This study is the first to provide valid information about the effects of music therapy in children with developmental speech delay. It was found that music therapy had an effect on fundamental qualities of speech development and resulted in significant improvements in phonological memory and the children's understanding of sentences. Furthermore, a positive shift in the memory of sentences and generation of morphological rules was observed. In particular, the difference between the developmental age and biological age of the children decreased significantly. These results were accompanied by a clinically significant effect of music therapy on the child-therapist relationship and musical communicative ability, as measured by the Nordoff-Robbins scales.
Empirical observations frequently described the phenomena of an "initial effect" of music therapy on speech development (i.e. that music therapy seemed to stimulate the speech development of developmentally delayed children even after a few music therapy sessions). In accordance with these findings, results from this pilot study illustrate which aspects of speech development are influenced specifically by music therapy and show to what extent an initial effect may be detected. Results show improvements in the development of speech and cognitive abilities even after the first block of music therapy treatment. Children may benefit specifically in the areas of relationship and communication as shown by the rating according to the Nordoff-Robbins scales.
It is assumed that a newly acquired ability, such as a basal social experience, has an initial effect on the further development of the sense of self-perception and the perception of others. Furthermore, it influences self-awareness and emotional mood. This 'snow ball effect' could possibly explain the relatively fast positive development of the participants. Since communication is a basic human need, it can be further assumed that the provided individual communication model 'music therapy' was simply claimed by the participants after a successful introduction and was also maintained.
The changes in phonological memory (PGN) and the understanding of sentences (VS) increased significantly with a parallel slope. Test results suggest that music therapy interventions may initially provide a boost in the development of these skills. Abilities covered by the subscale PGN are obviously related to prosodic abilities . We believe that the improvements occur because music therapy addresses listening, perception, processing, and the memorizing of sounds and musical structures. This corresponds to a study by Jungblut et al. , who reported positive improvements in speech development in patients with aphasia due to music therapy. Here, prosody was one of the parameters showing substantial progress. This corresponds with the theoretical considerations of Grimm , who regards prosody as a defining aspect of speech processing and language acquisition.
The improvement in the VS subscale is underlined by the empirical observations we made in the study. In the beginning, most of the children we worked with had difficulty focusing their attention with hand-eye-coordination and the concentration on a joint activity with the therapist. During music therapy sessions, most of the children enhanced their concentration and were increasingly able to direct their eyes and concentration to a joint activity with the therapist and to playing an instrument. The improvement in the Nordoff-Robbins scales especially underlines these observations. Children benefit specifically in the areas of forming relationships and enhancing their communication skills. Parents, speech therapists, and teachers reported that the children started to communicate more frequently and started to have more social contacts. According to a deeper single case analysis of two children in our study, we found that one important element in achieving linguistic understanding is the ability to relate to another person . Moreover, Grimm  describes three areas of so-called anticipatory abilities that are essential even in infants for acquiring speech: social cognition, perception, and cognition. These include the abilities to direct attention to objects and events, to differentiate between them, and to remember the differences. In addition, he determined that constructing a common point of focus has proved to be especially important in acquiring language skills.
The scores measured for SG also show a distinct increase over the study period. This may be explained through the inherent experience of structure and perspective in the process of active music making with the therapist. Studies of infants' abilities to perceive speech found that infants prefer well-structured speech patterns to less well-structured ones . Perception and grasp of structures seem to be important skills for acquiring speech. Form and general structure of a sentence must be understood in order to grasp the entire meaning. These qualities are exercised and targeted all the time when making music. They may be shortened or expanded step by step and thus be adapted to the individual making the music and his or her abilities.
It is interesting to note that the parameter "generation of morphological rules" shows improvements after music therapy blocks. This parameter, quite unrelated to music at first sight, seems to address recognition and understanding of structures, which is continuously practised in active music-making. Deficits in generation of morphological rules are considered as particularly distinct and obstinate. Music therapy may give support to the development of this ability.
Rhythmic-prosodic abilities seem to be central for acquiring language. Again, Grimm describes that children with developmental speech delay frequently display considerable difficulties in the rhythmic-prosodic area. An impaired ability to grasp the totality of prosodic structures means that larger parts of the working memory must be relied upon, thus limiting the amount of working memory available for the understanding and processing of language. Training a child's reproduction capabilities of phonologic (and therefore also of prosodic) structures could provide significant support in the child's development of language abilities .
Aldridge  emphasized the importance of rhythmic structures and abilities for infantile development: "rhythm plays a central co-ordinating role in the organization of human perception and action, and for the developmentally delayed child, a controlled - yet flexible - rhythmic structure found in musical playing seems to be an island of stability from which new initiatives can take place." According to Trevarthen & Aiken , music therapy from a neuropsychological point of view may support human communication skills that are organized rhythmically in accordance with neurological processes. Thus, active, creative music therapy works immediately with the contact and communication between the improvising participants. In such a setting, the integration of several senses, like hearing or seeing, motor abilities, and emotion, is of vital importance  because both verbal communication and joint musical improvisation require a meaningful integration of these senses. Thus, music therapy may offer a specific space to test and develop various senses on a level appropriate to the child's individual abilities and speed.
