Speaker basic questions

[akis]

Hello

Would like to ask two questions regarding speakers, especially those fitted in musical instrument cabs that will be gigged.

1) if my amplifier produces a voltage of say +/- 20 Volts, does that mean that I have to choose a speaker capable of converting this voltage into cone travel (pretty much linearly one would hope) ?

Is it possible for example that the speaker cone moves for up to +/-15 Volts, but then stays still for the remaining 5 volts having reached the end of its travel?

Can I assume that if a speaker is rated, say 30 Watts at 8 ohm, that this means it can travel +/-15 Volts and anything over it you are on your own?

2) if the amp is overdriven and clips is it likely to burn your speakers unless they can handle twice the rated power, since a square wave (clipped wave) carries twice as much power as the non-clipped wave?

[Dr.EM]

The travel (called excursion) of a speaker driver depends on the enclosure alignment and the frequency being played, as well as the drivers excursion capability (Xmax) and the power being fed to it.

To work out optimal drive you'll need to simulate a suitable enclosure. I reccomend the free software WinISD Alpha by Linear Team, which can show driver excursion based on the enclosure and power.

Power (Watts) is a more logical unit to use for this, most amplifiers are rated by thier power output rather than voltage drive. Using the software you'll create a graph showing where (ie, frequency) the driver will exceed Xmax with regard to a specified power input and enclosure type. Bear in mind below tuning in a bass reflex the driver is "unloaded" so will exceed Xmax easily and a very large sealed enclosure does little to limit Xmax too

Power ratings on drivers themselves are invariably thermal ratings, which are applicable outside of bass frequencies where Xmax comes into play (over 200Hz for instance, but varying dependant on many factors).

[AndrewT]

Originally Posted by Dr.EM

......Power (Watts) is a more logical unit to use for this, most amplifiers are rated by their power output rather than voltage drive.

and how I wish that Mr James Watt (a true Scotsman) had never invented the "watt" to help define the ability of his steam engines.

Logical to use power it is not. Common and misleading it certainly is.

A far better way of specifying an amplifier or pre-amp, would be voltage capability into a range of specified resistances and if they were really wanting to inform the customer Vrms or Vpk into R//C would be even more helpful.

Have you noticed that WinISD and the PRO versions all use Vrms to define the drive signal to the speaker being modeled?

[akis]

I was going to say, and Andrew pre-empted me.

So my questions remain:

1) if I buy speakers rated at P watts (RMS) at R Ohms, am I guaranteed that the cone can travel at least V = sqrt( 2 * P * R ) in either direction before hitting the buffers?

2) If I could somehow measure the excursion of the cone, would it measure the same at two different frequencies at the same input voltage?

3) Is perceived "loudness" directly proportional to the energy transfered in the air? Is this energy directly proportional to cone travel? How does frequency figure into this?

[AndrewT]

Xmax defines the almost linear cone travel in one direction.
Xmech is the damage limitation cone travel. If you hit this you will get an enormous bang from the speaker. Don't go (far) beyond Xmax with the rare transient peaks in a domestic situation.

Use a speaker modeling programme to investigate what your proposed speaker design will do at various frequencies and input powers and output SPLs.
WinISD pro does some very nice plots of these various parameters and is also fairly easy to use.

[akis]

Unfortunaltely most speaker specs I have read (Eminence, Celestion) do not specify xmax or xmech.

[chris661]

Ok - lower frequencies = more excursion.

Various speaker alignments change the excursion curves.

Percieved loudness is proportional not to the driver's excursion but to the steepness of the sound wave. For a long time I wondered why tweeters (small speakers for high frequencies) could reproduce far louder sounds than a woofer (large speaker for low frequencies), while using a fraction of a millimeter of excursion. The reason is (fairly) complicated, but the basic idea is: ears don't measure how far the air moves, but how quickly the sound pressure changes (ie, quicker sound pressure change is louder). This means that to play say 40Hz at the same level as 80Hz, the speaker must move further to make the steepness of the wave (when you see it on an oscilloscope) the same again. That is why frequency changes excursion.

A given voltage at two different frequencies would yield different SPLs, because the steepness of the waves would be different.

Chris

[preiter]

Originally Posted by akis

I was going to say, and Andrew pre-empted me.

So my questions remain:

1) if I buy speakers rated at P watts (RMS) at R Ohms, am I guaranteed that the cone can travel at least V = sqrt( 2 * P * R ) in either direction before hitting the buffers?

Not at all, the speaker's wattage rating is usually the power at which the voice coil is in danger of melting.

The travel of the cone is determined not only by the amount of power you feed it, but also the frequency being reproduced and also the cabinet that the speaker is mounted in.