Ahh urgent help please! Decibals SPL and stuff, cheers URGENT AHHHH!

R

raindrops

New member
Hi I've got a project coursework (A level) (17) "Testing and evaluating a sensor" Mr sensor is a AKG D770 microphone and using a signal generator I am recording a direct signal through
tone generator--->mixer----->pc
miced signal through
tone generator--->speaker--->mic---->mixer---->pc

The aim is to produce one of those frequency response charts. I started getting some results and then I wondered whether how far the mic is away from the speaker will have affect. I know some but not much about decibals and stuff but I suddenly seemed to remember something about distance involved.

The thing is due in on wednesday (tueday today)
Please help!

Thanks!
Nick
 
If I understand what you want then I'll explain.

There are several variables on distance effecting the sound and freq's.


I'm having some serious problems translating into english for this. I dont know the english equivelent for the matamatical term. so I will write the numbers in hope you understand.

2
2 = 4 meaning - as the distance is larger by 2 the energy is weaken by 4.
Or try this one.....
3
3 = 9 - as the distance is times 3 the sound weakens by 9.

Our ears are for sure going to be able to tell the difference in freq content as the low freq's slowly go under our abilty to hear them but the highs we can still hear.

Only problem is.... there are more factors like the room
and phase problems and the air in the room absorbing the freq content, (mostly around 1k and up). This depends mostly on the dampness in the air.....

The difference in the cold air hot air (floor celing) will effect as well although I'm not sure in a small room how much effect that would have ( I dont wear rubber gloves and a white Docters gown).

The combination of the reflections with the direct sound will effect the freq content as well becuase the reflections coming from different objects in the room contain different freq content due to some of the freq's being absorbed and therfore are not the same sound
as they started out.

Phasing will change your sound as well. every time you move you change the phase relation between the direct source and the reflections.

I could go on more but.... I have to be sure that this is what you need.
 
Last edited:
Wow thank you quick reply!

The english wording is

The 'energy' decreases proportional to the square of the distance.

It is useful information there but not quite what I mean. What I want to know is by how much the, lets say 1kHz frequency range is being cut by the microphone. It would be possible to have a relative chart but I would get more credit for have a scale.

What I am doing is running a direct line out of the generator as well as using it to power a speaker which the microphone picks up. These are then run into the computer inro cooledit pro where i use the spectrum analysis to look at the different frequencies and dB level of the two sources. I want to be able to compare these directly because I can obviously change the level of either of these. What I did is assume 1KHz as a flat frequency response and so level up the two at 1kHz. From this i can see the dB drop/increase across the differeent frequency ranges. But I'm not sure whether this is a REAL dB drob. I'm finding this a bit hard to explain. If somebody does understand what I'm asking but doesn't know the answer it would nice if you phrase it in a way that people who do know the answer could understand!!

Thanks alot
cheers
ta
nick
 
Yeah cheers, I already got that graph, the experiment is asking me to compare the product to th manufacturers claims as part of it.

I suppose to make understandable what I was saying in my question:

Does a 5dB drop say at 14kHz get affected by how far th mic is from the speaker and how loud the speaker is or is it all relative ie can just use that 5dB reading and compare it with results fro different frequencies. If not then what is the standard distance and speaker power?

I hope that makes more sense. And yes I am aware the speakers freq response will affect result but there is nothing I can do. there is also the other equipments freq response, mixer, soundcard, cable and for that I have constructed a graph which is subtracted from the mic results.

Cheers
Nick
 
As sound travels through air the high frequencies get attenuated faster than the lows, I dont know the exact mathematical formula but it is generally most problematic for frequencies above 8 khz, Most speaker/microphone analysis is done in the 'near field' to avoid too much attenuation and room interaction, I think that for a small speaker the near field is considered to be about 3 ft, if you are using a 2 way or 3 way speaker system (seperate bass, mid and high frequency speakers) then there will be a 'focal distance' where the sound from the multiple drivers seems to unify, anything closer will be biased towards one or other of the speakers. This 'focal distance' can be determined approximately by ear.
 
Ok still not what I am actually asking!!! cheers anyway.

The mic is about 1cm away from the speaker which is a single way speaker, the mic/speaker has been covered with a cloth to minimise outside disturbance.

I am comparing two sources of the signal, can I actually relate the two with dB scale (in cooledit pro). I CAN compare them, but is it valid?

Hope that works!!

Ta thanks for help!

Nick
 
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