It is only recently that I have come to realise how crucial bass is to the overall sound of an audio system. Over the years, I have had a succession of moderately-sized bass reflex speakers which never quite delivered the performance I might have hoped for. The bass reflex, or ported, configuration is extremely common, even in speakers that cost the price of a car, yet if you read about its pitfalls, it does seem amazing that anyone would consider it for serious use.
The big old speakers of our youth in the 1970s (if you’re as old as me!) were different: voluminous sealed enclosures with large woofers. The characteristics of these speakers are quite different from the modern ported bass reflex variety. Some of these differences are:
- For the same -3dB point, sealed enclosure speakers have to be considerably larger.
- Below the -3dB point, sealed speakers roll-off at 12dB/octave while bass reflex (ported) speakers roll off at 24dB/octave. A typical listening room can provide a ‘boost’ in the other direction that can approximately compensate for the sealed speaker’s roll-off if the speaker’s -3dB frequency is appropriate.
- Below resonance the response of the cone of a sealed speaker is still under control due to the restraining force of the air in the box, while the cone of a ported speaker is not restrained i.e. it is no longer under precise control.
- The bass reflex speaker’s output is augmented by the output of the port, which resonates in response to the movement of the back of the cone. This output is delayed, inverted and ‘smeared’ in comparison to the input signal.
- Around resonance, the sealed speaker’s cone has to move further than the ported speaker’s which implies higher distortion, but the output of the port may be heavily distorted itself, which would offset this advantage.
- A ported resonator’s output level is not completely linear, and it loses efficiency at high output levels. Overall bass output falls, and the cone’s displacement increases, resulting in higher distortion there. So the implication is that the speaker ‘pulls its punches’ on transients and sounds more brittle at higher output levels.
- There are fewer variables involved in the design of successful sealed enclosures compared to ported.
To my mind, and ears, there are few apparent disadvantages to sealed enclosures and plenty of advantages.
I’m no expert, but conventional speaker design often appears to be an exercise in ‘painting with frequency response’ and designers can be quite cavalier over what happens to the time response. If a particular technique like bass reflex or transmission line, can result in a lower -3dB point and a smaller box for the same amplifier power then they’ll use it without question. Testing with sine waves won’t show up the strange things that the port does to transients, and maybe there’s an element of not even wanting to question the orthodoxy.
As described in the article on building my own speakers, I decided to experiment with some pretty huge, sealed, active woofers. The signal is pre-corrected with DSP to linearise the phase, but there is no attempt to EQ the frequency response in-room at the bottom end.
The sonic results lead me to question many of my assumptions regarding the quality of speakers I have heard over the years. Is good bass, in fact, much more than a ‘nice-to-have’? Is it a crucial element without which an otherwise ‘perfect’ speaker simply cannot sound good? Without good bass, does the system’s frequency response appear to assume a critical importance that is, in fact, a ‘red herring’? And what of other variables? In the absence of good bass, can anyone make a meaningful judgement of whether valves are better than solid state, or vinyl better than digital, for example?
Bass is not merely a layer of paint underneath the music that gives it warmth or balance, nor is it an effect designed to shake the room. It is the foundation of the music’s dynamics and provides important information about the dimensions of the acoustic space the recording was made in. What makes us think we can discard most of it without incurring any penalty?
At the very least, good bass (and that means in the time domain, too) provides us with a realistic physical ‘hit’ from a symphony orchestra. And much of the excitement of a live rock performance lies in the sheer excessiveness of the volume and depth of the bass. By losing the bass we lose much of the excitement, plus we fail to hear some of what the performers are responding to in their playing.
There are sounds in some classical performances that are on the verge of being infrasonic. This sort of bass can be clean, with little in the way of accompanying harmonics, so that with typical speakers you simply wouldn’t be aware it was there at all. This has been a revelation to me.
These all seem like positive things, but in reality they are just a lack of negative things! Simply recreating the whole sound, rather than lopping a chunk off it or smearing it into a muddy rumble that obscures detail, makes it easier for our ears and brains to piece together the musical event that was captured in the recording.
My speakers have transformed my listening to recorded music. I no longer feel that part of the sound is wrong, or missing. I am no longer clutching at straws, worrying about non-existent cabinet resonances or whether I am using the wrong type of carpet spikes. I think that a fundamental part of this musical contentment is the completeness of the bass.