Under powering speakers

[Jono_GT]

Hi folks. Can an audio boff explain to me in simplistic terms, the effect of using uprated components that state a minimum 40w input, with the standard unit that I believe is a 35w output.

Discuss ...   s:D s:D

[AndyM]

Basically the speaker rating is designed to give you an indication of the power that is needed to get the speaker driver moving properly to produce it's range of sound. Generally a better speaker has a higher rating because it is heavier and has better magnets, these should be externally amped.

Incidentally it's also why the stock speakers in many cars sound a bit flat, due to the small on-board amps they match them with weedy drivers which make the noise and can even go loud but are flimsy and don't deliver rich sound reproduction.

If you under power a speaker they may actually work (although may not if the power is vastly lower) but most likely won't sound good and could be either a) dull/quiet, b) distorted, or c) both.

You also might find it's quiet but when you turn up the amp (in this case the one in the head unit) it could try to deliver power outside of it's range. This can cause the amp to 'clip' ( LINK ) the sound wave causing distortion. When clipping occurs it can have a few weird side effects (e.g. power surges, overheating, tripping fuses, etc...).

Absolute worse case scenario is it could burn the amp out with prolonged clipping.

Obviously it goes without saying that it's best to match the driver to the amplification available where possible.

[Jono_GT]

Nice - I get it. Thanks.

That could explain why my new components don't sound quite as good as I'd hoped.

Think I'll eventually have to invest in a better headunit... and underseat sub too! Miss the subs from my mk2.

[AndyM]

Think I'll eventually have to invest in a better headunit... and underseat sub too! Miss the subs from my mk2.

If it were me I'd consider looking at a compact 2 channel amp for the components as head unit amps never give decent power in real world terms (don't believe the label on the box) there just isn't enough space/cooling inside a din sized space to give a decent punch. Even more expensive head unit models normally focus on improving the SQ and features rather than amp power as they assume they will be externally amped. You'll notice an immediate improvement in volume and SQ by adding a dedicated amp for the comps.

Also just a heads up, you might struggle with an underseat sub (have a search on here), many have tried and many have found there isn't quite enough room to make it work well so go for a bins install or an underdash enclosure.

[Dambirch]

Unfortunately, the answer to this is not so easy to explain, without considering a number of factors.

In summary, a speaker manufacturer will place a rating on its speakers in order to ensure the best amplifier match for optimal sound reproduction.  Why..?  Each driver / transducer has a moving mass, which is comprised of the voice-coil, spider, cone and surround.  When energised the voice-coil moves within the magnetic field and radiates sound through the cone.  The spider and surround keep everything aligned and in-place.  The heavier the mass the more power is required to move it.  Therefore, as transducer size increases (with it the mass) Mo-Farah is required...  s:) s:)  There are exceptions to this, not generally in the car audio world but certainly in the HiFi world.

A speaker is only as good as its cone (the radiating body).  The requirements of good cone material are stiffness and low mass.  With the advent of materials science transducers have become stiffer and lower in mass – Aluminium, carbon-fibre, polypropylene, titanium, bonded paper, glass-fibre honeycomb, rohacell, magnesium, Kevlar are all materials that have desirable properties but will all bring some level of compromise to the sound quality.  Other aspects of speaker design will affect sound quality but for the purposes of this discussion have been left out.

Mass affects sensitivity.  The greater the mass, the lower the sensitivity.  Transducers in the 60 / 70s were generally made from paper cones and foam surrounds with a sensitivity of 95+ dBs.  Nowadays, with stiffer cone materials (and less compliant surround materials) the mass has increased and the typical sensitivity of a transducer can be as low as 83dBs, some 12 dBs lower.  But the benefits are less distortion, increased clarity and better articulation / detail.
Amplifier power is not necessarily volume.  Speaker sensitivity is more important especially if using lower powered amps.  For instance, a 5 watt amplifier will drive a 100 watt rated speaker system with no problem at low volume levels.  The problem arises when higher volume levels are played (coupled with transients - bursts of sound) where the amplifier will effectively run out of steam and clip as it tries to deliver the power required for the transducer to complete the wave form.  In this instance the amplifier can be damaged through overheating, and the cone of the transducer is forced forward and remains in this position until the clip passes and it returns back down the sine-wave.  Speaker damage will not normally occur.

The 12 dB loss in speaker sensitivity over the years is huge.  For every increase in 3 dBs amplifier power has to double.  Let's say 5 watts will provide a volume level of 83 dBs, for 86 dBs 10 watts power is required.  For 89 dBs 20 watts and at 95 dBs 80 watts will be the minimum requirement for the power of the amp.  Therefore, a more sensitive speaker system will give you more volume for less power.

The other consideration is the passive crossover.  Passive crossovers can rob up to 20% of amplifier power depending on complexity.

Personally, I will always ensure that the amplifier can deliver enough power to play at reasonable listening levels.  This will probably be double the input rating, which should be more than enough power to handle the transient sounds.  The only issue is if volume levels are high (overdriven) the transducer's voice-coil will blow just like a fuse.

To summarise, ensure you buy an amplifier with more power than is required to drive the speaker system you have in mind.  If the speaker system has a sensitivity of 90 dBs this will have been measured at 1 watt of amplifier power (at a distance of 1 metre from the cone).  If you like your volume levels at 120 dBs then you need to make sure the amp has at least 512 watts of power in reserve and this does not include the power required for the transient sounds and the power robbed in the crossover.

Methinks a separate amp is required.  I'm about to embark on this route in the next month or so.  I have all the equipment, just need the time...  s:) s:)

[StuC]

underseat sub too! Miss the subs from my mk2.

As Andy says underseat is not an option...
This is where it's at for subby goodness with package efficiency.  s8) s8)   s8) s8)
I believe that Nathan has one left too.  s:-D :-D s:-D

[Jono_GT]

underseat sub too! Miss the subs from my mk2.

As Andy says underseat is not an option...
This is where it's at for subby goodness with package efficiency.  s8) s8)   s8) s8)
I believe that Nathan has one left too.  s:-D :-D s:-D

Mmmmyeah - I know, but too much fitting effort required for my liking!

I dunno - Gonna change HU first but got to slow down the spending after recent expenditures (non-car). And I REALLY need a wheel refurb!   s:D s:D  That has to be the next job.