Rode NT1 okay for measuring?

I have a very small home recording studio which I acoustically treated last year with DIY rockwool panels according to Ethan Winer's and others' guidelines. The room is 10'x6.5'x8'.

I recently bought a Radio Shack 33-4050 SPL meter in order to measure the rooms acoustics / frequency response using REW. I have since discovered through reading some posts on this forum that this meter is only good for measuring the low frequencies. I presume that is not enough for my room and that I need something that will cope with the whole spectrum(?). As you can tell, I haven't used REW before.

I have a Rode NT 1 which is a good quality cardioid capacitor mic with a frequency response of 20Hz-20kHz. Will this suffice for taking the measurements or do I need to go out and buy a special measuring mic?

Thank you.

Do you have a calibration file or graph of the microphone? If so, you can create a calibration file that will work with REW.

brucek

Hi brucek. Thanks for your reply. That's good news but no, I don't have the file or graph. Is that something I can generate myself or do I have to get one from somewhere? How do I go about doing that? I will try and find this out myself by looking at the help files etc. but I would be grateful for any further help if possible.

Mark

Well, a calibration file something is that will come with a microphone to show it's frequency response.
If you use a mic or meter that we have tested on the site, then you can get away with using those calibration files (if you own one of those), with an assumption that they're accurate enough for home use. We have cal files for all the Radio Shack meters, Galaxy CM-140 meter and the ECM8000 microphone. The Radio Shack meters shouldn't be used much beyond 1000Hz as it seems as they aren't accurate past that. The ECM and Galaxy are full range. The ECM requires a preamp though.

If you don't have a cal file for the microphone you own, then you have no way of knowing its accuracy I'm afraid.

brucek

Hmm. I'll see if I can locate a cal file through Rode. Alternatively, I may pick up an ECM8000. I want to measure the room mainly to see how effective the treatment I have used is but also to find if there are any problem frequencies needing further attention, particularly in the lower end. I have a lot of bass trapping as it stands. Would you happen to know how necessary it would be to measure beyond the 1000Hz or would the fact that the RS meter will give me a good idea of what's happening in the lower frequencies be a good enough test for my purposes?

Thanks again.

Well, my understanding is that treatment doesn't do much above ~500Hz and does little for modal problems below ~80Hz (where eq is usually employed), so I see no reason not to use the Radio Shack meter. They're suitable for that area..........

brucek

That's great brucek. I'll do that and maybe post the results to get some help interpreting them. Meanwhile, I'll try and hunt down a file / graph for the NT1 to maybe compare the results obtained with both the mic and the meter.

many thanks,
Mark

Actually, I'm having a bit of a job finding a fre. resp. graph or a cal file for the NT1. The Rode website has one for the NT1A but it's a different design with an improved response so I don't think it's any good re. the NT1. I wonder if anyone knows where I can find one?

I will use the RS meter as brucek suggests but I would like to test a wider fre. band if possible as well without buying another mic.

Thanks, Mark

You can always have your microphone calibrated if you feel it's a decent mic. It can be expensive though. I suspect you could get a Galaxy meter for the same price. Sometimes stage mics don't make great measurment mics if the response is set for voice. I guess it needs external biasing, so some type of preamp is needed?

brucek

I don't think it's an option to get the mic calibrated. I could buy the Behringer or the Galaxy at that rate. The NT1 is a very good mic and is regarded as fairly flat; not sure what you mean by a 'stage' mic except it is of course a recording mic and is not specifically for vocals; it works well with most applications. I imagine it would do the job well if I could get a cal file together.

I'm going to contact Rode by phone just as soon as the time zone brings them into opening hours and do my best to get a graph. If not, I may have to work within the limitations of the RS meter.

Thanks brucek. I do appreciate your help with this.

Regards,
Mark

---

Hi Wayne. I'll check out that link. Meanwhile, here's a copy of the spec page for the NT1 and the fre. resp. graph for the more recent version, the NT1A. I spoke to a guy in Rode and he said the two mics are similar enough to use this graph with reasonable accuracy. He said the main difference is in the noise level and that the frequency response is similar. Also, it is not flat but I gather from reviews like this one http://www.soundonsound.com/sos/1997...7/rodent1.html (see the section immediately under the heading ¨Rode test¨) that the enhancements are quite subtle, which seems to be reflected in the graph.

Spec sheet:

nt1 specs.JPG

Frequency response graph:

NT1A frequency response graph.JPG

Does it look like I could use this mic? If so, can you tell me how to create a cal file from this graph, or else point me to where I might find that out?