From a music therapy perspective, Neugebauer  relates the steps of language development to musical qualities and concludes that music therapy works on those musical qualities and speech development can thus be enhanced. These findings are underlined by Papousek's research on infants . She analyzed mother-infant interaction and its relation to musical parameters, even if the use of the musical metaphors has to be taken into account critically . This might be an explanation of why music therapy can be effective in children with communication disorders. The therapeutic processes that took place in our study have been described comprehensively in the case report of two patients .
The non-verbal development test SON-R produced encouraging results. The SON-IQ of the entire group rose significantly and the difference between developmental age and biological age of the children decreased significantly. Even here our study showed an initial effect. For a majority of children it may be assumed that their intelligence potential had not been fully exploited prior to the study. In music therapy we see many children who seem to have no experience whatsoever with symbolism, imagining fantasy stories, or playing with sounds. At first these children seem to soak up our imaginative-musical proposals before they start to develop and share their own creative potential. Music therapy thus seems to evoke and reveal unused potential. During music therapy, children seemed to access their potential and were even able to adapt it to another setting such as in the test situation. This corresponded to a detailed analysis by Rittelmeyer , who emphasized the impact of creative abilities in the neurological, cognitive, and emotional development of children.
An analysis of the two subtests for cognitive structures and action patterns is also of interest. At first, the two scales diverge, converge closely after the first waiting period in order to diverge again, and approach each other on a significantly higher level at the end of the study (see Figure 2). Cognitive structures are the first subtests to advance. Action patterns keep up with the development during the waiting period. Again an initial effect of music therapy can be assumed in this case. It is remarkable that action patterns converge to the similar level with cognitive structures. Cognitive structures and action patterns are nearly integrated at that point. A child cannot do much with certain cognitive abilities without knowing how to use them actively. An integration of thinking and doing appears to be indispensable for the meaningful use of cognitive abilities. Music therapy may therefore provide an important contribution to the promotion of integrated thinking and doing and may reveal a child's hidden potentials.
Although the large effect sizes in the present study point to a potential impact of music therapy, the small number of participants in the study should be mentioned as an important limiting factor. The discussion of whether observational studies tend to overestimate the effects of a therapy compared to the results of controlled clinical trials is still vital  and the call for randomized controlled trials (RCTs) has already reached the borders of music therapy . Due to organizational and structural aspects we abstained from carrying out a RCT in this case. However, other researchers recommend more naturalistic, observational studies of patients in psychotherapy  and, with regards to external evidence, our findings give an impression of the real world effectiveness of music therapy. In order to verify the test results, further studies with a higher number of participants should be conducted to underline and specify the effects found in this setting [35, 36].
As there is no control group or a specific control condition we are of course aware that the changes observed here might not be attributed to the music therapy applied. It could, for instance, simply be the intervention itself associated with a high amount of care giving or the attention given to the kids. A further point is that the children might have improved their behaviour simply in response to the fact that they were being studied. This is also known as the Hawthorne effect . In his study on the effects of background music on quality of sleep in elementary school children, Tan also suggested, that children might be responding to music therapy treatment due to their awareness of participation .
Also, unblinding might be a potential source of bias. According to a study of Noseworthy et al., blinded outcome evaluators do assess outcomes less optimistically than unblinded evaluators . Thus, we tried to avoid unblinding of the external evaluators in our study. Consequently, they were not introduced in the therapeutic strategy at any time of the study to rule out that they would for example focus on a special subcategory of the psychological tests like phonologic memory. However as the evaluators in the case of the speech and intelligence tests directly interacted with the children and did see them five times within the course of the study, unblinding can not be ruled out.
Due to financial limitations, it was not possible for external therapists to conduct therapeutic music therapy ratings. Although the findings on inter-rater reliability in  give sufficient assurance on the reliability of the Nordoff-Robbins rating, an external rating would have probably been of higher internal validity with a more objective character.
Considering these limitations, our results should be interpreted with care.
Underlying working principles
Until now, working principles of music therapy have mostly been examined using qualitative research. Although single case studies tried to identify possible modes of action in music therapy in speech development, we are still not able to isolate elements of music therapy as the driving therapeutic force. The only aspect that is for certain is that a mutually created musical dialogue improves the child's perception of him or herself and of the person who is sharing the experience. As a result, according to  the "activity of listening, in a structured musical improvisational context, without the lexical demands of language" may improve the cognitive, gestural, emotional, and relational development of the child.
Neuro-physiological approaches might underline these results but, due to limited resources, they have only been marginally applied in music therapy research. Additionally, it is questionable whether neuro-physiological approaches would be able to show development in areas such as social communication and self-consciousness, which are essential preconditions for language acquisition.
In addition, some important parameters for the development of language skills are not covered by the test parameters. Vital information was provided by the additional qualitative data collected from parents and testers. Most of the children started to use their language with increasing confidence; social skills improved and children were motivated to communicate more intensively. Although these data often did not correlate with the test results, these findings seem to be evidence of very important parameters that cannot be checked by the speech test and development tests alone. The Nordoff-Robbins scales, however, do reflect these aspects.