We have all experienced being in rooms which sound very ‘lively’ (swimming pools, gymnasiums, train stations etc), but what is it that we are actually hearing? One of the things that allows us to judge the size of a room, even if we are blindfolded, is the decay time. Humans have a very acute sense of timing and how a sound energy dissipates over time. It is the rate of this decay or energy dissipation that lets us judge or ‘feel’ the size of the space we are in.
We don’t need to be taught this, it is a learned response that we develop as we grow. We all know that a sound will start out loud and reduce in volume until we can no longer hear it. The time that the sound takes to fade away is known as the decay time. The longer a sound takes to decay, the larger or more reflective the space is.
Conversely, sounds which decay quickly are associated with smaller or more heavily damped spaces like well furnished rooms, inside a luxury car or recording studios.
When the room is full of hard, reflective surfaces, the sound waves will bounce around with minimum dissipation, increasing the decay time. If a room has large curtains, soft furnishings, the sound waves are absorbed when they bounce around, reducing the decay time or time that we can hear them.
Now let’s focus on using this knowledge in a cinema. If we don’t treat a room with hard surfaces, all the energy from the loud speakers will bounce off these surfaces many times, causing distraction, loss of clarity and a harsh sound. At the opposite end of the spectrum if we treat every surface so it absorbs the energy of the speakers, we end up with a flat, dull sound, seriously lacking in dynamics.
Surely, we can just measure the sound from each speaker and adjust its response to suit the room, right? Well the answer to that is both yes and no. Room correction software can be incredibly effective in the low frequency range. Low frequency sound is made up of long wave lengths, (wave lengths are calculated by dividing the speed of sound at approx. 343m/s by the frequency so: 343m/s ÷ 30Hz = 10.6m wave length for 30Hz!) which are very difficult to absorb with room treatment materials. A 11.4m wave of energy needs an incredibly deep material depth to absorb the energy.
In the low frequency regions, you can see that it is largely impractical to treat the room with material, due to the sheer size of the sound waves. Using a processors room correction to attenuate the room modes of these frequencies at the listening position is a fantastic solution. Even more so if it looks at aligning the decay of the frequencies and not just the amplitude (or size).
The other factor to consider is the human hearings sensitivity to frequencies, this isn’t a linear response. That means that we don’t have the same level of ability to resolve or detect differences in frequencies across the frequency range. We all have our favourite loud speaker or brand of loudspeaker, right?
One of the key reasons we favour one brand over another is due to how the crossovers are designed by the manufacturer, where the midrange drivers hand over to the high frequency units and so on. A huge amount of time is spent by the engineers dialling these settings in to ensure the house ‘sound’. In DSP speakers, these changes can be in the time domain as well as frequency, bringing perfect time alignment between the drivers.
As we move up the frequency range, we can see that (343m/s ÷ 400Hz = 0.8m) the wave lengths get a great deal shorter and therefore easier to absorb with slimmer acoustic materials (50mm depth). Especially when we also consider that human speech (an area we are incredibly deft in understanding) is typically within the 100-3kHz range.
If we use a processors room correction algorithm in these higher frequencies, we alter the sound of the speaker, often quite significantly, literally changing its sound and tonality. In that frequency range we could easily use some room treatment such as scattering or absorbing materials to treat any excessive energy in these higher frequency regions. This means our speakers still sound as the manufacturer intended (and presumably the reason you chose them in the first place).
So, room correction or room treatment? Both! Room correction is at its most effective at lower frequencies where it isn’t affecting the tonal balance of the speaker. Room treatment is most effective and practical when it works above 250Hz as it doesn’t need to be especially deep and won’t change the sound of a loudspeaker. As always, if you aren’t sure just ask for advice. Our Design and Specification Service along with our calibration services have designed and calibrated many systems with our dealers so you can be confident of delivering outstanding performance to your client.