There are many myths and misconceptions surrounding the best way to handle bass in a listening room environment. Some think that because of the non-directional nature of bass frequencies, it doesn’t matter where a bass speaker is placed, and all too often in our industry we see bass speakers simply placed on the front wall for convenience. So, what's wrong with these ideas and what should we be doing?
In creating a quality immersive audio visual experience, such as a home cinema, media room, simulation experience or music listening room, the audio is the hardest thing to get right. There are a number of factors working against you (some of which we will explore here), and you’re dealing with peoples varying perceptions rather than a simple black and white situation. As such it’s important to allow a little time at the end of an installation to tune to taste, however the system should be built to provide a solid base to work from and in a way that avoids the traps that a simple ‘tune-up’ can’t fix.
Before we can answer the question of where to put a subwoofer in a home cinema or media room environment, we need a basic understanding how low frequency sounds interact with the space. More often than not a listening room is a rectangular shape, essentially made from 6 flat surfaces. Floor and ceiling are usually parallel and opposite each other and the same can be said about the front and rear walls as well as the side walls. This common feature of the vast majority of listening rooms presents a particular challenge when placing bass speakers, in that it’s pretty much guaranteed to give inconsistencies in bass performance as you move through the space, so what’s happening?
The speed of sound can be assumed to be around 344m/s (meters per second) dependant on air temperature and pressure. When we look at how this relates to wavelength we also start to see how this might react with the listening environment. Below you can see a basic sine wave of 80Hz. To find out the wavelength at 80Hz we simply divide the speed of sound (344m/s) by 80 (Hz = cycles per second). This gives you a wavelength of 4.3m and a half wavelength of 2.15m.
Sound can be thought of as acting in a very similar way to light, and the walls as mirrors. The walls will reflect sound that, at bass frequencies is radiated in all directions from the speaker. Where surfaces are parallel (as they often are), we can end up with sound being bounced back and forth, up and down or side to side between the parallel surfaces, decaying as it travels. Any dimension between parallel surfaces that divides by half a wavelength will then generate a ‘room mode’ or ‘standing wave’, causing loud spots where the energy is at its highest (peaks or troughs of the sine wave), and nulls where it is at its lowest (the centre or ends of the wave). This will happen throughout the frequency spectrum. Considering that there are often 3 sets of parallel walls, all with differing spacing, you can start to see how the experience at different locations within the listening space will change.
This is a phenomenon that is often easily experienced by moving around a room containing a single subwoofer, often bass is overpowering and ‘boomy’ in places, and in contrast left thin and lacking punch in others.
How do we overcome this?
First and foremost, armed with this understanding, it becomes clear that more than one subwoofer will be required to provide a consistent performance across the spectrum. Especially when dealing with listening environments in the home due to their dimensions. If bass notes coming from one location will have peaks and troughs as we move through the space, placing a bass speaker in another location may cause peaks and troughs in other locations, helping to offer a more consistent experience. Add in another subwoofer and the job gets a bit easier again, and it is possible to provide much improved consistency across a larger portion of the space.
Using multiple subwoofers isn’t necessarily about providing louder bass, it’s about providing a tight and consistent bass performance throughout the listening area.
There is more to this however, than simply adding subwoofers. Though that will be a step in the right direction, accurate room measurements and a knowledge of the room construction make it possible to simulate how bass frequencies will interact with the space before ever installing a speaker. This makes it possible to consider the positioning of the subwoofers within the room and how that will effect the experience across the listening area. All that is needed is a little considered design time where the appropriate calculations are carried out and room simulations run, more detailed room designs can then be based on this important performance information.
If this work is done it is possible to accurately calculate the number of bass speakers required to provide a flat response across the listening area, as well as determining suitable positioning. Ideal placement will change from room to room, as such there is no hard and fast rule as to where bass speakers will work best. One thing for sure though, placing bass speakers in the same way across a number of home cinema environments will produce wildly varying results and isn’t how you get the most bang for your buck!
If you would like to better understand how we can help in designing your home cinema or media room please Contact Us and a member of our team will be in touch.
We are audio visual specialists who have been delivering luxury technology solutions for residential spaces in the UK since 2010.