The Perfect Loudspeaker
Loudspeaker design nowadays is advanced to a point where loudspeakers have a pretty much flat frequency response between 20Hz to 20Khz, which are the lower and upper boundary of human hearing. In an ideal home cinema, the main speakers reproduce frequencies from 80Hz up while the subwoofer is reproducing frequencies from 80Hz down (in simplified terms). Sounds below 80Hz cannot be localised by human hearing (that is you cannot tell where the sound originated from) and because of the complexities of sound waves interacting below 80Hz in small rooms, it is best to have only one (or more) carefully placed subwoofers reproducing these lower frequencies.
Here Comes the Room
It’s great we have such advanced loudspeaker design, but once you put the speakers into a room, things get messy. The sound waves will reflect off the surfaces of the walls and furnishings and interact to create drips and peaks in the frequency response, making some frequencies louder, others quieter and yet others to echo and ring as they decay.
What is even more upsetting is that this is not uniform in the room. As you move your head, sit somewhere else or walk around, the dips and peaks change. If you have just spent 100’s or 1000’s on speakers, the situation is rather unacceptable. You obviously want to get the most out of your equipment.
EQ to the Rescue
There are two main ways to combat the above: digitally using equalisation and with acoustic treatments. The former is somewhat easier to implement and a lot more wife (or for our lady enthusiasts, husband) friendly.
Digital Room Equalisation is built into most receivers nowadays. Some are more effective than others. Let’s have a look at some of what is currently on the market:
Audyssey MultiEQ XT and MultiEQ XT32: Audyssey’s technology deals with the most problematic frequencies below 200Hz very well, especially when it comes to MultiEQ XT32. At the same time, higher frequencies may not sound natural to some listeners. Some manufacturers (such as Denon) allow you to apply it to only the bass frequencies for this reason. MultiEQ XT32 was also available in standalone products as SVS AS-EQ1 and Audyssey’s own branded subwoofer equalisers. They have been discontinued, however. It’s worth picking up a used one if you have a receiver that lacks a good subwoofer equaliser.
Yamaha YPAO and YPAO R.S.C: Yamaha YPAO and YPAO R.S.C are discussed in detail here. They both do a great job with frequencies above 80Hz. Fortunately, the implementation in most recent Yamaha receivers is fully editable using Parametric EQ, which gives you enough resolution to tackle most issues. The only exception to this is ringing of the modal frequencies (see below), which is important for subjective sound quality.
Parametric EQ solutions: a fully parametric EQ solution, such as the ones from MiniDSP, allow you to target the modal frequencies in the room precisely, therefore allowing the reduction of ringing / echoing of sound in the lower frequencies. A good parametric EQ is therefore essential for frequencies below 80Hz and good to have below 200Hz. Neither Audyssey or YPAO tackle ringing at these problematic frequencies.
Dirac Live: Dirac is at the forefront of audio equalisation research and seem to be overtaking even Audyssey. Their impulse response correction algorithms seem to be gathering great reviews. More recently, they are also available in MiniDSP NanoAVR DL product, which makes it affordable. It is also very easy to use for those with not a lot of experience in equalisation, but who want to get the most out of their equipment. The highest end home cinema kits feature Dirac Live.
How to EQ?
If you’re new to home cinema, at the very least read your receiver’s manual and run the automatic calibration routine with all its feature set. If you’re more adventurous, you can supplement your system with a fully configurable parametric EQ for your subwoofer to tackle the modal frequencies in your room. For this, you will need:
- A parametric EQ connected between your receiver and your subwoofer
- The free Room EQ Wizard (REW) software from HomeTheatreShack.com.
- A compatible USB microphone.
To know more about how to measure your room and configure a Parametric EQ, the MiniDSP or HomeTheatreShack websites offers some good guides, but in essence you will have to:
- Install all the software
- Measure the room response (in this case for the subwoofer)
- Calculate the EQ filters automatically or manually in REW
- Input the filters into your choice of EQ
- Re-measure the altered frequency response
- Repeat until you get satisfactory results
Modal Frequencies and What to Do with Them
Modal frequencies in effect are frequencies excited or affected by the size of a room. The modal region is the region below which modal excitation happens. This is normally somewhere below 250Hz for medium to large home cinema rooms. What happens in the modal region is that certain frequencies – very accurately predicted by room dimensions – will combine and re-enforce each other or cancel each other out in a way that creates big differences in loudness and decay time of the frequency. This will make the rest of the frequency spectrum – especially frequencies close to the mode – smeared, masking detail and transparency of the sound reproduction. To resolve this, the modes need to be calculated and measured using REW using the Room Simulation module, then checked using a frequency sweep and waterfall diagram. Once the offending frequencies are found, Parametric EQ filters need to be designed to EXACTLY match the mode’s frequency and Q to rob the mode of its energy. This will help reduce decay time also and make the waterfall diagram more even.
Waterfall plot showing increased decay time in the modal region. The most offending modal frequencies are clearly visible at 29.6Hz and 71Hz. Interestingly, speech intelligibility is affected by a modal at 83Hz, even though it is not as visible on the diagram.
We have really only touched on some basic concepts when it comes to equalisation. Nevertheless, the topic deserves the attention of anyone serious about home cinema. Learn it or get a friend or consultant who has the knowledge. It makes more of a difference to the perceived sound quality than your choice of speakers or amplifier. This is because the tonal quality that you like in your speakers is a lot to do with the above graphs. Even cheaper speakers can sound fantastic with the right equalisation and a good quality subwoofer.