April 14, 2011

Big speakers in a small room

Can I put big speakers in a small room and still get good sound?
Absolutely! It can be done, but there are a few factors that determine how effective this may be. 

From time to time you may hear things like "they're a great speaker, but they do need a big room." The next guy says "hell yeah, you don't want 'em in an average room, it wouldn't work." It's almost said as if self-evident truths are being shared. I used to hear comments like these and wonder why, half the time suspecting a solid reason didn't really exist. I have since learnt a few key issues that will determine if a speaker can work in a given room effectively. Firstly, let's dispell one myth - physical size alone tells us nothing except whether it will fit in the room and whether you can get it past the interior design committee.

So what does actually matter? There are a few critical factors involved.

Output capability

Bigger speakers tend to have either more bass extension or more output, or both. There are exceptions of course. A smaller speaker designed with limited extension for HT may have more output than a big speaker designed to go down to below 20 Hz. 

The output capability is determined by a couple of factors:

1. Sensitivity - rated as decibels measured at 1m with 1w input. Voltage sensitivity is often misleading and speakers are often not rated correctly.
2. Power handling. This is another misleading rating because nominal ratings often tell little of the real limits of a speaker.

Often the specs provided will mislead those who aren't able to judge if they are actually correct. In addition, the real world useable output is often less than expected. Many speakers will have a real world maximum output that is near the sensitivity rating of the speaker. In other words, many speakers rated at 90 db sensitivity will achieve about 90 db at the listening position in a typical setup.

However, if you are comparing speakers, these ratings if specified honestly will offer a guide. As a general rule, an output target is more likely to be achieved with high sensitivity rather than high power handling. The latter is limited by power compression, in which progressively less power is delivered since increasing levels are lost in heating up the voice coils of the drivers.

Bass integration

The way in which you intend to get the bass to work is crucial in this decision. Small enclosed rooms tend to have more bass gain. This is an acoustic boost that can make the bass sound boomy and exaggerated. In every room you also have room modes. It isn't unusual to have a big peak around 40 Hz with around 15 db of gain. A speaker with strong output there will sound boomy unless this effect is tamed. A speaker that has already rolled off at 65 Hz will simply gain some free extension but should not sound boomy.

Above: This speaker looks good before the room mode peak is included - then it looks ugly!

Above: This speaker appears very unimpressive until the room is considered. The rise at the low end will make the bass sound fuller, but it will not boom.

If you want an easy job of integrating a speaker into a room where you know there is a lot of gain and modal peaks, speakers with early roll off may make the task easier. My own preference is to arm yourself with more firepower than needed, then control it with bass traps and EQ. The result is greater headroom and accuracy. However, there are many that will be happy with the result of a more modest approach.

Vertical driver integration

Big speakers with very large distances between drivers sometimes require a certain listening distance for the drivers to integrate. Try listening up close. You hear separate sources, tweeting from the tweeter, woofing woofers, all that stuff. No sound stage. Move back and they start to integrated into a cohesive sound. At one point they are "nearfield" in that the speaker dominates the sound, not the room, but there is no sound stage. At a certain distance, the image floats into the room and you have the illusion of someone in the room singing at you.

The worst case scenario is a big horn speaker with a super tweeter coming in at 8k, being 1m offset from the mid vertically. That tends to mean a big room!

There is a bit of a rule of thumb here - the actual vertical offset is only relevant as far as it relates to the wavelength at the crossover point. You can have a big offset with a low xo point. The idea is to cross at 1/4 wavelength regarding vertical (or radial) driver to driver centres, but it's an ideal almost never achieved. A more common compromise is one wavelength. You can work it out

Distance in metres = 344 / frequency --- (1 wavelength)
Distance in metres = 344x4 / frequency --- (1/4 wavelength)

So let's say you have a Mini Monitor with a 1" dome tweeter and 5" mid and you know the xo point is 2.5k. The max distance apart to keep to the rule is 2500/344 = 138mm. That just happens to correspond to the typical offset between those drivers. It passes the rule. Now let's make the mid 8" in size and lower the xo point to 1.8k. Does it still behave?

Let's transpose .... f = 344 / D = 1.9k ... the answer is yes, we can cross as high as 1.9k anything lower is fine.

To satisfy the 1/4 wavelength rule, we'd like the crossover to be at 490 Hz - obviously that isn't going to happen!

All of this is behind my next project - a point source horn. It's huge in physical size, yielding around 100 db sensitivity compared to a more typical 85 db with conventional hifi speakers. Despite it's large size, it's a clever design that places all drivers within 1/4 wavelength apart. Not too many designs can pull this off.

So if we have a mini monitor and a big speaker with a 12" mid and horn, how do they compare? The mini monitor probably lives within the 1 wavelength rule, let's see about the big speaker. We'll say it uses a horn that is 12" across and 5" vertically. Vertical offset is 0.22m, therefore the crossover should be 1.6k. I would have it cross at 1.2 - 1.5k, so it can easily satisfy the rule, in fact more easily than the mini monitor. In that sense you might say it is acoustically smaller than the mini monitor.


This is the one that gets ignored. The smaller the room, the greater the need to control directivity to get good sound. Smaller rooms will tend to have a more coloured sound. Generally, size is needed to control directivity. In that sense, the 12" horn speaker can give us some control over dispersion so the room colours the sound less. A small speaker in a small room will have wide dispersion, suggesting we need treatment, but then the room quickly becomes dead. We have a problem.

No comments:

Post a Comment

All comments are moderated.