Thanks a lot,
Mark

It's very simple Mark.

It's up to you what type of frequency divisions you want to use because REW extrapolates between.

At the extremes REW will extend the last values.

If the mic reads low a negative value is entered and positive for positive.

You create a .txt file (with notepad) with frequency and decibel level. Then save the file and rename the extension to .cal

You'll have to decide from the graph what the values are.

For example here is a small sample of a cal file.

So decide what the vertical divisions on the graph are and use those values.

I would simply enter at the logarithmic values from the graph for the frequency divisions.

For example :

20 -2.00
30 -1.50
40 -0.50
50 0.00
60 (I'm sure you get what I mean):
70
80
90
100
200

create the file, and name it NT1.cal and load it into REW and see what it looks like. You'll see if it looks like your graph or not....

Questions?

brucek

That's excellent thanks brucek. Very detailed and clear.

I think I've got the idea. I opened up the cal file for the RS meter in notepad. Do I copy the format there in terms of spacing between the 2 values? Also, that file goes from 7.00 to 200.00 Hz. I presume I enter in all the values between 20Hz and 20kHz in the new file, using the divisions you suggest from my graph?

Does it look to you like I should be able to use this cal file ok with my mic to go ahead and take the measurements?

Many thanks again,
Mark

P.S. The other thing the guy from Rode said was that if I wanted more accuracy, I could test the mic myself and generate my own graph. How would I do that and would it be easy enough / worth it? I know this is being picky but I just want to get it right though I imagine the graph above would be good enough to use. Thanks.

You simply need a space between the frequency and amplitude values.

Yeah, just use the log values from the graph. That would be accurate enough. When you're done, and you take a look at it in REW, if you want more resolution, just add it, no problem.
At the extremes, REW will continue the last value. For example, if at 20Hz you have a -2.00, then that value is used down to 0Hz. Above the highest value it will also continue the last value - but after 20Khz, you don't really care.

Definitely.

brucek

Great! I'll get down to it tomorrow, given it's 1.30 am here.

Again, I am very grateful for your help brucek.

No doubt I'll be back again with more questions when I've done the measurements!

Mark

Hi brucek

I've created the cal file but when I load it into REW I can't see how to adjust the frequency and amplitude values so they look the same as my graph. Also, the 0 amplitude point seems to be fixed at 75 and I can't see how to change that. How do I change the horizontal and vertical values so they look the same as my graph? Sorry if I'm being slow here; it's just I can't see if the graph matches my own in order to confirm whether or not I've created the cal file correctly.

Thanks,
Mark

Edit: I thik I've got it working well enough now. I tried to delete this post but couldn't see how..

And so it should. It revolves about the target level. If the target is 75, then that is zero.


I guess you figured it out. The Graph Limits icon popup can be used to set any X or Y axis....

Remember when actually taking measures and posting graphs to use a standard of 45dB-105dB vertical.

brucek

I don't understand that, i.e. what the 'target level' actually means but I probably just need to dig further into the programme and the help files to find out. I'm working on it tonight and in my spare time so hopefully I'll get to the actual measuring soon!

Will do. I'll read through all the faqs etc. on the forum to find out the guidelines and do my best to apply them.

Thanks brucek,
Mark

Hi brucek, or whoever else is reading this. I have done some measurements using both the RS 33-4050 meter and my Rode NT1 mic. Here are the results. I hope they're in the correct format; I used a wide frequency range, 20Hz - 20kHz in order to cover the full spectrum.

RS meter:

room measured with rs meter only.jpg


NT1 mic:

room measured with nt1b only.jpg

I didn't apply any smoothing. I'm not sure if that would have been a good idea or not(?)

I'm also not sure how to interpret these results so I would be grateful for any help. It looks to me like there are quite a few dips and peaks but I've no idea how good or bad a response this is. I won't be applying any eq; this is to find out how my already-treated room is responding and to make any necessary changes to that treatment. So once again, any recommendations around that would be welcome.

If I need to go back and change the format / appearance of these graphs and upload them again, please let me know. Also, would it be useful for me to post the impulse response graphs or waterfalls as well?

Many thanks,
Mark

Actually, I decided to post the waterfalls as well since the low frequency behaviour of my room is of particular interest, given I have put up quite a lot of bass traps.

RS meter:

rs meter waterfall.jpg

NT1 mic:

nt1b waterfall.jpg

How do these look?