazine "CarSound". It was striking that even small bass drivers managed to radiate 20 Hz inside the vehicle without a major loss of level. I investigated the phenomenon in an article entitled "Car acoustics, part 1" and determined by measurement that the relatively small and somewhat dense interior of the vehicle increases the sound pressure at low frequencies more and more. The measurements on three different vehicle types showed an amplification of very low tones of almost 30 dB.
And at home?
Some people may now be wondering what mobile entertainment has to do with listening to music at home. The question is quite understandable, since, as mentioned above, this topic is rarely discussed in mainstream circles. Every room in which music is played, at least in this country, has a more or less closed volume, just like a car interior - albeit with a much larger volume.
But that does not mean that this effect is not present, it just shifts the effect towards lower frequencies, or the effect only begins to be noticeable at very low tones. Anyone familiar with simulation programs can simulate this effect. You will get an increase of around 12 dB per octave, below the cutoff frequency, i.e. from the frequency at which the effect begins to take effect.
The cutoff frequency, in turn, depends on the size of the room. In practice, however, the increase is somewhat flatter than 12 dB per octave. Values of around 8 to 10 dB per octave are realistic, I often see this in practice. The approximate corner frequency of this effect is around 50 to 80 Hz, depending on the size of the room. This effect can be easily determined with a simple measurement.
A measurement in the near field of the bass driver and a subsequent measurement in the room shows this effect very nicely. In this context, it is important to distinguish between the room gain and the room modes. The effect of room modes is narrow-band and appears as narrow peaks or dips in the frequency chart. The room gain is a gentle, even increase towards low frequencies.
Room gain in a medium-sized room. The green area shows the sound pressure “gain” through the room.
Impact on practice
Put simply, a loudspeaker should not have a linear transmission range down to 20 Hz or even lower, but rather a slightly declining curve towards low frequencies. Closed enclosures come closest to this behavior. Depending on the Qts (= total damping), the level in enclosures drops slowly (Qts = 0.5) to faster (Qts = 1.0) and approaches a 12 dB/octave asymptote.
This means that if the corner frequency of the room gain and the natural frequency of the bass driver in the closed enclosure are roughly in the same range, the room gain and the natural rolloff of the loudspeaker compensate each other. Classic bass reflex loudspeakers have a rolloff of 20–36 dB per octave, depending on the tuning. This means that there is often an increase in one area, but there is almost no sound level below that. Exceptions here are designs with very low tuning frequencies that are not tuned to the most linear level curve possible and therefore also have a very gentle rolloff.
The graphic now shows the level curve (red) of a loudspeaker X (in an anechoic chamber). The dashed curve (blue) shows the overall curve of the loudspeaker in the room. In this fictitious example, the curve is slightly falling.
The same graph with a steeper rolloff of a speaker Y produces a much less deep bass.
Conclusion
This short article makes it clear why pushing speakers has almost become a hobby. Very rarely does the room gain compensate for the falling curve of a loudspeaker perfectly. Most of the time there are peaks in the bass range and a drop at very low frequencies despite the room gain. However, the flatter the rolloff, the better the chance of a balanced bass curve. The room modes were deliberately ignored in this analysis.
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Situation in automobiles
Admittedly, the headline sounds a little strange, but eye-catching titles are ultimately meant to attract attention. And since this is a little-discussed topic, a little attention can't hurt. I first noticed this phenomenon when I was writing tests and taking measurements for the then magazine "CarSound". It was striking that even small bass drivers managed to radiate 20 Hz inside the vehicle without a major loss of level. I investigated the phenomenon in an article entitled "Car acoustics, part 1" and determined by measurement that the relatively small and somewhat dense interior of the vehicle increases the sound pressure at low frequencies more and more. The measurements on three different vehicle types showed an amplification of very low tones of almost 30 dB.
And at home?
Some people may now be wondering what mobile entertainment has to do with listening to music at home. The question is quite understandable, since, as mentioned above, this topic is rarely discussed in mainstream circles. Every room in which music is played, at least in this country, has a more or less closed volume, just like a car interior - albeit with a much larger volume.
But that does not mean that this effect is not present, it just shifts the effect towards lower frequencies, or the effect only begins to be noticeable at very low tones. Anyone familiar with simulation programs can simulate this effect. You will get an increase of around 12 dB per octave, below the cutoff frequency, i.e. from the frequency at which the effect begins to take effect.
The cutoff frequency, in turn, depends on the size of the room. In practice, however, the increase is somewhat flatter than 12 dB per octave. Values of around 8 to 10 dB per octave are realistic, I often see this in practice. The approximate corner frequency of this effect is around 50 to 80 Hz, depending on the size of the room. This effect can be easily determined with a simple measurement.
A measurement in the near field of the bass driver and a subsequent measurement in the room shows this effect very nicely. In this context, it is important to distinguish between the room gain and the room modes. The effect of room modes is narrow-band and appears as narrow peaks or dips in the frequency chart. The room gain is a gentle, even increase towards low frequencies.
Room gain in a medium-sized room. The green area shows the sound pressure “gain” through the room.
Impact on practice
Put simply, a loudspeaker should not have a linear transmission range down to 20 Hz or even lower, but rather a slightly declining curve towards low frequencies. Closed enclosures come closest to this behavior. Depending on the Qts (= total damping), the level in enclosures drops slowly (Qts = 0.5) to faster (Qts = 1.0) and approaches a 12 dB/octave asymptote.
This means that if the corner frequency of the room gain and the natural frequency of the bass driver in the closed enclosure are roughly in the same range, the room gain and the natural rolloff of the loudspeaker compensate each other. Classic bass reflex loudspeakers have a rolloff of 20–36 dB per octave, depending on the tuning. This means that there is often an increase in one area, but there is almost no sound level below that. Exceptions here are designs with very low tuning frequencies that are not tuned to the most linear level curve possible and therefore also have a very gentle rolloff.
The graphic now shows the level curve (red) of a loudspeaker X (in an anechoic chamber). The dashed curve (blue) shows the overall curve of the loudspeaker in the room. In this fictitious example, the curve is slightly falling.
The same graph with a steeper rolloff of a speaker Y produces a much less deep bass.
Conclusion
This short article makes it clear why pushing speakers has almost become a hobby. Very rarely does the room gain compensate for the falling curve of a loudspeaker perfectly. Most of the time there are peaks in the bass range and a drop at very low frequencies despite the room gain. However, the flatter the rolloff, the better the chance of a balanced bass curve. The room modes were deliberately ignored in this analysis.
Room Gain kurz erklärt - Der Raum als Verstärker?
Schnell ins Bewusstsein gerufen sind die Raum-Moden, wenn das Thema Raumakustik buchstäblich im Raum steht. Doch vielfach wird der Room Gain ausser Acht gelassen.
