SPL is a professional-grade sound level meter for your iPhone. It incorporates all the features of a real SPL meter -- ANSI modes Slow, Fast, Impulse, Peak, and Leq, A and C weighting, Octave band filters, and the ability to be calibrated.
NOTE: If you plan to purchase iProMic or iAudioInterface, you should instead download AudioTools and buy this app as an in-app purchase.
See our new demo video for SPL Pro. This video was recorded on the simulator, within AudioTools.
Also, included in version 1.3 (Mar 2008), an iPhone frequency compensation filter. This filter flattens the frequency response of the SPL meter. Apple rolls off the low frequencies, presumably for the purpose of pop and wind filtering. There is also a presence hump, and a high frequency roll-off. We have done lab comparisons with a type 1 microphone, and have created a proprietary DSP filter to cancel out these effects. Thus, you get the performance of a much better SPL meter, in your iPhone.
We do NOT recommend the use of the built-in mic with the first generation iPhone, as the frequency response is very limited, unless you are using our iAudioInterface. See our iPhone hardware page for more information.
How to Use SPL
Here is the SPL screen, in the aluminum skin.
Mode
SPL supports four standard ANSI decay times. All times are in terms of exponential decay, where the reading will decay by 20dB over the specified time.
SLOW -- this mode uses a 1-second decay time. Good for getting a general reading of the ambient sound level.
FAST -- this mode uses a 125ms decay time.
IMPULSE -- not used very much, normally Peak mode would be used. This mode has a 35ms decay time.
PEAK -- Peak mode show the peak value received since the last screen update.
Also, there is a mode call Leq. This mode does an equal-weighted time average of the incoming SPL level. A timer shows how long the Leq has been running. Touch Reset to start the averaging over from the start.
Filter
You can select unweighted, which is wide-band; all frequencies are represented equally.
A-weighting rolls off lows and highs fairly severely, and can be compared to voice band.
C-weighting rolls off lows and highs gently, and is sometimes thought of as music weighting.
Octave Band filters, 31, 63, 125, 250, 500, 1k, 2k, 4k, 8k, and 16kHz. These are Type 1 ANSI filters that in effect isolate a single octave band. Often used for noise analysis, or to test masking signals.
Calibration
You can use the Trim field to adjust the dB level that is shown when a particular sound level is received. There are actually two factors (starting in version 1.2), one for the internal (built-in) mic, and another for the external mic (this refers to the iAudioInterface microphone, not the headphone mic).
The typical iPhone 3G built-in mic Trim value is +7.0 dB. If you have access to a calibrated Sound Level Meter (note: we do NOT recommend using a Radio Shack meter for this purpose!), you can compare the sound level read on the meter to the sound level shown on the dB field, and adjust the Trim field until you get a match.
NOTE: Make sure that you select the same weighting on both the sound level meter and the SPL app, ie A-Weighted.
Store Settings
You can store the current reading in any of 9 slots for later reference. To do this, tap the Store button on the main screen. This screen will appear:
Select the slot number, and tap the "Store Current Values" button to save the results as of the time that you tapped "Store" into this slot. The values are saved, and can be recalled at a later time.
The value and time and date of the reading, along with the settings in effect are all stored.
Skins
To optimize the use of SPL in different lighting conditions, we have provided three skin selections. Aluminum, shown above, Dark Metal:
And Plain:
iPhone Hardware
See our our page on iPhone hardware that explains the differences between hardware models, and the effects on performance of our apps.
Audio Test Equipment
If you are looking for more advanced audio and acoustics test equipment, check the the SoundPro line at www.Sencore.com