‘Big sound’ may be a good thing, but can you have too much of that good thing? Anthony Grimani reports.

We are all usually faced with budget limitation issues in our proposals and systems and forced to lower the specifications on some of the gear. But what about when there is plenty of budget available? Is it possible to ‘over specify’ a system? Well, no. But then again, yes. Actually, it depends.

You can really never have too much quality in product build, sonic performance or reliability. But you can certainly hurt your end result if some of the components aren’t matched to the room, the user or the rest of the system.

Let’s look at some of the places where you need to be careful:

Speaker sensitivity
A big room needs speakers with higher sensitivity so as to play louder given a typical power amplification regime of, say, 200W. This usually means resorting to horn-loaded speakers or large line arrays with plenty of radiating surface area. Both types can sound very, very good and play very, very loud; however, in a smaller room with less acoustical volume where you sit closer to the speakers, high sensitivity will reveal a new little problem: hiss.

Yes, that dreaded memory from the days of analogue tape and vinyl records. It turns out that the typical surround decoder/pre-amp – plugged into a very necessary audio processor/equaliser in turn plugged into a power amplifier with high gain – will sit there and quietly but annoyingly reveal the residual hiss at the bottom of the typical electronics analogue stages.

There’s no way around it! And if you have seven or nine speakers surrounding the audience, you will certainly be confronted with a less than stellar noise floor in the room. So, be careful about the speaker sensitivity and match it to the room size, desired noise floor and desired sound pressure levels. A 30m² room will do fine with speakers purported to have 91dB/2.8V at 1m anechoic (the right way to describe the spec). A 40m² room typically needs 94dB speakers to get good and plenty sound pressure. A 60m² room will require about 97dB sensitivity to get to the engaging levels we all come to expect from film soundtracks.

Of course, if you under specify the speaker sensitivity, you will need to push more power into it and you risk that annoying failure mode where tweeters smell like toast with Vegemite.

Amplifier power
If a bit of power is good then more is better, right? Well, not always. If your application calls for an average power of 10W, with the occasional peaks at 120W, you may get into some trouble with 500W amplifiers.

Sound sordid? Check this out: high power amplifiers often have higher gain so that they appear to be louder. Fair enough. (Typical 100W amps have 29dB gain, while 500W amps will often have 35dB gain to basically get to clip with 1V of input). Having more gain may amplify the hiss from the analogue stages of the decoder or the equaliser – much like high speaker sensitivity as explained above. If the amp is one of these pro audio or commercial units with a gain control then none of this matters. Otherwise, be careful to not place your customer in the middle of what will sound like Niagara Falls.

Big amplifiers also generate more heat and take up more rack space. This can cause pointless challenges in the equipment room. Also, and this is more esoteric, typical large class AB amplifiers exhibit ever-so-slight distortion at low levels from the crossover conditions of the output stage. This may be audible at lower listening levels. Oh, I suppose that you also want to watch out for over-powering speakers rated at 100W with a 500W amp. That, too, may result in the dreaded toast-with-Vegemite smell!

Bass extension
Let’s face it; the best way to set up the main speaker configuration in a home cinema is to run them down to about 80Hz, then hand off the bass to a set of subwoofers strategically placed around the room. Playing the front speakers much lower than that will often result in bass errors from the room’s standing waves and – if you are the typical unlucky sort – you will get a giant dip in sound around 60Hz at the seating area. Not good, because that’s the major energy band for bass in action films and that’s also the usual frequency of rock kick drums.

The dip comes from the null in the second harmonic of the length standing wave in 6m long rooms. There is no real way around it.  If you sit down, the standing wave is still audible… tough!

A particularly interesting item of note is that properly done subwoofer crossovers in surround decoders and receivers provide low-pass filters that are fourth order (24dB/octave) so the subs don’t play any mid bass frequencies with directional energy. These crossover circuits also expect the main speakers to naturally roll off somewhere around 80 to 60Hz with a second order (12dB/octave) slope.  When a 2nd order high-pass filter is added to the speaker’s natural response, you get a net fourth order high-pass response that matches the subwoofer. If you are over-zealous and choose large speakers with response down to 30Hz for the mains, you will defeat the phase and amplitude alignment of the decoder’s subwoofer crossover and end up with a hole in the frequency response around 80Hz.

Funny, isn’t it? You put larger speakers that play louder bass, and you end up with less. Life can be so complicated…

Speaker wire size
We all know that thin speaker wire is a bad thing if you try to go any significant length between the amplifier and the speaker. But it’s actually pretty easy to figure out the necessary cable’s diameter. You just want the total resistance of the round trip of wire to be less than 0.2Ω. Any more than that and you will start to hear errors in frequency response that come from the interaction of the wire’s resistance and the speaker’s crossover components. You can simply measure it with a DC ohmmeter, with the wire on the spool, or you can try to get the specifications from the manufacturer.

If you measure, remember to get the total resistance by shorting the end of the wire and measuring between the + and – legs at the start of the wire. Here’s a rule of thumb: 16 gauge wire is OK for up to 10m, 14 gauge wire up to 15m and 12 gauge wire up to 20m.

What happens for longer runs? Do you go for a thicker 10 gauge wire? NO! Why not? Because of this pesky thing called inductance that comes along with larger diameter wires. A 30m run of 10 gauge wire has enough inductance to audibly attenuate the highs of most speakers. Of course, 8 gauge wire is even worse. So don’t start using car battery cables for your speaker connections. Instead, start doubling up on 12 gauge wires in parallel if you need a 30m run. You can hook them together at either end with thinner end leads that fit into the binding posts. No worries. Note that the 12/4 wires that several manufacturers sell are particularly useful for all this. Also, this has nothing to do with power levels. There is no more trouble at 1W than there is at 1,000W. Some people think they need heavy gauge wire for higher powered speakers. Not so!

Number of subwoofers
Some of you may have heard the news (from 10 years ago) that more subwoofers are better. Well, that’s true if you position them strategically around the room. Two subwoofers placed front and back or middle of the left and right walls can smooth out the bass bumps from standing waves. Four subwoofers in the four corners will further improve the situation.

But, research has shown that more than four subwoofer doesn’t further the cause very much. It just makes the room more difficult to tune. The sweet spot is four subs or eight subs clustered in the four corners if you need more SPL. If you also drive the four subs from four independent channels of digital EQ with independent level, delay and amplification, you can further improve the situation by fine-tuning the resonance response of the room. You can delay the back subwoofers a few milliseconds or change the levels between the units to smooth out the total response. In some cases you can get back a 15dB loss from the dreaded second harmonic of the length standing wave I mentioned earlier. That’s a lot of bass and your customers will love it.

So remember to specify everything right. Not too much and not too little. It’s an engineering art form that your customers expect you to know. In the end, just put the money where it matters!