How to compare Loudspeakers - What parameters really matter? - Avoiding the Hype
If you search on the internet using "hifi speakers" you will find literally hundreds of sites proclaiming that the speakers on offer are worth buying for any number of reasons. That is in addition to the hifi press, trade shows and other advertising mediums that, again, state convincing reasons why you should buy their particular product. The result - CONFUSION! Confused purchasers are then likely to buy on emotion rather than reason and get tricked into spending a small fortune. You end up with an expensive sewer pipe set into a block of concrete that her ladyship finds hard to dust!!!
This section aims to peel away the confusion like the layers of an onion to get at the core of what it takes to put a really good quality speaker system into your living space. It pulls no punches, spares no criticism but deals with the facts.
Speaker Facts and Myths.....Number of Drivers.....Point Source.....Dual Concentric.....Speaker Construction.....Speaker Size
So what determines speaker size selection? This depends what you want to achieve and can be summarised as follows:
- Digital Speakers - My speakers are a full 24Bit digital output - they must be good!?....MYTH
NO speaker is digital!!! Digital is a term that refers to a type of signal, a signal that is either ON or OFF with no inbetweens and sounds like that racket that comes from your modem speaker when you connect to the net. Some speakers DO have a digital INPUT ie 24Bit but they also have a power amplifier to convert this to an approximation of the original audio (analogue) input. It is also true that some power amplifiers work digitally (class D) to produce an approximation of audio at the speaker cone but it's a bit like trying to build a curved wall with ordinary straight bricks - a compromise with rough edges!.....Digital Vs Analogue - Technical
- Speaker Drivers - More = Better....MYTH
Many speakers are built with a bass driver and separate tweeter to get better sound. Some take this further to add a third and even fourth driver. The idea is to split up the audio spectrum and to let each driver handle the part it is best at reproducing. This is another idea that sounds fine in theory but is fatally flawed in practice - See Point Source Speakers below. Another factor which makes multiple drivers worse from a hifi perspective is that each speaker has to be connected via a crossover network and each network adds errors. See Crossover discussion for more detailed information.
- Point Source Speakers are Better....FACT....Back to Top
The next time you see a duck (sat on a pond) take two or three pieces of bread and throw them in the pond so that they land close together and at the same time. Watch the ripples produced and how they spead out - you will see they are very confused - hard to follow. Wait for the water to settle, then throw in a single piece of bread (away from the duck!) and observe how uniformly the ripples spread out in a pure circle.
This is an EXACT analogy of how sound waves travel in air. A single source (one speaker) produces a series of vibrations (ripples) that travel out from the speaker in a coherent (all together) fashion. The result - beautiful clear, direct sound with no confusion. Multiple speakers (2 or more) produce vibrations exactly like the confused ripples you saw when throwing multiple objects into the pond and produce lots of incoherent (confused) vibrations and is known technically as the Comb Effect.
This is complicated by the fact that as you change position (even slightly) in front of a multiple speaker, you hear different combinations of ripples (vibrations) that affect your perception of the sound - you lose focus.
Given the pond illustration, a single speaker must have the edge when it comes to producing a focused, coherent sound front and this is true. The difficulty is that no single driver can reproduce the full range of sounds contained within the audio spectrum and we need both a woofer and tweeter to achieve this. The point source problem is solved very cleverly by mounting the tweeter at the center and on the same exact axis as, the woofer. This Dual Concentric approach CAN and DOES produce the same clearly focused pattern as a single speaker, but crucialy, now across the whole audio spectrum. Hence we get the best of both worlds.
- Speaker Construction....Back to Top
All closed areas vibrate at certain frequencies - blow across the top of a milk bottle and see what happens. Change the enclosed volume (add water to the milk bottle) and the frequency of vibration (resonance) will change. This is true of ANY closed space, be it your speaker cabinet, fridge or living space itself. The larger the space, the lower the resonance produced. This is another law that cannot be changed. However, steps CAN be taken to control or reduce the effect of the resonance.
- Make the cabinet stronger - use thicker material
- Get the cabinet dimensional ratios right and try to eliminate edges
- Paint the cabinet inside and out with sound absorbing material - ie bitumin
- Put sound absorbing materials within the cabinet
- Add critically tuned ports to cancel out the last remnants of any cabinet resonance
- Make sure the cabinet cannot move about
So the trick is to minimise resonance by optimising the variables above. A sucessful well designed cabinet produces no unwanted or additional sounds - period. Sorry if that is a bit basic but.....
- Speaker Size - Bigger is Much Better.....MYTH....Back to Top
In all sound systems small to large, hifi or professional, the usual goal is to achieve a flat frequency response across the whole space to be covered. This produces the best quality of sound with no annoying resonant peaks or bits of the sound spectrum that you cannot hear. The basic size of speaker is a function of its job. If you want to produce nightclub levels of bass energy then you need larger cabinets BUT, since bass frequencies are omnidirectional (no direction information), you CAN use a single sub woofer with its own amplifier and tuck it out of the way. A classic place (where mine is) is to build it into the living space furniture - fit it under grannys chair (and watch her smile as she feels the music)!
- Small Residential Property - nice stereo sound required but not rock music.
In this case, you need to start with deciding how much bass you want to enjoy. If (like me) you love to hear the graunchy bits of a double bass playing the deepest notes then you need:
The subwoofer will provide coverage of the bass frequencies and a smaller type of main speaker with a higher bass cut off frequency will work well.
- A subwoofer
- Two smaller type loudspeakers
If you just want two speakers then in order to capture the low frequencies you need to opt for larger main speakers. In fact as large as she will allow! Any size will work well but be aware that the smaller the cabinet, the less low frequencies you will hear.
Turning up the Bass tone control will NOT improve the bass, rather it will boost ALL frequencies by a varying amount and starting at a surprisingly high frequency. You will therefore destroy the original sound quality. The bass may sound louder by doing this but any conception of hifi - truth - is now lost. You need to buy the right speakers in the first place!!
- Speaker size also relates to power handling. This can be very technical but in simple terms, if you want to listen to the hifi at moderate levels 85dB ish (as loud as an excited football crowd) then you need to add 3dB for every metre you are away from the speakers. So if sat 4m away, your speakers need to produce 85 + (3X3) 9 = 94dB output @ 1m. Consult the speaker specifications. If sensitivity is say 94dB at 1m for 1W input, then to produce your target volume, you need an amplifier of 1W ONLY!! If your speakers are rated at say 85dB sensitivity, then your amplifier needs to be 94-87=9dB louder and needs to be much more powerful to produce the same sound level. Complicated but in simple terms, the more efficient the speaker, the lower the power requirements of your amplifier to drive them. Of course loud bits demand MUCH more power - as a rule of thumb, for you to double the volume, you need to square the power (Watts). Notice that you can very quickly run out of power if you have a small amplifier (not many Watts output) driving inefficient speakers at high volumes.
An added complication is that speakers are difficult to drive electrically and put enormous stresses on the power amplifier, especially when being driven hard. Hence efficient speakers make it easier for your amplifier to deliver distortion free clean outputs.
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