Recording speech data in the field is a challenging process – factors such as location, background noise and environment, can all impact the quality of data collected. The VariCS team sought to find a method of recording children’s speech in primary schools in Scotland that would allow us to conduct acoustic analysis. While there is plenty of information on recording speech and high-quality audio on the internet, and from research studies, but this information largely pertains to recording in a lab setting. It is surprising that not much information is available on recording in the field, especially child speech. Research that did involve child speech either do not provide information on the microphone, or not enough information to find a microphone that meet similar criteria. A secondary problem the team faced in finding an appropriate microphone involved the impact of Brexit on access to certain company mics. For example, Beyerdynamic is a commonly used company for recording speech, but we were unable to get these shipped from Germany or would not be able to avail customer support on certain products. With technology evolving rapidly, any recommendations we did receive of potential microphones we could use were no longer manufactured or sold.
The resulting search for an appropriate microphone for recording child speech in the field took three months of research exploring various avenues such as previous research, the internet, and making enquiries with other academics. Various options needed testing to find a system that was compatible with an iPad, was quick to setup in schools, and easy to transport.
Guide on selecting recording equipment for field research with children
The microphone used for the VariCS project is the Shure SM35 and the Zoom U-22 interface for compatibility with recording our data through an iPad. There are several considerations to selecting the appropriate equipment for data collection in the field, specifically when recording children. Below are the criteria that we used for the VariCS project, that could be useful to other researchers considering recording child speech data.
Type of microphone
The frequency response of microphones
The frequency response of the microphone used for recording data should be wide enough to accommodate the frequency that speech sounds are made. This typically ranges from 100 Hertz (Hz) to 10,000Hz (Stevens, 1998). Although voice frequencies typically lie between 300Hz to 3,400 Hz (Baken, 2000), other vocal characteristics of voice are produced outside this narrow range. Recording children will require a wider frequency response as children produce speech that is both very quiet, but also higher pitched compared to adults (Stemple, 2014) whose vocal tract has fully matured.
Microphones that come with a windscreen are preferable. A windscreen is a flexible cover made from foam that is placed on the microphone basket to protect it from unwanted sounds like windblasts while recording and, in turn, produce a better-quality sound.
The Shure SM35, used for the VariCS project, has a frequency response of 40 Hz to 20,000Hz with a snap-fit windscreen.
Type of microphone
The type of microphone selected for speech recordings will need to be fit-to-purpose. An interview with multiple people in one room could benefit from the use of an omnidirectional microphone that will pick up sound from all directions. However, speech data collected from an individual, and using tasks that rely on capturing input accurately will be best served by a cardioid microphone. A cardioid microphone is unidirectional and picks up sound only from the front of the microphone capsule. Therefore, the placement of a cardioid microphone will need to be directly in front of the source of sound.
You may also want to consider between a head-mounted, lapel, or tripod microphone. As with the choice between an omni- and unidirectional mic, a head-mounted microphone would be better for isolating a single voice, ensuring other noises or voices are reduced. Lapel microphones can be convenient and more comfortable than a head-mounted option but is more likely to pick up on movement from clothing. Tripod microphones are best used in a studio environment when there is more control over any ambient sounds.
The type or fit of the microphone will also impact its weight. Field research would naturally benefit from a microphone that is lightweight, portable, and easy to setup efficiently. The VariCS project required a microphone that could isolate a child’s voice, would remain stable throughout the data collection procedure, had a flexible headband to ensure the correct fit and placement of the mic, and was lightweight to avoid discomfort for the child. The Shure SM35 headset microphone has a tight, unidirectional (cardioid) pickup pattern which provides rejection of off-axis sound sources to prevent feedback and any signal bleeding during recordings.
Our initial pilot and data collection period showed that children found the microphone comfortable and were told it was similar to when gamers play online or like singers wear on stage. The resulting data was as expected for conducting acoustic analysis.
For more information on the different types of microphones, there is an excellent article that explains them: https://www.popsci.com/reviews/types-of-microphones/.
Zoom interface for iPad compatibility
One major criterion for the VariCS project was the microphone compatibility with an iPad as our data collection would be through a designed iPad app. As most condenser, cardioid microphones require an external source for power, they are usually connected to a “phantom power” or a mixer or interface that can power the mic. For the purpose of field research, the team wanted to ensure the equipment was easy to transport and would not require too much space. This removed the option for a power source that required mains electricity, so any interface that required to be plugged into the mains was not an option for our project. There are several portable interfaces that are compatible with windows and mac laptops, or android devices, but are incompatible with iOS mobile devices.
The Zoom U-22 is an interface that allows compatibility with iOS devices as long as appropriate apple adaptors are used. The interface can provide phantom power to a cardioid microphone via two AA batteries, however the caveat to using phantom power is that batteries will need to be regularly replaced and the interface does not have an indicator for this. The interface light will show “red” to indicate any sound clipping that is occurring, but the source could be either an adjustment with settings on the interface, or the batteries, so both will need to be checked.
Based on a successful pilot with the above recording equipment, the project began data collection in May 2023. Watch this space for updates on our data collection!
References
Baken, R. J. (2000). Clinical Measurement of Speech and Voice, 2nd Edition. London: Taylor and Francis Ltd. (pp. 177), ISBN 1-5659-3869-0. That in turn cites Fitch, J.L. and Holbrook, A. (1970). Modal Fundamental Frequency of Young Adults in Archives of Otolaryngology, 92, 379-382, Table 2 (p. 381).
Stemple J. C. Roy N. & Klaben B. (2014). Clinical voice pathology : theory and management (Fifth). Plural Publishing.
Stevens, K. N. (1998). Acoustic Phonetics. Cambridge, MA: The MIT Press.
