The physics of volume

[jackcu 0]

Here's something that comes up from time to time and, much as I try (and I've read a fair bit too) my simple brain struggles to grasp the topic.

 

It's about volume.........power.........speakers........moving air.

 

I've read a bit about the physics of all this, but come from a starting point of having done no science at school (or anywhere else). So here's the thing - what are the different ways of achieving a certain volume? Facts like "to double the volume you need 10 times the power" confuse me.

 

To my simple mind, volume is about moving air. Let's say we have a 15" speaker cone. The volume you hear is a function of the amount of air being pushed by the cone towards your ears, right? The more air is pushed, the louder the sound you hear. But what happens if you want an even louder sound? Are two speakers twice as loud as one?

 

For simplicity, let's say I have an amp with 100W per side pushing 2 x 15" speakers. That gives a certain volume. If I want it to be twice as loud, what do I do? Could I simply have an identical second set-up? That doubles the amount of air being pushed, so I would have thought it was twice as loud. But is it?

 

When I look at the PA at rock gigs I notice they use loads of speakers - is this because they are just running multiples of the same system - a bit like having 20 stereos all playing the same thing?

 

What about if I have a 200w amp? From what I read this wouldn't be twice as loud - I would need 1000w to be twice as loud - but why? Are bigger speakers louder 'cos they push more air (all things being equal)?

 

I'm sure you gurus will be able to explain this - but please remember, you'll need to explain it in very simple terms :)

 

Thanks,

 

Jack.

[Gary 0]

In simple terms... (as requested)

 

 

 

 

 

 

 

 

 

 

 

 

 

"Can you hear me OK at the back?" http://www.dj-forum.co.uk/html//emoticons/533.gif

 

 

 

There!

 

 

 

If "Yes"....perfect.

 

If "No"....turn the volume a little higher, or bring another amp'n'speakers next week.

 

 

Taaa daaa!

 

 

 

 

 

Although its not the case in this topic, its possible to get too bogged down in the maths and equations and loose sight (or sound) of the reason for being there - within reason. OK, if you turn up with 50watts @ 8ohms for 1000 people, then you might get more than the occasional "Oi! Turn it up mate!". Similarly, if you play 10000watts at 8ohms to 50 people - you might not get re-booked - even by those whose eardrums are still on the correct sides of their heads.

 

Some parts of db and sound pressure are useful such as:

 

"Placing two bass bins together, rather than apart can add 3db of bass"

 

or

 

"Bass travels mainly through surfaces, so placing a bass bin in a corner (3 surfaces) will give you more bass than placing the bass bin centrally"

 

also

 

knowing to look for impedence (ohms) ratings, and RMS, rather than being taken in by PMPO (ha ha ha) and Max watts, is a good hint too.

 

But acoustic physics can be a bit like food warnings...try to follow all the advice and you'll end up with more questions than answers.

Edited September 5, 2005 by Gary

[jackcu 0]

I know it's tough Gary, but I keep getting stumped.

 

I found yet another useful reference that partly answers the "number of speakers" question:

 

http://www.musiccenters.com/vol.html

 

Apparently doubling the speaker area (moving from a 10" to a 20", or adding another 1 x 10") increases the sound by 3db. This is about the smallest increase in volume we can pick up (so it says). Unfortunately you can't just keep on adding speakers and amps as phase cancellation negates some of the extra volume.

However, I'd have thought the more amps and speakers you add the louder it gets - just that the increments get smaller and smaller.

 

I also think this helps to understand the "10 times as much power to double the volume" equation:

 

http://www.epinions.com/content_3626016900

 

Perhaps my question should be, "what is a reasonable SPL for a medium sized room with, say, 100 people in it?"

 

How many of you have measured your SPL?

 

Answers on a postcard please :)

 

Thanks,

 

Jack.

[spinner 0]

Speaker sensitivity has a part to play too.

 

If I remember correctly +3db in speaker sensitivity will double the output power from the amplifier although this will result in an increase in loudness of 1.25 times ( or something like that ).

 

I can say that when I changed the mid/bass speakers in my mid/top cabinets from the originals, rated at 99db to B&C 12PE32 drivers rated at 101.5 db there was a noticeable increase in output ( as well as detail and clarity ).

[jackcu 0]

Thanks Spinner - that makes sense. +3db takes twice as much power, if I've understood correctly, but only increases the volume by about 25%. Also, having a higher rated speaker will make the starting point louder.

 

Cheers,

 

Jack.

[spinner 0]

QUOTE (jackcu @ Sep 5 2005, 03:09 PM)

Thanks Spinner - that makes sense. +3db takes twice as much power, if I've understood correctly, but only increases the volume by about 25%. Also, having a higher rated speaker will make the starting point louder. Cheers, Jack.

Produces ( from the amp ) rather than takes, if you see what I mean.

[jeffwall 0]

Now i know why i just bought powered mackies! Forget all the above and just turn them on! http://www.dj-forum.co.uk/html//emoticons/biggrin.gif

[jackcu 0]

QUOTE (Jeffwall @ Sep 5 2005, 03:50 PM)

Now i know why i just bought powered mackies! Forget all the above and just turn them on! http://www.dj-forum.co.uk/html//emoticons/biggrin.gif

That's not really my point Jeff - it's not about matching amps to speakers, it's about "is your system loud enough?". Let's say your powered Mackies are fine in a certain room with a certain number of people in it. What would you need for an increase in volume of, say 40%, for a larger room / more people?

 

Thanks guys,

 

Jack.

[spinner 0]

This puts it more succinctly:-

 

SPL represents one of the most useful specifications published for any transducer. It is a representation of the efficiency and volume you can expect from a device relative to the input power. This is important because it requires twice the power to

increase the volume of a speaker by 3dB. For instance, to increase the volume of a 50W guitar amplifier 3dB (an audible, but relatively small amount), it would require 100W of power. The same outcome could be achieved more economically by

replacing the speaker with a model that is 3dB more sensitive.

[jackcu 0]

That's more like it - I think I need a SPL meter LOL

[Award Entertainment 0]

The spectacular-sounding and visually stunning Bose PAS system I recently trialled threw almost everything I knew to be true about the physics of sound reinforcement completely out the window. It was very humbling and a little frustrating because now I KNOW I need to buy a couple of them.

 

http://www.bose.com/images/musicians/products/p_l1double_s_b.jpg