Frequency correction for 1/3 octave

HansJAvanLeeuwen

If you use the standard iPhone microphone, you'll have lower sensitivity at low frequencies. For example, at 80 Hz, this is about 4 dB and at 63 Hz, about 5 dB.

My question is whether it's possible to use a frequency correction every 1/3 octave. One solution is to use a file with this correction as a standard for a typical microphone.

Thanks in advance!
Greetings

benfaber

SoundMeter does not currently offer any kind of frequency response correction (FRC).

For correction of the low-end rolloff of an iPhone microphone response, 1/3-octave resolution may be insufficient for an acceptable solution. The steep spectral slope at the lowest frequencies would demand higher resolution for FRC, depending on the nature of the sound being measured and the desired accuracy of the corrected result.

Of course, SoundMeter results can be exported to a CSV or MAT file and manually corrected according to your needs in third party software.

HansJAvanLeeuwen

OK Thanks.

I found a graph of the frequency response in a blog which is very, very interesting!

The shape is what I would expect, but the magnitude drop-off at low frequencies is lower than I expected. When I look at the description on the axis, I see the ratio of magnitudes in Pa/Pa. Is this correct? At a lower magnitude, the value will be below 1. Perhaps these values are decibels, and are these decibels of pressure squared?

I am very interested and curious about your response.

Regards, Hans J.A. van Leeuwen

(Same comment as in Linkedin)

benfaber

Hans,

If you're referring to one of the iPhone microphone measurements I published on our blog (such as the recent iPhone 17 Pro measurements), then, yes, the magnitude is indeed expressed in decibels. The response is between 2 microphones which are calibrated to measure acoustic pressure in pascals (Pa). The response is unitless (since both signals have the same units of pascals), so 0 dB indicates a perfect match between the 2 microphones.

The low end rolloff of built-in iPhone microphones has been fairly consistent for at least the last 3 generations, with the response at -3 dB at about 50 Hz and falling to -12 to -15 dB at 20 Hz.

HansJAvanLeeuwen

Thanks!

But my question is about the 'magnitude Pa/Pa', which is on the Y-axis title of the graph. The dB relation is clear otherwhere you should have a positive value, 1 and a fraction value on Pa/Pa.
The question is on the Pa/Pa and/or Pa2/Pa2. The relation with energy (so on dB) is on Pascal squared and not on Pascal!
We found for example at 100Hz -2.5 dB and at 50 Hz -6 dB. This is your dB value times 2 so the Pa value squared….
So the question is whether you have really determined 20*log(p2/p2) (p is pressure in Pa)

benfaber

The units of frequency response (transfer function) between 2 microphones are Pa/Pa, not Pa²/Pa². The dB values you see in the graphs on our blog are correct (20*log10(Pa/Pa)). The measurements were made using a dual FFT analyzer.

How were your measurements made?

HansJAvanLeeuwen

Ben,

As you know 20log(p/p) is equal to (10log(p2/p2. So it is a relation on p2.......

(p is pressure in Pa, log is the log10 and p2 is p squared, superscript is nog working)

I have measurements of an old iPhone, so this is the real explanation!